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#1
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I built a typical phono amp with 3 12AX7s, the last tube a cathode follower
(low impedance). It has plenty of gain (70db) and can easily drive any amp directly. I like using a 100k step resistor volume control instead of a line amp. My problem is that when I use any of my tube amps, the sound has limited bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is rather impressive to me (for silicone). With my limited know-how, I'm swagging that an impedance mismatch is the culprit. If my contention is correct, is there a simple cure besides using a line amp? Perhaps some kind of buffer is called for. All comments welcomed and thank you. Cordially, west |
#2
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"west" said:
I built a typical phono amp with 3 12AX7s, the last tube a cathode follower (low impedance). It has plenty of gain (70db) and can easily drive any amp directly. I like using a 100k step resistor volume control instead of a line amp. My problem is that when I use any of my tube amps, the sound has limited bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is rather impressive to me (for silicone). With my limited know-how, I'm swagging that an impedance mismatch is the culprit. If my contention is correct, is there a simple cure besides using a line amp? Perhaps some kind of buffer is called for. All comments welcomed and thank you. You already have a buffer in the form of the 12AX7 cathode follower. Just put the pot in front of it, with a switch for phono/CD if you like. No need for another amplifying element in the chain, less is more. Better yet, get a 6SN7 for that CF function and power the heater separately, with DC offset to about cathode level. -- - Maggies are an addiction for life. - |
#3
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west wrote: I built a typical phono amp with 3 12AX7s, the last tube a cathode follower (low impedance). It has plenty of gain (70db) and can easily drive any amp directly. I like using a 100k step resistor volume control instead of a line amp. My problem is that when I use any of my tube amps, the sound has limited bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is rather impressive to me (for silicone). With my limited know-how, I'm swagging that an impedance mismatch is the culprit. If my contention is correct, is there a simple cure besides using a line amp? Perhaps some kind of buffer is called for. All comments welcomed and thank you. Cordially, west Seems like you don't know how to make your tube amps have full bandwidth. But all the well made tube amps have full BW, so make your tube amps well and no problems, OK. If the tube sound seems like there isn't full bandwidth when in fact there is, then your statement about SS amps having more subjective? BW than tube amps could be because the SS has more distortion. Usually SS has less measured THD/IMD. Have you conducted a proper AB test, set up two amp systems, one SS, the other tubed, and with same source from the same CD played feeding both amp systems, then matched the level very carefully, then switched the same speakers from one amp system to the other? Usually such a test reveals very little difference between tube or SS especially if the THD/IMD measurements are the same figures, Rout are equal, and BW is -1dB down at 20Hz to 20kHz or better. Many audiophiles hate this test because it tests their ability to tell tube and SS apart, and its a big blow to their ego when they find they cannot tell good samples of SS and tube amps apart. Similary, ppl prejudiced in favour of SS or who hate tubes especially SET amps are also given an uncomfortable lesson about reality when given the switch to switch the speakers, they cannot tell to which amp system the speakers are connected. Patrick Turner. |
#4
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"Patrick Turner" wrote in message ... west wrote: I built a typical phono amp with 3 12AX7s, the last tube a cathode follower (low impedance). It has plenty of gain (70db) and can easily drive any amp directly. I like using a 100k step resistor volume control instead of a line amp. My problem is that when I use any of my tube amps, the sound has limited bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is rather impressive to me (for silicone). With my limited know-how, I'm swagging that an impedance mismatch is the culprit. If my contention is correct, is there a simple cure besides using a line amp? Perhaps some kind of buffer is called for. All comments welcomed and thank you. Cordially, west Seems like you don't know how to make your tube amps have full bandwidth. But all the well made tube amps have full BW, so make your tube amps well and no problems, OK. If the tube sound seems like there isn't full bandwidth when in fact there is, then your statement about SS amps having more subjective? BW than tube amps could be because the SS has more distortion. Usually SS has less measured THD/IMD. Have you conducted a proper AB test, set up two amp systems, one SS, the other tubed, and with same source from the same CD played feeding both amp systems, then matched the level very carefully, then switched the same speakers from one amp system to the other? Usually such a test reveals very little difference between tube or SS especially if the THD/IMD measurements are the same figures, Rout are equal, and BW is -1dB down at 20Hz to 20kHz or better. Many audiophiles hate this test because it tests their ability to tell tube and SS apart, and its a big blow to their ego when they find they cannot tell good samples of SS and tube amps apart. Similary, ppl prejudiced in favour of SS or who hate tubes especially SET amps are also given an uncomfortable lesson about reality when given the switch to switch the speakers, they cannot tell to which amp system the speakers are connected. Patrick Turner. First thing Patrick ... tone down some, OK? If you have 150X an impedance mismatch between a preamp out & amp's input, i.e. preamp out 10K; amp input 150K, does that not account for any aberrations in the overall bandwidth passing on to the amp? Is this OK? My amps' bandwidths check out to specs and the numbers are excellent. What is an advantage to a line stage over a passive stage? Stop picking fly **** out of pepper and don't put me to the ringer to ask perfect questions. If you do not understand me, then move on to another thread. Thank you. west |
#5
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west wrote: First thing Patrick ... tone down some, OK? If you have 150X an impedance mismatch between a preamp out & amp's input, i.e. preamp out 10K; amp input 150K, does that not account for any aberrations in the overall bandwidth passing on to the amp? Firstly there is no such thing here as a 'mismatch' ! Matching in audio is only an issue when you have kilometres of cable. All that matters here is the source impedance, load impedance, cable capacitance and the resulting transfer characteristic. You built yourself a sloppy low pass filter. That's all. Get yourself a modelling program and you can see the effect of cable capacitance on a 100k attenuator for yourself. Plus the influence of load impedance on same. Graham |
#6
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Eeyore wrote: Get yourself a modelling program and you can see the effect of cable capacitance on a 100k attenuator for yourself. Plus the influence of load impedance on same. Better still, learn how to calculate it yourself ! The modelling program just makes it easy for you. A demo version will do the job. Graham |
#7
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west wrote: "Patrick Turner" wrote in message ... west wrote: I built a typical phono amp with 3 12AX7s, the last tube a cathode follower (low impedance). It has plenty of gain (70db) and can easily drive any amp directly. I like using a 100k step resistor volume control instead of a line amp. My problem is that when I use any of my tube amps, the sound has limited bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is rather impressive to me (for silicone). With my limited know-how, I'm swagging that an impedance mismatch is the culprit. If my contention is correct, is there a simple cure besides using a line amp? Perhaps some kind of buffer is called for. All comments welcomed and thank you. Cordially, west Seems like you don't know how to make your tube amps have full bandwidth. But all the well made tube amps have full BW, so make your tube amps well and no problems, OK. If the tube sound seems like there isn't full bandwidth when in fact there is, then your statement about SS amps having more subjective? BW than tube amps could be because the SS has more distortion. Usually SS has less measured THD/IMD. Have you conducted a proper AB test, set up two amp systems, one SS, the other tubed, and with same source from the same CD played feeding both amp systems, then matched the level very carefully, then switched the same speakers from one amp system to the other? Usually such a test reveals very little difference between tube or SS especially if the THD/IMD measurements are the same figures, Rout are equal, and BW is -1dB down at 20Hz to 20kHz or better. Many audiophiles hate this test because it tests their ability to tell tube and SS apart, and its a big blow to their ego when they find they cannot tell good samples of SS and tube amps apart. Similary, ppl prejudiced in favour of SS or who hate tubes especially SET amps are also given an uncomfortable lesson about reality when given the switch to switch the speakers, they cannot tell to which amp system the speakers are connected. Patrick Turner. First thing Patrick ... tone down some, OK? What tone do I turn down? bass, treble, both perhaps? Do I turn them up? That'd really mean turning the midrange down. The phono amp with 12AX7 CF output and with only 0.7mA of idle current should easily produce 0.4Arms into a load as low as 10k, which is 4Vrms. So the 100k attenuator after the CF won't cause a problem because of its value. If you have 150X an impedance mismatch between a preamp out & amp's input, i.e. preamp out 10K; amp input 150K, does that not account for any aberrations in the overall bandwidth passing on to the amp? It depends on the impedance character of the amp following the attenuator and the attenuator resistance. You have not given us any technical advice about the exact input-output resistance impedance networks which exist, but just say all your tube amps hve little bass and treble. Kindly submit the facts please when asking for a detailed technical opinion. For simplicity, let's assume you have cathode followewr, dc blocking cap = 1uF feeding 100k attenuator with output to the outside world taken directly from the wiper of the attenuator. There will be ab LF pole at xxxx Hz. Please work this out. I refuse to spoon feed you by doing calculations you should be prepared to do yourself. Hint, F, -3dB = 159,000 / ( R x C uF ) Hz. Assume the SS amp has "10k" input resistance, and dc blocking cap of 10uF to keep out the DC from any previous amp. What is the F pole? Assume you base your calulations of a setiing of the attenuator at the 12oclock position, ie, -20dB, so that the attenutaor is 90k from CF to wiper, 10k from wiper to OV. What is the actual network that you have? what is the F response at LF. What is the source resistance looking back into the attenuator? What happens to the answers to all questions as you move the wiper up the attenuator? how about downwards? Now that you have answered all those very basic questions about your gear's LF behaviour, let's consider the HF. Knowing that the following amp has 10k input resistance, we assume it is shunt R, ie, measured from input terminal after the blocking cap to 0V, and we assume it is 10k. Is there any R beyound this point that is in series after the blocking cap to the first base of the first input transistor? What is the R value? Is there any C from this base to 0V? what is the C value? If we assumed the signal source resistance was 100ohms, what is the F pole at HF caused by the series R and shunt C? Knowing that you would have signal source R = 10k, what happens to the pole, and why? What is the actual R & C filter at the input that you have when source R = 10k? What happens to the HF pole when you change the source R by raising or lowering the wiper along the attenuator track, or steps on seriesed resistors? I want you to spend more time working out anwers to your own questions. "Work it out yourself" is a statement that all members of the group should not ask, but command themsleves to do onece you roll out of bed each morning. Otherwise it won't happen, and we have to waste all our time telling you yet again and again the same old answers to the same old questions, because you are lazy. Is this OK? My amps' bandwidths check out to specs and the numbers are excellent. I have no idea, because there is not enough data upon which to make an assement. What is an advantage to a line stage over a passive stage? In my designs of preamps I often have a deletable gain stage. Often this a single µ-follower using 12AU7/6CG7 with open loop gain reduced from a maximum of about 18 down to about 5, using a shunt NFB loop. This reduces the gain block Rout also to less than 1k. See my website for details. The gain stage feeds gain pot or attenuator of at least 50k, and the wiper goes to a cathode follower grid, and output is from the cathode. What is the impedance change effeced by having a CF after the gain attenuator? Why is it put there? Stop picking fly **** out of pepper and don't put me to the ringer to ask perfect questions. I'm not picking flu **** out of peper, and insist i have the right to say what is thought provoking on a discussion group. I will NEVER give up the right to ask dozens of questions if I can so that you and all the others have to actually use your god given brain to work out the answers or remain dumb, just fed answers by me acting like a damn parrot. I am happy to give you just enough of a guide to work out answers on your own. If you do not understand me, then move on to another thread. Thank you. You are not alone in not understanding the interactions between a given preamp, and a given power amp, neither of which was PERHAPS designed to run optimally with each other. The question you have asked has been asked before in news groups. Rather than give a short generic one statement fits all answer, except its never enough, I will give you the full story, depending on where I think you are according to skill, abilities as I see them, and sometimes that means you have to go and take all the covers off your amps, trace all the circuits, do it all over to make sure there is no mistake, measure responses involved, and then calculate out what you should be getting and then work out a fully detailed cure to problems, then carry out the mods and check everything. Don't just post a winge about poor bandwidth on a NG concerning your own gear about which we assume you are responsible for correct working. I am not there to look over your shoulder to guide your soldering or to be your tutor with the most very basic frequency response issues with R&C networks. And you have not offered to pay me for my professional services. The weather here today was a glorious autumn day of about 21C. I rode by bicycle about 85km, not sure how far; I was out for about 4.5 hours. It seemed to go a bit better after cleaning off a pound of oily grunge on the chain and gears before i left. Afterwards the pool beckoned, and a short swim of 300M got the arms and back re-aliged after the ride. I sometimes ask, Why do they have the swim as the first item in an triathlon? It'd make more sense to make it the last thing, so that when contestents got to the podium finish, they'd be clean and refreshed, but probably embarrassed appearing in a swimsuit, or pair of budgy smugglers as we call them here. Men are such fickle creatures. I'm too old and decrepit to run 26 miles after a swim of 3 miles and ride of 100 miles. The run is the real killer; the other two can almost be done by age challenged personel such as meself, who I know ain't as good as I was. I'm happy doing 80kM thse days by bike, only 50 miles, and not very fast, I like to see the scenery, and not worry about competion, or what anyone else might think about what i am doing, or not doing. Being Easter Sunday, not many others were going very fast and I passed a large number on my way around the town. One guy I know cycled nearly every day until he was about 84, I think a bad hip eventually crumbled. But my sporting hero is Hubert Opperman, see http://canberrabicyclemuseum.com.au/...he_cyclist.htm This man makes me look like a fool. I bet he asked a lot of questions of those around him, and of himself. Patrick Turner. west |
#8
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Patrick:
Something to keep in mind, a bit of poetry from Schiller: Against stupidity, the gods themselves labor in vain. In a slightly lighter vein, from Dorothy Parker: You can lead a horticulture, but you can't make her think. What makes *you* think that you can do any better than these two against invincible ignorance? Peter Wieck Wyncote, PA |
#9
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Peter Wieck wrote: Patrick: Something to keep in mind, a bit of poetry from Schiller: Against stupidity, the gods themselves labor in vain. In a slightly lighter vein, from Dorothy Parker: You can lead a horticulture, but you can't make her think. What makes *you* think that you can do any better than these two against invincible ignorance? Nothing makes **ME** think I can do any better than the persuasive Dorothy Parker types of this world, or any gods in another world. I know I cannot really have any great effect here on this planet, I am but one very insignificant male gender person of an unromantic character and undesirable age and un-photogenic appearance who farnarkles around with tubes in audio amp circuits, guided by a tiny amount of applied intelligence. I have zero claims to fame, and have gotten use to the fact I will never be famous, never be rich, so I may as well try to teach my craft if I cannot sell it much or obtain breathtaking reviews in Stereophile, for which I would have to pay an obscenely large amount of money. I still get a nice sense of satisfaction when someone plays some music through my gear and they look at me and say things like, "why did I waste all that money on that other crap?" I sometimes feel like the circus performer still doing the trapeze at age 60.... But at least if I fall, its only off the chair to the ground, not from 50 feet up. I do hope its not an unwanted sustained shock from 500 volts that sends me to the floor. One can only hope, try to be good.... People should not take my apparent terseness as a sign I am fed up with ppl, I ain't, and I still enjoy talking about the audio manufacturing experience. It seems only natural to me to try to encourage the best efforts from everyone I meet, so they benefit the most. Is hard to treasure quality based on cost cutting and absent minded design, and I urge tinkerers to consider the long term effects of what they are doing. I once was extending the house of a university arts professor in 1982. He'd had me build a couple of beautiful rooms, then he engaged a pool builder to build a nice 12 metre swimming pool in the large yard beside his tennis court. He had the most stunningly attractive daughter, age 17, and when the pool was filled, she liked to soak up sun quite topless which slightly swayed my gaze and delayed completion of the work I did on the house nearby. Her father came to me one day to ask where he should place a pool fence and what height it should be. He said he'd like to use a brush fence, which would be a screen for privacy. So I said let's take a walk around the outside of your property, and look back at it from the point of view of understanding the mind of a passing person looking in, as we might look back upon ourselves, and so that we may better undertsand the situation, and ourselves. The guy got the drift of what I was saying. Very soon, an optimal solution about where the fencing was to be placed, and a fencing contractor was hired, and Bridgette could bask in all her glory without passerbuyers peering in. I had to feel sorry for the fencing guy, a young fellow I knew well. Bridgette "hung out" while he tried to work, and I comiserated, but mentioned casually that if he laid a finger on the girl he'd be in deep dodo. And so would I have been, had I tried anything. So there are times in life when we must be wise, step outside of ourselves, and look at ourselves from afar, and ask, who is that man? Then we must let the thoughts about ourselves flow. And there are times we must step firmly on our tongue, and sit heavily on our hands, lest a cock drag us to regret. I recall that after I completed work for that family, and moved on, I used to occasionally meet Bridgette in the disco bars around town, and we often had a chat, and a dance, a drink, but she was a rich man's young daughter, and I was the slightly old swarthy hired help from across town, and simply not amoung the right class of person for her to **** with. I've never cared that I have spent most of my life working in dirty gear, and being lower caste. I feel no shame. But at least i knew where to put a fence up, and how to speak to his highness about it, without mentioning his princess for a moment, and without either of us being embarrased, and without the slightest hint of a fee, because for a lot of the time I just enjoy talking to people. Patrick Turner. Peter Wieck Wyncote, PA |
#10
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west wrote: I built a typical phono amp with 3 12AX7s, the last tube a cathode follower (low impedance). It has plenty of gain (70db) and can easily drive any amp directly. I like using a 100k step resistor volume control instead of a line amp. My problem is that when I use any of my tube amps, the sound has limited bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is rather impressive to me (for silicone). With my limited know-how, I'm swagging that an impedance mismatch is the culprit. If my contention is correct, is there a simple cure besides using a line amp? Perhaps some kind of buffer is called for. All comments welcomed and thank you. You're using a 100k 'pot' effectively to drive a cable.. With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz ! The transistor amps will have a lower input impedance that loads this which will push the turnover frequency out of the audible area (just). You should never drive a cable from a pot or attenuator of high value. Try using a 10k stepped attenuator instead. The whole point of cathode followers is to drive lower impedances. Graham |
#11
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Eeyore wrote
You're using a 100k 'pot' effectively to drive a cable.. With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz ! Little? 1nF is over 10 metres of ordinary audio coax. The transistor amps will have a lower input impedance that loads this which will push the turnover frequency out of the audible area (just). Equally, reducing cable length to less than 1m would be just about OK. You should never drive a cable from a pot or attenuator of high value. High is entirely relative. Try using a 10k stepped attenuator instead. The whole point of cathode followers is to drive lower impedances. Yes, assuming all sources have output resistances of less than 1k, but doesn't your explanation equally lead to the conclusion that the problem could be solved by reducing the input resistance of the valve amps? That would be a much cheaper solution, but not a good one IMO. All things in proportion. cheers, Ian |
#12
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Ian Iveson wrote: Eeyore wrote You're using a 100k 'pot' effectively to drive a cable.. With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz ! Little? 1nF is over 10 metres of ordinary audio coax. That depends what you call ordinary. What do you use and what's its capacitance ? Graham |
#13
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Eeyore wrote
You're using a 100k 'pot' effectively to drive a cable.. With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz ! Little? 1nF is over 10 metres of ordinary audio coax. That depends what you call ordinary. What do you use and what's its capacitance ? Erm...it's pearlescent and thick, so it may be this: http://www.maplin.co.uk/module.aspx?...2&doy=8m4#spec but around 90pF/m seems common. I am using a 100k pot too...but with only 1/2m of cable, until I have finished my remote controlled pre. 10k will be a better load for my sources. cheers, Ian |
#14
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PS
You just helped me solve a serious problem. On inspection, I found I had one of my cables pointing the wrong way. Now I have turned it round the sound is...er...hmm...perhaps it wasn't so serious after all. Apart from the arrows, the cable says "Shark wire - 80 * 0.1 - OFC - Aud/Vid" regularly, three times per metre. It seems quite proud of itself, even though it is too thick for Neutrik plugs unless you dispense with the collets. cheers, Ian |
#15
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Ian Iveson wrote: Eeyore wrote You're using a 100k 'pot' effectively to drive a cable.. With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz ! Little? 1nF is over 10 metres of ordinary audio coax. That depends what you call ordinary. What do you use and what's its capacitance ? Erm...it's pearlescent and thick, so it may be this: http://www.maplin.co.uk/module.aspx?...2&doy=8m4#spec but around 90pF/m seems common. I am using a 100k pot too...but with only 1/2m of cable, until I have finished my remote controlled pre. 10k will be a better load for my sources. cheers, Ian With a 100k log pot set at -20dB, R source = 9k. 1.2M of cable at 90pF/M = 108pF. The pole caused by pot and cable = 163kHz. The worst HF losses are where pot is set for -6dB, R source = 25k, so the pole is at 59kHz. So a 100k pot would offer no audible losses over a 10k pot. The 10k pot would load a previous CF with too low a load if tubed. Just because we have a triode set up as a CF which gives low Rout 1k, it doesn't mean we are permitted to use loads lower that the common cathode gain stage. Ideally a 47k load on a 1/2 6SN7 should be used for eithe ranode of cathode load. Where the THD may have been 0.1% at 1V out with anode loading, it becomes 0.0066% when the CF is used, but if the load = 10k, perhaps THD becomes 0.02% at a volt output. So thus the gains brought by the CF are eroded if the load is reduced. The bandwidth won't change at the CF; its Rout = 1/gm in parallel with RL, or about 500 ohms and whether RL =10k or 100k does not make any great difference. Patrick Turner. |
#16
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"Eeyore" wrote in message ... west wrote: I built a typical phono amp with 3 12AX7s, the last tube a cathode follower (low impedance). It has plenty of gain (70db) and can easily drive any amp directly. I like using a 100k step resistor volume control instead of a line amp. My problem is that when I use any of my tube amps, the sound has limited bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is rather impressive to me (for silicone). With my limited know-how, I'm swagging that an impedance mismatch is the culprit. If my contention is correct, is there a simple cure besides using a line amp? Perhaps some kind of buffer is called for. All comments welcomed and thank you. You're using a 100k 'pot' effectively to drive a cable.. With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz ! The transistor amps will have a lower input impedance that loads this which will push the turnover frequency out of the audible area (just). You should never drive a cable from a pot or attenuator of high value. Try using a 10k stepped attenuator instead. The whole point of cathode followers is to drive lower impedances. Graham Your answer is what I suspected but not having much experience, I asked this NG. I was looking for a Tube Basic 101 generic answer that I did not find in my reference books. I don't understand where Patrick is coming from. I have never asked for specific help. That's the most enjoyable aspect of this hobby (for me), is rolling your own, having some problem, and ultimately solving it. Thanks again Graham. west |
#17
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west wrote:
I built a typical phono amp with 3 12AX7s... CF... 100k step resistor volume control ,,,My problem is that when I use any of my tube amps, the sound has limited bandwidth. A SS amp does not exhibit this problem You're using a 100k 'pot' effectively to drive a cable... With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz ! Your answer is what I suspected but not having much experience, I asked this NG. I was looking for a Tube Basic 101 generic answer that I did not find in my reference books. I don't understand where Patrick is coming from.... (1) the limited bandwidth problem being only there is presumably due to the high output impedance (100k step resistors not the same as 100k potentiometer, which is not the same as a 100k output impedance though - you might not even need 100pF to get a serious high frequency cut). The test is to measure (on a scope or whatever, with a low capacitance probe) directly at the output socket, with no cable connected, what is going on with the frequency response. The trouble with this test is that you said it is a phono preamp, so you have to rig up a reverse RIAA circuit for your signal generator's output. Another, easier, test would be to shove a 22k resistor or so across the output and then see if the valve amps are then happy. Presumably the solid state amp's low input impedance is reducing the effects of the RC filter you make with the interconnect cable. Or the valve amps have lots of capacitance/Miller effect at their input and want a lower input impedance (not so likely to the a significant factor I guess). If the oscilloscope test shows a high frequency cut is still going on, maybe the RIAA values are wrong and maybe the SS amp has some slight high frequency lift (some nfb circuits do this) that is compensating - two wrongs making a right?? (pretty unlikely). (2) I get the feeling Patrick sometimes growls about the questioners as a sign of friendship. Mark A |
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Mark Aitchison wrote: 100k step resistors not the same as 100k potentiometer Would you care to explain your philosphy behind that statement ? Graham |
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Eeyore wrote:
Mark Aitchison wrote: 100k step resistors not the same as 100k potentiometer Would you care to explain your philosphy behind that statement ? Graham 100 k step resistors means step resistors of 100k each, so total resistance = N times 100k; 100k potentiometer means 100k from end to end. The important question was whether the potentiometer is 100k end-to-end, and my initial reading of the posting was that the step resistors are 100k each (which makes a v-e-r-y high value potentiometer!)... everyone else worked out it was short-hand for a 100k potentiometer, which makes more sense. If this is a stepped volume control, with 100k between steps, the h-f has to be really bad, so bad as to be obviously wrong. |
#20
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Mark Aitchison wrote: west wrote: I built a typical phono amp with 3 12AX7s... CF... 100k step resistor volume control ,,,My problem is that when I use any of my tube amps, the sound has limited bandwidth. A SS amp does not exhibit this problem You're using a 100k 'pot' effectively to drive a cable... With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz ! Your answer is what I suspected but not having much experience, I asked this NG. I was looking for a Tube Basic 101 generic answer that I did not find in my reference books. I don't understand where Patrick is coming from.... (1) the limited bandwidth problem being only there is presumably due to the high output impedance (100k step resistors not the same as 100k potentiometer, which is not the same as a 100k output impedance though - you might not even need 100pF to get a serious high frequency cut). The test is to measure (on a scope or whatever, with a low capacitance probe) directly at the output socket, with no cable connected, what is going on with the frequency response. The trouble with this test is that you said it is a phono preamp, so you have to rig up a reverse RIAA circuit for your signal generator's output. Another, easier, test would be to shove a 22k resistor or so across the output and then see if the valve amps are then happy. Presumably the solid state amp's low input impedance is reducing the effects of the RC filter you make with the interconnect cable. Or the valve amps have lots of capacitance/Miller effect at their input and want a lower input impedance (not so likely to the a significant factor I guess). If the oscilloscope test shows a high frequency cut is still going on, maybe the RIAA values are wrong and maybe the SS amp has some slight high frequency lift (some nfb circuits do this) that is compensating - two wrongs making a right?? (pretty unlikely). (2) I get the feeling Patrick sometimes growls about the questioners as a sign of friendship. Indeed, so GGGRRRRRRR! If only petioners would apply what they were supposed to have learnt.... I'd like people to not stagger around in the dark. Patrick Turner. Mark A |
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west wrote:
I built a typical phono amp with 3 12AX7s... CF... 100k step resistor volume control ...My problem is that when I use any of my tube amps, the sound has limited bandwidth. A SS amp does not exhibit this problem [re-editing my previous post that contained errers] My understanding of "step resistor" is the resistance between volume control positions* - but does "100k step resistor volume control" mean a volume control made from 100k step resistors (perhaps around 2M total?!) or "a 100k volume control that contains step resistors"? I jumped to the conclusion it was the first, which isn't a sensible conclusion, thinking about it. You're using a 100k 'pot' effectively to drive a cable... With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz ! Your answer is what I suspected but not having much experience, I asked this NG. I was looking for a Tube Basic 101 generic answer that I did not find in my reference books. I don't understand where Patrick is coming from.... (1) the limited bandwidth problem being only there _when driving a valve amp_ is presumably due to the high output impedance (not the same as a 100k output impedance though - depends on volume setting). The test is to measure (on a scope or whatever, with a low capacitance probe) directly at the output socket, with no cable connected, what is going on with the frequency response. The trouble with this test is that you have to rig up a reverse RIAA circuit for your signal generator's output. Another, easier, test would be to shove a 22k resistor or so across the output and then see if the valve amps are then happier (albeit with a low volume). If the test shows a high frequency cut is still going on, maybe the RIAA values are wrong and maybe the SS amp has some slight high frequency lift (some nfb circuits do this) that is compensating - two wrongs making a right?? (pretty unlikely). How much "limited bandwidth" are we talking about, by the way? *defined at http://ww1.microchip.com/downloads/e...es/01080a.pdf: The Step resistance. This is the change in resistance that occurs between two adjacent wiper register values. It is also the RAB resistance divided by the number of RS resistors (resolution) in the Resistor Ladder. Mark A |
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west wrote
I built a typical phono amp with 3 12AX7s, the last tube a cathode follower (low impedance). It has plenty of gain (70db) and can easily drive any amp directly. I like using a 100k step resistor volume control instead of a line amp. My problem is that when I use any of my tube amps, the sound has limited bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is rather impressive to me (for silicone). With my limited know-how, I'm swagging that an impedance mismatch is the culprit. If my contention is correct, is there a simple cure besides using a line amp? Perhaps some kind of buffer is called for. You don't say which part of the bandwidth goes missing. A resistance ratio of at least 1:10 generally does the job, as a rule of thumb. The max output resistance of a 100k attenuator is 25k + source resistance/4. Its minimum input resistance is its load resistance in parallel with 100k. What are the input resistances of the SS and valve power amps? Bandwidth isn't hard to measure. Input resistance not so easy, but in the case of the valve amps you can look at the circuits. Incidentally, don't be conned by the AB test thing, West. A difference cannot be heard, only inferred. For a meaningful inference, the listener must be trained in AB testing. Training is a self-fulfilling prophecy. All comments welcomed and thank you. Hostage to fortune...you have failed to keep this promise. cheers, Ian |
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west wrote:
I built a typical phono amp with 3 12AX7s, the last tube a cathode follower (low impedance). It has plenty of gain (70db) and can easily drive any amp directly. I like using a 100k step resistor volume control instead of a line amp. My problem is that when I use any of my tube amps, the sound has limited bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is rather impressive to me (for silicone). With my limited know-how, I'm swagging that an impedance mismatch is the culprit. If my contention is correct, is there a simple cure besides using a line amp? Perhaps some kind of buffer is called for. All comments welcomed and thank you. Cordially, west It's not an impedance problem, West. You've built a filter consisting of the 100K pot and the following cable's capacitance. Reduce the pot to 10k, shorten the cable to no more than a meter (3ft 3in) or preferably 18 inches, and your problem should go away. Or you can move the pot to before the cathode follower to break the link with the cable capacitance. Don't worry about the wretched kibbitzers who will now wield their little bit of knowledge on you like a club. This is an error that is often made by professional designers and sometimes even gets into production. As a matter of taste rather than electronics, you should also lose the 12AX7, which are suitable only for guitar amps, and build your preamp with octals, preferably 6SL7 and 6SN7. The CF in particular should be a 6SN7. If you don't want to reengineer the chassis for the bigger sockets, at least try the 6CG7. Another taste tip: A 12AT7 for practical purposes has as much gain as the 12AX7 and sounds more precise. You can also, if you have the gain in another stage, engineer in more warmth with the 12AU7. HTH. Andre Jute Visit Jute on Amps at http://members.lycos.co.uk/fiultra/ "wonderfully well written and reasoned information for the tube audio constructor" John Broskie TubeCAD & GlassWare "an unbelievably comprehensive web site containing vital gems of wisdom" Stuart Perry Hi-Fi News & Record Review |
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Andre Jute wrote: It's not an impedance problem, West. You've built a filter consisting of the 100K pot and the following cable's capacitance. Reduce the pot to 10k, Good Lord. You followed Poopie's advice ! Graham |
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Eeyore wrote: Andre Jute wrote: It's not an impedance problem, West. You've built a filter consisting of the 100K pot and the following cable's capacitance. Reduce the pot to 10k, Good Lord. You followed Poopie's advice ! Graham Who is Poopie? What a disgusting name! Andre Jute Our legislators managed to criminalize fox-hunting and smoking; when they will get off their collective fat backside and criminalize negative feedback? It is clearly consumed only by thickoes like Graham Stevenson. |
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Andre Jute wrote: Our legislators managed to criminalize fox-hunting and smoking; when they will get off their collective fat backside and criminalize negative feedback? You wouldn't understand the value of well-applied negative feedback if it stood up and smacked in you in the face. Graham |
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Eeyore wrote: Andre Jute wrote: Our legislators managed to criminalize fox-hunting and smoking; when they will get off their collective fat backside and criminalize negative feedback? You wouldn't understand the value of well-applied negative feedback if it stood up and smacked in you in the face. Graham Dear Graham, Andre comes at you with the mildest of proposterous wit, and you talk of smackings. Get thee thither, find a sense of humourator before 'tis two late. Patrick Turner. |
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Andre Jute wrote: This is an error that is often made by professional designers and sometimes even gets into production. No it isn't. Certainly not any designer I know. Graham |
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Eeyore wrote: Andre Jute wrote: This is an error that is often made by professional designers and sometimes even gets into production. No it isn't. Certainly not any designer I know. In this particular case I think there are too many one liner replies which are less than very informative. The original poster complained of poor bandwidth and probably it is a case of the R component being too high to get decent BW with respect to whatever C is present. But as I pointed out before, the R with an attenuator varies. And a 100k pot will, when hooked to a low Rout CF have maximum Ro at the -6dB position and Rout = 25kpot. But at -20dB, its 10k in parallel to 90k = 9k, and at -26dB, its much lower again, and as you reduce the Rout of the attenuator, HF banwidth gets higher since the same C causes a rising pole with lower shunt R. My previous long post was about the whole issue, not just parts of it, and made strong, even obnoxious hints that anyone with such response problems need to analyse carefully before applying any solution, and teach themselves some basic R&C network theory in the process, and they'd then be well equipped for fixing the next curly problem that arrises, and about which they might post. Patrick Turner. Graham |
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Patrick Turner wrote: Eeyore wrote: Andre Jute wrote: This is an error that is often made by professional designers and sometimes even gets into production. No it isn't. Certainly not any designer I know. In this particular case I think there are too many one liner replies which are less than very informative. You aren't so innocent, Patrick, that you imagine Poopie is capable of an informative answer, are you? Poor bugger is still trying to master joined-up writing, never mind joined-up thinking! Just in case you want to send him an example of a complete answer, with subsidiary thoughts. here is what I sent again: ******** Andre Jute wrote: It's not an impedance problem, West. You've built a filter consisting of the 100K pot and the following cable's capacitance. Reduce the pot to 10k, shorten the cable to no more than a meter (3ft 3in) or preferably 18 inches, and your problem should go away. Or you can move the pot to before the cathode follower to break the link with the cable capacitance. Don't worry about the wretched kibbitzers who will now wield their little bit of knowledge on you like a club. This is an error that is often made by professional designers and sometimes even gets into production. As a matter of taste rather than electronics, you should also lose the 12AX7, which are suitable only for guitar amps, and build your preamp with octals, preferably 6SL7 and 6SN7. The CF in particular should be a 6SN7. If you don't want to reengineer the chassis for the bigger sockets, at least try the 6CG7. Another taste tip: A 12AT7 for practical purposes has as much gain as the 12AX7 and sounds more precise. You can also, if you have the gain in another stage, engineer in more warmth with the 12AU7. HTH. Andre Jute Visit Jute on Amps at http://members.lycos.co.uk/fiultra/ "wonderfully well written and reasoned information for the tube audio constructor" John Broskie TubeCAD & GlassWare "an unbelievably comprehensive web site containing vital gems of wisdom" Stuart Perry Hi-Fi News & Record Review ******** On second thoughts, better not to confuse poor Poopie. He might ask you what "thought" is, and you will never finish explaining to him... Andre Jute A little, a very little thought will suffice -- Lord Keynes The original poster complained of poor bandwidth and probably it is a case of the R component being too high to get decent BW with respect to whatever C is present. But as I pointed out before, the R with an attenuator varies. And a 100k pot will, when hooked to a low Rout CF have maximum Ro at the -6dB position and Rout = 25kpot. But at -20dB, its 10k in parallel to 90k = 9k, and at -26dB, its much lower again, and as you reduce the Rout of the attenuator, HF banwidth gets higher since the same C causes a rising pole with lower shunt R. My previous long post was about the whole issue, not just parts of it, and made strong, even obnoxious hints that anyone with such response problems need to analyse carefully before applying any solution, and teach themselves some basic R&C network theory in the process, and they'd then be well equipped for fixing the next curly problem that arrises, and about which they might post. Patrick Turner. Graham |
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Andre Jute wrote: Poor bugger is still trying to master joined-up writing, never mind joined-up thinking! Projecting again I see ! Graham |
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On Apr 8, 10:16 am, "Andrew Jute McCoy" distilled:
... a bunch of good stuff cribbed from the rest of the thread. As always. When are you going to have an original thought or be the 'first' with good advice? Typically, a snippet here and a snippet there... Ah, well. Your little Timmee will lap it up and thank you for it. Patrick gets in in one and is criticized for detail. You say nothing in highlights... Let's see if West actually gets it right, and if so, based on which set of suggestions. Peter Wieck Wyncote, PA |
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Peter Wieck wrote: "Andrew Jute McCoy" distilled: ... a bunch of good stuff cribbed from the rest of the thread. As always. When are you going to have an original thought or be the 'first' with good advice? Android McCoy hasn't had an original thought in the entire time I've been posting here. Graham |
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"Eeyore" wrote in
message Peter Wieck wrote: "Andrew Jute McCoy" distilled: ... a bunch of good stuff cribbed from the rest of the thread. As always. When are you going to have an original thought or be the 'first' with good advice? Android McCoy hasn't had an original thought in the entire time I've been posting here. The style of writing, if you call what McCoy does *style*, seems origional enough. ;-) |
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On Apr 7, 5:57 pm, "west" wrote:
I built a typical phono amp with 3 12AX7s, the last tube a cathode follower (low impedance). It has plenty of gain (70db) and can easily drive any amp directly. I like using a 100k step resistor volume control instead of a line amp. My problem is that when I use any of my tube amps, the sound has limited bandwidth. A SS amp does not exhibit this problem, if fact the SS sound is rather impressive to me (for silicone). With my limited know-how, I'm swagging that an impedance mismatch is the culprit. If my contention is correct, is there a simple cure besides using a line amp? Perhaps some kind of buffer is called for. All comments welcomed and thank you. Not exactly an impedance mismatch. Say an amp (doesn't matter if it's tube or solid state) plus interconnect cable has an input capacitance of 100pF. This is on the high end of reasonable but it doesn't take many feet of coax to get there. And many power amps (especially tube ones) will have a input circuit that looks like a few hundred k ohms in series with a capacitor of 50 pF or so. Then at 20kHz, it will have a reactance of 1/2*pi*f*C, which is 80k. So if your preamp has an output impedance of 100K, you are a little more than 3db down at 20kHz. No need for spice modeling. No need for anything other than pencil and paper. If you have a cathode follower as the last stage of your preamp, you could probably be using a 10K attenuator at the output, but I don't know what else you might doing to spoil an already low output impedance stage. Tim. |
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