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December 2016

Speaker Sharing

Does anyone have a simple circuit that will allow me to mix my iPod audio out with my computer’s audio to play through the same speakers? I want to avoid un-plugging/re-plugging just to hear some music.

#12162
Jonnie Vanalstyne
Manchester, NH



Answers

I was disappointed in the answer that missed the obvious. The question asked about mixing the audio, not switching it.

Most powered computer or pod speakers have more gain than necessary and could make due with the loss of a passive mixer. The signals from earphone outputs can be combined by using a series resistor on each before combining them in a small junction box. Use the resistors like wires between the jacks forming a "Y" connector. To each source, the other device looks like ground, so the two resistors form a 50% or 3 dB divider. The series resistance forms another divider with the input impedance of the speaker/amp, so it should be as small as safety permits. I frequently see a couple hundred ohms of resistance on headphone outputs just to limit the power to small headphones, so my guess is that something around 2K would be reasonable, but 5K would give extra isolation if you still have enough signal.

The idle device is not a perfect ground and could introduce some distortion, so better isolation matters if you are using Hi-Fi speakers (but we're talking MP3's from a pod). Use 5% or better values to maintain channel balance. You may want to check the outputs for DC leakage, which would suggest using a blocking capacitor (10-20 MFD). Microphone inputs on computers often provide a positive polarizing voltage for electret microphones, and some jacks are multi-purpose and electrically switched by sensing software. If the computer misidentifies the connected device, you probably don't want that voltage fed back to the pod or phone.

Dennis Green
Farmington Hills, MI

I use a simple DPDT toggle switch for something like this. I switch my computer output between speaker and headphones without wearing out the headphone jack.

In your case, the audio to the speaker would be switched between the iPod or the computer. The speakers must be amplified, and the computer’s output volume adjusted down to be close to the level coming out of the iPod headphone jack. You can mount the jack and switch on the speaker.

Bert
Toronto

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