I'm working on an audio amplifier project and I'm trying sort out the solutions to certain problems.
The problem that I most need to get solved is that I want to digitally control certain operations within the amplifier via a digitally controlled resistor and/or resistor divider.
I would need a voltage controlled resistor that is both as linear and noiseless as possible and it would need to function on voltages as high as 600v at 100ma with a resistance ranging from near nothing to multiple megaohms.
The only thing I can remotely think of is octocouplers but I know little about them other then they are inherently non-linear. I've heard that they can be linearized through certain techniques but I just don't know anything about the subject.
The linearity and noise factor are paramount here, it might even be worth it to use a mechanical solution to control a trimmer.
Does anyone have any ideas?
The resistors that I need to control are circled in red.
I intend for the CVS to be adjustable from 10v to near 600v and the CCS to be adjustable from 1ma to 100ma, possibly more. The intention is to be able to manually shift the load line operating point and accept a wide range of tube types reaching up to high voltage tubes such as the 300b, which I would want to be controlled digitally and with safety measures so I don't shoot 350v and 70ma into a 6SL7 or something.
Best Answer
In other tube amplifier designs the standard approach is to use one or more vactrols along with an ordinary digital to analog converter.
These devices are a led coupled with one or two light dependent resistors.
https://en.wikipedia.org/wiki/Resistive_opto-isolator
Pro: No problems with high voltage, low distortion.
Con: It can take a moment to go from low to high resistance. Light dependent resistors are very slow.