062702
Theremax 27
The Volume CV is also used to derive a Velocity Control Voltage. Natural instruments are
sensitive to how hard you play them and in this MIDI age this quality has come to be know as
Velocity . In Theremax, Velocity is proportional to the rate of increase of Volume, the “velocity”
with which you hand is approaching the Volume Antenna. C26 couples changes in the Volume
CV through current limiting resistor R28 to the base of the emitter follower Q7. When the
Volume CV is decreasing D14 forward biases to clamp the Velocity CV to ground and provide
a high current recharge path for C26. Panel mounted pot R84 serves as a load resistor for
emitter follower and an attenuator for the CV.
The Velocity CV is available at the front panel Jack J3 and goes a couple of places internally
as well. It routes to the base of Q11 in the VCA differential pair where it makes the response
non symmetrical, which adds even harmonics to the output to give the sensation of being
played hard. The Velocity CV also is routed to the Schmitt trigger consisting of IC1:c and
associated components, when velocity exceeds a threshold the Schmitt changes state to
provide a triggering pulse to external equipment. Since many vintage analog synths use a
switch closure to ground (what is known as “S” trigger) to initiate a musical event, the final
comparator in IC1 is used as an open collector to ground turned on by the Gate. Both of these
signals appear at the Stereo Phone jack J2 with the gate connected to the tip and the open
collector to the ring. A mono plug may be used to access the Gate since the open collector tied
to the ring can be grounded with no problem.
D1, the 8.2V zener stabilizes the voltage from the wall wart power supply so that power line
transients and sags don’t produce pitch glitches. Finally, the funny wiring on the power switch?
That’s just so the “spare” lug can be used as a ground and provide mechanical support for the
Resistor and LED.
