|
A
little on how it works
The
Super Psycho Modulation Source is a fairly simple
circuit with a lot of repetition. The circuit consists
of several distinct blocks. The first are the oscillators,
each based on a section of a 40106 hex schmitt inverter
and their associated components. The switch allows
extra capacitance to be added, reducing the speed
of the oscillator to a lower range. The same switch
allows the capacitor to be bypassed to the negative
rail via a protection resistor, disabling the oscillator,
and setting its output to near 0 volts. The 1M RATE
pots allow the upper frequency range to extend over
a large portion of the audio spectrum, and in the
lower frequency position, to extend from sub-audio
to low audio frequencies.
Two
of the oscillators are equipped with voltage followers
that follow the the, roughly, triangular shape of
the wave present on the oscillator capacitors. The
100k and 360k resistors on the output, when coupled
to the virtual ground summing node of the following
mixer, via the switch, correct the amplitude and offset
of the triangle wave.
The
next block is a traditional op amp inverting mixer
stage. Signals from the six oscillators are mixed
through 470k resistors, the overall gain controlled
by a 100k pot in the feedback path of the op amp.
What
is interesting here is that in order to keep the output
signal of the mixer positive without the need for
an additional inverting stage, the chip used to build
the six oscillators is powered from the negative rail,
it's positive power pin connected to 0V and its earth
pin connected to -15V. Take special note of this,
because it is an unusual way to power a digital chip,
and inadvertent poking with a logic probe powered
from the positive rail could cause you grief!
The
final stage is a simple glide circuit consisting of
a potentiometer, a capacitor and unity gain voltage
follower. The switch is to allow easy switching in
and out of a pre-adjusted glide setting.
|
