Hardware Hacking 93cut a lot of highs and the sound will be very bassy. In contrast, a large value of
CI (0.1uF) has almost no effect on the tone, but a very small one (100pF or so) will
cause much of the bass to disappear and the sound will be very tinny. This
simple equalizer is affected by its gain as well as the values of the capacitors --
for simple tone control purposes, it is best to have unity gain, i.e., use the same
value for RI and RF. At unity gain (RI=RF=100k), with a moderate capacitor in
CF (0.01uF), this circuit can roll off the high edge of a square wave oscillator (like
those we’ve built), making it sound smoother, more like a triangle wave.
Distortion
Amplifier sections can also be cascaded to produce greater gain, i.e., 2 sections of
a gain of 10 = gain of 100.. But this simple circuit is not “perfect” and by adding
a lot of gain in series we introduce distortion, the guitarist’s friend. The circuit
below, based on a venerable design by Craig Anderton (the godfather of musical
hacking,) is simple, versatile and good sounding. By varying the gain (via RF)
and substituting different capacitors for CF one can produce a wide range of
distortion timbres. For RF try a 100k resistor in series with a 5 meg pot for
variable distortion
As Robert Poss says:
Rule #20: Distortion is Truth. (Poss’s law)
But truth nonetheless comes in many flavors -- if you are interested in distortion
you should spend some time substituting different values of resistors and
capacitors in this circuit until you find perfection.
Unlike more sophisticated equalizers and tone controls, the frequency response
in both the preamplifier and distortion circuits can only be varied in steps, by
substituting capacitor values, and not with a continuous turn of a pot. But if you
want more control you can incorporate a multi-position switch for this purpose
(switching different capacitors in and out of the circuit) when you go to solder
up.