Quote:
Originally posted by ToneLux
You're parsing words. The chokes are there to tame the high frequencies.
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I am stating this in laymans terms, so others will be able to understand more about opamp design, since this is a public forum,
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Dean used the chokes in the emitter side to do the same thing.
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So, not to sound like a broken record, I prefer opamps with lower gain because they are naturally more stable, and that is why I said the 990 is like a lose cannon, because it has so much gain.
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Paul,
I am indeed parsing words. It was my mentor Deane Jensen that first taught me that when addressing scientific issues, the words one uses are critically important to successful communication. Deane always insisted that he see the final print galley of any article he wrote in order to prevent editors from botching it. In one article he was discussing an IR drop (which is the voltage drop that happens in a wire when current flows through it). Some non-technical editor who was apparently unfamiliar with Ohm's Law, in the desire to be helpful, substituted the word "InfraRed" for "IR", which made the whole sentence into nonsense.
I believe that most "laymen" can understand a lot about all kinds of technical stuff if the person who really understands it well can explain it simply, but accurately. There is no need to mis-state what is really going on in order to "simplify" it. Usually the gory details (like the math) are not necessary to convey the principles. The book "The Art of Electronics" by Horowitz and Hill is a great example of how complicated electronic subjects can be explained in simple, understandable language.
I will once again try explain clearly for you and anyone else interested that the chokes in the emitter circuit of the 990 have no influence on its high-frequency compensation. They do NOT serve the same function as the RC network between the collectors of other opamp designs. The chokes bypass the high frequency compensation in order to make the 990 perform better in the audio band.
Your comments about high frequency instability of opamps, including overshoot, ringing, and oscillation degrading their sound quality are exactly correct. That is the reason that Deane worked so hard to adjust the internal compensation in the 990 so that it did not have overshoot, ringing and oscillations. The 990 is a great sounding amplifier precisely because it is so stable. In addition, its high gain in the audio band gives it superior distortion performance to amplifiers with lower gain. The trick with high gain amplifiers is that they take a lot more design skill to properly compensate than lower gain types.
To say that "high gain" as a property, makes any opamp better or worse for audio is like saying all audio transformers are bad, when in fact some of them are bad and some of them are good. It depends on how well they are designed and made.
The truth is that some high gain opamps are notoriously unstable and some, like the 990, are very stable and exhibit no overshoot, ringing, or oscillations. IMHO the stable ones sound much better.
I have no beef with your preference for lower gain opamps. You're welcome to that preference with my blessings. My desire in responding on this forum is hopefully to help those who read it distinguish between preferences and facts.
Best wishes,