how to build custom RCA stereo to mono cable?

[peterw]

I need some help designing custom stereo-to-mono cables. Has anybody here built one?

I'm experimenting with a kitchen rig of a Squeezebox2, old 2-way loudspeaker, and a Sonic Impact T-amp. Currently the interconnect is a mess of three parts, all Radio Shack:
* RCA M-to-M "Y" adapter turns the SB stereo analog out to one mono RCA M
* RCA F-to-F turns that into an RCA F
* stereo RCA M to 1/8" stereo plug so that the mono sound is fed as the L channel to the T-amp 1/8" input

I'd like to build a custom cable that would be shorter & cleaner.

Also, I'd like to feed mono (L + R) into both the L and R channel of the 1/8" plug, so that I could occasionally connect a second speaker to the other T-amp speaker output (yes, for dual mono). I've got similar desires for another room, where I've got an old mini-stereo that will drive a single speaker but takes dual RCA inputs.

Web searches suggest designs with resistors of various sizes:
http://209.85.165.104/search?q=cache...o%22+RCA+cable
http://groups.google.com/group/rec.a...04dd78ef9f6bb1

and my multimeter shows me that my cheap old RadioShack Y adapter is *not* a simple cable, but has some sort of circuit. The resistance between L+/M+, R+/M+, L-/M-, and R-/M- seems to fluctuate -- it's not the constant 0/low impedance I see on the F-to-F adapter or the stereo RCA-to-1/8" cable.

Do any of you have experience building such stero-to-mono cables?

Thanks,

Peter

[tingtong5]

I'd say simply use a resistor for each channel and then tie them together. The resistors function is to protect the line drivers of analog outputs.

I used 390R on my Lyngdorg amp pre outs (subwoofer!).