062702
Theremax 25
DESIGN ANALYSIS
The complete Theremax schematic is shown in Fig 6. At the heart of the circuitry are four
oscillators, two of which are mixed to produce the pitch signal and two of which are mixed to
produce the volume control signal. If it occurs to you that one oscillator could serve as refer-
ence for both pitch and volume sensing you have a good designer’s instincts, this would be
more economical. The difficulty is that multiple oscillators operating at close to the same
frequency have a tendency to “pull” and lock to exactly the same frequency, just like the swing
of multiple compound pendulums will tend to synchronize. It’s not too difficult to minimize this
tendency in a single pair of oscillators by physically isolating them from one another, putting
guard bands around them on a circuit board and decoupling them from their common power
supply, but three oscillators, all heading for essentially the same frequency is considerably
more difficult.
If the oscillators lock, there is no more difference frequency so the output goes to zero. It
would be bad enough if at the lowest notes the sound suddenly stopped, but the worst part is
that prior to locking the oscillators go through an unstable region where the synchronization is
chaotic. Instead of just suddenly going quiet, you first hear a burst of noise. These are very
unmusical characteristics and having four oscillators allows us to offset the frequency ranges
of the pairs so that they do not interact.
The basic oscillator is a classic Hartley type as typified by transistor Q2 and associated
circuitry. The primary of Oscillator coil L2 and capacitor C10 from a resonant “tank” circuit
load for the transistor, which is configured as a common-base amplifier stage. A tap on the
inductor is coupled by C11 to the input of the amplifier (the emitter of Q2) for feedback.
Resistors R7 and R10 set the operating point of Q2 to provide the gain necessary to maintain
oscillation. R8 and C8 provide decoupling between the power supply and the oscillator to
minimize unwanted interactions between the four oscillators. In this oscillator, and the identical
one built around Q3, the frequency is set solely by the combination of C10 and the inductance
of the primary of L2 and is adjusted by varying the ferrite slug in the oscillator coil.
The sensing oscillators, typified by the one for pitch comprising Q1 and associated compo-
nents, have a couple of tweaks. The capacitive reactance of the tank has the additional
component of the pitch sensing antenna, which is effectively in parallel with capacitor C6. The
parasitic capacitance of this antenna is greatly affected by the presence of objects, particularly
flesh and blood objects. As an object approaches the antenna, capacitance increases causing
the resonant frequency of the circuit to go down.
The sensing oscillator also provides for vernier control of frequency using the potentiometer
*R79 which allows a variable setting of the operating point of Q1. Varying the operating point
changes frequency by increasing or decreasing the DC current flow through the Primary of
L1, which changes the permeability of the core slightly and consequently the reactance of the
inductor. The volume sensing oscillator (Q4, etc.) follows this same design.
