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Servo Amp Isolation

Posted by Triode_Kingdom on August 9, 2017 at 13:19:34:
So I've designed several filters with cutoffs around 1 Hz for the servo loop. It's pretty simple to construct a low pass with attenuation of 60dB or more at 20 Hz. This means there's no reason to be concerned regarding unwanted audio signals making it through the loop. That only leaves the issue of isolating the output of the servo amp from the audio on the grid. With the 10M resistor and a 500K volume pot, rejection in the midrange is about 26dB. With a 250K pot, it increases to roughly 32dB. I'm not 100% certain this is sufficient to prevent some form of audible loading of the grid by the output of the opamp.

One possible solution for this might be to interface the loop to the grid by means of analog optical isolators. These devices use a light-sensitive resistor that can change its resistance from roughly 1k to 1meg depending on input current. This would totally isolate the grid from contact with any silicon devices, and in conjunction with the 10meg grid resistor, should create a totally transparent system. I'll try to work up a circuit for all this soon.