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#1
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
This one's for Patrick in Oz.
Hi, Patrick, I've looked at your "standard" VLF stabilization coupling network for this amplifier class (0.05 parallel 1Mohm, with 220Kohm grid leak on the o/p tubes.) It appears to provide a forward gain roll-off below about 15 Hz with a shelf at about 1/6 of the normal gain (-15.5 dB) below about 3.2 Hz. Assuming my calculations are correct, what is the purpose of the 1 Mohm across the 0.05 uF? Surely just letting the roll- off continue at 6 dB/octave below 15 Hz would scotch any VLF oscillation. Is this a phase shift issue? The simple -6dB /octave roll-off leave a 90 degree phase advance in place below, say, 10 Hz whereas the shelf would appear to avoid that (I'm not certain as, regrettably, no pspice to hand at present!) I stabilized my Williamson clone with just 0.05 uF coupling caps to the 6L6's with an "aggressive" 100K grid leak (- 3dB at 32 Hz, but I may increase the 100K a bit....) Do I really need a VLF shelf below that? Thanks and cheers, Roger |
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#2
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Quote:
I've successfully used the parallel RC with differential amp driver to avoid the additional phase shift at LF. But with the split load phase inverter is a problem. Patrick, your thoughts? Cheers, John |
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#3
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jun 18, 2:39*am, Engineer wrote:
This one's for Patrick in Oz. Hi, Patrick, I've looked at your "standard" VLF stabilization coupling network for this amplifier class (0.05 parallel 1Mohm, with 220Kohm grid leak on the o/p tubes.) *It appears to provide a forward gain roll-off below about 15 Hz with a shelf at about 1/6 of the normal gain (-15.5 dB) below about 3.2 Hz. *Assuming my calculations are correct, what is the purpose of the 1 Mohm across the 0.05 uF? Surely just letting the roll- off continue at 6 dB/octave below 15 Hz would scotch any VLF oscillation. *Is this a phase shift issue? *The simple -6dB /octave roll-off leave a 90 degree phase advance in place below, say, 10 Hz whereas the shelf would appear to avoid that (I'm not certain as, regrettably, no pspice to hand at present!) *I stabilized my Williamson clone with just 0.05 uF coupling caps to the 6L6's with an "aggressive" 100K grid leak (- 3dB at 32 Hz, but I may increase the 100K a bit....) *Do I really need a VLF shelf below that? Thanks and cheers, Roger The Wiiliamson has two RC coupled stages and OPT in the loop surrounded by NFB so it is very prone to LF oscillations, especially if you have a preamp powered off the pwr amp PSU and then you try to boost the bass with a tone control network. Williamson stipulated that the OPT have at least 100H at Vaa = 5Vrms to get LF stability. Nearly all OPTs made since 1949 have been by ppl who thought Williamson was a ****** who made the amp cost of production too high. These ppl didn't use enough P turns or big enough core size so lots of Williamsons actually do oscillate at LF but it went un-noticed by the dumb DIYers of yesteryear because they had no oscilloscopes to see the small oscillations at below 10Hz. OPT core permeability rises with applied voltage and so does Lp so the amplitude of LF oscillations may be kept low if Lp rises enough to prevent any increase in amplitude. Many people used CR coupilng values which encouraged phase shift caused oscillations. This is the background theory of how the Williamson and many other tube amps become a phase shift oscillator at LF. In a williamson with V1 direct coupled to V2 concertina and CR coupled balanced amp with CR coupled OP tubes, the best place to put a shelving network is between concertina and balanced amp, and there must be TWO networks, one from concertina anode and one from cathode. OK, the 0.47 + 1M strapped with 0.047 + 220k acts like this :- At 1 kHz, all C have low Z so the phase shift is low and the driver, ie concertina or whatever you have "sees" a load of 220k. As F is reduced, there is a pole between 0.047 and 220 at 15.4Hz which is usually above the F at which the amp may want to oscillate. Sometimes I have used 0.022uF, so pole is at 32Hz. phase shift caused is less than 90 degrees. As F is further reduced, the network response tends to flatten to a shelf formed by 1M and 220k, ie, signal flattens at roughly -15db, 0.18 times the 1kHz level. The 0.047 has gone to an open circuit by 2Hz with little effect. But the 0.47 the rolls the response off at the pole between 0.47uF and 1.22M ohms, ie at 0.27Hz, below which there is an ultimate phase shift of 90d, but at such a low F as to not cause bothers because its well below the poles of all other sages. The Williamson amp published in August 1949 has CR before balanced amp with 0.05uF + 470k and before KT66 0.25uF + 100k. So poles are at 6.76Hz an 6.36Hz and so by 3Hz phase shift because of the two networks would have been about 120d and with another 60d from OPT, maybe it oscillated if the OPT didn't have enough Lp, and open loop gain 1 where 180d phase shift exists. Ppl tend to try using larger C values only to find oscillation frequency Fo just goes lower, or smaller C and Fo rises. But the GAIN at where Fo is likely to occur must be lowered. The only good way is with a shelving network. This means the open loop gain without GNFB, OLG, seems to roll off at say -3dB at 30Hz and ppl panic and say "OMG, I've lost the bass." But when they connect the GNFB its nice and stable and -3db occurs at 7Hz and there is no peak in the response below 30Hz. What is happening with GNFB applied is that there is 20dB NFB applied to all F down to about 35Hz below which the amount of NFB is reduced. Maybe only 10dB at 10Hz, and hardly any at 5Hz, because OLG has been reduced so much by the shelving network. But there is ENOUGH NFB being applied at 10Hz to still get amp Rout quite low enough, and reduce THD etc. Nobody has much of anything below 20Hz in music so signals below 20Hz are tiny so they don't create much THD/IMD so lots of NFB at 10Hz is NOT required. This is especially valid where the OP stage is a triode type which has Rout RL even with no GNFB. Williamson's original KT66 triodes had Ra-a of 3k2, plus maybe Rw = 400 ohms, so DF without NFB = 10k/3k6 = 2.77, and not bad, needing only 10dB NFB to get DF to 10. On page 346, RDH4, there is a Wlliamson with 807 in beam tetrode mode which is VERY LIKELY to oscillate with the higher tetrode gain. CR networks have 0.05 + 470k and 0.05 + 500k before 807. Looks like an oscillator to me! The Williamson rule was that there should be 100H for a load of 10k, even at low Va-a levels. This meant RLa-a = Lp reactance at 15.9Hz. In otherwords the amp will show less than 1 dB of response roll off due to load reduction at 15.9Hz. Everyone mostly ROTFL at Willy, and they made OPTs where ZLp = RL at 40Hz at low Va-a, and RLa-a was raised to say 5k and tubes pushed into class AB with little class A, LF fidelity dissappeared, power doubled though, bean conters were happy, and the brandname sold yet another fraud to the public. One can build an amp with only 20H of Lp, and the NFB will desperately try to compensate the response towards flatness but don't expect the best bass performance at loud levels and without core saturation effects. Unless shelving networks are placed in such amps they will oscillate for sure. Some makers avoided the issue by using only two amp stages, ie, like Quad-II. This removed one of the 3 places where there could be an ultimate 90d phase shift at F which is too high, ie, where OLG 1.0 and where total OLG phase shift 180d. So Quad-II has a Tiny Toy OPT with 3,180 turns around a core Afe = 25mm x 25mm, while the Williamson original had 4,400t around 44 x 32. I'd suggest the Quad-II has a lot less Lp than Williamson does, and I know that the Quad-II saturates at 49Hz at 420Va-a while the Williamson saturates at 15Hz at 420Va-a. The ratio of Lp to RL in the original Williamson is much BETTER than in the Quad-II. But Quad-II have 0.1uF + 680k between EF86 drivers and KT66, so as years pass Eg1 goes positive without the 100k needed to stop the Eg1 rise. The Rout of EF86 is rather high, determined by the 180k and 680k and EF86 Ra all in parallel, about 140k, so the pole in CR is at 1.9Hz, and low enough. Walker couldn't use less tha 680k for KT66 Rg because that'd load the EF86 down an prevent the gain he needed. So QUad-II has a really bad bodgie designed network in place. The triode driver of the williamson allows the Rg = 100k without gain loss, and the low Rg makes the KT66 last a long time. Leak also had bad design problems. None of those old British brands got everything right. Most of what I have said is incomprehensible. I bet you are entirely baffled. You'll never learn until you build and measure everything while asking questions and while sitting with a dual trace oscilloscope to SEE the phase shifts and gain changes in networks. Its real basic stuff and to build good amps you must know all about it. After building 10 amps, maybe the penny will drop. Most Diyers build one amp, and then forget the way they muddled through the process. Read this page carefully, http://www.turneraudio.com.au/basic-tube-%283%29.htm Its about time I renovated the page. I dunno how the "-%283%29" got into the title line, years go by, I change ISP, and **** happens to titles and text and formatting. I am re-editing pages now, lots to do. Patrick Turner. |
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#4
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jun 18, 12:24*pm, flipper wrote:
On Sat, 18 Jun 2011 00:19:29 +0000, John L Stewart wrote: 'Engineer[_2_ Wrote: ;933456']This one's for Patrick in Oz. Hi, Patrick, I've looked at your "standard" VLF stabilization coupling network for this amplifier class (0.05 parallel 1Mohm, with 220Kohm grid leak on the o/p tubes.) *It appears to provide a forward gain roll-off below about 15 Hz with a shelf at about 1/6 of the normal gain (-15.5 dB) below about 3.2 Hz. *Assuming my calculations are correct, what is the purpose of the 1 Mohm across the 0.05 uF? Surely just letting the roll- off continue at 6 dB/octave below 15 Hz would scotch any VLF oscillation. *Is this a phase shift issue? *The simple -6dB /octave roll-off leave a 90 degree phase advance in place below, say, 10 Hz whereas the shelf would appear to avoid that (I'm not certain as, regrettably, no pspice to hand at present!) *I stabilized my Williamson clone with just 0.05 uF coupling caps to the 6L6's with an "aggressive" 100K grid leak (- 3dB at 32 Hz, but I may increase the 100K a bit....) *Do I really need a VLF shelf below that? Thanks and cheers, Roger One needs to be a bit careful with these cct values since they may be in order for Patrick T's OPTs. Your Hammond will probably benifit from a shelf at a higher f since Pat Ts transformers are sure to have a much larger primary inductance. The loudspeaker resonance will complicate things since it is in the same region you are trying to fix. You're missing his question, which is why 'stop' the roll off with a shelf? Why not just let it continue since we don't intend to amplify anything that low anyway? Roger does not appear to deeply understand the cause of problems due to phase shift. Its easier for me to explain my circuit techiniques and ideas rather than to labour away and say why someone is wrong. Once ppl learn **at least** what I know, they'll ALWAYS have completely stable stable amps with fabulous bass sound. Even if an amp is stable without the shelving network, shelving should be used especially where there is some peaking in the region where "we don't intend to amplify anything that low anyway" as you say. The shelving improves overload charactecter and saturation effect behaviour. I will never ever say you too need to learn a lot more. There, I didn't say it now did I? Patrick Turner. |
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#5
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
"flipper" wrote in message ------------------- Even if an amp is stable without the shelving network, shelving should be used especially where there is some peaking in the region where "we don't intend to amplify anything that low anyway" as you say. The shelving improves overload charactecter and saturation effect behaviour. Yes, I'm sure the OP will appreciate improved overload characteristics at frequencies that will never be presented to the amplifier. Do you want rumble (from say a pick-up playing a warped record) driving your amp to saturation, if your amp is close to LF oscillation? |
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#6
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
"Engineer" wrote in message ... This one's for Patrick in Oz. Hi, Patrick, I've looked at your "standard" VLF stabilization coupling network for this amplifier class (0.05 parallel 1Mohm, with 220Kohm grid leak on the o/p tubes.) It appears to provide a forward gain roll-off below about 15 Hz with a shelf at about 1/6 of the normal gain (-15.5 dB) below about 3.2 Hz. Assuming my calculations are correct, what is the purpose of the 1 Mohm across the 0.05 uF? Surely just letting the roll- off continue at 6 dB/octave below 15 Hz would scotch any VLF oscillation. Is this a phase shift issue? The simple -6dB /octave roll-off leave a 90 degree phase advance in place below, say, 10 Hz whereas the shelf would appear to avoid that (I'm not certain as, regrettably, no pspice to hand at present!) I stabilized my Williamson clone with just 0.05 uF coupling caps to the 6L6's with an "aggressive" 100K grid leak (- 3dB at 32 Hz, but I may increase the 100K a bit....) Do I really need a VLF shelf below that? Thanks and cheers, Roger Your agressive LF frequency compensation might cause undesirable effects. 1. 100K bias is bit too low -- loading preceeding stage (driver, phase splitter) unnecessarily. Perhaps you can achieve the same result by 220K and 0.022uF coupling. 2. By agressively cutting LF from 32Hz you reduce loop gain in the working range (20Hz) thus increasing output impedance, reducing speaker damping, increasing intermodulation and distortion (if it matters at 20Hz?) This is aggravated by the fact that the driver stage has to labour 3dB harder at 20Hz because of the attenuation in your agressive circuit.. So I bet you will get overall results twice worse than Mr Turner would have done with his smart shelving. 3. Though your amp might appear stable, most likely it will be peaking close to oscillation at 8Hz or so where the +90deg lead from your agressive circuit will combine with +90deg lead from your OPT at the 0dB loop gain crossing. Any LF rumble might drive your amp into overload. So I need to admit, shelving is wiser, because it does not take out precious dBs from the loop gain in the whole audio range and gives a better phase margin, and no peaking. Mr Turner makes a deep 12dB/octave nose dive below 15...20Hz by combining OPT effect with 220K/0.05uF effect, but then, closer to 0dB crossing, he gently goes out of the deep nose dive, shelves the beast out and happily crossing 0dB at 8dB/octave. Past that he can dive deep again -- does not carem as no oscillation will occur below 0dB. However, both shelving and "agressive" compensation have one drawback in common. NFB gain is constant at all frequencies since NFB is simply a resistive divider. Thus the NFB tries to have the amp's responce flat (at low level of course) down to possibly 3Hz or so. This is crasy and unnecessary. For that reason, instead of shelving interstage coupling I shelf the NFB. Instead of a resisive divider I would put a resistor (typically 100R) in series with a capacitor (order of 47...100 uF). Interstage coupling is kept a simple semi-agressive RC circuit (say 0.022uF and 330K). Overall loop gain shelving effect is the same as in Mr Turner desihns, but because of the deepening feedback at VLF the amp becomes a rumble filter itself. A drawback of that method is that you need an electrolytic in the feedback, Some people do not "trust" electrolytics as a frequency shaping components. Regards, Alex |
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#7
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jun 18, 6:02*pm, "Alex Pogossov" wrote:
"flipper" wrote in message ------------------- Even if an amp is stable without the shelving network, shelving should be used especially where there is some peaking in the region where "we don't intend to amplify anything that low anyway" as you say. *The shelving improves overload charactecter and saturation effect behaviour. Yes, I'm sure the OP will appreciate improved overload characteristics at frequencies that will never be presented to the amplifier. Do you want rumble (from say a pick-up playing a warped record) driving your amp to saturation, if your amp is close to LF oscillation? And its not uncommon for there to be 10% difference in Ia at idle for each output tube and then any very low frequencies can easily push the core into saturation and distortion. So what is wanted is a reduction of gain at such very low F below 15Hz, and the NFB cannot become a real pest at such LF. I'll always get blasted by those who think shelving networks are a bodgie way of building tube amps, they have no other better solutions though. Make the OPT better they scream. OK, then you want it to weigh tonne? OK, the screamers have crummy OPTs pulled from something designed by a bean counter and unless they understand critical damping and shelving networks they'll always have an inferior amp. Often it takes me 1/2 a day to get a given amp channel optimised experimentally, with my mind driven by the metal picture, not guesswork. If the whingers are so ****ed off about me saying you gotta do this, you gotta do that, and they just can't understand, they can always take the contraption they have assembled to someone who does know about these things. And the HF shelving networks and Zobel networks are also very important to get right. Its not uncommon for OP tubes to give bursts of RF when the F goes real low ans saturation is approached. AND nobody wants the amp to break into strong RF oscillations during a bias failure event. **** DOES happen. Patrick Turner. |
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#8
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jun 18, 7:01*pm, "Alex Pogossov" wrote:
"Engineer" wrote in message ... This one's for Patrick in Oz. Hi, Patrick, I've looked at your "standard" VLF stabilization coupling network for this amplifier class (0.05 parallel 1Mohm, with 220Kohm grid leak on the o/p tubes.) *It appears to provide a forward gain roll-off below about 15 Hz with a shelf at about 1/6 of the normal gain (-15.5 dB) below about 3.2 Hz. *Assuming my calculations are correct, what is the purpose of the 1 Mohm across the 0.05 uF? Surely just letting the roll- off continue at 6 dB/octave below 15 Hz would scotch any VLF oscillation. *Is this a phase shift issue? *The simple -6dB /octave roll-off leave a 90 degree phase advance in place below, say, 10 Hz whereas the shelf would appear to avoid that (I'm not certain as, regrettably, no pspice to hand at present!) *I stabilized my Williamson clone with just 0.05 uF coupling caps to the 6L6's with an "aggressive" 100K grid leak (- 3dB at 32 Hz, but I may increase the 100K a bit....) *Do I really need a VLF shelf below that? Thanks and cheers, Roger Your agressive LF frequency compensation might cause undesirable effects. 1. 100K bias is bit too low -- loading preceeding stage (driver, phase splitter) unnecessarily. Perhaps you can achieve the same result by 220K and 0.022uF coupling. But with 6SN7, and dc carrying RL = say 39k then total RL = 28k, and this is about 3 x Ra of the tube which is fine. Bias R up to 150k in OP stage is about right. I've seen too many tubes with several volts dc across the high value bias resistance, so the tube is being turned on by this positive bias and things only get worse if the tube heats up more - there is a positive bias effect. Just using 220k and 0.022uF from driver anode to output grid gives pole at 32Hz, and 88d phase shift at say 8 Hz, maybe it oscillates. 2. By agressively cutting LF from 32Hz you reduce loop gain in the working range (20Hz) thus increasing output impedance, reducing speaker damping, increasing intermodulation and distortion (if it matters at 20Hz?) This is aggravated by the fact that the driver stage has to labour 3dB harder at 20Hz because of the attenuation in your agressive circuit.. So I bet you will get overall results twice worse than Mr Turner would have done with his smart shelving. The shelving leaves some gain available and reduces phase shift. So usually there is enough FB operative at 20Hz, and with lower phase shift at 20Hz its more effective than otherwise. 3. Though your amp might appear stable, most likely it will be peaking close to oscillation at 8Hz or so where the +90deg lead from your agressive circuit will combine with +90deg lead from your OPT at the 0dB loop gain crossing. Any LF rumble might drive your amp into overload. That's why I have referred him to some typical response graphs at my website. So I need to admit, shelving is wiser, because it does not take out precious dBs from the loop gain in the whole audio range and gives a better phase margin, and no peaking. Mr Turner makes a deep 12dB/octave nose dive below 15...20Hz by combining OPT effect with 220K/0.05uF effect, but then, closer to 0dB crossing, he gently goes out of the deep nose dive, shelves the beast out and happily crossing 0dB at 8dB/octave. Past that he can dive deep again -- does not carem as no oscillation will occur below 0dB. However, both shelving and "agressive" compensation have one drawback in common. NFB gain is constant at all frequencies since NFB is simply a resistive divider. Thus the NFB tries to have the amp's responce flat (at low level of course) down to possibly 3Hz or so. This is crasy and unnecessary. Indeed. The trick is to get the shelving right, neither over done or uderdone. Same goes for HF shelving. The other way to do LF shelving is to reduce the amount of NFB at LF so you have a parallel network of C&R in series with the FB resistance in the NFB divider network. I've never needed to use this method. Once a shelving network is connected and a plotted F response shoes no peaking outside the 20Hz to 20kHz band, it is wise to plot the response at the anode output of V1 just before any shelving networks in the input/driver line up between 1Hz and 1MHz. The signal from V1 output to OP tube grids is called the ERROR SIGNAL, because it contains a pure version of the music signal PLUS a fraction of a phase inverted version of the THD/IMD phase shift and any other artifact generated by the amp. The Error Signal will never be a flat response except in the middle of the AF band. The peaks in this signal at the ends of the band and beyond the band should ideally not exceed 3dB above the level in the centre of the band. Such peaking is inevitable in most amps as the NFB causes more signal to be applied at band ends to maintain the output level as flat. But never should the peaking ever cause any input tube or driver tubes to become overloaded, work into grid current, become cut off, etc. Hence driver amps should always be able to make TWICE the voltage one needs at the OP grids. When testing with a 5kHz square wave, then you may see some huge peaks appear in the error signal. And especially at the anodes of OP tubes. The amp is having troubles dealing with HF. The shelving tends to prevent the amp from bothering to fix the HF part if square waves above 20kHz, which is fine, lots of NFB above 20kHz, say at 60kHz, a typical HF oscillation frequency, is entirely pointless. For that reason, instead of shelving interstage coupling I shelf the NFB. You have read my mind a bit. Instead of a resisive divider I would put a resistor (typically 100R) in series with a capacitor (order of 47...100 uF). But what of the phase shift of that C? Isn't it better to have R&C in parallel inserted from FB take off at OPT sec to the feedback R? Say you have 1k0 and 100r as the normal FB divider so that 1/11 of the OPT signal is applied to V1 cathode. Say one adds 3k3 so you then have 3k3, 1k0, then 100r at k to 0V at V1. Then ß becomes 0.022, much less than 0.09, and at very low F there is no phase shift, so with less NFB its probably going to be stable. But there isn't enough FB at higher F so you shunt the 3k3 with say 6.8uF. So at 100Hz the 6.8uF = 233 ohms reactance and 3k3 is well shunted. Peaking still has to be checked in output and in error signal. Interstage coupling is kept a simple semi-agressive RC circuit (say 0.022uF and 330K). Overall loop gain shelving effect is the same as in Mr Turner desihns, but because of the deepening feedback at VLF the amp becomes a rumble filter itself. A drawback of that method is that you need an electrolytic in the feedback, Some people do not "trust" electrolytics as a frequency shaping components. You can use 63V rated plastic caps OK. I doubt you need 47uF caps. Switched networks in FB loops were often used for tone controls, not my scene in power amps though. Patrick Turner. Regards, Alex- Hide quoted text - - Show quoted text - |
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#9
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
"Patrick Turner" wrote in message ... Patrick: But what of the phase shift of that C? Isn't it better to have R&C in parallel inserted from FB take off at OPT sec to the feedback R? Say you have 1k0 and 100r as the normal FB divider so that 1/11 of the OPT signal is applied to V1 cathode. Say one adds 3k3 so you then have 3k3, 1k0, then 100r at k to 0V at V1. Then ß becomes 0.022, much less than 0.09, and at very low F there is no phase shift, so with less NFB its probably going to be stable. But there isn't enough FB at higher F so you shunt the 3k3 with say 6.8uF. So at 100Hz the 6.8uF = 233 ohms reactance and 3k3 is well shunted. Peaking still has to be checked in output and in error signal. Alex: I suggested something quite opposite. The NFB divider looks like: - 1K from the speaker terminal to cathode of the driver stage; - 100R from the cathode to 100uF capacitor; - the other end of the 100uF capacitor is tied to GND. Thus the feedback "beta" increases (!) at low frequencies (below 10Hz in this case), giving -90deg phase lag in the loop below 10Hz. This is in effect turning the amp into a anti-rumble filter. From the first glance it might sound crazy to increase the loop gain at VLF where we want an overal reduction of yje loop gain, but consider this: - OPT typically gives +90deg lead below 15...20Hz; - interstage coupling (simple RC with no shelving, 220K and 0.033uF) is calculated to give -3dB corner at say 15...20Hz and also gives +90 deg lead below; - but this "funny" NFB with a 100R and 100uF gives -90deg LAG below 10Hz! And this lag maintains down until 1Hz! At LF one lag subtracts from two leads and in combination we have only +90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be crossed safely at only +90deg! Killing many birds with one stone: - perfect transient with no peaking; - natural low cut off in the whole amp (sort of built-in anti-rumble filtering); - reduced error signal at VLF as more feedback is applied; - no need to use two identical shelving circuits in push-pull amps -- just one extra electrolytic. A drawback - a despised electrolytic as a shaping component. |
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#10
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jun 18, 9:56*pm, "Alex Pogossov" wrote:
"Patrick Turner" wrote in message ... Patrick: But what of the phase shift of that C? Isn't it better to have R&C in parallel inserted from FB take off at OPT sec to the feedback R? Say you have 1k0 and 100r as the normal FB divider so that 1/11 of the OPT signal is applied to V1 cathode. Say one adds 3k3 so you then have 3k3, 1k0, then 100r at k to 0V at V1. Then ß becomes 0.022, much less than 0.09, and at very low F there is no phase shift, so with less NFB its probably going to be stable. But there isn't enough FB at higher F so you shunt the 3k3 with say 6.8uF. So at 100Hz the 6.8uF = 233 ohms reactance and 3k3 is well shunted. Peaking still has to be checked in output and in error signal. Alex: I suggested something quite opposite. The NFB divider looks like: - 1K from the speaker terminal to cathode of the driver stage; - 100R from the cathode to 100uF capacitor; - the other end of the 100uF capacitor is tied to GND. My apologies, I didn't understand you. And the reason I didn't understand was because the technique you are suggesting is completely unknown in the range of traditional means of applying NFB in tube amps. Your method here is normal procedure in solid state amps which are all dc coupled and without any large phase shifts at very low frequencies down to DC. Thus the feedback "beta" increases (!) at low frequencies (below 10Hz in this case), giving -90deg phase lag in the loop below 10Hz. This is in effect turning the amp into a anti-rumble filter. Yes, I see you are against rumble. But in any tube amp with 2 CR couplings and an OPT the gain below 10Hz quickly falls to zero at DC and there is no need to roll off LF any more than it naturally is rolled off. So if a signal at 1Hz enters the amp, the FB fed back is extremely small because the output signal is so small. So I cannot see your method would improve LF behaviour. I've read most of the texts about NFB applications in tube amps and I don't recall a single instance where your idea has been applied by any commercial manufacturer nor have I read of anyone supporting it. From the first glance it might sound crazy to increase the loop gain at VLF where we want an overal reduction of yje loop gain, but consider this: - OPT typically gives +90deg lead below 15...20Hz; - interstage coupling (simple RC with no shelving, 220K and 0.033uF) is calculated to give -3dB corner at say 15...20Hz and also gives +90 deg lead below; - but this "funny" NFB with a 100R and 100uF gives -90deg LAG below 10Hz! And this lag maintains down until 1Hz! Well, your NFB method does not increase OLG. OLG remains what it is regardless of the NFB network. But th closed loop gain, CLG, could be boosted with positive FB. 3 lots of reactive phase advances in the amp will add to a rapid phase turnover per octave below 10Hz. So maximum phase shift can be more than 180d then because the Lp inductance falls to near zero near dc, you have only dc winding resistance in the OP stage so phase shift lessens a bit towards DC. But exactly what anyone measures depends on voltage applied and fiddling around with trying to increase NFB as one gets close to DC makes no sense to me at all. When you have measured and demonstrated your technique and analysised it all with maybe 10 detailed pages on a website with photos of the amps, then ppl might say you have something to offer. People here are very difficult people. We cannot agree with anything anyone says unless they offer the truth, the whole truth, and nothing but the truth, so help them in the eyes of the God Of Triodes :-) At LF one lag subtracts from two leads and in combination we have only +90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be crossed safely at only +90deg! I'll believe works when I see it. Killing many birds with one stone: - perfect transient with no peaking; - natural low cut off in the whole amp (sort of built-in anti-rumble filtering); - reduced error signal at VLF as more feedback is applied; - no need to use two identical shelving circuits in push-pull amps -- just one extra electrolytic. A drawback - a despised electrolytic as a shaping component. Perhaps there are other drawbacks you have not thought about. I suggest you embark on a course of soldering in your laboratory to prove your idea works. We all look forward to results. Patrick Turner. |
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#11
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jun 18, 7:22*pm, Patrick Turner wrote:
On Jun 18, 9:56*pm, "Alex Pogossov" wrote: "Patrick Turner" wrote in message .... Patrick: But what of the phase shift of that C? Isn't it better to have R&C in parallel inserted from FB take off at OPT sec to the feedback R? Say you have 1k0 and 100r as the normal FB divider so that 1/11 of the OPT signal is applied to V1 cathode. Say one adds 3k3 so you then have 3k3, 1k0, then 100r at k to 0V at V1. Then ß becomes 0.022, much less than 0.09, and at very low F there is no phase shift, so with less NFB its probably going to be stable. But there isn't enough FB at higher F so you shunt the 3k3 with say 6.8uF. So at 100Hz the 6.8uF = 233 ohms reactance and 3k3 is well shunted. Peaking still has to be checked in output and in error signal. Alex: I suggested something quite opposite. The NFB divider looks like: - 1K from the speaker terminal to cathode of the driver stage; - 100R from the cathode to 100uF capacitor; - the other end of the 100uF capacitor is tied to GND. My apologies, I didn't understand you. And the reason I didn't understand was because the technique you are suggesting is completely unknown in the range of traditional means of applying NFB in tube amps. Your method here is normal procedure in solid state amps which are all dc coupled and without any large phase shifts at very low frequencies down to DC. Thus the feedback "beta" increases (!) at low frequencies (below 10Hz in this case), giving -90deg phase lag in the loop below 10Hz. This is in effect turning the amp into a anti-rumble filter. Yes, I see you are against rumble. But in any tube amp with 2 CR couplings and an OPT the gain below 10Hz quickly falls to zero at DC and there is no need to roll off LF any more than it naturally is rolled off. So if a signal at 1Hz enters the amp, the FB fed back is extremely small because the output signal is so small. So I cannot see your method would improve LF behaviour. I've read most of the texts about NFB applications in tube amps and I don't recall a single instance where your idea has been applied by any commercial manufacturer nor have I read of anyone supporting it. From the first glance it might sound crazy to increase the loop gain at VLF where we want an overal reduction of yje loop gain, but consider this: - OPT typically gives +90deg lead below 15...20Hz; - interstage coupling (simple RC with no shelving, 220K and 0.033uF) is calculated to give -3dB corner at say 15...20Hz and also gives +90 deg lead below; - but this "funny" NFB with a 100R and 100uF gives -90deg LAG below 10Hz! And this lag maintains down until 1Hz! Well, your NFB method does not increase OLG. *OLG remains what it is regardless of the NFB network. But th closed loop gain, CLG, could be boosted with positive FB. 3 lots of reactive phase advances in the amp will add to a rapid phase turnover per octave below 10Hz. So maximum phase shift can be more than 180d then because the Lp inductance falls to near zero near dc, you have only dc winding resistance in the OP stage so phase shift lessens a bit towards DC. But exactly what anyone measures depends on voltage applied and fiddling around with trying to increase NFB as one gets close to DC makes no sense to me at all. When you have measured and demonstrated your technique and analysised it all with maybe 10 detailed pages on a website with photos of the amps, then ppl might say you have something to offer. People here are very difficult people. We cannot agree with anything anyone says unless they offer the truth, the whole truth, and nothing but the truth, so help them in the eyes of the God Of Triodes :-) At LF one lag subtracts from two leads and in combination we have only +90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be crossed safely at only +90deg! I'll believe works when I see it. Killing many birds with one stone: - perfect transient with no peaking; - natural low cut off in the whole amp (sort of built-in anti-rumble filtering); - reduced error signal at VLF as more feedback is applied; - no need to use two identical shelving circuits in push-pull amps -- just one extra electrolytic. A drawback - a despised electrolytic as a shaping component. Perhaps there are other drawbacks you have not thought about. I suggest you embark on a course of soldering in your laboratory to prove your idea works. We all look forward to results. Patrick Turner. Thanks for all the valuable comment. Here's what I've gleaned from this so far... A VLF shelf has merit in that it leaves some Global NFB at VLF and mitigates the -90 deg phase shift at critical instability frequencies that adds to the -90 deg in the OPT and the -90 in the other RC coupler... easy to get to 180 ! This suggest use large C's (and normal Rg's) inside the NFB loop except for ONE shelf network. My Fisher OPT is presumably "medium quality" (is that more or less than the Hammond 1620, 1650F, 1645, etc. series?) in that it came from a claimed 30 watt RMS Fisher amp (model KX-200PP, not mine... OPT bought on eBay.) My 100K grid resistor on the 6L6's is too low... I'll up it to 220 Kohms and recalculate the C for the shelf I need. Rumble will be cut off in the pre-amp so none hits the PA. BTW, I do know about phase shift... also gain and phase margins, Nyquist diagrams, PID control, feedback controller tuning and stability, open and closed loop Bode plots... and the whole ball of wax of stochastic sampled data control (off topic!), as I was a practicing control systems engineer for some 30 years. But the last 18 years were spent in engineering management (pays better!), so you get a bit rusty, but I still knew when any staffers tried to BS me... fortunately, not often, as I never forgot the basics! Now retired. However, tube audio is pretty straightforward in principle but, of course, needs close design attention or it will bite you. Again, my thanks to all, particularly Patrick. We shall overcome! Cheers, Roger |
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#12
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
"Patrick Turner" wrote in message news:2169d98c-5935-4e7c- When you have measured and demonstrated your technique and analysised it all with maybe 10 detailed pages on a website with photos of the amps, then ppl might say you have something to offer. People here are very difficult people. We cannot agree with anything anyone says unless they offer the truth, the whole truth, and nothing but the truth, so help them in the eyes of the God Of Triodes :-) At LF one lag subtracts from two leads and in combination we have only +90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be crossed safely at only +90deg! I'll believe works when I see it. Killing many birds with one stone: - perfect transient with no peaking; - natural low cut off in the whole amp (sort of built-in anti-rumble filtering); - reduced error signal at VLF as more feedback is applied; - no need to use two identical shelving circuits in push-pull amps -- just one extra electrolytic. A drawback - a despised electrolytic as a shaping component. Perhaps there are other drawbacks you have not thought about. I suggest you embark on a course of soldering in your laboratory to prove your idea works. We all look forward to results. Alex: I am not into building audio tube amps, because it is crazy to do while SS works (or can potentially work) much better in all respects, apart from creating a warm fuzzy feeling. But I restore and improve radios, and use this feedback increase at LF technique. Of course there is no rumble in an AM receiver, but fadings, beating of two stations on close but not synchronised carriers and simply skimming the band makes lots of VLF. As you know to minimise AF load on the AM detector, it is not uncommon to have grid leak in the first audio stage of 10M with 0.05uF of coupling cap. This huge time constant makes the first tube (6SJ7, e.g.) virtually open to DC. VLF undulations result in 30V swings on the plate of the 1-st stage bringing it close to saturation. Part of this reaches the grid of the power stage (say, 6V6) unnecessarily swinging its bias and plate current. The later in turn cause undulations on the lightly filtered +B rail, and even might fall into resonance with the supply CLC filter. I am not very impressed with the shelving approach, because it is only an attenuator. While it would cut off 6V6 grid excursions, it will not prevent from 6SJ7 front overloading. Instead I use a circuit which helps reduce VLF voltage applied between grid and cathode of the first stage 6SJ7. You would say: Use a low cut-off filter. But it is a complication. Much better is to turn the amp into an active high pass filter by means of the NFB. |
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#13
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
snip for brevity,
Perhaps there are other drawbacks you have not thought about. I suggest you embark on a course of soldering in your laboratory to prove your idea works. We all look forward to results. Patrick Turner. Thanks for all the valuable comment. *Here's what I've gleaned from this so far... A VLF shelf has merit in that it leaves some Global NFB at VLF and mitigates the -90 deg phase shift at critical instability frequencies that adds to the -90 deg in the OPT and the -90 in the other RC coupler... easy to get to 180 ! *This suggest use large C's (and normal Rg's) inside the NFB loop except for ONE shelf network. Basically, you have it right. measurements of response with and without GNFB and networks should confirm the ideas I have presented. My Fisher OPT is presumably "medium quality" (is that more or less than the Hammond 1620, 1650F, 1645, etc. series?) in that it came from a claimed 30 watt RMS Fisher amp (model KX-200PP, not mine... OPT bought on eBay.) Most OPTs in generic mass made stuff is not state of the art. Its usually well short of that, like Quad-II which saturates at too high a frequency and which has very high winding losses. Nice effort by Quad though, real nice, just marred by using wire too thin, and not having enough iron; thus the amp retail price could compete with all the other mass made muck in the market place. To expect any brand name to make real good stuff is expecting far too much unless the brands happen to be ARC, Conrad Johnson, Manley Labs and a few others, and you will never find spare non ****ed up OPTs on E-bay taken from some old ARC amp which someone canobolised to sell the parts. Hammond PP 1650F for 60W for 6k6:4,8&16 ohms have about 50mH of leakage inductance. They are barely any better than the horror OPTs fitted to Jolida amps. But I have fitted Hammonds into Jolidas which had shorted turns due to utterly attrocious Chinese winding methods. The critical damping networks and shelving networks are 100% necessary to get the Jolidas to be where they should be. Sound is OK, despite quality shortcomings. i My 100K grid resistor on the 6L6's is too low... I'll up it to 220 Kohms and recalculate the C for the shelf I need. Once you have the shelving networks and a response that is say -3dB at 7Hz and its stable and no peaking in bass response below 30Hz, AND you put in a passive HP filter using 0.22uF and 100k at the amp input, rumble should never be a problem with vinyl. Rumble will be cut off in the pre-amp so none hits the PA. BTW, I do know about phase shift... also gain and phase margins, Nyquist diagrams, PID control, feedback controller tuning and stability, open and closed loop Bode plots... and the whole ball of wax of stochastic sampled data control (off topic!), as I was a practicing control systems engineer for some 30 years. *But the last 18 years were spent in engineering management (pays better!), so you get a bit rusty, but I still knew when any staffers tried to BS me... fortunately, not often, as I never forgot the basics! *Now retired. However, tube audio is pretty straightforward in principle but, of course, needs close design attention or it will bite you. Again, my thanks to all, particularly Patrick. We shall overcome! Cheers, Roger- The tube amp must be considered to be an active pass band filter which includes a loop of GNFB which extends the open loop band width. All sorts of traps exist for the un-wary who have never studied phase shift. I'm no real expert and dealing with reactance equations with J factor and square root of -1 and vectors. All the high-end maths is all incomprehensible to me. I've never studied at a university course covering such stuff. But I have enough hands on experience to know what BS behaviour to expect from amps and how to get maximum stable bandwidth with any sort of load at the highest possible power. There are limits to what may be had, mostly defined by the OPT. In my new amps the open loop bandwidth at full power without GNFB is designed to be 14Hz to 65kHz. When I use GNFB without any shelving networks the darn things will often oscillate at very low F and at very high F, depending on the load. But with 65kHz of BW to begin with these oscillations are easy to tame and get a stable closed loop BW = 65kHz, same as the open loop, and with unusually high "magin of stability". With the most inferior OPTs I have used in re-engineering efforts, maybe I get full PO BW from 50Hz to 25kHz, and the F at which these amps try to oscillate at are close to or within the 20 to 20kHz band. At 1/4 full PO, ie, 1/2 Vo, bandwidth then becomes about 25Hz to 30kHz and as long as low average levels are used the sound is passable. Patrick Turner. |
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#14
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jun 19, 7:47*am, "Alex Pogossov" wrote:
(snip) ... But I restore and improve radios, and use this feedback increase at LF technique. Of course there is no rumble in an AM receiver, but fadings, beating of two stations on close but not synchronised carriers and simply skimming the band makes lots of VLF. As you know to minimise AF load on the AM detector, it is not uncommon to have grid leak in the first audio stage of 10M with 0.05uF of coupling cap. This huge time constant makes the first tube (6SJ7, e.g.) virtually open to DC. VLF undulations result in 30V swings on the plate of the 1-st stage bringing it close to saturation. Part of this reaches the grid of the power stage (say, 6V6) unnecessarily swinging its bias and plate current. The later in turn cause undulations on the lightly filtered +B rail, and even might fall into resonance with the supply CLC filter. I am not very impressed with the shelving approach, because it is only an attenuator. While it would cut off 6V6 grid excursions, it will not prevent from 6SJ7 front overloading. Instead I use a circuit which helps reduce VLF voltage applied between grid and cathode of the first stage 6SJ7. You would say: Use a low cut-off filter. But it is a complication. Much better is to turn the amp into an active high pass filter by means of the NFB. Alex, this interests me as I'm an avid AM receiver "improver", too, but it's not clear to me just what you are referring to. Yes, the common 0.05 uF and 10 meg grid leak (or even the more common 4.7 megs) gives the triode audio stage "near DC amp" behavior (to as low as 0.3 Hz -3dB) but the DC comes from the detector diode, proportional to RF signal level, and feeds back via the AVC line (to the IF and converter tubes) to alleviate this. Also, I don't see this VLF getting to the o/p tube unless someone has used a huge coupling cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz. Perhaps there's a good reason for this little 0.005 uF... it also stops the tiny speakers from rattling too much! Could you please describe the cct fix you allude to? Sorry, I can't access binaries so it has to be words or .jpg pics to Flickr, or somewhere! Thanks and cheers, Roger |
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#15
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jun 20, 11:46*am, Engineer wrote:
On Jun 19, 7:47*am, "Alex Pogossov" wrote: (snip) ... *But I restore and improve radios, and use this feedback increase at LF technique. Of course there is no rumble in an AM receiver, but fadings, beating of two stations on close but not synchronised carriers and simply skimming the band makes lots of VLF. I also am always working on AM radio restorations. Even in old radios with working original circuits there is very little trouble with anything you are saying about VLF. Makers normally manage to make their radios stable at VLF and fading and variations in applied AVC voltage cause no audible problems. As you know to minimise AF load on the AM detector, it is not uncommon to have grid leak in the first audio stage of 10M with 0.05uF of coupling cap. This huge time constant makes the first tube (6SJ7, e.g.) virtually open to DC. The 10M and 0.05 is used to bias a triode-diode or pentode-diode tube with cathode taken to 0V. Its grid leak bias. Bias will wobble a bit when tuning but once tuned I've never encountered VLF bothers. SW signals dafe all over the place but generate very small carrier levels so the AVC hardly varies at all. VLF undulations result in 30V swings on the plate of the 1-st stage bringing it close to saturation. Not always, and it usually does not matter. But I never just repair a damn awful bean counter designed radio where the part list has been minimised to maximise the profits. I always dump the tube rectifier, put in large PSU caps, reduce hum by 30dB at least, then dump 6V6 and put in EL34 in triode, add a couple of tubes to make a decent detector and generally make the sound quality far better than the day they made the radio 50 years ago. Part of this reaches the grid of the power stage (say, 6V6) unnecessarily swinging its bias and plate current. The later in turn cause undulations on the lightly filtered +B rail, and even might fall into resonance with the supply CLC filter. After fixing maybe 50 AM radios, I've never encountered your problems, and I'm one of the most observant techs around. I am not very impressed with the shelving approach, because it is only an attenuator. While it would cut off 6V6 grid excursions, it will not prevent from 6SJ7 front overloading. Not if the whole amp has been designed right! Instead I use a circuit which helps reduce VLF voltage applied between grid and cathode of the first stage 6SJ7. You would say: Use a low cut-off filter. But it is a complication. Much better is to turn the amp into an active high pass filter by means of the NFB. Good luck, you might need it. Alex, this interests me as I'm an avid AM receiver "improver", too, but it's not clear to me just what you are referring to. Yes, the common 0.05 uF and 10 meg grid leak (or even the more common 4.7 megs) gives the triode audio stage "near DC amp" behavior (to as low as 0.3 Hz -3dB) but the DC comes from the detector diode, proportional to RF signal level, and feeds back via the AVC line (to the IF and converter tubes) to alleviate this. *Also, I don't see this VLF getting to the o/p tube unless someone has used a huge coupling cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz. Perhaps there's a good reason for this little 0.005 uF... it also stops the tiny speakers from rattling too much! Could you please describe the cct fix you allude to? *Sorry, I can't access binaries so it has to be words or .jpg pics to Flickr, or somewhere! Thanks and cheers, Roger- Hide quoted text - There is often AF signal down to about 20Hz in transmitted AM signals. Old radio OPTs often saturate at 80Hz, full power, and speakers don't go low so depending on the radio one has to ensure the CR couplings have poles which are a lot higher than what's used in a hi-fi amp. The typical AM radio might have a 6AV6 triode-diode first AF tube coupled to the 6V6 output and NO GNFB whatsoever. Use of any FB at all often makes oscillations happen because of very poor OL bandwidth. And because AF is limited by the sideband cutting, ie, the AF BW is defined by the total response of 5 tuned circuits then the rising speaker AF output without GNFB gives a wanted boost to F between 1kHz and 3 kHz before the AF drops away at 20dB per octave. But if the IFTs are well made and some other tricks are used to reduce selectivity while maitainining skirt selectivity over 30kHz away from some other station then audio BW can be 8kHz and then a flat amp with NFB makes good sense. If proper rules are followed as one does for a hi-fi amp you can get AM that is excellent sound and none of the VLF instabilities being cited here. I can listen to the news broadcast on AM radio and then switch to the FM tuner which is also broadcasting the same news from the same feed and I hear better sound via the AM radio I have. The tuner is an old Marantz chip based thing from late 1970s, not bad in its own way, but it don't way very much :-) Patrick Turner. |
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VLF stability in Williamson-type amplifiers
"Engineer" wrote in message ... Alex, this interests me as I'm an avid AM receiver "improver", too, but it's not clear to me just what you are referring to. Yes, the common 0.05 uF and 10 meg grid leak (or even the more common 4.7 megs) gives the triode audio stage "near DC amp" behavior (to as low as 0.3 Hz -3dB) but the DC comes from the detector diode, proportional to RF signal level, and feeds back via the AVC line (to the IF and converter tubes) to alleviate this. Also, I don't see this VLF getting to the o/p tube unless someone has used a huge coupling cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz. Perhaps there's a good reason for this little 0.005 uF... it also stops the tiny speakers from rattling too much! Could you please describe the cct fix you allude to? Sorry, I can't access binaries so it has to be words or .jpg pics to Flickr, or somewhere! Thanks and cheers, Roger Alex: Nice to meet another radio man on the tube amp list. Of course the easiest improvement of a tube radio would be getting rid of the tubes and replacing evething, in the first place, an audio amp with an IC, source follower after the detector, op-amp based tone control, etc. A simple op-amp based audio amp powered from ~6.3V winding would work far better than a boring 6AV6 + 6AQ5. However, it is not the way. The trick is to improve a radio keeping the tubes and approximately the same topology, not adding much. There are several issues, e.g.: 1. How to make AM detector to handle 95% of modulation, not 60...70% as in most of the boring radios; 2. Reduce distortion and booming in audio. Part 1 would cover optimisation of the AM detector, including unveiling of the "three-germanium-diodes-in-series" witchcraft of Mr.Turner. Which detector is the best? Vacuum diode? Silicon? Germanium? And why. Part 2 would cover adding NFB to the audio amp for distortion reduction and speaker damping, at the same time turning the amp in a high-pass filter (not to overload a lousy OPT and speaker with bass and DC level fluctuations). I will try to write up something on these issues from both theoretical and practical experience. |
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Alex, this interests me as I'm an avid AM receiver "improver", too,
but it's not clear to me just what you are referring to. Yes, the common 0.05 uF and 10 meg grid leak (or even the more common 4.7 megs) gives the triode audio stage "near DC amp" behavior (to as low as 0.3 Hz -3dB) but the DC comes from the detector diode, proportional to RF signal level, and feeds back via the AVC line (to the IF and converter tubes) to alleviate this. Also, I don't see this VLF getting to the o/p tube unless someone has used a huge coupling cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz. Perhaps there's a good reason for this little 0.005 uF... it also stops the tiny speakers from rattling too much! Could you please describe the cct fix you allude to? Sorry, I can't access binaries so it has to be words or .jpg pics to Flickr, or somewhere! Thanks and cheers, Roger[/quote] Has anyone ever tried the attached cct? Shows up in some form in the front of all the RCA Tube Handbooks. It avoids the AC coupling between the diode detector & 1st AF amp. Earlier apps used the low mu 85 triode but more recently the 6SR7 or 6BF6 have been recommended. Cheers, John |
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Alex, this interests me as I'm an avid AM receiver "improver", too,
but it's not clear to me just what you are referring to. Yes, the common 0.05 uF and 10 meg grid leak (or even the more common 4.7 megs) gives the triode audio stage "near DC amp" behavior (to as low as 0.3 Hz -3dB) but the DC comes from the detector diode, proportional to RF signal level, and feeds back via the AVC line (to the IF and converter tubes) to alleviate this. Also, I don't see this VLF getting to the o/p tube unless someone has used a huge coupling cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz. Perhaps there's a good reason for this little 0.005 uF... it also stops the tiny speakers from rattling too much! Could you please describe the cct fix you allude to? Sorry, I can't access binaries so it has to be words or .jpg pics to Flickr, or somewhere! Thanks and cheers, Roger[/quote] Here is another originally posted by John Byrns a few years ago. Not sure but looks like no DC return is possible for the carrier so the 180 pf cap would charge up & detection stops. Might work if the IF transformer had a center tap. Perhaps John B can fill us in. Cheers, John S |
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#19
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: On Jun 18, 9:56*pm, "Alex Pogossov" wrote: "Patrick Turner" wrote in message ... Patrick: But what of the phase shift of that C? Isn't it better to have R&C in parallel inserted from FB take off at OPT sec to the feedback R? Say you have 1k0 and 100r as the normal FB divider so that 1/11 of the OPT signal is applied to V1 cathode. Say one adds 3k3 so you then have 3k3, 1k0, then 100r at k to 0V at V1. Then ß becomes 0.022, much less than 0.09, and at very low F there is no phase shift, so with less NFB its probably going to be stable. But there isn't enough FB at higher F so you shunt the 3k3 with say 6.8uF. So at 100Hz the 6.8uF = 233 ohms reactance and 3k3 is well shunted. Peaking still has to be checked in output and in error signal. Alex: I suggested something quite opposite. The NFB divider looks like: - 1K from the speaker terminal to cathode of the driver stage; - 100R from the cathode to 100uF capacitor; - the other end of the 100uF capacitor is tied to GND. My apologies, I didn't understand you. And the reason I didn't understand was because the technique you are suggesting is completely unknown in the range of traditional means of applying NFB in tube amps. Your method here is normal procedure in solid state amps which are all dc coupled and without any large phase shifts at very low frequencies down to DC. Thus the feedback "beta" increases (!) at low frequencies (below 10Hz in this case), giving -90deg phase lag in the loop below 10Hz. This is in effect turning the amp into a anti-rumble filter. Yes, I see you are against rumble. But in any tube amp with 2 CR couplings and an OPT the gain below 10Hz quickly falls to zero at DC and there is no need to roll off LF any more than it naturally is rolled off. So if a signal at 1Hz enters the amp, the FB fed back is extremely small because the output signal is so small. So I cannot see your method would improve LF behaviour. I've read most of the texts about NFB applications in tube amps and I don't recall a single instance where your idea has been applied by any commercial manufacturer nor have I read of anyone supporting it. From the first glance it might sound crazy to increase the loop gain at VLF where we want an overal reduction of yje loop gain, but consider this: - OPT typically gives +90deg lead below 15...20Hz; - interstage coupling (simple RC with no shelving, 220K and 0.033uF) is calculated to give -3dB corner at say 15...20Hz and also gives +90 deg lead below; - but this "funny" NFB with a 100R and 100uF gives -90deg LAG below 10Hz! And this lag maintains down until 1Hz! Well, your NFB method does not increase OLG. OLG remains what it is regardless of the NFB network. But th closed loop gain, CLG, could be boosted with positive FB. 3 lots of reactive phase advances in the amp will add to a rapid phase turnover per octave below 10Hz. So maximum phase shift can be more than 180d then because the Lp inductance falls to near zero near dc, you have only dc winding resistance in the OP stage so phase shift lessens a bit towards DC. But exactly what anyone measures depends on voltage applied and fiddling around with trying to increase NFB as one gets close to DC makes no sense to me at all. When you have measured and demonstrated your technique and analysised it all with maybe 10 detailed pages on a website with photos of the amps, then ppl might say you have something to offer. People here are very difficult people. We cannot agree with anything anyone says unless they offer the truth, the whole truth, and nothing but the truth, so help them in the eyes of the God Of Triodes :-) At LF one lag subtracts from two leads and in combination we have only +90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be crossed safely at only +90deg! I'll believe works when I see it. Killing many birds with one stone: - perfect transient with no peaking; - natural low cut off in the whole amp (sort of built-in anti-rumble filtering); - reduced error signal at VLF as more feedback is applied; - no need to use two identical shelving circuits in push-pull amps -- just one extra electrolytic. A drawback - a despised electrolytic as a shaping component. Perhaps there are other drawbacks you have not thought about. I suggest you embark on a course of soldering in your laboratory to prove your idea works. We all look forward to results. Hi Patrick, I like Alex's idea. As you point out, this feedback scheme was widely used in solid state amps, although as you allude to, it didn't have anything to do with feedback stability, my take was that the reason for its use was to increase the amount of feedback at DC in order to minimize the DC offset at the output of the amp. The thing I always liked about these solid state amps is the built in rumble filter, which Alex also seems to like. So this discussion gets me thinking, how could I apply Alex's idea to a tube amp without using the despised electrolytic capacitor. My solution, multiply the impedance of the feedback network by 220 X, making Alex's 1K resistor 220k, his 100 Ohm resistor 22k, and his 100uF capacitor 0.47uF. Of course this network will no longer drive the cathode of V1 because its impedance is too high, so I propose to add an additional triode operating as a cathode follower driven by the high impedance feedback network and direct coupled to the cathode of V1. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#20
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VLF stability in Williamson-type amplifiers
In article ,
"Alex Pogossov" wrote: "Engineer" wrote in message ... Alex, this interests me as I'm an avid AM receiver "improver", too, but it's not clear to me just what you are referring to. Yes, the common 0.05 uF and 10 meg grid leak (or even the more common 4.7 megs) gives the triode audio stage "near DC amp" behavior (to as low as 0.3 Hz -3dB) but the DC comes from the detector diode, proportional to RF signal level, and feeds back via the AVC line (to the IF and converter tubes) to alleviate this. Also, I don't see this VLF getting to the o/p tube unless someone has used a huge coupling cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz. Perhaps there's a good reason for this little 0.005 uF... it also stops the tiny speakers from rattling too much! Could you please describe the cct fix you allude to? Sorry, I can't access binaries so it has to be words or .jpg pics to Flickr, or somewhere! Thanks and cheers, Roger Alex: Nice to meet another radio man on the tube amp list. Of course the easiest improvement of a tube radio would be getting rid of the tubes and replacing evething, in the first place, an audio amp with an IC, source follower after the detector, op-amp based tone control, etc. A simple op-amp based audio amp powered from ~6.3V winding would work far better than a boring 6AV6 + 6AQ5. However, it is not the way. The trick is to improve a radio keeping the tubes and approximately the same topology, not adding much. There are several issues, e.g.: 1. How to make AM detector to handle 95% of modulation, not 60...70% as in most of the boring radios; 2. Reduce distortion and booming in audio. Part 1 would cover optimisation of the AM detector, including unveiling of the "three-germanium-diodes-in-series" witchcraft of Mr.Turner. Which detector is the best? Vacuum diode? Silicon? Germanium? And why. Part 2 would cover adding NFB to the audio amp for distortion reduction and speaker damping, at the same time turning the amp in a high-pass filter (not to overload a lousy OPT and speaker with bass and DC level fluctuations). I will try to write up something on these issues from both theoretical and practical experience. There is nothing wrong with discussing radio's on this "tube amp list". If you lookup the charter for rec.audio.tubes, you will find that the group is not restricted to amps, "Radio Circuits" are explicitly listed as an allowed topic of discussion! I too noticed Patrick's mention of "three-germanium-diodes-in-series" and wondered what this was all about? The last time Patrick discussed his AM detector designs, IIRC the Turner standard AM detector was an RF cathode follower driving a single germanium diode. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#21
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VLF stability in Williamson-type amplifiers
On Jun 20, 10:04*am, "Alex Pogossov" wrote:
"Engineer" wrote in message ... Alex, this interests me as I'm an avid AM receiver "improver", too, but it's not clear to me just what you are referring to. Yes, the common 0.05 uF and 10 meg grid leak (or even the more common 4.7 megs) gives the triode audio stage "near DC amp" behavior (to as low as 0.3 Hz -3dB) but the DC comes from the detector diode, proportional to RF signal level, and feeds back via the AVC line (to the IF and converter tubes) to alleviate this. *Also, I don't see this VLF getting to the o/p tube unless someone has used a huge coupling cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz. Perhaps there's a good reason for this little 0.005 uF... it also stops the tiny speakers from rattling too much! Could you please describe the cct fix you allude to? *Sorry, I can't access binaries so it has to be words or .jpg pics to Flickr, or somewhere! Thanks and cheers, Roger Alex: Nice to meet another radio man on the tube amp list. Of course the easiest improvement of a tube radio would be getting rid of the tubes and replacing evething, in the first place, an audio amp with an IC, source follower after the detector, op-amp based tone control, etc. A simple op-amp based audio amp powered from ~6.3V winding would work far better than a boring 6AV6 + 6AQ5. However, it is not the way. The trick is to improve a radio keeping the tubes and approximately the same topology, not adding much. There are several issues, e.g.: 1. How to make AM detector to handle 95% of modulation, not 60...70% as in most of the boring radios; 2. Reduce distortion and booming in audio. Part 1 would cover optimisation of the AM detector, including unveiling of the "three-germanium-diodes-in-series" witchcraft of Mr.Turner. Which detector is the best? Vacuum diode? Silicon? Germanium? And why. Part 2 would cover adding NFB to the audio amp for distortion reduction and speaker damping, at the same time turning the amp in a high-pass filter (not to overload a lousy OPT and speaker with bass and DC level fluctuations). I will try to write up something on these issues from both theoretical and practical experience. Alex, you too! I regularly replace the tube diode detector with a 1N34A and use NFB to the cathode of the audio triode from the OPT secondary (47 to 100 ohms to GND and whatever FB resistor I need to get about 10 dB loss in sensitivity in the audio stages), but I don't try to steer the audio frequency response - it just extends a bit under NFB with less distortion and I haven't had a speaker rattle problem from too much bass extension. Looking forward to your schematics (NOT on binaries, please!) Cheers, Roger |
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#22
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VLF stability in Williamson-type amplifiers
snip for brevity,
Alex said... A drawback - a despised electrolytic as a shaping component. I said..... Perhaps there are other drawbacks you have not thought about. I suggest you embark on a course of soldering in your laboratory to prove your idea works. We all look forward to results. Hi Patrick, I like Alex's idea. *As you point out, this feedback scheme was widely used in solid state amps, although as you allude to, it didn't have anything to do with feedback stability, my take was that the reason for its use was to increase the amount of feedback at DC in order to minimize the DC offset at the output of the amp. *The thing I always liked about these solid state amps is the built in rumble filter, which Alex also seems to like. With SS and total direct coupling there is good LF stability and LF BW is limited by the rail caps. But one sure doesn't want any DC entering the differential input pair of any SS amp because it will appear at the output. DC gain = unity. I cannot think of any reason to apply the idea in any tube amp. So this discussion gets me thinking, how could I apply Alex's idea to a tube amp without using the despised electrolytic capacitor. My solution, multiply the impedance of the feedback network by 220 X, making Alex's 1K resistor 220k, his 100 Ohm resistor 22k, and his 100uF capacitor 0.47uF. *Of course this network will no longer drive the cathode of V1 because its impedance is too high, so I propose to add an additional triode operating as a cathode follower driven by the high impedance feedback network and direct coupled to the cathode of V1. Its very easy to have a differential pair at the input of any PP OR SE amp, and the two high impedance grid inputs are used as ports for the input and the FB signal. So the FB network can all be high impedance as you say, but why you'd have a series cap in there is really unknown to me. An adequate rumble filter can be made with a single passive C&R HPF at input, or two in series so although it's -3dB at say 10Hz, ultimately roll off becomes -12dB/octave below 3Hz. Patrick Turner. |
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#23
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VLF stability in Williamson-type amplifiers
On Jun 21, 2:45*am, John Byrns wrote:
In article , *"Alex Pogossov" wrote: "Engineer" wrote in message ... Alex, this interests me as I'm an avid AM receiver "improver", too, but it's not clear to me just what you are referring to. Yes, the common 0.05 uF and 10 meg grid leak (or even the more common 4.7 megs) gives the triode audio stage "near DC amp" behavior (to as low as 0.3 Hz -3dB) but the DC comes from the detector diode, proportional to RF signal level, and feeds back via the AVC line (to the IF and converter tubes) to alleviate this. *Also, I don't see this VLF getting to the o/p tube unless someone has used a huge coupling cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz. Perhaps there's a good reason for this little 0.005 uF... it also stops the tiny speakers from rattling too much! Could you please describe the cct fix you allude to? *Sorry, I can't access binaries so it has to be words or .jpg pics to Flickr, or somewhere! Thanks and cheers, Roger Alex: Nice to meet another radio man on the tube amp list. Of course the easiest improvement of a tube radio would be getting rid of the tubes and replacing evething, in the first place, an audio amp with an IC, source follower after the detector, op-amp based tone control, etc. A simple op-amp based audio amp powered from ~6.3V winding would work far better than a boring 6AV6 + 6AQ5. However, it is not the way. The trick is to improve a radio keeping the tubes and approximately the same topology, not adding much. There are several issues, e.g.: 1. How to make AM detector to handle 95% of modulation, not 60...70% as in most of the boring radios; 2. Reduce distortion and booming in audio. Part 1 would cover optimisation of the AM detector, including unveiling of the "three-germanium-diodes-in-series" witchcraft of Mr.Turner. Which detector is the best? Vacuum diode? Silicon? Germanium? And why. Part 2 would cover adding NFB to the audio amp for distortion reduction and speaker damping, at the same time turning the amp in a high-pass filter (not to overload a lousy OPT and speaker with bass and DC level fluctuations). I will try to write up something on these issues from both theoretical and practical experience. There is nothing wrong with discussing radio's on this "tube amp list". *If you lookup the charter for rec.audio.tubes, you will find that the group is not restricted to amps, "Radio Circuits" are explicitly listed as an allowed topic of discussion! I too noticed Patrick's mention of "three-germanium-diodes-in-series" and wondered what this was all about? *The last time Patrick discussed his AM detector designs, IIRC the Turner standard AM detector was an RF cathode follower driving a single germanium diode. I tried 3 germanium diodes in series after an IFT secondary with a recent radio job with following C = 200k and a small cap, maybe 68pF I recall. following this there was an emitter follower comprising 2 generic darlington connected signal bjts operating from a +50V supply The 200k biases the bjt base from +35V. Anyway, I found the 3 series Gw diodes gave a much higher reverse current leakage because their backward resistance is in series. Forward voltage drop is still low, but at very low level signals the THD due to diode turn on was still low, and low level short wave audio was OK, as good as with tube diodes. Ge diodes I used were unknown types in my parts bins; maybe those with highest reverse voltage ratings have least reverse leakage, but you guys must try lots and lots of things to make things work they way you want, like I do during nearly every day of my life. The emitter follower convert the output pf the diode + RC circuit to low Z for driving a volume control and it buffers the detector output to prevent high THD when modulation exceeds 70% I get very low detector distortion. Its necessary to have a low distortion detector because radio stations use high percentage mod for efficiency, wheras in old days mod % was quite low. I usually connect last IFT winding directly to a grid of cathode follower with IFT winding biased at about +30Vdc. From CF low Z output I then use Ge diode to charge 220pF with say 500k taken to 0V, so Ge diode is slightly biased with 0.07mA, ie, its "on" a bit, and the leakage doesn't cause trouble in such a low Z drive circuit. See the AM section of the AM/FM radio at.... http://www.turneraudio.com.au/am-fm-...ex-decoder.htm The CF shows 6AU7, but should be 6AU6,- typo - but could also be 1/2 12AU7, then detector circuit, and then another 1/2 12AU7 CF to buffer detector output. The 330k and 100k in my schematic keep the AM radio output about level with what comes from the FM circuit. Patrick Turner. |
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#24
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VLF stability in Williamson-type amplifiers
"Patrick Turner" wrote in message ... OK, the 0.47 + 1M strapped with 0.047 + 220k acts like this :- At 1 kHz, all C have low Z so the phase shift is low and the driver, ie concertina or whatever you have "sees" a load of 220k. As F is reduced, there is a pole between 0.047 and 220 at 15.4Hz which is usually above the F at which the amp may want to oscillate. Sometimes I have used 0.022uF, so pole is at 32Hz. phase shift caused is less than 90 degrees. As F is further reduced, the network response tends to flatten to a shelf formed by 1M and 220k, ie, signal flattens at roughly -15db, 0.18 times the 1kHz level. The 0.047 has gone to an open circuit by 2Hz with little effect. But the 0.47 the rolls the response off at the pole between 0.47uF and 1.22M ohms, ie at 0.27Hz, below which there is an ultimate phase shift of 90d, but at such a low F as to not cause bothers because its well below the poles of all other sages. Alex: Shelving network does a good job, but it has a drawback. It is an attenuator for low frequencies. If low frequency signal or DC step gets applied to the input of the amp, the first stage will or might overload, while the shelving circuit will protect the next stage from the overloading. To prevent the front stage from overloading at VLF it is better to apply the shelving as a local feedback, i.e. in the cathode circuit of the 1-st stage. Imagine the cathode of the 1-st stage is not directly connected to a feedback divider (say 1K/100R) but through a RC circuit of paralleled 10K and 5uF (approx.) This cathode degeneration at LF will act similar to the attenuator shelving, but will prevent overloading of the first stage by VLF. Then you do not need 1M||0.05uF interstage attenuator. Of course in this cathode shelving the grid DC bias voltage has to be elevated, so the grid can not be directly connected to the volume control. However it is not a big deal. there are three known ways of elevating grid bias voltage: - fixed divider from upply voltage; - divider cathode-grid-ground; - split cathode resistor and from the tap thus formed throw 1M to the grid. Regards, Alex |
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#25
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: So this discussion gets me thinking, how could I apply Alex's idea to a tube amp without using the despised electrolytic capacitor. My solution, multiply the impedance of the feedback network by 220 X, making Alex's 1K resistor 220k, his 100 Ohm resistor 22k, and his 100uF capacitor 0.47uF. *Of course this network will no longer drive the cathode of V1 because its impedance is too high, so I propose to add an additional triode operating as a cathode follower driven by the high impedance feedback network and direct coupled to the cathode of V1. Its very easy to have a differential pair at the input of any PP OR SE amp, and the two high impedance grid inputs are used as ports for the input and the FB signal. So the FB network can all be high impedance as you say, but why you'd have a series cap in there is really unknown to me. An adequate rumble filter can be made with a single passive C&R HPF at input, or two in series so although it's -3dB at say 10Hz, ultimately roll off becomes -12dB/octave below 3Hz. The reason for the "series cap" is because it provides the same function as your low frequency gain stepping network, but does the job more effectively with fewer drawbacks, as Alex points out. I suppose that Alex's scheme has the drawback of affecting the amplifiers frequency response if the gain change must start in the audible range to achieve stability. The rumble filter effect is not the point, it is just a bonus that comes with the circuit. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#26
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VLF stability in Williamson-type amplifiers
Its very easy to have a differential pair at the input of any PP OR SE amp, and the two high impedance grid inputs are used as ports for the input and the FB signal. *So the FB network can all be high impedance as you say, but why you'd have a series cap in there is really unknown to me. An adequate rumble filter can be made with a single passive C&R HPF at input, or two in series so although it's -3dB at say 10Hz, ultimately roll off *becomes -12dB/octave below 3Hz. The reason for the "series cap" is because it provides the same function as your low frequency gain stepping network, but does the job more effectively with fewer drawbacks, as Alex points out. *I suppose that Alex's scheme has the drawback of affecting the amplifiers frequency response if the gain change must start in the audible range to achieve stability. *The rumble filter effect is not the point, it is just a bonus that comes with the circuit. -- Regards, John Byrns I've made a considerable number of phono amps and I've never had to deliberately include a rumble filter in any of them nor in any power amp. I've seen speaker cones wobbling around especially with bass boost on a tone control turned up on some amps made by others but not much of that happens in my amps. The shelving idea is not mine though; it was invented and used long ago by all the brightest engineers and they published and I agreed they were right. The worst effects of having bass response going too low in tube amps is best witnessed if one uses pink noise which has a flat bandwidth from say 1Hz to 20kHz, and as one cranks the volume you'll hear knocking noise in the OPT from core saturation before clipping occurs. Just put in first order HPF with pole at 14Hz and the knocking noise vanishes, and level can be raised to clipping OK. Usually there is no music or vinyl artifacts worse than VLF in full range pink noise. I get fabulous bass from my phono amps but the VLF don't make it past the amp. No global FB is used anywhere. No peaks in VLF response. Patrick Turner. |
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#27
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ëþáîü çëà - ïîëþáèøü è êîçëà.
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#28
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: Its very easy to have a differential pair at the input of any PP OR SE amp, and the two high impedance grid inputs are used as ports for the input and the FB signal. *So the FB network can all be high impedance as you say, but why you'd have a series cap in there is really unknown to me. An adequate rumble filter can be made with a single passive C&R HPF at input, or two in series so although it's -3dB at say 10Hz, ultimately roll off *becomes -12dB/octave below 3Hz. The reason for the "series cap" is because it provides the same function as your low frequency gain stepping network, but does the job more effectively with fewer drawbacks, as Alex points out. *I suppose that Alex's scheme has the drawback of affecting the amplifiers frequency response if the gain change must start in the audible range to achieve stability. *The rumble filter effect is not the point, it is just a bonus that comes with the circuit. I've made a considerable number of phono amps and I've never had to deliberately include a rumble filter in any of them nor in any power amp. I've seen speaker cones wobbling around especially with bass boost on a tone control turned up on some amps made by others but not much of that happens in my amps. You are fixated on the "rumble filter" aspect, which isn't the point of Alex's circuit, it is just an extra feature that comes for free. And when we are talking about phono reproduction, rumble isn't really the issue even there, it is the VLF signals caused by record warp. The record warp signals can be largely mitigated by the proper choice of arm mass and cartridge compliance. The shelving idea is not mine though; it was invented and used long ago by all the brightest engineers and they published and I agreed they were right. Yes, we know the shelving idea isn't yours, it was used in many amps, but wasn't commonly used in the typical USA built amplifiers which usually had only two low frequency poles, and so didn't require a low frequency shelving network for stability, high frequency shelving networks were another mater though, and were commonly used. Here is a simplified schematic of an AM broadcast transmitter from the 1940s that includes a low frequency shelving network http://www.qsl.net/wa2whv/XT1A/xt1schem.jpg Notice that the audio section includes no fewer than 10 low frequency poles, 7 of them within the negative feedback loop, no wonder a low frequency gain stepping network was required! The worst effects of having bass response going too low in tube amps is best witnessed if one uses pink noise which has a flat bandwidth from say 1Hz to 20kHz, and as one cranks the volume you'll hear knocking noise in the OPT from core saturation before clipping occurs. Just put in first order HPF with pole at 14Hz and the knocking noise vanishes, and level can be raised to clipping OK. I would expect that Alex's feedback circuit also helps with your "knocking noise in the OPT", for real world signals although not to quite the same extent as your first order HPF does. If that isn't enough, your "first order HPF" can always be used in combination with Alex's circuit. But this "knocking noise in the OPT" is really beside the point of Alex's circuit which is to stablize the feedback loop at low frequencies. Usually there is no music or vinyl artifacts worse than VLF in full range pink noise. I get fabulous bass from my phono amps but the VLF don't make it past the amp. No global FB is used anywhere. No peaks in VLF response. I take it your are referring to the phono preamp only in this last paragraph? -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#29
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VLF stability in Williamson-type amplifiers
You are fixated on the "rumble filter" aspect, which isn't the point of Alex's circuit, it is just an extra feature that comes for free. *And when we are talking about phono reproduction, rumble isn't really the issue even there, it is the VLF signals caused by record warp. *The record warp signals can be largely mitigated by the proper choice of arm mass and cartridge compliance. I doubt I am fixated about rumble because I never have any trouble with it. I question if there is any benefit in Alex's circuit. Has anyone tried it? where are measured results? The shelving idea is not mine though; it was invented and used long ago by all the brightest engineers and they published and I agreed they were right. Yes, we know the shelving idea isn't yours, it was used in many amps, but wasn't commonly used in the typical USA built amplifiers which usually had only two low frequency poles, and so didn't require a low frequency shelving network for stability, high frequency shelving networks were another mater though, and were commonly used. Here is a simplified schematic of an AM broadcast transmitter from the 1940s that includes a low frequency shelving network http://www.qsl.net/wa2whv/XT1A/xt1schem.jpg Notice that the audio section includes no fewer than 10 low frequency poles, 7 of them within the negative feedback loop, no wonder a low frequency gain stepping network was required! Sometimes two lots of shelving are needed. But notice that the two loops of balanced GNFB come from a pair of dividers at the output but there's a cap in series with the FB, so there **cannot be dc FB**. FB reduces as F goes low because of the cap. The worst effects of having bass response going too low in tube amps is best witnessed if one uses pink noise which has a flat bandwidth from say 1Hz to 20kHz, and as one cranks the volume you'll hear knocking noise in the OPT from core saturation before clipping occurs. Just put in first order HPF with pole at 14Hz and the knocking noise vanishes, and level can be raised to clipping OK. I would expect that Alex's feedback circuit also helps with your "knocking noise in the OPT", for real world signals although not to quite the same extent as your first order HPF does. *If that isn't enough, your "first order HPF" can always be used in combination with Alex's circuit. *But this "knocking noise in the OPT" is really beside the point of Alex's circuit which is to stablize the feedback loop at low frequencies. I am having trouble seeing how DC FB around a tube amp could make anything better at all. Where is the evidence? While an OPT is making knocking noises, the core inductance is intermittently becoming negligible and the RL becomes = to Rw, thus causing gross distortions to all signal F. Any kind of GNFB makes it worse. I'd like to see a schematic of what Alex proposes before making up my mind finally about it. Usually there is no music or vinyl artifacts worse than VLF in full range pink noise. I get fabulous bass from my phono amps but the VLF don't make it past the amp. No global FB is used anywhere. No peaks in VLF response. I take it your are referring to the phono preamp only in this last paragraph? Well yes. I sure don't need rumble filters in my power amps. Not even in AF amps in radios, with say 12AX7 driving EL34 in triode with 12dB FB. I've been doing all this for years and never have had bothers with VLF. I guess I know enough about CR time constants to get by. Patrick Turner. |
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#30
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VLF stability in Williamson-type amplifiers
On Jun 23, 6:11*am, flipper wrote:
On Wed, 22 Jun 2011 03:50:30 -0700 (PDT), Patrick Turner wrote: You are fixated on the "rumble filter" aspect, which isn't the point of Alex's circuit, it is just an extra feature that comes for free. And when we are talking about phono reproduction, rumble isn't really the issue even there, it is the VLF signals caused by record warp. The record warp signals can be largely mitigated by the proper choice of arm mass and cartridge compliance. I doubt I am fixated about rumble because I never have any trouble with it. It's obvious you are because it's the only aspect of his circuit you ever talk about. I talked about other issues, although perhaps you didn't notice. Rumble filters are not needed unless you have an extremely poor TT, recording etc, and I ain't got either, so I don't have rumble filters. I question if there is any benefit in Alex's circuit. He provided a list. Maybe all totally invalid. I've asked for EVIDENCE that the claims he makes are valid. Deafening silence ever since. Patrick Turner. |
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#31
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VLF stability in Williamson-type amplifiers
At LF one lag subtracts from two leads and in combination we have only
+90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be crossed safely at only +90deg! Interesting idea but I'm having trouble wrapping my head around the lead/lag analysis because, if I understand your circuit correctly, that cap is at the summing junction so shouldn't it introduce poles into both the NFB and the input signal? I'm not the only one questioning Alex's FB network. You are having trouble accepting Alex's claims. Where's the evidence? its no ****ing good making claims without backup evidence. Patrick Turner. |
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#32
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: You are fixated on the "rumble filter" aspect, which isn't the point of Alex's circuit, it is just an extra feature that comes for free. *And when we are talking about phono reproduction, rumble isn't really the issue even there, it is the VLF signals caused by record warp. *The record warp signals can be largely mitigated by the proper choice of arm mass and cartridge compliance. I doubt I am fixated about rumble because I never have any trouble with it. I question if there is any benefit in Alex's circuit. Has anyone tried it? where are measured results? The why do you keep on about the rumble issue? "Rumble Filter" was an unfortunate choice of words by Alex, and I have unfortunately helped perpetuate it. A better description of the effect would be a HPF with a VLF cutoff. It is really not correct to call it a rumble filter because rumble is usually in the audible band and hence a "Rumble Filter" has a much higher cutoff frequency. As I said in an earlier post the effect of the filter is more to eliminate subsonic signals resulting from record warp. So lets just call it a "VLF HPF that has useful properties, in addition to its primary purpose of helping to stabilize the feedback. The shelving idea is not mine though; it was invented and used long ago by all the brightest engineers and they published and I agreed they were right. Yes, we know the shelving idea isn't yours, it was used in many amps, but wasn't commonly used in the typical USA built amplifiers which usually had only two low frequency poles, and so didn't require a low frequency shelving network for stability, high frequency shelving networks were another mater though, and were commonly used. Here is a simplified schematic of an AM broadcast transmitter from the 1940s that includes a low frequency shelving network http://www.qsl.net/wa2whv/XT1A/xt1schem.jpg Notice that the audio section includes no fewer than 10 low frequency poles, 7 of them within the negative feedback loop, no wonder a low frequency gain stepping network was required! Sometimes two lots of shelving are needed. But notice that the two loops of balanced GNFB come from a pair of dividers at the output but there's a cap in series with the FB, so there **cannot be dc FB**. FB reduces as F goes low because of the cap. Yes, that would appear to result in several undesirable consequences. The worst effects of having bass response going too low in tube amps is best witnessed if one uses pink noise which has a flat bandwidth from say 1Hz to 20kHz, and as one cranks the volume you'll hear knocking noise in the OPT from core saturation before clipping occurs. Just put in first order HPF with pole at 14Hz and the knocking noise vanishes, and level can be raised to clipping OK. I would expect that Alex's feedback circuit also helps with your "knocking noise in the OPT", for real world signals although not to quite the same extent as your first order HPF does. *If that isn't enough, your "first order HPF" can always be used in combination with Alex's circuit. *But this "knocking noise in the OPT" is really beside the point of Alex's circuit which is to stablize the feedback loop at low frequencies. I am having trouble seeing how DC FB around a tube amp could make anything better at all. It's not the "DC feedback", it is compensating phase shift introduced by Alex's network. I believe a similar compensation scheme is widely used at high frequencies in the form of a capacitor across the part or all of the feedback resistor. Where is the evidence? While an OPT is making knocking noises, the core inductance is intermittently becoming negligible and the RL becomes = to Rw, thus causing gross distortions to all signal F. Any kind of GNFB makes it worse. How many people have a need to reproduce "pink noise" that extends down to DC, if such a signal even exists? I'd like to see a schematic of what Alex proposes before making up my mind finally about it. Alex described the circuit, and I improved the idea by adding a cathode follower. Usually there is no music or vinyl artifacts worse than VLF in full range pink noise. I get fabulous bass from my phono amps but the VLF don't make it past the amp. No global FB is used anywhere. No peaks in VLF response. I take it your are referring to the phono preamp only in this last paragraph? Well yes. I sure don't need rumble filters in my power amps. Not even in AF amps in radios, with say 12AX7 driving EL34 in triode with 12dB FB. I've been doing all this for years and never have had bothers with VLF. I guess I know enough about CR time constants to get by. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#33
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
flipper wrote: On Sat, 18 Jun 2011 21:56:00 +1000, "Alex Pogossov" wrote: "Patrick Turner" wrote in message ... Patrick: But what of the phase shift of that C? Isn't it better to have R&C in parallel inserted from FB take off at OPT sec to the feedback R? Say you have 1k0 and 100r as the normal FB divider so that 1/11 of the OPT signal is applied to V1 cathode. Say one adds 3k3 so you then have 3k3, 1k0, then 100r at k to 0V at V1. Then ß becomes 0.022, much less than 0.09, and at very low F there is no phase shift, so with less NFB its probably going to be stable. But there isn't enough FB at higher F so you shunt the 3k3 with say 6.8uF. So at 100Hz the 6.8uF = 233 ohms reactance and 3k3 is well shunted. Peaking still has to be checked in output and in error signal. Alex: I suggested something quite opposite. The NFB divider looks like: - 1K from the speaker terminal to cathode of the driver stage; - 100R from the cathode to 100uF capacitor; - the other end of the 100uF capacitor is tied to GND. Thus the feedback "beta" increases (!) at low frequencies (below 10Hz in this case), giving -90deg phase lag in the loop below 10Hz. This is in effect turning the amp into a anti-rumble filter. From the first glance it might sound crazy to increase the loop gain at VLF where we want an overal reduction of yje loop gain, but consider this: - OPT typically gives +90deg lead below 15...20Hz; - interstage coupling (simple RC with no shelving, 220K and 0.033uF) is calculated to give -3dB corner at say 15...20Hz and also gives +90 deg lead below; - but this "funny" NFB with a 100R and 100uF gives -90deg LAG below 10Hz! And this lag maintains down until 1Hz! At LF one lag subtracts from two leads and in combination we have only +90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be crossed safely at only +90deg! Interesting idea but I'm having trouble wrapping my head around the lead/lag analysis because, if I understand your circuit correctly, that cap is at the summing junction so shouldn't it introduce poles into both the NFB and the input signal? I'm having some trouble wrapping my mind around this summing junction issue. What is the problem if it does introduce a pole into the input signal? That would not affect the feedback signal, and at worst would only enhance the "Rumble Filter" effect, if it even does anything additional to the input signal at all. I will have to write out the transfer function and see if there is any additional affect on the input signal. If it does have some additional effect on the input signal, which is considered undesirable, my cathode follower modification described in an earlier post should eliminate the unwanted effect. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#34
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jun 24, 5:20*am, flipper wrote:
On Wed, 22 Jun 2011 16:57:27 -0700 (PDT), Patrick Turner wrote: At LF one lag subtracts from two leads and in combination we have only +90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be crossed safely at only +90deg! Interesting idea but I'm having trouble wrapping my head around the lead/lag analysis because, if I understand your circuit correctly, that cap is at the summing junction so shouldn't it introduce poles into both the NFB and the input signal? I'm not the only one questioning Alex's FB network. You aren't 'questioning'. You've got your fingers stuck in your ears. You are having trouble accepting Alex's claims. I asked only one specific question about the phase analysis. That it prevents peaking, the POINT of the suggestion, is self evident. Where's the evidence? its no ****ing good making claims without backup evidence. The evidence is in the analysis that you keep snipping and ignoring. What sort of fish gave you the idea of calling yourself FLIPPER? Doncha feel embarassed most days to be seen by others as a half brained fish? Where is the evidence you can think about anything? NOBODY HAS POSTED EVIDENCE THAT THE NFB IDEA AS DESCRIBED BY ALEX WILL WORK TO TO ANYTHING WORTHWHILE. I do not have my hands across my eyes, hands across my ears, or hands across my mouth like the three unwise monkeys who would not see the truth, hear the truth, or speak the truth. SO WHERE IS THE TRUTH? Truth needs to be proven to be true. So Flipper, perhaps you can spend a few days to prepare a lecture on the issue and explore all possibilities but I won't accept simulations. You shall build a circuit, you shall apply a healthy self critical attitude, and shall carefully measure all voltages and currents and waveforms and distortions and phase shifts. You shall not dither about on news groups throwing mud and accusing ppl of crapping when they don't. When that's all done we all might learn and beneft. Patrick Turner |
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#35
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: Truth needs to be proven to be true. So Flipper, perhaps you can spend a few days to prepare a lecture on the issue and explore all possibilities but I won't accept simulations. You shall build a circuit, you shall apply a healthy self critical attitude, and shall carefully measure all voltages and currents and waveforms and distortions and phase shifts. You shall not dither about on news groups throwing mud and accusing ppl of crapping when they don't. When that's all done we all might learn and beneft. I am not an NFB expert, you are the reining expert on that topic, however I do have some knowledge of the design of equalizer networks and the poles and zeros involved. Before I get into that and what I think is its relationship to the negative feedback issue. Aside from the feedback stability issues we are mainly concerned with, it appears obvious by inspection that in the infrasonic region below the pole frequency of Alex's network, the network reduces the potential for overload in the first stage, while your network increases the potential for first stage overload. Now as far as how Alex's feedback network might help improve amplifier stability, from my perspective it is just a tool in your toolbox that requires some sophistication to apply usefully. The first and most obvious thing Alex's network does is increase the loop gain, I think that is the correct term correct me if I have the wrong term. Normally increasing the loop gain increases stability problems so that at first blush Alex's network might appear to be counter productive. However the network introduces both a zero and pole into the response, with the zero at a higher frequency than the pole. Remember this network is just another tool in your toolbox; it is not a cure all and requires some sophistication in its application. Now the one thing I know about stabilizing the low frequency response of a feedback system is that it is all about correctly placing the poles, and zeros if there are any. If the only constraint on the pole frequencies is achieving stability, then there is no problem in the first place; we can simply choose the pole frequencies to insure stability. Unfortunately in the real world there are other constrains and requirements on the choice of pole frequencies, so we can run into problems trying to meet all the requirements at the same time. The zero in Alex's network can be used to cancel one of the original low frequency poles, and replace it with a new pole at a lower frequency. If you are a clever designer you may be able to use this to affect a better compromise in the choice of pole frequencies. Your network works in a similar way except that the pole frequency is higher than the frequency of the zero, and that it decreases the loop gain rather than increasing it. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#36
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
After deleting Flipper's evasions and denials and weaseling out of
providing any truth about a NFB idea **which may,** or **may not** be of any use, I'll leave the brain dead fish response to my questions... I don't give a tinker's damn what you deign to 'accept' and "shall" not waste my time with your shalls. I didn't expect anything different from you. You are allergic to any truth you don't believe in. You just don't understand what Alex is proposing either. And you cannot convince a living soul that Alex's method works to provide any benefit to a tube power amp. Patrick Turner. |
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#37
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jun 24, 3:34*pm, John Byrns wrote:
In article , *Patrick Turner wrote: Truth needs to be proven to be true. So Flipper, perhaps you can spend a few days to prepare a lecture on the issue and explore all possibilities but I won't accept simulations. You shall build a circuit, you shall apply a healthy self critical attitude, and shall carefully measure all voltages and currents and waveforms and distortions and phase shifts. You shall not dither about on news groups throwing mud and accusing ppl of crapping when they don't. When that's all done we all might learn and beneft. I am not an NFB expert, you are the reining expert on that topic, however I do have some knowledge of the design of equalizer networks and the poles and zeros involved. * I am NOT an expert. I'm just a ****en bloke who does a bit with a soldrin' iron. In my work shed where I spend days and months and years fixing up the shortcomings of one tragic amp after another, ideas I may addopt and employ are good because they are found to be and can be proven to be, or they are better left well alone. If I was the expert, I would be up there with Peter Baxandal an Williamson and 1/2 a dozen others who would be able to mathematically calculate whether or not The Alex NFB loop is of any benefit or not. I've realised that asking or persuading anyone here to give some proof of their ideas and circuit techniques doesn't work. Ppl make claims, then won't prove them to be true. I've an open mind. But I question every damn thing I come into contact with. I try to assume nothing. Before I get into that and what I think is its relationship to the negative feedback issue. *Aside from the feedback stability issues we are mainly concerned with, it appears obvious by inspection that in the infrasonic region below the pole frequency of Alex's network, the network reduces the potential for overload in the first stage, while your network increases the potential for first stage overload. I know for a fact that in my amps the input tube has an unused dynamic headroom of perhaps +20dB. Ie, the input tube could produce 50Vrms easily, but in my circuits maybe only has to make 5Vrms at CLIPPING! The gain shelving network I use may indeed result in V1 having to procuce a peaked response in order to keep the V0 at a constant voltage level. The peak in response is below 20Hz, and usually less than a few dB, and so the input tube NEVER EVER overloads, ie, is forced into cut off ot grid current - unless the amp is taken to clipping - perhaps, because usually the middle driver stage overloads before the input tube. The output stage usually overloads first at clipping. Now as far as how Alex's feedback network might help improve amplifier stability, from my perspective it is just a tool in your toolbox that requires some sophistication to apply usefully. *The first and most obvious thing Alex's network does is increase the loop gain, I think that is the correct term correct me if I have the wrong term. * I welcome any new ideas, but some ideas turn out to not be very good. I need to ss the proposer of any new ideas is ready to prove the worth of any new idea. Not too much to expect, now is it? Ppl hate proving claims. They are so often allergic to spending a day in their workshop soldering and measuring something then recording it all and writing up the experiments complete with nicely drawn circuit diagram which all of us can try to verify their claims. I do not see how NFB "increases loop gain". Which gain do you mean? There is Open Loop Gain, OLG, ie, the gain without a stitch of any GNFB, and there is the "Closed Loop Gain," CLG, ie the gain when GNFB has been connected. Normally increasing the loop gain increases stability problems so that at first blush Alex's network might appear to be counter productive. The OLG is whatever it is and cannot be boosted by GNFB. The CLG at LF *can be* boosted at LF if the FB network is designed to reduce applied NFB. I want to see a schematic with all test results before I make up my mind on Alex's FB "trick." It could be a clever trick, or a swindle. However the network introduces both a zero and pole into the response, with the zero at a higher frequency than the pole. *Remember this network is just another tool in your toolbox; it is not a cure all and requires some sophistication in its application. *Now the one thing I know about stabilizing the low frequency response of a feedback system is that it is all about correctly placing the poles, and zeros if there are any. In any amp where there are say 2 CR coupled stages and a final stage with LR then you have a recipe for LF instability and a poor margin of stability at LF. If there were ways of putting LF phase shift tailoring and/or shelving networks in GNFB networks to better control LF behaviour in tube amps, then one would think thay have all been invented and would have been widely addopted bt brand names. But I ain't seen anything different to what I know and to what the many amp designers of the past have done. If the only constraint on the pole frequencies is achieving stability, then there is no problem in the first place; we can simply choose the pole frequencies to insure stability. All of what you say is jargon. In practice, getting good amp behaviour at LF is more complex, and a few sentences here land well short of providing anyone with a "how-to-do-it" better method they may follow. Unfortunately in the real world there are other constrains and requirements on the choice of pole frequencies, so we can run into problems trying to meet all the requirements at the same time. *The zero in Alex's network can be used to cancel one of the original low frequency poles, and replace it with a new pole at a lower frequency. *If you are a clever designer you may be able to use this to affect a better compromise in the choice of pole frequencies. I'll let Alex or anyone else proove that the idea works as well as forecast. Why should I waste my time doing their R&D experiments? Your network works in a similar way except that the pole frequency is higher than the frequency of the zero, and that it decreases the loop gain rather than increasing it. When ppl say such a sentence, I remain unable to gain meaning. "frequency of the zero?" Who on earth knows what that means? What this discussion needs but has not got is a means of being able to post schematics, then Alex or anyone else can post up a scanned schematic with ***ALL*** test results. REAL progress is made, rather than clouds of meaningless hot air. Patrick Turner. |
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#38
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
flipper wrote: On Wed, 22 Jun 2011 19:24:10 -0500, John Byrns wrote: In article , flipper wrote: On Sat, 18 Jun 2011 21:56:00 +1000, "Alex Pogossov" wrote: I suggested something quite opposite. The NFB divider looks like: - 1K from the speaker terminal to cathode of the driver stage; - 100R from the cathode to 100uF capacitor; - the other end of the 100uF capacitor is tied to GND. Thus the feedback "beta" increases (!) at low frequencies (below 10Hz in this case), giving -90deg phase lag in the loop below 10Hz. This is in effect turning the amp into a anti-rumble filter. From the first glance it might sound crazy to increase the loop gain at VLF where we want an overal reduction of yje loop gain, but consider this: - OPT typically gives +90deg lead below 15...20Hz; - interstage coupling (simple RC with no shelving, 220K and 0.033uF) is calculated to give -3dB corner at say 15...20Hz and also gives +90 deg lead below; - but this "funny" NFB with a 100R and 100uF gives -90deg LAG below 10Hz! And this lag maintains down until 1Hz! At LF one lag subtracts from two leads and in combination we have only +90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be crossed safely at only +90deg! Interesting idea but I'm having trouble wrapping my head around the lead/lag analysis because, if I understand your circuit correctly, that cap is at the summing junction so shouldn't it introduce poles into both the NFB and the input signal? I'm having some trouble wrapping my mind around this summing junction issue. I don't know why. I wrote it at the exact spot in the message where it matters, the lead/lag analysis, and said "lead/lag analysis." If you don't know why, then please explain exactly where the summing junction is located in this circuit? In inverting amplifiers the summing junction is usually explicit and easy to see in the schematic diagram. Non inverting amplifiers like this one are a different matter, and the summing junction is buried somewhere inside the active devices, be they tubes, transistors, or ICs. What I was trying to say is that I was trying to figure out where the "summing junction" is actually located in this circuit. What is the problem if it does introduce a pole into the input signal? The whole point of that paragraph is adding the lead/lags and if there's *another* lead/lag it changes the phase analysis. Does, it? If there is a pole added to the input circuit, it doesn't affect the "phase analysis" around the feedback loop, or the feedback stability issue, although it would affect the closed loop gain of the entire amplifier circuit, and hence what we see on the CRO after we solder the circuit together and apply a test signal to the input. That would not affect the feedback signal, and at worst would only enhance the "Rumble Filter" effect, First Patrick was obsessed with the "Rumble Filter" and now you are. I *specifically* said my question was about "the lead/lag analysis." And now it seems to be you that is obsessed with the "Rumble Filter"! In any case it's not a "Rumble Filter", it's a high pass filter with an infrasonic cutoff, it wouldn't do a thing for rumble! -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#39
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
In article ,
Patrick Turner wrote: On Jun 24, 3:34*pm, John Byrns wrote: In article , *Patrick Turner wrote: Before I get into that and what I think is its relationship to the negative feedback issue. *Aside from the feedback stability issues we are mainly concerned with, it appears obvious by inspection that in the infrasonic region below the pole frequency of Alex's network, the network reduces the potential for overload in the first stage, while your network increases the potential for first stage overload. I know for a fact that in my amps the input tube has an unused dynamic headroom of perhaps +20dB. Ie, the input tube could produce 50Vrms easily, but in my circuits maybe only has to make 5Vrms at CLIPPING! The gain shelving network I use may indeed result in V1 having to procuce a peaked response in order to keep the V0 at a constant voltage level. The peak in response is below 20Hz, and usually less than a few dB, and so the input tube NEVER EVER overloads, ie, is forced into cut off ot grid current - unless the amp is taken to clipping - perhaps, because usually the middle driver stage overloads before the input tube. The output stage usually overloads first at clipping. I can believe that the first stage of your amps never overloads at infrasonic frequencies when feed with a signal at the level needed to produce a full power output at midband frequencies, as a result of the headroom provided in the first stage, however it doesn't seem credible that the "peaked response" in the first stage is "less than a few dB". How much is "a few dB"? Have you ever measured this and made note of it in your notebook along with the relevant parameters of the amp in question, such as the amount of negative feedback applied and the characteristics of your gain stepping network? Now as far as how Alex's feedback network might help improve amplifier stability, from my perspective it is just a tool in your toolbox that requires some sophistication to apply usefully. *The first and most obvious thing Alex's network does is increase the loop gain, I think that is the correct term correct me if I have the wrong term. * I welcome any new ideas, but some ideas turn out to not be very good. I need to ss the proposer of any new ideas is ready to prove the worth of any new idea. Not too much to expect, now is it? Ppl hate proving claims. They are so often allergic to spending a day in their workshop soldering and measuring something then recording it all and writing up the experiments complete with nicely drawn circuit diagram which all of us can try to verify their claims. You yourself appear to be guilty of this very sin as far as documenting the effects of your gain stepping network on first stage overloading! I do not see how NFB "increases loop gain". I mean the sum of the "open loop gain" of the amplifier and the gain, actually negative, of the feedback network. Which gain do you mean? There is Open Loop Gain, OLG, ie, the gain without a stitch of any GNFB, and there is the "Closed Loop Gain," CLG, ie the gain when GNFB has been connected. And there is the Loop Gain, the gain around the feedback loop. Normally increasing the loop gain increases stability problems so that at first blush Alex's network might appear to be counter productive. The OLG is whatever it is and cannot be boosted by GNFB. The CLG at LF *can be* boosted at LF if the FB network is designed to reduce applied NFB. And of course the "CLG" at LF can be reduced if the FB network is designed to increase the applied NFB. Increasing the NFB at LF is the same thing as increasing the Loop Gain at LF, and decreasing the NFB at LF is the same thing as decreasing the Loop Gain at LF. I want to see a schematic with all test results before I make up my mind on Alex's FB "trick." It could be a clever trick, or a swindle. I am having trouble believing that you can't visualize Alex's circuit!! It is simply a series RC network connected between ground and the cathode of the input stage where the feedback resistor from the oPT is commonly connected, the value of the feedback resistor being adjusted to maintain the same degree of feedback at midband. However the network introduces both a zero and pole into the response, with the zero at a higher frequency than the pole. *Remember this network is just another tool in your toolbox; it is not a cure all and requires some sophistication in its application. *Now the one thing I know about stabilizing the low frequency response of a feedback system is that it is all about correctly placing the poles, and zeros if there are any. In any amp where there are say 2 CR coupled stages and a final stage with LR then you have a recipe for LF instability and a poor margin of stability at LF. If there were ways of putting LF phase shift tailoring and/or shelving networks in GNFB networks to better control LF behaviour in tube amps, then one would think thay have all been invented and would have been widely addopted bt brand names. But I ain't seen anything different to what I know and to what the many amp designers of the past have done. Remember that Alex's circuit has one drawback that displeases both bean counters and audiophiles alike, it requires adding an electrolytic capacitor to the circuit. That could explain why it was not widely used except in SS amps. If the only constraint on the pole frequencies is achieving stability, then there is no problem in the first place; we can simply choose the pole frequencies to insure stability. All of what you say is jargon. One might say the same thing about your discussions of OPTs here. Jargon is the price you pay for discussing specialized esoteric subject area. In practice, getting good amp behaviour at LF is more complex, and a few sentences here land well short of providing anyone with a "how-to-do-it" better method they may follow. I think I made the very same point in an earlier post and pointed out that Alex's network is only another tool in the designers toolbox that can be used to help assure stability, it doesn't eliminate the need for a designer know what he is doing. Unfortunately in the real world there are other constrains and requirements on the choice of pole frequencies, so we can run into problems trying to meet all the requirements at the same time. *The zero in Alex's network can be used to cancel one of the original low frequency poles, and replace it with a new pole at a lower frequency. *If you are a clever designer you may be able to use this to affect a better compromise in the choice of pole frequencies. I'll let Alex or anyone else proove that the idea works as well as forecast. Why should I waste my time doing their R&D experiments? Who has asked you to do any R&D experiments? Your attention has been called to the idea, there is no need to put this tool in your toolbox, and indeed you have chosen not to include it, hence no need for you to do any R&D. Your network works in a similar way except that the pole frequency is higher than the frequency of the zero, and that it decreases the loop gain rather than increasing it. When ppl say such a sentence, I remain unable to gain meaning. "frequency of the zero?" Who on earth knows what that means? As a wise sage once said "Google is your friend". Without taking my own advice, if you write out the transfer function of a network, IIRC the poles are basically the factors in the denominator and the zeros are the factors in the numerator, Google could doubtless give you the precise definition. What this discussion needs but has not got is a means of being able to post schematics, then Alex or anyone else can post up a scanned schematic with ***ALL*** test results. REAL progress is made, rather than clouds of meaningless hot air. I believe that there are numerous places that one could post such information. If anyone has information on this subject but doesn't want to use the photo posting services that are out there, if they send the images to me I will be happy to creat a page at my web site to display the submissions. I won't suggest that you simply post them in the relevant USENT binary group, because I know that you are using Google Groups. Many people go so far as to block posts like yours that come through Google Groups. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
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#40
Posted to rec.audio.tubes
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VLF stability in Williamson-type amplifiers
On Jun 25, 8:54*am, flipper wrote:
On Thu, 23 Jun 2011 23:11:11 -0700 (PDT), Patrick Turner wrote: After deleting Flipper's evasions and denials and weaseling out of providing any truth about a NFB idea **which may,** or **may not** be of any use, I'll leave the brain dead fish response to my questions... Oh grow up. The 'diversions' were yours. I simply corrected the errors I don't give a tinker's damn what you deign to 'accept' and "shall" not waste my time with your shalls. I didn't expect anything different from you. You are allergic to any truth you don't believe in. No, I am 'allergic' to arrogant jackasses who think they can 'order' people around. You just don't understand what Alex is proposing either. You're the odd monkey out, Patrick. And you cannot convince a living soul that Alex's method works to provide any benefit to a tube power amp. Not my job. Then you have very little to contribute to this thread, no? Patrick Turner |
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