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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

Showing 4 responses by andy2

andy2 OP
1,563 posts
 
Self actually says in his book that one should use as little of nfb as possible. He advices that each stage (which in itself has its own natural feedback) should be designed to give as small a distortion as possible. Then after each gain stage has been designed, as a final step, a nfb loop should be added to suit one's needs.

His argument is that global feedback is unfairly criticized by amplifier makers since each gain stage already has its own natural feedback built in. For example, an output stage complementary pair of PNP/NPN transister operating in class A/B has an inherent feedback with its emiter output resister ... So since you cannot avoid feedback in the first place, then why it is so bad?
My takes on this is that global feedback does have by far a larger impact on the behavior of the entire amp as compared to each gain stage feedback.

My one problem (among others) main problem is with his position on distortion measurement.
andy2 OP
1,563 posts
 
Just want to add my two cents:

So far, as far as I can tell, there are no specs that could tell me how well an amp will perform in the following categories:

1. Soudstage width
2. Soundstage depth
3. Instrument separation
4. Treble sweetness
5. Tight and tuneful bass
6. Non-fatige
6. Well, I can go on and on.

My conclusion is: specs are pretty much useless. For example, every amp that I know of have a spec of frequency response from 20hz - 20khz. Now, that tells a lot.

That was my original point that distortion is not the only parameter to judge an amp. There are so many variables that it is impossible to come up a set of parameters that can tell you everything about an amp.

That's why those guys are making big bucks. I mean there are more secrets in amp design than those surrounding the whole Irag WMD things.
andy2 OP
1,563 posts
 
Sean: what kind of specs that can tell you how good tight and tuneful the bass of an amp?
andy2 OP
1,563 posts
 
Sean: Let's do a thought experiment. Suppose there is an amp that everyone would agree it has nice and tight bass. A set of measurements are then made on this amp and these measurements are to be taken as benchmark specs for tight and tuneful bass. So by definition, any amp that has these exact specs must then have tight and tuneful bass.

1.Then comes along an amp designer who sets out to disprove those specs by designing a amp with those exact specs but has subjectively very lousy bass. According to the law of physics, I don't see anything that can prevent this from happening.

Of course, this is highly theoretical but I guess you can see my points.

Intuitively, I understand what you say but as someone on this thread pointed out that every set of specs has a loop hole.
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