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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
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hammy
176 posts
 
Well, I never tried it, as I am a former musician and liberal artsy/ fartsy guy. My ears have never failed me- but they have cost me a bundle. Probably the most spot- on post in this whole affair was:
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.

Yes, all things in moderation....something less and less common in politics. I suppose if you can interpret sound from specs. they can be a handy aid.
andy2 OP
1,541 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.
marakanetz's avatar
marakanetz
5,473 posts
 
Sunfire line of Bob Carver isn't crap at all by specs and sound as well.
eldartford
4,262 posts
 
Andy2...If the specs that describe the good amp were properly chosen, the laws of physics would DICTATE that a circuit having the same specs would perform in the same way. The electrons would not know whose circuit they were traveling through.
sean
6,231 posts
 
El: Thanks for summing that one up in fewer words than i would have : )

Marakanetz: I was referring to "older" Bob Carver designs i.e. Phase Linear, Carver Corporation, etc..., not his current Sunfire offerings. Having said that, even some of the spec's on the Sunfire aren't all that fabulous and it can be heard in the sonics too. That is, the lack of bandwidth results in a lack of high frequency air, clarity and detail as compared to the finest of amps. While the Sunfire's are no slouches, i don't consider them to be "show-stoppers" either. They do offer very solid bang for the buck though, especially where GOBS of power are required and thermal losses are a concern. Sean
>

PS... A nice sounding and "somewhat reasonably priced" combo that i stumbled into by accident was a Marsh preamp running into a Sunfire amp. It was one of those combo's where each piece on it's own is a very reasonable performer, yet something "magical" happens when you put them together. Can't remember what i had for interconnects and power cords though, so i won't be much help in trying to "re-create the magic" if someone else were to try a similar set-up.
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