Randall RM20 master volume mod

Warning! There are lethal voltages inside your RM20.  If you don't know what you are doing, then get a qualified amp tech to make these mods.


  • lift the right side (looking from the front) of C9 from the board;
  • solder to pin 3 of an A1M pot (1M log / audio taper) pot;
  • solder an 1nF cap from pin 2 of the pot back to the vacant hole from C9;
  • ground pin 1 of the pot (a convenient ground point is the ground pin at R20;
  • drill a hole in the back of the head for the master volume pot (somewhere around the serial number is a good location). Alternatively, on one of my amps I've relocated the boost pot up a little and put the master volume below it so its on the front of the amp. Relocating the boost pot does involve cutting the PCB though.

Explanation: we're basically bleeding signal to ground before it goes to the power amp.

Here's what it looks like if you mount the master volume pot at the rear of the amp:

Master volume mod for RM20

In case its not clear from the picture, the 1nF cap goes to the middle of the pot (pin 2) and the 0.1uF cap goes to pin 3.

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