Your mission, should you choose to accept it: Opto-FETs in the audio path

[artifus]

is it not the distortion present in the path to ground? see control elements table here: http://www.geofex.com/article_folders/ecftech/ecftech.htm

[merlinb]

Not too dig this up, but is Fig. 12 not the same idea as using it as an input attenuator?

Not exactly. Notice that even if the FET is completely off, the voltage across it is not equal to the total input voltage; it is instead attenuated by the divider formed by the two series resistors. If you size those resistors correctly, then even with your maximum input signal amplitude the voltage across the FET will still be small enough to avoid distortion.

[midwayfair]

I note that this discussion has gone a bit beyond my understanding.

Carry on.

[samhay]

Not too dig this up, but is Fig. 12 not the same idea as using it as an input attenuator?

Not exactly. Notice that even if the FET is completely off, the voltage across it is not equal to the total input voltage; it is instead attenuated by the divider formed by the two series resistors. If you size those resistors correctly, then even with your maximum input signal amplitude the voltage across the FET will still be small enough to avoid distortion.

Neat solution. If you make the resistor to the (-) input of the op-amp say 10x smaller than the resistor preceding it, then you only have 10% of the signal voltage across the FET.

[Jazznoise]

O.k, that's a clever trick! Summers approaching, though I've a recording gig at the end of May: expect me to actually get off my ass and do something with this information by June!