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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
wywhcan
39 posts
 
It's been a while since college. But I'll put in my 2 cents here.

For a close-loop amplifier design, one is mostly concerned with GBW (gain-bandwith product), which is a constant for an amp. You wouldn't run an amp in open-loop where the gain is max and bandwidth is min (half-power bandwidth). You extend the bandwidth by reducing the gain (dB) with measured amount of feedback (i.e. close-loop). Of course, bandwidth enhancement may not be an issue for audio frquency range.

Remember that there is always some parasitic cap in solid state devices that causes Miller-Feedback which is unavoidable. As for local negetive feedback in a multi-stage amp, it is usually employed for, amongst other reasons, attaining certin amount of gain per stage as well as slew-rate control. How the step response will look like can be predicted by the slew rate of the amp, or by how fast the amp is.

Global feedback, which I guess is taken from the very last stage, can be seen as error-correction from a Control System point of view. It is also a way of reducing the effective output impedence of the amp.
eldartford
4,262 posts
 
Audioengr...You may be clever enough to design "an amp that has bandwidth to 1 MHz, but has lousy step reponse (overdamped). " But why would you, or anyone else, do that. In practice there is correllation.
bomarc
501 posts
 
Why make an amp like that? Because somebody will buy it. Why else?
sean
6,231 posts
 
I think that what Audioengr was getting at is that there are loop-holes to every generalization. That is, something can deliver flat response over a wide bandwidth at a given amplitude on a steady-state basis, but that doesn't mean that it will respond correctly to changes in amplitude in a linear manner. While i agree with that, it all boils down to speed and impedances. If you've got the speed, the circuitry can respond as fast or slow as needed. If the impedances are right, power transfer is maximized and dynamic variations in amplitude don't present a problem. It really is simple if you think about it and that's why i can't understand why most of the products on the market are the way that they are i.e. under-designed and over-priced.

All of this stuff was talked about and dealt with 30 years ago by a designer that never got the credit he deserved. That person's name is David Spiegel. Too bad he only ever marketed one product as i'm sure that he would be an "audio legend" had he kept at it. His ideas were light-years ahead of most, yet he was humble enough to admit that his ideas weren't original at all when i spoke to him about them. Sean
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eldartford
4,262 posts
 
Sean...."there are loop-holes to every generalization" is one.
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