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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
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
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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.
newbee
4,767 posts
 
Sean, Was that Marsh preamp the Tube version? :-)
tedbu
1 posts
 
In some of the am[plifiers I have designed and built on my own I have founf that the consistency of feedback with frequency makes the biggest difference.I fin d that if a negative feedback loop is usede,if a squarewave is applied to the input a sqarewave should be observes at the feedback terminal of tghe fronr end differential amplifier.Anotherwards,the amou8nt of feedback should be constant with ffrequency and roll off at the same rate as the open loop frequency response rolls off.Using this parameter,open loop freq.resonse and closed loop are equal.
sean
6,231 posts
 
Newbee: It was the original SS model i.e. the non-balanced version. Given that i'm using my Sunfire's in my HT system, and the Marsh doesn't have a processor loop, it would have been a pain to try and make it work there. I tried the Marsh with other amp / speaker combo's and i didn't really like it. In that specific system though, it sounded very, very good. That's why i had originally called up Marsh and asked about the availability of their Pre-Pro. Since this was probably about two years ago, they were advertising such a unit, but it wasn't available for a LONG period of time after that. As such, i purchased another Pre-Pro but can't find any reliable sources pertaining to the sonics / performance of the Marsh unit.

Tedbu: That's a good observation. Glad you shared it with us. If the feedback circuit itself isn't fast enough or cover a wide enough bandwidth, there's no doubt that it will hinder performance. While this should be a matter of common sense design, i bet that a lot of engineers / designers overlook this factor. Sean
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