I am not sure if I should be bothering you folks in this group with
my amateurish audio problems...also, because it is actually part of
my HO train hobby audio circuitry.

But if anyone would care to throw in his or her two cents worth.

http://i168.photobucket.com/albums/u172/dcrane_2007/C.jpg

What I have made here is a box containing six audio circuits to play
the audio out of six HO train sound decoders. These are little 1 W
amplifiers that can be passed through small audio transformers if
desired, to jack into a stereo amp, for example, to provide room
filling audio of the locomotives. (These decoders are used normally
with up to 4 " speakers as stationary sound devices, or mounted in
model rail cars.)

Now, every time I connect an individual decoder via a transformer to
the stereo, things work great.

However, when I get ambitious and wire them all up as shown in
the sketch, with the DPDT switches to allow me to alternate between a
8 ohm internal test speaker and the audio out jack, I get terrific
hum....which as an amateur I take to be horrible ground loop.

Note, this hum has nothing to do with the power circuit (not shown in
the sketch) because I didn't even get ot the point of switching the
track power on.

I'm sure this will seem like a dunderhead's bit of wiring. But just
for the record, the hum rises and falls with my placing a hand on the
surface of the DPDT switches, and/or on the metal case of the stereo
amplifier.

Disconnecting the internal speaker from the terminal strips solved
nothing.

I imagine I have created a ridiculous bit of circuitry. Back to the
drawing board.

I may just have to create six individual little boxes and jack them
in and out as needed instead of trying have all the decoders in one
locaction "ganged" up on the terminal strips.

I thought I had isolated everything. Did I use the wrong switches or
wire them incorrectly? They are center off. I have the decoder
audio leads to the center tabs. Connections to the internal speaker
via a terminal block on two bottom tabs, and connections to the input
of the audio transformers on the top two tabs.

You see, this post is in a way, one about improving home audio.