# Linear voltage controlled resistor divider

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.

• If you can float the power controlling the effective resistor (as I have done with a high voltage project so that the control circuitry only ever sees 20V), then a JFET makes an excellent voltage controlled resistor, albeit with a minimum resistance of some 10s to a few hundred ohms. Feb 8, 2016 at 14:22
• Yeah, a Jfet was my first thought. I have some leeway in the minimal resistance I need but from my understanding a Jfet will not be linear between 50v-600v at low resistances to multi Mohms which is what I need. Feb 8, 2016 at 14:27
• Do you have an architecture diagram? Feb 8, 2016 at 14:30
• Effectively speaking I am just trying to control a CVS and a CCS. I could post a full schematic if you want. Feb 8, 2016 at 14:33
• Ehmmm, why are you running an audio amp at 600 V above ground? Feb 8, 2016 at 14:41

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.

• I thought they are very nonlinear, especially from one unit to another? Look at the effort this dude put into LDR linearization tortugaaudio.com/evolution-of-ldr-volume-control Feb 8, 2016 at 15:54
• @coinmaster yes, the LED current to resistance ratio needs calibration. The R = V/I relationship aka the linearity of the resistor at a constant LED current is pretty good though. Feb 8, 2016 at 16:01
• I'm assuming I'd need a similar solution to the one the guy in the link is using? Feb 8, 2016 at 16:11
• LDRs are tempting. I've read that they are better for audio than normal resistors when calibrated properly. I wouldn't know where to start with the calibration though unless I reverse engineered that tortuga guys mechanism. The mechanically controlled trimmer is still on the table as well. It seems like it could be the simplest solution. Feb 8, 2016 at 16:50
• @coinmaster do you need this for few devices only or do you want to go into production? Feb 8, 2016 at 17:20

You should probably use a power transistor which you modify the voltage at the gate to drain the specified current. You will surely need a negative feedback since the VGS is variable through different transistors and a current sense amplifier to modify the feedback loop. Very difficult circuit since you need high voltage (600V) !!!

• I know, the 600v is a real problem. I'm wondering if a mechanical control would be easier, I'm trying to think of the pitfalls of such a thing. It's only for manual adjustment of operating parameters so I don't need it to be incredibly fast. Feb 8, 2016 at 14:28