Here is a little project I got off the old Ampage forum ,
I think it was
suggested it. My friend Maynard wanted to bridge two bass amps
together, using the preamp of one to drive them both. When we tried it
we found there was a volume disparity between the two amps, so I asked
for ideas at Ampage and Enzo suggested this box. It worked great, it's
easy to build, and Enzo is right in saying it's nice to have one or two
around for general purposes. This is pretty much the same thing as what
is selling on Ebay as a "Volume Box". It also is claimed to be an
attenuator, a power soak, a tone and dynamics control, and penis
enlarger. Well, maybe not the last, but there's as little validity in
the latter as in the former three. Those Ebay "Volume Boxes" sell for
twenty bucks, pretty good profit for a pot, a couple of jacks and a
How to build a Volume Box
If you want it to pad down the preamp out signal, it is easy. Get a
metal box and mount a couple jacks in it for in and out. Then mount a
high value pot in there - oh, 100k or higher, it doesn't matter much on
the value. Wire the input jack to the top of the pot and the output
jack to the wiper. Wire the commons together and to the bottom leg of
the pot. There you go, a volume control in a box.
These circuits are high impedance, so the 100k or 250k or whatever will
not load it down. You have made a simple voltage divider, so you can
dial down the level to any percentage of full volume you like.
Now run a cord from the preamp out of the master amp to the input of
our box, then another cord from the output of our box to the slave
power amp in.
I have a couple of these boxes in my drawer of tricks for general use.
You never know when you want to modify a signal level.