The circuit shown is the most interesting of several drum oscillators on an analog sound board that appeared as part of an article in the June 1993 issue of Electronics Now.
Like most drum tone oscillators, the circuit is a very high Q resonant filter that is only slightly overdamped and on the verge of oscillating. Triggering pulses (coupled through C5 and R5) provide an excitation signal that “rings” the oscillator — an analog of the rapidly decaying envelope of percussion sounds. This circuit topology is sometimes called a “Twin-T” because of the arrangement of components R1, C1, R2 and C2, R4, C3 and it’s the values of these parts that set the characteristic pitch of the percussion sound — bass, tom, bongo, clave, etc. Decay rate (“Q”) is adjustable with R8, which in conjunction with R6 sets the gain of the amplifier stage. Rotating R8’s wiper toward the “+5V” end increases “Q” for a slower decay rate. The circuit will self-oscillate at high “Q” settings.
This circuit has the embellishment of a simple Voltage Control mechanism. The diodes D1 and D2 are effectively in parallel with R4 (one of the components that determines pitch). As the CV-in voltage increases, the exponentially increasing current flow through these diodes causes their effective impedance to decrease, which raises the frequency of the Twin-T network.
Such a simple Voltage Control implementation has an interesting secondary characteristic — as the CV increases, “Q” also increases and above some threshold value the circuit acts like a Voltage Controlled Oscillator. A classic synth-drum down-scale gliss can be quite long since at the higher pitches the circuit is essentially a VCO which “quenches” when lower CVs produce lower effective “Q”s. This single simple circuit combines the functions of VCO, EG and VCA and is easy to mod and tweak.
On the PAiA Drum Tone Board, the downward sweeping ramp CV is generated by using the trigger pulse to charge C4 through D3. The voltage on C4 is the Control Voltage, which decays over time as it discharges through R3. The amplitude of the trig-in pulse determines the peak voltage on C4 and consequently how far upscale the down-scale gliss begins.
The circuit was designed to work with a power source having 12VDC and 5VDC rails. It can be converted to use a bi-polar supply by connecting all +5V points to ground and pin 11 of the IC to the negative supply rail. Bipolar supply voltages can be from +/- 5V to +/- 15V. Here’s how to tap power from the 9700 MIDI2CV8 MIDI>CV converter and use the pair for a MIDI controlled Drum Card.
Places to Tweak
Change the glide time with C4. Increasing values increase time proportionally.
Change gross frequency range with C1, C2 and C3. Keep their values proportional to those shown, e.g. taking C1 to 0.02uF and C2 and C3 to 0.002uF lowers the frequency range an octave.
Sensitivity to CV should be adjusted at R3, but be aware that this will also shift the center frequency somewhat. It’s best to only increase the value, it’s presently optimized for about a 0-5V range and doubling it to 2.2Meg would take this up to 0-10V. The response is exponential (V/Oct), more or less, but is by no means chromatic in any given scale.
Removing C1 changes the response to low-pass with resonance, which is often more useful musically, but doesn’t ring quite as well for drum sounds. It could be switched in and out.