I would hope Mr. Dick Sequerra is a clever chap. Being the might behind Day Sequerra Broadcast monitors is pretty impressive in itself. But if you look back on Mr. Sequerra's history, he had a major hand in the design of the Marantz 10 tuner, among other products. I would put him down in the category of legendary audio designers myself. I envy Mr. Curl, Mr. Wurcer, etc. for enjoying his company and learning from him. Of course meeting and learning from either of those two, among others would be a very enlightening honor for a lowly audio/electronics fool as I. 🙂
Peace,
Dave
Peace,
Dave
Ah! You mean the ultimate audio amp guy! Of course I know his site and project. Very interesting with intimidating specs.
Have fun, Hannes
Have fun, Hannes
Sid Smith, was one of the best amplifier designers, ever.
I only met him once over dinner in NY with Dick Sequerra and JC Morrison. We got along great!
I did not know at the time, but between 1968-70, the reference tube amp that I was comparing my new comp-diff input design with, was designed by Sid Smith. IT beat me! Yet both amps had essentially the same output power, damping factor, distortion characteristic,etc. That is what encouraged me to try harder and find out why there was a difference. Please remember, we noted damping factor, distortion, output level, stability with a cap load, and frequency response in the 1960's. Yes, 40 years ago. Many here think this is something new, for some reason.
I only met him once over dinner in NY with Dick Sequerra and JC Morrison. We got along great!
I did not know at the time, but between 1968-70, the reference tube amp that I was comparing my new comp-diff input design with, was designed by Sid Smith. IT beat me! Yet both amps had essentially the same output power, damping factor, distortion characteristic,etc. That is what encouraged me to try harder and find out why there was a difference. Please remember, we noted damping factor, distortion, output level, stability with a cap load, and frequency response in the 1960's. Yes, 40 years ago. Many here think this is something new, for some reason.
john curl said:
Hi John,
I think the reason is that it is in your head. You don't give other designers on this board enough credit, especially those who do not share all of your design philosophy.
I take it your solid state amplifier had a pretty low damping factor. 🙂.
Cheers,
Bob
john curl said:
Pretty low for a solid state design; pretty high for a tube design. Sid must have used a lot of negative feedback 🙂. Was Sid's design a triode?
Cheers,
Bob
I did not use a Darlington driver stage, that is why my damping was so low.
Sid's amp WAS triode, and that is why it was so high. I measured it myself.
I used about 40dB of negative feedback, Sid used about 20dB. That was the DIFFERENCE that lead me to Otala, and his conclusions.
Sid's amp WAS triode, and that is why it was so high. I measured it myself.
I used about 40dB of negative feedback, Sid used about 20dB. That was the DIFFERENCE that lead me to Otala, and his conclusions.
Someone forwarded me a link to a Doug Self article in which he says he can get good results without grotesque levels of feedback. No, he doesn't inform the reader what he regards as too much NFB. He then goes on to deem 'eccentric' an amplifier with 'only' 20dB of feedback. If you track down his reference, the 'eccentric' amplifier is an Otala design.
One man's floor is another man's ceiling, and all that...I regard 20dB NFB as an upper limit.
Grey
One man's floor is another man's ceiling, and all that...I regard 20dB NFB as an upper limit.
Grey
John,
is that your helpful way of saying it's closed loop gain bandwidth limitations coming in that impinge on stability?
or am I completely wrong?
is that your helpful way of saying it's closed loop gain bandwidth limitations coming in that impinge on stability?
or am I completely wrong?
I rather guess it could have something to do with the parasitic capacitances, those of tubes are more stable (not signal dependent) compared to silicon.
- Klaus
- Klaus
AndrewT said:
When somebody gives “advice” in such cryptic terms, it’s probably because they don’t really know what they are talking about or are being intentionally disingenuous.
In valve power amplifiers it is the open loop bandwidth and phase limitations imposed by the output-coupling transformer that ultimately puts a practical limitation on the amount of global negative that can be applied before the amplifier becomes an oscillator (typically 20-30dB).
It is silly to infer that solid state amplifiers should therefore be designed with valve amplifier levels of global negative feedback due to the performance limitations of an obsolete technology.
Motor boating is a low frequency oscillation caused by the lousy power supply rejection of typical valve amplifier stages (often caused by supply rail fluctuations of the power output stage being fed back to the supply rail for the low power/level stages).
It is a potential problem regardless if the amplifier uses no (zero, nil, nada) global negative feedback or 30dB of global negative feedback.
Heavy RC decoupling of the low level (input stage / phase splitter / driver stage) plate supply rails avoids motor boating. Motorboating is a common fault that develops in valve power amplifiers in which the supply rail decoupling capacitors have dried up and gone high impedance with age (I’ve got one on my workbench ATM).
Applying high levels of global negative feedback to a valve amplifier can incur low frequency or subsonic oscillation if there are too many interacting low-frequency roll-offs in addition to that of the output transformer in the open loop response (each inter-stage RC coupling provides one high-pass pole). But this is simply solved by using large coupling capacitors and using direct coupling as much as possible, so that the high-pass pole of the output-coupling transformer is dominant.
For more info read this:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=118901
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