audio / EE question - multiplexing audio channels

Factual Questions
1

I have 8 audio channels that I would like to multiplex together (4 headsets from cell phones, 2 channels per headset (speaker/mic)). Only one cellphone will be operating at a time.

Would it be electrically sound if I simply connected all the outputs(earphone) together, all the inputs(mic) together, and all the grounds together? I’m thinking something like a poor man’s multiplexer.

Only one of the cell phones will be operating at a time, so only one will output to the headphone, correct? Can anyone see any impedance or other issues with this scheme?

I suppose the right way to do this would be to use an analog multiplexer, but that has been introducing noise on the audio.

Thanks for the help,
MS

2

To clarify, I meant I wanted to splice all of the individual lines together to one single headset output.

3

I don’t know if it will work or not.

Will the other three cell phones be completely off? If so, here are some issues:

  1. Conventional wisdom says it’s not a good idea to source an active signal to an output. In this case, the audio output of one phone will be sourced to the output amps of three other phones.

  2. You run the chance of offering too low of a load impedance to the “on” phone’s audio output amp. When a phone is off, is its audio output amp at high or load impedance? I don’t know.

Similar issues are in play for the microphone channels.

But who knows? Maybe it will work. I mean, if an “off” phone offers a high impedance on its microphone and audio output stages, and if an active voltage sourced to either (while the phone is off) will not hurt it, then it might work.

4

‘Might work’ is good enough for me! I’ll give it a try on the lab bench and see what happens.

5

Any luck yet?

Just to be clear, you have just one headset that you want to work with four different cell phones, right?

A step up, but still cheap, would be a 2 (or 3)-pole rotary switch. You’d have to physically change it to the phone you want, however.

6

I realized that I don’t have any headphone plugs to hack apart. But, CrafterMan’s advice seems pretty sound and I don’t want to run the risk of damaging the phones in the long run. I think I will have to go the route of tying an opamp to the output of each speaker before tying all the outputs together. This should present a high impedance at each output and thereby prevent driving the output with an active signal, correct?

7

You don’t ever want to tie active outputs together, since they will end up fighting each other for what the actual output value should be. Instead of damaging the phones, you will end up damaging your op amp outputs.

Here’s a typical mixing circuit: Music Device Circuit: Mixer op-amp

8

The mixing circuit can be as simple as a resistor (say 4.7 ohms) in series with each output. The output level will be reduced by about 1/n where n is the number of devices connected. (“about” 1/n depends on your headphone impedance relitive to the series resistors)

I’ve done some limited avionics work for fellows with thier own airplanes. It is normal to tie the outputs of multiple navcoms togethor, or to provide switching provisions to do that. I didn’t like doing it, but because it is common practice, most coms include the above mentioned resistor internally…so it works fine.

9

Summing amplifier… Of course! Thanks, it’s been a long time since I learned about opamp circuits in school. And just to be sure, inverting the audio signal shouldn’t make a difference in output, correct?

10

Inverting the signal is no problem (though there are active noninverting summing circuits). But as Kevbo said, you may be fortunate enough to not need the op-amp; the passive mixer may be good enough.

Check out All About Circuits for more on both topics.