Thanks, Larry. But nope. That didn't work. Still a great hum that
increases if I place my hand anywhere on the stereo amp metal cabinet
or controls etc. etc. I think I will forego the switches and wire
every decoder to one of six separate output jacks, and physically
change plugs as needed.

--- In diy_audio_tweaks@yahoogroups.com, Larry Owens <larryo@...>
wrote:
>
> Only the unasked question is a poor one.
>
> Try eliminating the (6) audio transformers from where they are
wired
> in - reconnecting the wires where the transformers used to be.
Then
> insert one of the transformers into the lines between the
aggregating
> terminal strip and the audio output jack on the left side.
>
> Cheers,
> LarryO
>
>
> At 05:34 PM 5/25/2008, you wrote:
>
> >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>http://i16
8.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.
>