82 Nicolas Collins
may need to add pull-up resistors to the inputs of the circuits to start them
oscillating: connect a resistor in the 100kOhm – 1mOhm range between the input
pin and + 9 volts of the various stages of your circuit. Arranged in matrices,
sometimes the oscillators produce unstable, complicated patterns of pitch and
rhythm not displeasing to the ear (or brain.)
Volume Control
These oscillator circuits are really loud: they put out 9 volts peak-to-peak, versus
about 0.7 volts peak-to-peak for the output of a typical piece of consumer audio
gear, such as a CD player. They may overload the input to your amplifier or
mixer, causing distortion (not always a bad thing) and limiting the useful range
of your faders. If you want to drop the level down to a more reasonable volume,
try adding this circuit to the output of each oscillator or the mix of several
oscillators:
Alternatively, if you want fade the volume up and down, rather than just drop it,
you can add a volume control at the output of the oscillator, as shown below.
Any pot whose value is 10kOhm or greater will do. If you can find a pot
designated as having an “audio taper” it will make the fade sound smoother (see
Chapter 27 for a discussion of pot characteristics.)