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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
eldartford
4,262 posts
 
As a test design engineer (now happily retired)I have a different understanding of the purpose of specs.

Performance of the equipment, we all agree, is determined by its design. A prototype is built and its performance is good. Now we want to put it into production. How can we be sure that each unit that comes off the line is as good as the prototype? We certainly cannot do, on each production unit, the exhaustive performance evaluation that was done on the prototype.

The designer identifies the parameters that he believes are critical to his design, and limits (maximum values) are defined for these parameters. Now, when production units are to be evaluated it is straightforward to measure these parameters. It can even be automated.

So, specs are important...they assure that the unit you buy performs like the one that the designer evaluated (and liked well enough to put into production).
andy2 OP
1,541 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.
sean
6,231 posts
 
Hammy: That's why i said ALL the spec's have to be kept in perspective and properly obtained, not just "some of the spec's have to be good" or "some of the spec's were obtained in a manner that isn't representative of real world conditions", etc...

El: Nice explanation.

Andy2: Some of the things that you say aren't available from spec's are "kinda sorta" interpretable via spec's. I agree about soundstage width and depth though. Sean
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andy2 OP
1,541 posts
 
Sean: what kind of specs that can tell you how good tight and tuneful the bass of an amp?
sean
6,231 posts
 
Andy: How's this for a list? Slew Rate, Rise & Fall Time, Rated Power Bandwidth, Rated Frequency response, Current Capacity ( steady state and peak ), Output Impedance, Square Wave Tilt @ 20 Hz, Power Output @ clipping into various impedances, THD, IMD, S/N Ratio, Quantity of Global Feedback used, Phase Response, Time Delay, etc...

Like i've said before, if one has access to a wide variety of spec's and those spec's were derived using proper testing procedures, one can have a good idea of what to expect from an amp and / or how consistent it will be with different loads. Some amps are sonic chameleons because they don't achieve a high level of electronic stability and this will be reflected in the various spec's / test results. If one familiarizes themselves with the proper interpretation of these spec's, they can get by with just a portion of the above information and make a relatively well educated "guesstimate" in terms of sonic performance. Sean
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PS... Since some spec's are influenced by others, it's possible that a manufacturer could leave out specific spec's that would explain why others would look so good on paper, yet sound so bad in real life. That's why more info is always a good thing as it gives you more of an explanation as to what is going on in the grander scheme of things.
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