Simple Mixer Schematics

Virtual Earth 

Circuit 2 shows a basic active mixer. It uses 2 virtual earth preamps. One for the summing node and 1 to re-invert the phase of the signal.

The summing node (The point at which all the resistors meet) enteres into the inverting input of the op-amp. A feedback resistor is connected between the output of the op-amp and the inverting input. The function off this feedback loop is essentially to limit the open-loop gain of the op-amp.

Any signal entering the inverting input of the op-amp will appear at the output but it will be upside down. That is to say 180 degrees out of phase. In other words if you put 2 volts in you'd expect -2 volts out. To achieve unity gain (that is no gain or amplification at all) the feedback resistor must be the same as the summing resistor. In this case 10K. All the summing resistors are 10K and the feedback resistor is 10K. Because the feedback resistor feeds the output signal back to the inverting input of the op-amp @ 180 degrees out of phases it cancels out any gain. It also means that the inverting input of the op-amp is held pretty close (If not exactly) at zero volts. or earth potential. Thus the term "virtual earth".

Any signal coming in through the summing resistor is like dumping it to ground via 10K. It theoretically has the same loss. However the feedback resistor of 10K gives the exact opposite in gain. So if you feed 2 volts in you will get 2 volts out only it will be upside down.

Because the summing node (The inverting input of the op-amp) is at virtually earth potential, there is little chance that this signal will bleed it's way out to any of the other inputs. Essentially speaking all the audio sources are isolated from each other.

However we're still left with the problem of the phase being wrong. If the output of the first op-amp were recombined with one of the other signals at a later stage it would cancel out rather than mix. So we have to re-invert the phase with yet another op-amp. This is a unity gain amplifier just like the first except that there is no summing node as such. (Except for the feedback resistor of course) The output of these two stages will now be the summ of all the inputs with the correct phase. Because of the inherent compensation of the feedback/op-amp/summing node, there is virtually no limit to the number of inputs you can put on this. Most modern op-amps have enough drive capability that 128 inputs would be just peanuts.

However it must be remembered that you are summing the inputs so if you had a powersupply of say +/- 15 volts, and 4 inputs of +5 volts each, The result would be 20 volts mathematically speaking. But the op-amp can only produce +15 volts so you would be clipping by 25%. Distortion occurs. Most op-amps can't swing exactly to the supply rails so clipping and distortion would be even worse. In practice however most audio signal wouldn't exceed a few hundred milivolts. A 2 volt peak to peak signal is considered to be a very high level.