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#1
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Hi Z Buffer Distortion
What are the dominant distortion mechanisms when one tries to build a
unity-gain (or greater than unity gain) buffer to buffer a signal with a source impedance of, say, 100 megohms, at audio frequencies? Do vacuum tubes suffer from the capacitance-modulation effects that FETs do? Thanks, Sean B |
#2
Posted to rec.audio.pro
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Hi Z Buffer Distortion
On Apr 6, 1:18*pm, "
wrote: What are the dominant distortion mechanisms when one tries to build a unity-gain (or greater than unity gain) buffer to buffer a signal with a source impedance of, say, 100 megohms, at audio frequencies? *Do vacuum tubes suffer from the capacitance-modulation effects that FETs do? Thanks, Sean B is this for a condensor mic buffer? Mark |
#3
Posted to rec.audio.pro
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Hi Z Buffer Distortion
On Mon, 6 Apr 2009 16:12:24 -0400, "Soundhaspriority"
wrote: I do not have expertise in this area, but there is probably wide agreement that vacuum tube solutions have higher distortion than FETs. This is widely expressed, but fundamentally incorrect. An appropriately used vacuum valve can give linearity with transfer exponents less than 1.05, and exteremely low distortion, and at high signal levels, and monotonically. Many years of deliberate abuse have given vacuum valves a reputation as distortion generators, but it comes from the abuse, and not as a fundamental of the device. For example, a classic type 12AX7 made back in the good old days (when men were men and women liked it that way) by real craftswomen, can deliver +20dBu outputs with less than -40dBu total distortion products (before any feedback). No single semiconductor can deliver even remotely similar performance in linearity. Type 5687 in larger devices and type 211 in really big devices are also very linear. Vacuum valves have an additional advantage for use from capacitive sources because signal voltages don't modulate their input capacitance, ehich is purely parasitic. Many hi-Z sources (the OP didn't specify) tend to the capacitive. I seem to always be defending some old technology or another, but, hey, audio's old tech. I can live with that. Much thanks, as always, Chris Hornbeck |
#4
Posted to rec.audio.pro
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Hi Z Buffer Distortion
On Mon, 6 Apr 2009 21:23:50 -0400, "Soundhaspriority"
wrote: It would seem that, among commercial mike designs, the following is generally true (and I will be very interested in the exceptions!) 1. Solid state is used by the top makers, DPA, Schoeps, Neumann and Sennheiser, for designs that are admired, more or less, for neutrality and low noise. Some or most of the designs may no longer use the single FET impedance converter, which you have by implication impugned. 2. Tube mikes are noisy, colored, and favored for the human voice.. I am sure that other members of this forum can point out interesting exceptions. There would be little point, however, in saying that a high-spec measurement mike can be built better with a vacuum tube impedance converter, unless there is a product to point to. Well, if you want to get all practical, and ever'thing like that, well, yeah. Doesn't mean I have to stop listening to the Beach Boys! Or something like that... Said by someone who may own one of the last working specimens of a Tektronix 555 oscilloscope, which has around 129 tubes and draws 1250 watts. At least, it worked the last time I checked My (inherited) 503 is still used for some loudspeaker measurements - excellent differential input sensitivity and "common mode rejection". Still nothing better in my low-dollar world (and I have the complete battery of Sound Technology gear from the 1970's in mostly working condition. Keeping stuff from the 1950's and 1960's going is actually easier than the 70's stuff right up to the point of death). Also still useful, and sometimes used, is an old HP 120B that can do 10mV/cM differential - good enough for most loudspeaker work, immune, like all vacuum valve stuff, from over-voltage issues, and probably free these days. Within the audio range, a buffer stage and a sound card's conversion to computer manipulation is a huge step beyond squinting at an old scope's screen, but above the audio range important things still happen, and a real 'scope is still needed to try (!) to see what they are. Much thanks, as always, Chris Hornbeck |
#5
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Hi Z Buffer Distortion
On Mon, 6 Apr 2009 23:08:09 -0400, "Soundhaspriority"
wrote: Chris, the Beach Boys are obsolete, having been replaced by superior forms of music. I suspect you probably listen to Simon & Garfunkel as well. Please, get a hold of yourself and stop the slide toward deviancy. You can go slow. Start by listening to some of these three or four hours a day: http://knowledgerush.com/kr/encyclop...amous_rappers/ I work with a young man who makes great rap music (folks willing to sign into MySpace (I think - not personally willing) can hear him as Chuck Sweet. But I'm too old to put it into the same category as the Beach Boys. Chuck probably wouldn't either; I'll ask him tomorrow. We'll know in forty years. (Except, I'll be dead - maybe that's the difference...) Much thanks, as always, Chris Hornbeck |
#6
Posted to rec.audio.pro
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Hi Z Buffer Distortion
"Soundhaspriority" wrote in message
... wrote in message ... What are the dominant distortion mechanisms when one tries to build a unity-gain (or greater than unity gain) buffer to buffer a signal with a source impedance of, say, 100 megohms, at audio frequencies? Do vacuum tubes suffer from the capacitance-modulation effects that FETs do? Thanks, Sean B I do not have expertise in this area, but there is probably wide agreement that vacuum tube solutions have higher distortion than FETs. Not necessarily, and particularly not when high source impedances are involved. As the OP notes, there are major problems with capacitance-modulation in FETs, which can lead to remarkably high levels of distortion if the source impedance is above a few tens of kilohms. Vacuum tubes typically have extremely low levels of capacitance-modulation, which makes them good candidates to buffer very high impedances. A *properly-designed* vacuum tube circuit can have very low levels of distortion; 0.01% harmonic distortion at audio frequencies isn't particularly hard to achieve, depending on the level you want to put out. Tubes aren't very good at driving low impedances; if the load on the buffer is going to be lower-impedance, than a good solid-state driver circuit following the buffer is in order. So the first two questions for the OP a what exactly do you expect to be buffering? And what will the buffer have to drive, and at what level? Also, no active device exists that has zero capacitance, because such a device would have the theoretically impossible ability to measure without disturbance. This is forbidden by physics. True but irrelevant. The issue isn't capacitance by itself, but capacitance which is modulated by signal level. That adds distortion to the signal at high frequencies when the impedance is high, sometimes plenty of it (like 2-3% THD, and presumably comparable levels of high-frequency IMD). Peace, Paul |
#7
Posted to rec.audio.pro
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Hi Z Buffer Distortion
" wrote: What are the dominant distortion mechanisms when one tries to build a unity-gain (or greater than unity gain) buffer to buffer a signal with a source impedance of, say, 100 megohms, at audio frequencies? Do vacuum tubes suffer from the capacitance-modulation effects that FETs do? You might get a good technical answer in sci.electronics.design. Graham |
#8
Posted to rec.audio.pro
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Hi Z Buffer Distortion
Soundhaspriority wrote: Correction, Should read: Vacuum tubes disappeared from instrumentation with the advent of the FET back in the late 60's. I can still remember my Tek 545 scopes ! And the fan at the back. Graham |
#9
Posted to rec.audio.pro
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Hi Z Buffer Distortion
Chris Hornbeck wrote: On Mon, 6 Apr 2009 16:12:24 -0400, "Soundhaspriority" wrote: I do not have expertise in this area, but there is probably wide agreement that vacuum tube solutions have higher distortion than FETs. This is widely expressed, but fundamentally incorrect. An appropriately used vacuum valve can give linearity with transfer exponents less than 1.05, and exteremely low distortion, and at high signal levels, and monotonically. If used as a buffer stage ( cathode follower ) the linearity would be quite good. Not sure how it'd fare noise wise though. Ultra low noise and THD solid state circuitry could follow it. Graham |
#11
Posted to rec.audio.pro
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Hi Z Buffer Distortion
Paul Stamler wrote: "Soundhaspriority" wrote in message Also, no active device exists that has zero capacitance, because such a device would have the theoretically impossible ability to measure without disturbance. This is forbidden by physics. True but irrelevant. The issue isn't capacitance by itself, but capacitance which is modulated by signal level. That adds distortion to the signal at high frequencies when the impedance is high, sometimes plenty of it (like 2-3% THD, and presumably comparable levels of high-frequency IMD). Believe it or not I've seen this effect in mic amps designed to handle high signal level with the now unobtainium bipolar Rohm 2SB737 input device. Being large geometry to get thermal noise down, Rbb was only about 4 ohms ! Ccb was also large and acted like a varicap diode and you could see the distortion increasing over about 10kHz. Graham |
#12
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Hi Z Buffer Distortion
On Apr 7, 6:45*am, Eeyore
wrote: Soundhaspriority wrote: Correction, *Should read: *Vacuum tubes disappeared from instrumentation with the advent of the FET back in the late 60's. I can still remember my Tek 545 scopes ! And the fan at the back. Graham That was a great scope and very rugged! :-) |
#13
Posted to rec.audio.pro
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Hi Z Buffer Distortion
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#14
Posted to rec.audio.pro
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Hi Z Buffer Distortion
1. The distortion issues with high-Z input followers and gain stages are all
the same as with conventional follower and gain stage circuits, PLUS a whole raft of other problems which are all related to parasitic capacitance. 2. The input capacitance of both tubes and FETs is modulated by signal but in different ways. For an introduction to how the effect works for tubes, look at the data sheet for any reactance modulator tube. 3. One good solution to deal with capacitances that change with modulation is to reduce the modulation. Look at the B&K 2615 as an example of an extremely-low-distortion tube design. Notice what is done to sacrifice noise floor for low distortion. 4. Even the worst input stage has far less distortion than even the best output transformer. 5. With any competently-designed microphone today, the distortion due to mechanical and acoustical issues will be far, far higher than the distortion due to any of the electronics. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#15
Posted to rec.audio.pro
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Hi Z Buffer Distortion
In article ,
Eeyore wrote: Soundhaspriority wrote: Correction, Should read: Vacuum tubes disappeared from instrumentation with the advent of the FET back in the late 60's. I can still remember my Tek 545 scopes ! And the fan at the back. I have one on my bench right now and it's just fine, even though the cal guys are always making fun of it and trying to give me an Agilent digital scope to replace it. The reason tubes disappeared from instrumentation had more to do with stability than anything else... the 545 really needs to warm up for an hour before it's reasonably stable and you can't use anything in it for quantitative measurement directly. A hell of a lot of work went into making tube circuits stable at all over long time periods. Korn and Korn's book on analogue computer circuits is fascinating to read to look at some of the stuff they did. A lot of it is still applicable today, too. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#16
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Hi Z Buffer Distortion
In article 49e22ecd.76189031@localhost, Don Pearce wrote:
There should be no capacitance modulation in a unity gain buffer since the voltage between gate and source does not change with the input signal. This is true, but most microphones do not have the front end operating in unity gain. Many older designs in fact use a huge amount of gain on the front end, driving a step-down transformer. All active devices have inherent non-linearities, but the degree to which they appear in a complete circuit depends inversely on the degree of feedback that corrects them. That in turn depends on the amount of open loop gain before feedback. Generally that is much higher for the FET than the tube, so the linearity of the FET circuit is likely to be better. Well, the thing is that although the gain of the FET is probably higher so that you can put more feedback on it, the straightline section of the curve isn't as flat as with, say, a frame grid tube. So you pays your money and you takes you chance. (Once you get out of the straightline section of the curves, all bets are off too). Of much greater concern is the fact that the source impedance is of the order of 100 megohms. Even the odd pF or so of capacitance will kill the top end of the audio - a cable connection is certainly out of the question. Yes, and you can have all kinds of parasitic issues too... with 1G to 10G input impedances, if you have wires shaking around even that can create modulation. Poor wiring layout and PC board layout are big issues. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#17
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Hi Z Buffer Distortion
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#19
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Hi Z Buffer Distortion
On 7 Apr 2009 11:50:42 -0400, (Scott Dorsey) wrote:
In article 49ec730e.93657937@localhost, Don Pearce wrote: On 7 Apr 2009 11:29:17 -0400, (Scott Dorsey) wrote: In article 49e22ecd.76189031@localhost, Don Pearce wrote: There should be no capacitance modulation in a unity gain buffer since the voltage between gate and source does not change with the input signal. This is true, but most microphones do not have the front end operating in unity gain. Many older designs in fact use a huge amount of gain on the front end, driving a step-down transformer. For a microphone preamp sure - the source is grounded and capacitance modulation will happen. But not in the OP's unity gain buffer. No. Open up any typical microphone and you will usually see the front end is a gain stage and not a cathode follower. You will sometimes see followers, and it's a good idea for the reason you mention, but most of the classic microphone designs are gain stages. I think you misread me. I am saying that you are right about a microphone, which is a gain stage, hence prone to capacitance modulation, but the OP's post was about a unity gain buffer, which isn't. d |
#20
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Hi Z Buffer Distortion
Thanks for all the responses!
Yes it is a condenser capsule I am buffering. One with around 80 pF of capacitance. I am messing around with a mic design. I would like to drive loads as low as 1500 ohms, in order for a user to be able to use mic preamps with input transformers. But my personal philosophy is that the mic preamp should be near the performer. It seems ridiculous to knock the impedance down with a transformer in the mic and then up in the preamp; to have 2 transformers in the path that aren't really necessary if you just place the preamp close to the performer. What would be a good tube type and bias current to drive 1500 ohms to about +14 dBu on the very highest peaks or so? I'd like to point out that there is a small voltage change between grid and cathode, or gate and source, in any real-world follower that has a load attached and signal on it's input. If there were no change in voltage difference there would be no output signal, since the device's open-loop gain is finite. I don't know in practice if this small a wiggle is big enough to produce capacitance-modulation related distortion with JFETs (Mosfets are too noisy for this app) or tubes. I also wouldn't say that Jfets always have higher Gm than tubes. There are Jfets that have a Gm of only a couple thousand microMhos, and tubes with 5000 or more. Gm is quite dependent on bias current as well. Scott, what is the capacitance modulation behavior for a typical tube like a 12AU7? I've never seen this mentioned on a tube data sheet, but then I've only looked at data sheets for the most common types. One more question: grid to plate, grid to cathode, and cathode to plate are all obvious terms for a tube's capacitances, but what do the terms "input" capacitance and "output" capacitance really mean? I've never found a proper definition for these terms. Check out page 3 for capacitance versus voltage behavior for a high Gm Jfet: http://www.datasheetcatalog.org/data...shiba/1027.pdf Thanks! Sean B |
#21
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Hi Z Buffer Distortion
wrote:
Scott, what is the capacitance modulation behavior for a typical tube like a 12AU7? I've never seen this mentioned on a tube data sheet, but then I've only looked at data sheets for the most common types. You'll have to measure it. The HB-3 has numbers for some devices, but not too many. The 12AU7 is a crappy microphone tube for a lot of different reasons, including microphonics. One more question: grid to plate, grid to cathode, and cathode to plate are all obvious terms for a tube's capacitances, but what do the terms "input" capacitance and "output" capacitance really mean? I've never found a proper definition for these terms. Input capacitance is the capacitance seen at the input of the circuit... that it, it's the grid-to-cathode capacitance, plus grid-to-plate capacitance, plus and minus some stuff on the outside. Likewise output capacitance is the capacitance seen at the output of the circuit.... it's plate to cathode capacitance plus and minus some stuff. There is a discussion of a lot of this stuff in Terman's book _Radio Engineering_ as well as in the Radiotron Handbook. Check out page 3 for capacitance versus voltage behavior for a high Gm Jfet: http://www.datasheetcatalog.org/data...shiba/1027.pdf This is a lot wider variation than you will ever see with a tube, which is unfortunate for people designing reactance tube modulators for FM. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#22
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Hi Z Buffer Distortion
Hey guys -- we still don't know what the original poster wants to buffer.
Everyone's assuming it's a condenser mic capsule -- but he didn't say that, and we don't know it. Peace, Paul |
#23
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Hi Z Buffer Distortion
wrote in message
... Yes it is a condenser capsule I am buffering. One with around 80 pF of capacitance. I am messing around with a mic design. I stand corrected -- now we know what's going on. I would like to drive loads as low as 1500 ohms, in order for a user to be able to use mic preamps with input transformers. But my personal philosophy is that the mic preamp should be near the performer. It seems ridiculous to knock the impedance down with a transformer in the mic and then up in the preamp; to have 2 transformers in the path that aren't really necessary if you just place the preamp close to the performer. What would be a good tube type and bias current to drive 1500 ohms to about +14 dBu on the very highest peaks or so? The thing is, tubes don't really like that kind of impedance. The maximum current you'd be swinging is about +/- 3.7 mA, so in theory something biased at about 5mA would work -- maybe a 6SN7. But it wouldn't really be low distortion. My suggestion: use a tube to buffer the capsule, then something like a Schoeps solid-state circuit to create a balanced output that'll happily drive 1500 ohms. Peace, Paul I'd like to point out that there is a small voltage change between grid and cathode, or gate and source, in any real-world follower that has a load attached and signal on it's input. If there were no change in voltage difference there would be no output signal, since the device's open-loop gain is finite. I don't know in practice if this small a wiggle is big enough to produce capacitance-modulation related distortion with JFETs (Mosfets are too noisy for this app) or tubes. I also wouldn't say that Jfets always have higher Gm than tubes. There are Jfets that have a Gm of only a couple thousand microMhos, and tubes with 5000 or more. Gm is quite dependent on bias current as well. Scott, what is the capacitance modulation behavior for a typical tube like a 12AU7? I've never seen this mentioned on a tube data sheet, but then I've only looked at data sheets for the most common types. One more question: grid to plate, grid to cathode, and cathode to plate are all obvious terms for a tube's capacitances, but what do the terms "input" capacitance and "output" capacitance really mean? I've never found a proper definition for these terms. Check out page 3 for capacitance versus voltage behavior for a high Gm Jfet: http://www.datasheetcatalog.org/data...shiba/1027.pdf Thanks! Sean B |
#24
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Hi Z Buffer Distortion
On Tue, 07 Apr 2009 18:45:02 GMT, "Paul Stamler"
wrote: Hey guys -- we still don't know what the original poster wants to buffer. Everyone's assuming it's a condenser mic capsule -- but he didn't say that, and we don't know it. We know it isn't a condenser mic capsule - he said it had a source impedance of 100 megohms. Having said that, I have no idea what that might be. d |
#25
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Hi Z Buffer Distortion
The 12AU7 is a crappy microphone tube for a lot of different reasons, including microphonics. * What are the other reasons? I often like the sound of low-mu tubes, and the 12AU7 has a decent current drive capability for a noval tube. Low capacitance, too. I don't think a tube like a 6SN7 will fit in this mic. Also, a lower mu gives you more headroom at the grid, if you do want to use a plate- loaded configuration. Especially in a mic, where you're inclined to use lower plate voltages for noise reasons. Sean B |
#26
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Hi Z Buffer Distortion
On Apr 7, 10:23*am, (GregS) wrote:
In article , wrote: On Apr 7, 6:45=A0am, Eeyore wrote: Soundhaspriority wrote: Correction, =A0Should read: =A0Vacuum tubes disappeared from instrument= ation with the advent of the FET back in the late 60's. I can still remember my Tek 545 scopes ! And the fan at the back. Graham That was a great scope and very rugged! :-) I had to fix a 565 recently and several plug ins. I hate the multi component stuff. transistors, Nuvistors, and tubes. I had to put in a nos crt in the scope. Funny thing, the rear connector had corosion around a couple pins only, and ate the pins alltogether. Couple weak tubes and was ready to go. Troubleshooting the HV was the main problem, and was an internal crt short. greg Parts, especially tubes, will wear out. Given the age of the hardware, some things will go bad. These scopes are antiques. However, the switches seemed to be almost indestructable compared to what followed in the later scope series. |
#27
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Hi Z Buffer Distortion
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#28
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Hi Z Buffer Distortion
On Apr 7, 1:31*pm, "
wrote: Thanks for all the responses! Yes it is a condenser capsule I am buffering. *One with around 80 pF of capacitance. *I am messing around with a mic design. I would like to drive loads as low as 1500 ohms, in order for a user to be able to use mic preamps with input transformers. *But my personal philosophy is that the mic preamp should be near the performer. *It seems ridiculous to knock the impedance down with a transformer in the mic and then up in the preamp; to have 2 transformers in the path that aren't really necessary if you just place the preamp close to the performer. *What would be a good tube type and bias current to drive 1500 ohms to about +14 dBu on the very highest peaks or so? I'd like to point out that there is a small voltage change between grid and cathode, or gate and source, in any real-world follower that has a load attached and signal on it's input. *If there were no change in voltage difference there would be no output signal, since the device's open-loop gain is finite. *I don't know in practice if this small a wiggle is big enough to produce capacitance-modulation related distortion with JFETs (Mosfets are too noisy for this app) or tubes. I also wouldn't say that Jfets always have higher Gm than tubes. There are Jfets that have a Gm of only a couple thousand microMhos, and tubes with 5000 or more. *Gm is quite dependent on bias current as well. Scott, what is the capacitance modulation behavior for a typical tube like a 12AU7? *I've never seen this mentioned on a tube data sheet, but then I've only looked at data sheets for the most common types. One more question: grid to plate, grid to cathode, and cathode to plate are all obvious terms for a tube's capacitances, but what do the terms "input" capacitance and "output" capacitance really mean? *I've never found a proper definition for these terms. Check out page 3 for capacitance versus voltage behavior for a high Gm Jfet: http://www.datasheetcatalog.org/data...shiba/1027.pdf Thanks! Sean B Sean, the reason i asked about the condensor mic is that the condensor mic ITSELF suffers from a (very small) distortion due to capacitance modulation... so if you connect it to a device that also has some capacitance modulation and you get the polarities correct, you might even get some first order cancellation. But this is all theory, in reality these effects are very very small and unless you are dealing with some really loud sounds and/or very critical instrument type application, I would say you shuold probably have bigger fish to fry than the distortion caused by capacitance modulation from a singal of a few mV. Mark |
#29
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Hi Z Buffer Distortion
On Apr 7, 2:06*pm, (Scott Dorsey) wrote:
wrote: Scott, what is the capacitance modulation behavior for a typical tube like a 12AU7? *I've never seen this mentioned on a tube data sheet, but then I've only looked at data sheets for the most common types. You'll have to measure it. *The HB-3 has numbers for some devices, but not too many. The 12AU7 is a crappy microphone tube for a lot of different reasons, including microphonics. * One more question: grid to plate, grid to cathode, and cathode to plate are all obvious terms for a tube's capacitances, but what do the terms "input" capacitance and "output" capacitance really mean? *I've never found a proper definition for these terms. Input capacitance is the capacitance seen at the input of the circuit... that it, it's the grid-to-cathode capacitance, plus grid-to-plate capacitance, plus and minus some stuff on the outside. Likewise output capacitance is the capacitance seen at the output of the circuit.... it's plate to cathode capacitance plus and minus some stuff. There is a discussion of a lot of this stuff in Terman's book _Radio Engineering_ as well as in the Radiotron Handbook. Check out page 3 for capacitance versus voltage behavior for a high Gm Jfet: http://www.datasheetcatalog.org/data...shiba/1027.pdf This is a lot wider variation than you will ever see with a tube, which is unfortunate for people designing reactance tube modulators for FM. --scott -- "C'est un Nagra. *C'est suisse, et tres, tres precis." the input capacitance of tube (triode) amp CIRCUIT will be dominated by the grid to plate cap which is magnified by the Miller effect. So the effective value of CIRCUIT input capacitnace is not simply determined by the capacitance between the elements in the tube alone but the amount of gain is also a factor. Mark |
#30
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Hi Z Buffer Distortion
On Apr 7, 2:58 pm, (Don Pearce) wrote:
We know it isn't a condenser mic capsule - he said it had a source impedance of 100 megohms. Having said that, I have no idea what that might be. Probably a piezoelectric instrument pickup. He must have heard the term "buffer" applied to interfacing a high impedance source with the real world, and didn't want to use the old, tired, "Hi Z guitar input" terminology that we'd all understand. |
#31
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Hi Z Buffer Distortion
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#32
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Hi Z Buffer Distortion
wrote in message ... The 12AU7 is a crappy microphone tube for a lot of different reasons, including microphonics. What are the other reasons? It's less linear than the 6SN7 family, for one. And microphonics in this application are a deal-breaker. I often like the sound of low-mu tubes, and the 12AU7 has a decent current drive capability for a noval tube. Low capacitance, too. I don't think a tube like a 6SN7 will fit in this mic. The 6CG7/6FQ7 is a 6SN7 in a noval envelope. It's taller than a 12AU7, but the same diameter. Very, very linear, and if you bias it up to a high enough current, low-noise as well. Another alternative, popular among tube mic manufacturers, is the EF86, which I gather is back in production, or the somewhat similar 5879. These are pentodes, but were popular in microphones when triode-wired. Don't know the linearity specs, but they're quiet and very non-microphonic, and they're both about the size of a 12AU7. Peace, Paul |
#33
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Hi Z Buffer Distortion
On Apr 8, 1:29*am, "Paul Stamler" wrote:
The 6CG7/6FQ7 is a 6SN7 in a noval envelope. It's taller than a 12AU7, but the same diameter. Very, very linear, and if you bias it up to a high enough current, low-noise as well. Thanks, Paul! Have you ever heard the type 5687 triode? The curves look similar to the 6CG7. |
#34
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Hi Z Buffer Distortion
wrote in message
... On Apr 8, 1:29 am, "Paul Stamler" wrote: The 6CG7/6FQ7 is a 6SN7 in a noval envelope. It's taller than a 12AU7, but the same diameter. Very, very linear, and if you bias it up to a high enough current, low-noise as well. Thanks, Paul! Have you ever heard the type 5687 triode? The curves look similar to the 6CG7. I haven't myself, but it has a good reputation; I gather each section behaves approximately like two 6CG7 (=6SN7) sections in parallel, making it a pretty powerful tube for a small-signal triode. However, IIRC it runs quite hot, making it perhaps a poor choice for a microphone. And the input capacitance is on the high side, again IIRC. Peace, Paul |
#35
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Hi Z Buffer Distortion
On Sun, 12 Apr 2009 07:25:12 GMT, "Paul Stamler"
wrote: wrote in message Thanks, Paul! Have you ever heard the type 5687 triode? The curves look similar to the 6CG7. I haven't myself, but it has a good reputation; I gather each section behaves approximately like two 6CG7 (=6SN7) sections in parallel, making it a pretty powerful tube for a small-signal triode. However, IIRC it runs quite hot, making it perhaps a poor choice for a microphone. And the input capacitance is on the high side, again IIRC. The 5687's are a dual, with huge cathodes and twice the heater power of the already hefty 6SN7/6CG7/6FQ7 family. The big cathodes and other good geometry gives them very sharp low-current knees and excellent linearity, maybe the best of the 3 to 10 mA/V bottles (and!, they're at the top end of that gm group). And when the military dumped their supply a few years ago, they were a stone bargain. But as you say, they're not a first choice for stuffing into an existing mic body. How important is gm to this gig? Valves with 1.5 mA/V (with 1.0 mA plate current) and exceptional linearity are still available in a 7-pin as type 6AV6. Its triode is a half of a 12AX7, probably the most linear device (into a suitably high impedance, but also into a somewhat reactive load) that *can* be made, and "NOS" devices shouldn't cost much of anything. (Selection for microphonics will still be a project for *any* valve for this gig, natch). Random thoughts, Much thanks, as always, Chris Hornbeck |
#36
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Hi Z Buffer Distortion
Chris Hornbeck wrote:
But as you say, they're not a first choice for stuffing into an existing mic body. How important is gm to this gig? Valves with 1.5 mA/V (with 1.0 mA plate current) and exceptional linearity are still available in a 7-pin as type 6AV6. Its triode is a half of a 12AX7, probably the most linear device (into a suitably high impedance, but also into a somewhat reactive load) that *can* be made, and "NOS" devices shouldn't cost much of anything. (Selection for microphonics will still be a project for *any* valve for this gig, natch). 6AV6 and the ever-popular 6AU6 pentode are both used in AA5 table radios and therefore are quite plentiful. The bad news is that there are a lot of noisy ones out there so you may have to do some selection. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#37
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Hi Z Buffer Distortion
"Chris Hornbeck" wrote in message
... But as you say, they're not a first choice for stuffing into an existing mic body. How important is gm to this gig? Valves with 1.5 mA/V (with 1.0 mA plate current) and exceptional linearity are still available in a 7-pin as type 6AV6. Its triode is a half of a 12AX7, probably the most linear device (into a suitably high impedance, but also into a somewhat reactive load) that *can* be made, and "NOS" devices shouldn't cost much of anything. Naah; the 12AX7 isn't anything like the most linear tube out there. A 6SL7 can beat it, and so can a 5751, but if you want real clean behavior, a good 6SN7 or its relatives beats the crap out of any higher-mu tube. Peace, Paul |
#38
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Hi Z Buffer Distortion
On Tue, 14 Apr 2009 05:26:21 GMT, "Paul Stamler"
wrote: type 6AV6. Its triode is a half of a 12AX7, probably the most linear device (into a suitably high impedance, but also into a somewhat reactive load) that *can* be made, and "NOS" devices shouldn't cost much of anything. Naah; the 12AX7 isn't anything like the most linear tube out there. A 6SL7 can beat it, and so can a 5751, but if you want real clean behavior, a good 6SN7 or its relatives beats the crap out of any higher-mu tube. I'll not argue too much - the last vacuum valve amplifier that I built (in 1994) used 1/2 of a type 6SL7 driving paralleled sections of a type 6SN7 driving a type 845, famous for its drive neediness (over 300 volts peak-to-peak; yikes). Good bottles of both these types are really, really good. My favorites are the old Sylvania's with cylindrical cathodes, but these days the hot tip is to use the 12 volt heater versions (for availability!) or other un-hip voltages. And plain vanilla Russians are surprisingly good - not much to go wrong with these types really. A good 12AX7/ECC83 is in this same ballpark, and IME more reliably linear (into very high Z optimised loading, yada yada). "Good" here means Telefunken's and large geometry Amperex's. Also the old large geometry TungSol's and RCA's if sorted for noise are great. There are lots of crappy, inexcusable 12AX7 type devices these days - they are *not* 12AX7's - anything that looks Chinese is probably a 12AT7 geometry (and worthless for linear audio). Never seen a really linear 5751, but then I've sure not seen everything! And they have an excellent rep. Much thanks, as always, Chris Hornbeck |
#39
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Hi Z Buffer Distortion
Chris Hornbeck wrote:
There are lots of crappy, inexcusable 12AX7 type devices these days - they are *not* 12AX7's - anything that looks Chinese is probably a 12AT7 geometry (and worthless for linear audio). As far as I know, all of these tubes are from the same Chinese factory. It's the same one that makes the "KT88" tubes that have a maximum 250V plate voltage on the data sheet. Never seen a really linear 5751, but then I've sure not seen everything! And they have an excellent rep. To be honest, there are a LOT of compactrons out there that are intended for use in TV tuners, with very low noise and very low input capacitance, which sell for nothing and are excellent performers in high-Z circuits. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#40
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Hi Z Buffer Distortion
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