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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
marakanetz's avatar
marakanetz
5,473 posts
 
Hammy,
and you're missing mine:
The one that truely specs out great MUST sound great.
If the one that specs out great and doesn't sound great it means the specs are NOT true.
Nowdays specs mean completely NOTHING unless you're purchasing a professional equipment.
The marketing assumption is that consumer is dumb, therefore needs nothing to know about specs.
sean
6,231 posts
 
Hammy & Marakanetz: I'm somewhere between the two of you. That is, i believe that you CAN hear specs, if the tests are performed in the proper manner and ALL the spec's are taken into consideration and properly interpreted. Having said that, Pro gear typically won't give you the spec's that you need, so they too are about as useless as the lack of spec's that most "hi-end" manufacturers offer. Sean
>
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hammy
176 posts
 
Welllll...I'll agree to disagree, because I know that "The one that truely specs out great MUST sound great" is not true. It only means the unit performs well on that test. Music is much more complex than audio industry specs are good for. Furthermore, many companies don't test specs uniformally. Some test 20-20. Some with a true load. Some some with a test resistor. Specs show CDs sound better than LPs. They don't. They are more convenient. Researchers recenly found 2nd and third order harmonics above the range of human hearing affect audible tones and thus are perceived by listeners. Specs don't test for that. How many other unresearched areas of acoustic and psychacoustic interactions are not addressed by "slew rate" , "damping factor" and "Total Harmonic Distortion". Plus, you DID miss my point that in designing to ACE spec A, a designer might screw up spec B, C, and D.
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.
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