The Chirper 3FW is a distillation of the original Red Chirper boiled down & modified to a compact module version. The pre-arranged configuration of two cross-coupled voltage-controlled oscillators and one voltage-controlled filter seems like a simple setup but it hides a rich world of sonic surprises – even more so when combined with the Chirper Expander!
The Block Diagram shows how the Chirper is put together:
- the two Oscs are identical, with wide range (sub audio to c.40kHz) and sweepable from Triangle to Square.
- each Osc has a 1V/Oct input, an External Modulation input with polarity switch and depth control, plus an Internal Modulation depth control sourced from the other oscillator.
- in addition, an in-phase squarewave is generated for each Osc (mainly for use with the Expander) – the Square wave can be selected via a jumper on the rear as Norm(al) or InPhase.
- the State-Variable Filter can be fed from Osc1 or an external signal (typical +/-5V) and resonance goes up to self-oscillation. The output is switchable from LowPass to either Band or High Pass (selectable on the rear via jumper). The filter has an External Modulation input with polarity switch & depth control (approx 1V/Oct at full depth), along with Internal Modulation from Osc2.
The design is centred around a single 2164 Quad-VCA chip (obviously with much supporting circuitry!) – one section is used for each VCO and the other two for the VCF. Note that the 1V/Oct response of the Oscs & Filter are only quite accurate – fine for tracking over a few octaves, but not temperature compensated nor as accurate as the more advanced VCO modules. The overall design aim is more at surprise than being a well-pitched synth!
Current: +ve 65mA, -ve 50mA