Using resistors (which we discussed last time) we can build a very basic audio mixer. The schematic looks like this:
All resistors should be the same value. You can generally use any value from 1k to 5k.
Theoretically this mixer can be extended to as many channels as you would like by adding additional inputs and resistors. The problem is that each additional channel reduces the volume of the output. If all resistors are the same value, the formula for determining the loss in dB is
loss in dB = log(n) * 20
Where n is the number of inputs.
A 2 input mixer has a loss of 6.02059991 dB, a 4 input mixer has a loss of 12.0411998 dB, an 8 input mixer has a loss of 18.0617997 dB etc…. You can compensate for some of this loss by raising the input signal levels. You can also raise the level of the output. However, the resistors add a small amount of noise to the signal – this is called thermal noise. Raising the level of the output, will also amplify thermal noise.
Another limitation of this mixer is that you cannot change the relative volume level of the inputs. All inputs are mixed together at the same relative level. This is the same as using a traditional mixer with all faders set to the same level.
The faders on a traditional mixer are called potentiometers. Potentiometers are variable resistors. You cannot replace the resistors in the above circuit with variable resistors. Because of the way this circuit works, if you use potentiometers, changing the level of any one channel will affect the volume levels of the other channels. We will look at ways around this later when we discuss potentiometers.
So that is it, a very simple audio mixer. Next time we will look at building this circuit.