Can I add an audio out jack to a desktop radio (stereo) with isolated left / right channels?

The plan was to add a stereo output jack and a switch to turn the speakers on/off. This way I could leave a cord plugged in and still have the option of switching the unit's speakers on or off. Anyway, a visual inspection of the circuit board shows that the two grounds are isolated from each other. The plan was to tie the grounds together and switch with an spdt between speakers and the jack, then add another set of positive leads to the jack for left and right. Now that I see they are on isolated and separate circuits, I am hesitant to tie them together. Maybe I just need to go with RCA jacks and a dpdt? I guess my real question is "am I going to blow up my radio?" Do I need to worry about dummy loads or about frying the driver circuit? I realize the audio level will not be "line-out" and I have read that a 1k resister will help, but I don't mind turning down the volume before running the signal to another amp. Thanks!?

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lemonie6 years ago

You  could probably do this with a bit of splicing on jacks. If you get those jacks that have a switch that opens when you put a jack in it.

elNorm (author)  lemonie6 years ago
Thanks lemonie. I get your thinking, but I need to keep everything isolated, so even if I switch both positives, I still need two separate negatives! Argh!
lemonie elNorm6 years ago

A pair of mono-jacks?
Or a big switch I suppose.

elNorm (author)  lemonie6 years ago
Haha, yes, I had thought about the "big switch" but Radio Shack doesn't have any "quad-throw" switches! My main problem with switched jacks is that I don't want to keep plugging and un-plugging the cable.
framistan6 years ago
All you need is an isolation transformer on each speaker. see the picture below. If you have any old answering machine or old computer MODEM, you can salvage an isolation transformer out of them at zero cost. or you can buy them at any radioshack. This will also CURE most "hum" problems when you do something like connect a computer to a stereo and have AC-hum. I would place a 1K resistor in series with the 47k just so you never place full speaker volume into the transformer.
elNorm (author)  framistan6 years ago
Oh, boy... The other thing about "these modern amps" is they don't waste a lot of space! So what I need to do is put in two jacks (or something with four conductors) so I can add a break-out box. Altoids anyone? What I don't get in the diagram is why does R1 look like it is tapped in the middle? Is it bridging the pos and neg, sorta like a dummy load? I am wondering if I can parallel the speakers, leaving them running, and omit R1? Would I still need the 1K? My eagerness to include a switch for the jacks diminished as the other components started adding up! Thank you!
These modern amps are "push-pull" outputs, and are basically an H bridge. If you don't have them isolated, somethings gonna fry. Try taking half the signal off each side, and a ground from the power supply, sure, you'll lose volume, but you'll save your tuner.
elNorm (author)  steveastrouk6 years ago
Do you mean that I can divide the positive with a path to ground on one side and a resistor that halves the signal before sending it to the jack on the other? I wouldn't know where to begin to get the value for that resistor.

I can live with the little speakers running all the time, so... If I parallel in some RCA jacks (that's halving the signal?) and promise never to use a y-cable... will that work? I am guessing that the negatives will still get coupled in the outboard stereo amp anyway. Another thing to consider is that I would never do this at full volume, maybe 1/3 max. Those components "look" awfully delicate, would that much signal loss be a problem? Thanks so much for the help.