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Douglas Self on Negative feedback and distortion

I've been reading Douglas Self book on amplifier design and something he said that really makes me think twice.

As you have seen most amplifier makers claim that their amps either does not use global NFB at all or very little of it to improve dynamic (or transient response).

According to Self, the only parameter that matters is distortion and nothing else. I supposed he measures the extra harmonics that the amp produces given a sinusoidal input. In other words, distortion is measured in the frequency domain.

If I remember correctly in my Control Theory course way back in my college days, the frequency domain reponse cannot tell how the amp will response for a given step input. And the STEP RESPONSE is what can tell a lot about the behavior of an amp dynamic and transient response.

In his book, he is very adamant about his position that the only thing that matters is the amp frequency response.

I don't thing frequency response contains information about how any amp would respond to a step input but I could be wrong. Frequency response is only a steady state behavior of the amp. It cannot tell how much the amp would over-shoot, under-shoot, tendency to ringing, and so and so, given a step response. I don't think you can look at the frequency response and make any conclusion about the amp tendency to overshoot, undershoot, ringing and so on...

What do you think?

By the way, I think his book is excellent read into the theory an amplifier design if you can ignore some of his more dogmatic position.
andy2
sean
6,231 posts
 
You're asking the right questions. I haven't read his books but i've seen enough quotes from them and read enough on his website to know that he and i don't see eye to eye on too many subjects.

I will say that to achieve ultra-wide bandwidth with excellent linearity typically entials fast response out of the circuitry. The faster the circuitry, the less likely you are to suffer from ringing and / or under / overshoot. Combining this with the proper impedances lends stability to the circuit. When you've got speed and stability, you've got good performance that is consistent without the need for a lot of error correction. The more error correction that you have has to do with the lack of speed or consistency in processing the signal. Rather than make the circuit slower and more complex, why not just design the circuit right in the first place? Wouldn't that be better than trying to band-aid it after the fact???

As a side note, i've never read but have also heard good things about Randy Sloan ( or Sloane's ) books. Anyone familiar with those and if so, how do they compare to Self's books? Sean
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marakanetz's avatar
marakanetz
5,473 posts
 
You're right,
The control theory states that negative feedback MAINLY exists to keep amplifier stable.
An amplifier at the same time can have multiple stages of amplifications which we also can consider as an amplifiers.
A global negative feedback i.e. over the whole amplifier not only should exist to decrease distortions over the larger freequency bandwidth but to prevent an amp from self-oscillations.
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hammy
176 posts
 
I don't trust ANYONE who is dogmatic about specs. We're not trying to reproduce sine waves at 1000Khz. It's ridiculous- all you have to do is compare a cheap yamaha with 100 watts and ultra low distortion to a 25 watt audiophile amp. How many compromises did yamaha engineer in to get those specs right? A little Naim Nait sounds better and plays louder- but the yamaha is better on paper.
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