"Mr Macaw" wrote in message news
On Fri, 13 May 2016 11:29:21 +0100, Wayne Chirnside wrote:

On Sun, 08 May 2016 00:47:57 +0100, Mr Macaw wrote:

I know what a servo motor is, and that you can get servo amplifiers (or
servo drives) to work with them, but what does it mean when a stereo
system says it has a servo amplifier?

There is active circuitry to keep the bias at a point where the output
tracks symmetrically between positive and negative voltage excursions.

I see. Is this to prevent unnecessary DC current through the speaker
creating heat in the coil? Or does it improve sound quality by leaving
the cone centred so it doesn't hit the ends of its movement?

That's more difficult in a direct coupled amplifier - drift is always a
problem.

There's no way out of incorporating loads of DC only nfb. That has to be
decoupled against the AC signal, the capacitors tend to be bigger than you'd
use for AC coupling.

That means aluminium electrolytics, probably about as bad as it gets for
colouring the audio signal.