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Re: sound card input

If you just connect the speaker outputs in parallel, no wonder you're
hearing the audio on all the speakers! You may also damage the audio
output stages of the other radios by backfeeding them.

An audio mixer as suggested by others would work, but you don't really
need to get that fancy. Just build a resistive combiner. Put a
resistor in series with the hot speaker lead from each radio to a
common junction that feeds the sound card. Run another resistor to
ground from this point.

Just as a starting point, I'd suggest 1Kohm resistors. That's
sufficiently high compared to the usual 4-8 ohm speaker impedance to
keep the radios from driving each others' speakers, but not so high
that the signal into the sound card would be too low. Chances are you
already have far more signal than you need to drive your sound card,
so this will also help reduce any hum and/or noise problems you may be
having. Adjust the level by varying the resistor to ground from the
common tie point.

Check also that your radio speaker outputs all have common grounds
before you tie them together. Some audio amplifiers, notably those in
some car stereos, have bridge-type outputs where neither speaker
terminal is (or can be allowed to be) grounded without damage to the
amplifier. I don't know if such amplifiers are used in any amateur
transceivers, but it's worth checking.


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