from Dana Scott:

ARP power supply modification:
Most ARP power supplies can be made much quieter with the following
modification.
This change appeared on Omni power supplies around 1978. Products that use the
Omni power supply will of course already have this mod. Omni power supply is
found on (duh) Omni's, Avatars and some of the later 2600's.
This change adds qty2  0.01uF capacitors to the PS unit.
ARP power supply connections are red for +15V and Violet for -15V
If you have a service manual, locate the 33.2K1% (R3?)on the power supply board.
solder a 0.01uF cap across this resistor. If you don't have a service manual,
the 33.2K is the only metal film resistor that connects directly to the +15V
output. It also connects to one leg of the +15V voltage adjustment trimmer.
There is only one 33.2K on the board.
The other 0.01uF cap connects across a 10K 1% ( R12?). One end of this resistor
connects directly to one end of the -15V adjustment pot and also to the -15V
output and is the only metal film resistor that ties to the -15V supply.
For 2600 power supplies, the 2 resistors in question are both 6.98K metal films.
Likewise they are directly connected to the voltage outputs and also one leg of
the voltage adjustment pots. Be careful , there are a total of (4) 6.98K
resistors on the 2600 supply.

This mod actually made the Avatar supply too quiet. The ADSR would not kick into
the decay phase when the attack slider was at maximum. It seems that the circuit
was relying on PS noise to work properly. A small capacitor was then added to
the ADSR circuit to fix the problem. I can describe that change too, but that's
another email for another day.