Voltage Controlled 3-Pole State Variable Filter
This is an interesting twist on the traditional 2-pole state variable filter,
as used in the Oberheim SEM and OB-X, Roland Jupiter-6 and Yamaha CS-50 and other
analogue synths. This novel filter, based on an Analog Devices SSM2164 quad VCA, offers
- 3-pole, -18dB/octave response
- simultaneous Low Pass, Band Pass and High Pass outputs
- full voltage control of both cutoff frequency and resonance
- frequency range spanning 12 octaves (4Hz to 16kHz), with
1V/octave frequency control response, 0-10V control range
- resonance range includes self-oscillation, 0-10V control range
- 100k input impedance
- short-circuit protected output buffers
- input levels up to ±3V
Sound Samples (MP3)
White Noise (modified Jen SX1000, reverse-biased transistor noise source) sweeps:
Note: you can load these into an audio editor (for example,
Audacity) and view the waveforms and spectra.
Here is a Simple sequence
played on my modified Jen SX1000, through VCF low pass, Lexicon MPX-1 delay/detune.
Starts without effects, then enable effects and play with filter cutoff and Q.
Finishing off back without effects for closing sweep.
For interested parties the following documentation is provided.
Please note that all these documents are Copyright © 2008 Neil Johnson.
- The signal connections are marked on the PCB as follows:
IN: S = Signal, F = frequency CV, Q = Q CV
OUT: L = Lowpass, B = Bandpass, H = Highpass
- The white area near the power connector is for you to mark the board with some
identifier (I usually write the date of completion).
- Setting up procedure is quite simple: set maximum Q (into self-oscillation) and adjust SCALE until
you have the desired frequency response, e.g. 1V/octave. Then set TUNE to give you the desired
range of control.
- I have specified polypropylene capacitors for the integrators. You can also use polystyrene
capacitors if you have them available.
- Components marked 'DNF' are Do Not Fit - I put them in for experimentation (I find it easier to not
fit a component rather than to bodge it on later if I change my mind).
In particular, R38 sets the bias current in the VCAs. In this design I find the default
behaviour (Class AB) is suitable.
Here are some links to other people's work with this filter.
- Dave Brown's MOTM-compatible panel.
Dave has put together a very nice looking MOTM format front panel design, using FrontPanelExpress.
You can download the design file from Dave's page for your own use.