Dual Filter
This module offers a pair of State-Variable Filters with
Low, Band or High Pass responses and variable Resonance
up to self-oscillation. The Filters are largely identical
except that the left-hand one features a range switch while
the right-hand one offers output phase inversion.
Each Filter has a DC-coupled Input typically expecting a
b/-5V input signal. The Output is also DC-coupled,
with in-phase unity gain when no filtering or resonance are
applied.
The Low and High pass responses are 2-pole 12dB/Oct,
while the Band pass is 1-pole 6dB/Oct.
The Cutoff Frequency is determined by a sum of the
Frequency dial and the two Exponential Control Sources –
CV A via Polarizing FMod Attenuator and V/Oct.
With Resonance turned up full, the Filters will self-
oscillate, producing a pure sine-wave which will track over
several octaves via the V/Oct input. Note that the
oscillation amplitude increases with frequency:
20 Hz – 3V P-to-P,
200 Hz – 4V P-to-P,
2 kHz – 6V P-to-P,
12 k Hz – 8V P-to-P,
The Sub mode on the left-hand section switches in extra filter capacitors to shift the range into sub-
audio frequencies.
Low-pass corresponds to integration and functions like a Voltage-Controlled Glide when resonance
is set to minimum.
High-pass corresponds to differentiation and can create variable exponential decay (instant attack)
envelopes from a Squarewave input.
[Who knows what the Band-pass corresponds to.?. experiment]
Adding resonance will cause oscillations to be superimposed on the resulting waveform – akin to
waveform wobble, it gives strange results in the VC-Glide areas.
The output will oscillate as in Audio mode, though you may need to turn the frequency up first to
allow oscillations to build up to full amplitude.
The Phase switch on the right-hand section is a simple polarity inverter for the Main Path output.
You can experiment with the switch while mixing (externally) the input/dry signal with the Filter
output.
Calibration
Note that the module is not temperature compensated and that best tracking stability is achieved
after around 2 hours warm-up time.
Each filter has a multi-turn trimmer to adjust the V/Oct response.
