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I'd like to make a simple audio mixer that takes in line level signals from various mp3 players, computers, etc. I found a simple schematic online, but I'm wondering if this will be okay for my purposes. Do I need some sort of DC blocking in this circuit? I just want to ensure this won't damage any components I plug into it.

Simple Audio Mixer

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That will probably work well enough. Most devices are going to output a average level of 0, and 4.7 kΩ is high enough that I don't think any device will have trouble with it. This method will allow the other signals to back drive each output a bit, but at 4.7 kΩ that is unlikely to be a problem.

For extra security, you could put 2 µF caps in series with each resistor to guarantee DC blocking. 2 µF and 4.7 kΩ have a high pass rolloff frequency of 17 Hz, so won't cut into your audio.

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Such a simple passive summing circuit with too many inputs has another problem - its inputs are weighted and each of them depends on the number of the connected devices.

If all devices are connected, the input gain of each of them will be only 1/6. If some of them are connected (other inputs are unconnected), it will vary up to 1/2 (when only 2 devices are connected)... or even up to 1 (when there is only one device). I assume a high-resistive load... but if it is low-resistive, the gains futher decrease.

There will be no such problem if you improve this passive summer by the help of an op-amp. It will compensate (zero) the output voltage by equivalent opposite voltage and will use the latter as buffered output voltage to drive the low-resistive load. This circuit is known as "op-amp summing amplifier".

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