Thanks for all your comments :)

I'll be out for a couple days, and make the changes when I come back.

+12V ----------------+----------------------+--------
              OPTO   |                 220  |
   +--------+        |              +\/\/\/-+  _ /|
   |       _|_     |/C              |       +-| | |
 INPUT     /_\ --> |               -+--| |----|_| |
   |        |      |\E            |/C  1uF       \|
   +-\/\/\/-+        |         +--|
       1K            |         |  |\E
                     |         |    ----+
                     |         | 10K    |
                     /     ----+-\/\/\/-+------+
           INPUT POT \   |/E                   |
                 10K /<--|      +----PWM INPUT |
                     \   |\C    /              |
                     |     |    \ PWM POT      |
                     |     +--->/              |
                     |          \ 10K          |
GND -----------------+----------+--------------+-----

As far as I know all the transistors are NPN (!) (BC547) The circuit
diagram layout is a bit strange, but the idea was for the switch
transistor to allow current to pass from the PWM input to the
amplifier's base (plus a 10K resistor to ground) when the opto
conducted. Have I done something terribly wrong? (Obviously I'm not
too good at analog circuits :-)

-- 
Hector Martin (hector@marcansoft.com)
Public Key: http://www.marcansoft.com/hector.asc

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