Simple Mixer Schematics

PREAMBLE 

I've been cooking audio circuits for so long now I no-longer need a recipe. A lot of the theory I have forgotten over the years because I've just gotten to know the circuits by instinct. But this should serve as something of a guide to designing mixers from scratch.

The idea: 
Most people reading this would be well aware of what a mixer is used for but I'll reiterate here. The job of an Audio mixer is to combine various audio signals into a single audio signal. It is better known in electronic terms as a summing circuit. That is to say that the output is the sum of all of the inputs. A summing note is often represented as a circle with a PLUS (+) symbol in it.

Audio is of course an AC (Alternating current) signal but if we look at the incoming signals as a frozen moment in time we can represent it as 2 or more DC voltages. This is only useful to illustrate the point.

If we had two signals to be mixed. The first was 2 volts and the second was 3, the output should be the sum of these two voltages. 2+3=5. If on the other hand the two voltages were 2 volts and -3 volts then the output would be -1 volt. We are now subtracting 3 volts from the +2 volts leaving -3. It is important to recognise that we are dealing with what is known as a bipolar signal. That is one that can be positive or negative around a zero base-line.

When you get to the stage of adding many signals together, the complexity grows. In1 + In2 + In3 + In4 + .... and so on.

Because each incoming signal has it's own load impedance it is impractical just to wire all of them together and hope for the best. Especially when the following device you are trying to mix into also represents it's own load impedance. Sometimes you may be able to get away with it because the combined impedance is quite high. However most of the time it drags the whole network down and causes one or more devices to fail or distort or what ever. Usually no damage is done but it just won't work.

What is required is a little load isolation. (See Circuit 1: Passive mixer) The trade off is that you can't use terribly high value resistors because of the losses that they may cause. Especially if the load impedance of the following device is a little low. This will give the effect of severely attenuating some or all of the signals. A practical trade off has to be reached and this is as much trial and error as anything because the conditions change with each new device added or changed.

The device used at the summing node, IE: an amplifier or tape deck should be able to provide enough gain to compensate for the combined losses through the resistors and the combined loading of the system. But the loading will change depending on the combination of devices you have hooked into it.

This approach also creates another side effect. That is that a signal flowing into the summing node via one source can pollute the audio signals of other devices. Say you had two cassette decks that you wished to mix. However you also wanted to send the audio from cassette deck #1 to an effects processor. The audio from cassette deck #2, although attenuated slightly, will find it's way back to the audio from cassette deck 1 and also go to the effects processor.