062702
26 Theremax
For both pitch and volume, the outputs of the reference and sensing oscillators are taken off
of the secondaries of their respective inductors to buffer them from the effects of loading by
the rest of the circuitry. Taking the pitch circuitry as typical, the oscillators are mixed in the
modulator consisting of D3 and D5. The output of the modulator consists almost entirely of
the sum and difference frequencies with some small leakage at the frequencies of the
oscillators. The higher frequencies are rejected by the low pass filter consisting of R26 and
C22 and only the audible difference frequency passes. Q8 and associated components
comprise a single stage amplifier that boosts the output of the modulator to a more usable
level with C27 providing a second pole of filtering for further suppression of the higher
frequencies. The comparable circuit elements in the Volume Sensing side of things should
be apparent from inspection of the schematic.
Pitch and Volume Control Voltages are each produced in the same way: A comparator
converts the sine wave difference frequency to square waves, which are differentiated to a
string of pulses, which are then integrated to a control voltage. Taking the Pitch CV as typical,
the output of the amplifier Q8 is coupled by C30 to the Schmitt Trigger wired around IC1:A. The
inverting input of the comparator is tied to a half-of-supply reference, Vr, that comes from R22
and R23. R38 ties the non-inverting input to Vr and R43 and R44 combine to provide a slight
hysteresis that speeds switching and prevents “chattering” when the Schmitt trigger fires.
The output appears across the load resistor R51 and is coupled by C28 to R24 so that the
rising edge of the square wave produces a positive-going pulse. On falling edges of the
square wave D11 forward biases and quickly charges C28 for the next pulse while also
clamping to ground the negative spike that would be produced. As the frequency increases, the
constant width pulses come closer together so the equivalent DC value of the pulse train
increases. The average value of the pulse train is recovered by charging C24 through D10.
The voltage on the capacitor is “read out” by the high impedance emitter follower consisting of
Q5 and R82, which is also the panel control that sets the control voltage available at the Pitch
CV Jack. The Volume CV is generated in the same way using the comparator built around
IC1:b and associated components.
The potentiometer R81 allows either the sine wave at the collector of Q8 or the square wave at
the output of the IC1:a or a mix of the two to serve as the audio signal. At the Counterclock-
wise end of the rotation of this pot, it’s grounded wiper shorts out the junction of R40 and R42
allowing only the sine wave to pass to the next stage through C29, R41 and R39. At the other
end of it’s rotation the wiper shorts out the sine wave and at intermediate settings the two are
mixed. C36 couples this audio signal to the Voltage Controlled Amplifier.
In the VCA the gain of a differential pair of transistors (Q10, Q11) is controlled by setting the
current flow through them with a third transistor, Q12. The Volume CV, as set by the panel
control R83, is converted to a current by R70 and this current sets the collector current of
Q12. As this current increases the gains of Q10 and Q11 increase as well. The significant
shift in DC voltage at the collectors of these transistors is canceled out in the differential amp
built around the 748 opamp IC2. The output of IC2 is coupled by C38 to the audio output Jack
J1.
