Friday, February 24, 2006

"no-feedback" nonsense

I saw this diagram on this page found via Digg, which is a pretty good source of fresh new stuff to read about. I wanted to read about the big subwoofers... which are good to gawk at.

The part that gets in my craw is the misunderstanding of feedback.

The quote that goes with it the diagram contains an error

FIGURE BELOW: shows the Electromotive Energy produced by the speaker that thru the F.B. path reaches even your turntable cartridge. Amp and preamp are shown as Operational Amplifier for semplicity. Any linear amplifier is by definition an Operational amplifier with linear trasfer function used as a constant multiplier.
There is a hidden assumption, left unwritten. This is that the transfer function is a constant multiplier in both directions. It's not true. The forward transfer function is almost perfectly a constant gain (multiplier), but the reverse function isn't.

The amplifier uses a low impedance output to supply whatever current is necessary to the load, with the feedback resistor along for the ride. The result is that any possible contribution to the current through the feedback resistor is shunted through the output stage to ground. This means that even a single stage amplifier will provide attenuations on order of Rout/Rfeedback * Open Loop Gain. This means that even a single stage is sufficient to completely isolate an input.

The insistance on "no-feedback" amplification makes for a nice market niche, but is not based on fact.


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