DIY Digital pot for tube amps

[GainFreak]

Hi,

There was a thread about using LDRs as pots in tube amps. I recently noticed that Maxim offers high voltage CMOS switches that can handle up to 200V. Since they can take such voltages I was wondering if a digitally controlled pot with external resistors can be designed around those chips:

MAX4800A, MAX4802A Low-Charge-Injection, 8-Channel, High-Voltage Analog Switches with 20MHz Serial Interface - Overview

MAX14800*, MAX14801*, MAX14802, MAX14803 Low-Charge-Injection, 16-Channel, High-Voltage Analog Switches - Overview

[Steve Conner]

Very interesting, I'll add that one to our list of useful parts

I can't see any reason not to, say, use the 16-channel one to selectively short out 16 resistors with binary weighted values, making a 16-bit 200V variable resistor.

[GainFreak]

I can't see any reason not to, say, use the 16-channel one to selectively short out 16 resistors with binary weighted values, making a 16-bit 200V variable resistor.

That's what I was thinking about.
Or if we put aside the digital part of the chip for a while is it possible to use some high voltage FETs as switches? For example some high voltage version of J174 or J175 because the prices on those chips are not very low?

[Steve Conner]

I don't know how you would use FETs. The problem is driving the gates, without introducing a lot of charge injection, and/or exceeding the gate voltage rating. Low charge injection is important because the charge injected by each switching event causes a pop, and the result is nasty "zipper noise" in the audio when you adjust the pot.

For all I know the charge injection in those chips might be unacceptable too. The datasheet claims low charge injection, but low compared to what?

I don't know of any high-voltage JFETs, either. There's the LND150 rated at 500V, but that's a depletion-mode MOSFET, so it has the body-drain diode that would make it useless as an analog switch. You can overcome that by connecting two back-to-back, but that doubles the cost. 32 FETs, plus 16 lots of drive circuitry, might end up costing more than the 16-channel Maxim chip.

[GainFreak]

I can live with some zipper noise especially when switching from one preset to another on stage. The question is how much those switches will influence the sound. I'll put one of those chips in my list for future experiments.