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Old 12th March 2012, 07:41 AM   #101
M Gregg is offline M Gregg  United Kingdom
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Originally Posted by cnpope View Post
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By the way, I am a great fan of OTL amps, and have built three of them, using the designs of Hans Beijner, Tim Mellow and Alan Kimmel.

Chris
Which one is your favorite amp and why?

Just for interest the CT of the power supply has a big impact on the sound of the amp and also any values of series resistance such as R33 used in the Tim Mellow. It seems to "shift" the frequency response that is heard. I never tested this but it is easy to hear. Not like is this cap better than that cap. The bass changes. This may be to do with damping factor and speaker control of the OP stage.

Only my opinion, I think the power supply used in most of the OTL's is over simplified and more attention should have been spent on the design. IT seems to be one of the highest complaints on the web.

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M. Gregg
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Old 12th March 2012, 02:26 PM   #102
cnpope is offline cnpope  United States
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Originally Posted by M Gregg View Post
Which one is your favorite amp and why?

Just for interest the CT of the power supply has a big impact on the sound of the amp and also any values of series resistance such as R33 used in the Tim Mellow. It seems to "shift" the frequency response that is heard. I never tested this but it is easy to hear. Not like is this cap better than that cap. The bass changes. This may be to do with damping factor and speaker control of the OP stage.

Only my opinion, I think the power supply used in most of the OTL's is over simplified and more attention should have been spent on the design. IT seems to be one of the highest complaints on the web.

Regards
M. Gregg
I think probably I like the Tim Mellow amp the best. It seems to be the most stable (in the sense of the output offset drifting the least, and the quiescent current through the output tubes staying steady). Possibly my implementation of the Hans Beijner design is a bit flawed, with the transformers, capacitors, etc. of the bias supplies getting rather too hot because of proximity to a slightly inadequate heater transformer for the 6C33Cs, and that might explain the tendency of the HB amp to drift a bit more.

When I built the Tim Mellow amp I was uneasy with the imbalance between the positive and negative main supply rail voltages, and so i just shorted out R33 (meaning the centre-tap of the transformer is connected to ground). More recently, I tried reintroducing an R33, but with a much lower value than in Tim Mellow's schematic (82 ohms rather than 1K). I can't say I noticed any difference in the sound, but I have to admit that my ears seem not to be very good at picking up the differences that others apparently can hear. I would tend to believe measurements more than my own ears, I think! I will try making a few comparative measurements.

It is, by the way, interesting if you do have a resistor between the centre-tap and ground to try putting a scope on the centre-tap. The crud that one sees there is quite something!

It is true that the power supply is typically less than ideal. With 2000 uF capacitors, there is something of order 1.5V "ripple" (in line with what is expected from I = C dV/dt) even under quiescent conditions. But things must balance out pretty well, because even with my ear next to my 98dB Lowthers, hum is essentially inaudible.

Chris
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Old 13th March 2012, 12:07 AM   #103
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Originally Posted by cnpope View Post
I think then we are in complete agreement; there is more to a power supply than just the audio signal path. My previous comments were specifically addressing the restricted question of the audio path, and I was pointing out that regardless of the centre-tap connection or not, the capacitors are part of the audio signal path.

By the way, I am a great fan of OTL amps, and have built three of them, using the designs of Hans Beijner, Tim Mellow and Alan Kimmel.

Chris
It is good that we agreed on something that is a given , I mean about the power supply , and it would be nice if we saw one of your creations on the forum .
As for the CT , Ιt is a part of the power supply and if it's connected ( to the capacitors and to earth of course ) the amp will have lower output impedance and therefore higher damping factor , this happens because with this way the capacitors are not in series with the output tubes ( like output capacitors ) and the speaker but they are in parallel with these ( the power supply capacitors are always in parallel with the output stage and the speakers ) .
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Old 13th March 2012, 01:05 AM   #104
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Originally Posted by cnpope View Post

When I built the Tim Mellow amp I was uneasy with the imbalance between the positive and negative main supply rail voltages, and so i just shorted out R33 (meaning the centre-tap of the transformer is connected to ground). More recently, I tried reintroducing an R33, but with a much lower value than in Tim Mellow's schematic (82 ohms rather than 1K).

It is, by the way, interesting if you do have a resistor between the centre-tap and ground to try putting a scope on the centre-tap. The crud that one sees there is quite something!


Chris
Your own words Evidenced that the CT is a part of the power supply and a very important one , I wonder how you don't see that .
Αnd if you put an oscilloscope on the edges of R33 ( CT and earth ) you will see a waveform exactly similar to the audio output signal but certainly on a smaller scale , this mean that this resistance increases the impedance of the power supply ( because it is inserted between the secondary windings and the power supply capacitors ) and thus increases the output impedance of the amp .
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Old 13th March 2012, 04:20 AM   #105
cnpope is offline cnpope  United States
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Originally Posted by Dimitris AR View Post
Your own words Evidenced that the CT is a part of the power supply and a very important one , I wonder how you don't see that .
Αnd if you put an oscilloscope on the edges of R33 ( CT and earth ) you will see a waveform exactly similar to the audio output signal but certainly on a smaller scale , this mean that this resistance increases the impedance of the power supply ( because it is inserted between the secondary windings and the power supply capacitors ) and thus increases the output impedance of the amp .
No, actually the waveform on the CT is rather horrific; not at all like the audio output. That is the most striking feature. It is dominated by a pulse related to the mains frequency. When I get the chance, maybe next weekend, I will try measuring the output impedance with the centre-tap connected and with it not connected. But since Tim Mellow reports about 0.4 ohm output impedence with a 1Kohm resistor for R33, and I measured about 0.3 ohm output impedance with a direct short circuit in place of R33, then I suspect that within experimental error/difference between his amp and mine, there is essentially no perceptible change from connecting the centre-tap or not. I don't think the calculated value of the output impedance is affected in any significant way by the connection of the centre tap.

Chris
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Old 13th March 2012, 09:09 AM   #106
M Gregg is offline M Gregg  United Kingdom
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Originally Posted by cnpope View Post
No, actually the waveform on the CT is rather horrific; not at all like the audio output. That is the most striking feature. It is dominated by a pulse related to the mains frequency.

Chris
I have not measured,

However I guess these are the charge pulses to each side of the supply to caps via the diode bridge. So without the CT the caps charge across the +/- as one supply ie 300V. With CT each side charges as a seperate supply via half of the bridge.
Theoretically the charge cycle is dependant on ESR of supply caps or balance resistors for charge without CT.
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Old 13th March 2012, 11:32 AM   #107
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Originally Posted by M Gregg View Post

However I guess these are the charge pulses to each side of the supply to caps via the diode bridge. So without the CT the caps charge across the +/- as one supply ie 300V. With CT each side charges as a seperate supply via half of the bridge.
Theoretically the charge cycle is dependant on ESR of supply caps or balance resistors for charge without CT.
Regards
M. Gregg
All that you say I've said in previous post ( post #92 e.t.c ) with different words , it would be better to read all the posts before and then subscribe , there is no need to say the same thing twice .
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Old 13th March 2012, 11:38 AM   #108
cnpope is offline cnpope  United States
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Originally Posted by M Gregg View Post
I have not measured,

However I guess these are the charge pulses to each side of the supply to caps via the diode bridge. So without the CT the caps charge across the +/- as one supply ie 300V. With CT each side charges as a seperate supply via half of the bridge.
Theoretically the charge cycle is dependant on ESR of supply caps or balance resistors for charge without CT.
Regards
M. Gregg
Yes, absolutely. Here are two photos showing what I am talking about. In the first photo, the upper trace is the audio output signal, and the lower trace is the signal on the centre-tap of the transformer (with 82 ohm resistor to ground). Both are 2V/div. There is no sign of the audio signal on the transformer centre-tap.

In the second photo, the upper trace is showing the voltage on the +160V line, showing how it decays (by about 1.3V) during each half cycle of the mains frequency, and then rapidly recharges. The lower trace is again the voltage on the transformer centre tap, illustrating how the ugly pulse correlates with the recharging phase.

Chris
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File Type: jpg trace2.jpg (129.2 KB, 288 views)
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Old 13th March 2012, 09:00 PM   #109
M Gregg is offline M Gregg  United Kingdom
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Originally Posted by cnpope View Post
Yes, absolutely. Here are two photos showing what I am talking about. In the first photo, the upper trace is the audio output signal, and the lower trace is the signal on the centre-tap of the transformer (with 82 ohm resistor to ground). Both are 2V/div. There is no sign of the audio signal on the transformer centre-tap.

In the second photo, the upper trace is showing the voltage on the +160V line, showing how it decays (by about 1.3V) during each half cycle of the mains frequency, and then rapidly recharges. The lower trace is again the voltage on the transformer centre tap, illustrating how the ugly pulse correlates with the recharging phase.

Chris
Very interesting,

It would be interesting to see a trace of center point of caps with and without CT. Then a duel trace of CT connected each side of R33.
The traces you show don't even show a significant modulation from the audio out which would seem to say that the caps are in circuit for both situations, this would not account for the change in sound...except that the charge pulse " is ?" present at the cap Center point?

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M. Gregg
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Old 13th March 2012, 09:44 PM   #110
cnpope is offline cnpope  United States
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Originally Posted by M Gregg View Post
Very interesting,

It would be interesting to see a trace of center point of caps with and without CT. Then a duel trace of CT connected each side of R33.
The traces you show don't even show a significant modulation from the audio out which would seem to say that the caps are in circuit for both situations, this would not account for the change in sound...except that the charge pulse " is ?" present at the cap Center point?

Regards
M. Gregg
Yes, the power supply capacitors are in the signal path equally much in either case. Also, it is obvious that there must be a "flatlining" of the signal on the centre tap for most of the mains frequency cycle (as is very clear in the traces), since for most of the mains cycle the output voltage from the power transformer is less than the DC voltages across the capacitors, the rectifier diodes are then completely non-conducting, and so the "hot" ends of the power transformer secondary are effectively completely isolated. Clearly, therefore, no current can flow out from the centre-tap during the majority of the mains cycle. This is one way to see why the centre-tap cannot possibly have a clean copy of the audio signal on it.

By the way, I suspect that the reason why in my traces the signal on the centre tap is a definite upward pulse is because of an imbalance between the capacitance of the B+ capacitors and the capacitance of the B- capacitors. (Electrolytic capacitors are notoriously imprecise in their tolerances.) I'll try to check this when I get the chance. I suspect that if I add a bit more capacitance to the one that is at present smaller, then the height of the pulse should reduce, and it should be possible to make it become instead a negative pulse if enough capacitance is added. If this is right, then it would seem that if one is going to have a non-zero R33 then it would be good to "pad" the smaller capacity bank so as to balance it with the other bank. I'll report back after I get the chance to check this.
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