Maybe amps need a third terminal?

A long time ago, I worked for a company that made power supplies for film audio. This DC supply was responsible for a high precision lamp that would, via lenses, shine a slit through the film's sound track. Photosensors at the opposite end would convert that to an electrical signal, which then would get picked up by the preamp, often a Dolby unit.

Anyway, we used 3 wires. The standard (+) and (-) plus a sensor wire. The sensor fed a high impedance feedback input. That's how we measured the voltage at the lamp, which was kept rock steady, regardless of cable, current or anything else.

Looking at how traditional feedback based amps are made, I wonder if we shouldn't do the same? Use a separate wire for the feedback circuit, thus eliminating the possible contributions of the cable?

Most feedback circuits are high impedance, 12k or so. It is connected to the output inside the amp. Maybe we should be disconnecting it and feeding it from the speaker terminal instead?


I have often wondered the same thing about remote sensing feedback.   Out here in CA, many years ago, a small start up called Street's Electronics built a 200W amp (I think it was 200W) and it used a small percentage of current feedback to compensate for the cable losses.   It had three terminals for speakers, the typical two terminal voltage feedback but if you connected the speaker return to the third terminal, the current feedback compensated for the cable losses.   It produced a noticeable improvement to the sound.