Hardware Hacking 105By adding some switches and another output jack you can make a stereo version
of the same circuit (Figure 3.) Each summing resistor is connected to the switch’s
C; NO is connected to one output jack, NC to the other; throwing the switch
swaps the signal between the left to right outputs.
Figure 3Because this circuit is completely passive it is also bi-directional -- like the
photocell circuits in Chapter 22. If you plug a CD player into the output jack of
the circuit in Figure 1 you can use the pots to send it to any number of the
“input” jacks, which in this case become the outputs -- useful for distributing a
signal to a number of different amplifiers or signal processors. Sound confusing?
Just try it.
The pots can be of any value between 10-100kOhm. It’s best if all the pots are the
same. Pots are specified as “linear taper” (what we’ve been using so far) and
“audio taper”, which are optimized for adjusting audio level. You may find that
the curve of volume change sounds smoother with an audio taper pot. If you
find yourself mixing a lot of stereo signals (like the outputs of CD players) you
may want to use stereo pots, which let you trim two signals together from a
single knob. You can use ordinary rotary pots for these circuits, or slide pots
(easy to find through on-line sources), which make your circuit look more like a
“real” mixer, but be warned:
Rule #21: It is easier to drill round holes than slots.
Because it contains no amplifiers or other gain circuits, this circuit is best for
mixing signals of more or less similar levels (ie, CD and tape players, oscillators),
and is not so useful with low-level signals like microphones. What it lacks in
gain it makes up for in versatility and audio quality, but you can incorporate
preamplifier circuits such as those in Chapter 23 if you get ambitious -- insert
them between the input jack and the pot.