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In Reply to: Re: Opamp feedback question posted by bilbobaggins on March 16, 2003 at 05:51:49:
That means that the 47 Gaincard or pretty much any op amp based amplifier with closed loop gain of around 20-30 would constitute a high feedback design. The feedback loop is kept as short as possible in the gaincard circuit.Correct.
Now, the current trend in the audiophile press seems to be a dismissal of high feedback designs. By that standard, the 47 Gaincard should be ruled out by any one wanting a low or no feedback design.
Yes. But then the prevailing "conventional wisdom" has always been a moving target. Soon as a given conventional wisdom settles in, someone comes along and tips over the apple cart.
The Gaincard has done more than most in that regard. The conventional wisdom was not just about global feedback or the amount, it also eschewed opamps. Monolithic integrated circuits. Non-class-A operation. Small amounts of power supply filter capacitance. Lack of bypass capacitors. Thin gauge interconnect and speaker cables.
So prior to the Gaincard, what much of the conventional wisdom crowd was looking for was totally discrete design in a non-opamp configuration, pure class-A operation, hundreds of thousands of uF of power supply filter capacitance, gobs of bypass capacitors, garden hose size interconnects and fire hose size speaker cables.
It seems to me that the emphasis in the press on the amount of feedback as opposed to the relative speed of the feedback circuitry is an emphasis on the wrong parameter, if the gaincard is any evidence.
One could look at it that way, yes.
The way I look at it, I think it simply illustrates how silly it is to get dogmatically hung up on ANY particular "parameter." Well, other than the only truly meaningful parameter of "how do you like the end result?"
Feedback properly applied would have a much smaller characteristic time in the feedback loop than in the open loop, i.e. the feedback loop would be much faster than the open loop circuit. One part of doing this is to keep the feedback loop as short as possible.
Well, sure, you could look at it like that in an absolute objective technical sense. Keeping the feedback loop as small as possible would be beneficial. Though the transit time in the loop would perhaps be the last reason to do so.
I mean, the EM fields are going to propagate through the loop at speeds approaching the speed of light. Even if you assume some bizzare worst case scenario at half the speed of light, you have to realize that the wavelength at 20kHz is about 9 miles. Differences of fractions of an inch or even a few inches just dissappear into insignificance.
That doesn't mean that it's not a good idea to keep feedback loops short. The feedback loop, just as all other conductive loops in a system, is susceptible to picking up noise and interference and keeping them all as small as possible would be beneficial in that regard.
Please give me some [feedback] on what I wrote. A little bit of knowledge [on my part] is a dangerous thing.
Hehehe. Well as long as you realize that, then you're not in much danger. :)
se
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Follow Ups
- Re: Opamp feedback question - Steve Eddy 09:35:27 03/16/03 (0)
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