CdS VCF/A


The CdS VCF/A is a vactrol-controlled module with switch selectable responses. It can function as a 1-pole VCF, a 2-pole VCF, or a VCA.

CdS VCF/A front panel
There are three distinct responses selected by a switch:
  • LP2
  • LP1
  • VCA
Two CV inputs
  • CV1 with a Push/Pull potentiometer to attenuate and/or invert the CV signal
  • CV2 is unattenuated
Two modes for the CV inputs:
  • CV1(CV2) internal VCA mode
  • CV1+CV2 - mix / sum
Resonance control pot that ranges from 0 to self oscillation
The module is built and trimmed for:
  • IN jack: +/-5V audio or CV
  • OUT jack: +/-5V typical
  • CV1 and CV2: 0-5V span the full response range. Responds to negative voltages when the OFFSET pot is anywhere other than full CCW (7').
Various pcb headers to easily normallize two CdS modules together.
MU format (will fit in synthesizers.com and STG cabinets and rack frames)
Built with love and care in Chicago using high quality parts including:
  • Switchcraft 112 jacks
  • NKK toggle switches

The module is based on the LPG (lowpass gate) circuitry pioneered by Don Buchla in the 1960's and 70's and cloned by many others since. However, the CdS VCF/A implements various changes and enhancements to the original circuit:

Resonance control
Ranges from zero/nothing to classic at ~10 o'clock to self oscillation at ~12 o'clock and beyond. Further rotation past 12 deals in greater amounts of oscillation, resulting in a more distorted oscillating waveform. The output level will also rise when the module is oscillating but a technique is used to keep this level reasonable (approx. +/-7.5V when Res pot is at max).

4th response: LP2 without resonance
The module achieves a fourth mode with the response switch set to LP2 and with the Res pot fully off (CCW). This is similar to LP1 but with a steeper slope. Check out the sound samples for a comparison.

This response is not achievable with the original LPG or the various clones that I am aware of.

Internal VCA for the control voltage inputs
When the CV mode switch is in the CV1(CV2) position, the internal control VCA is activated. CV2 is the input of the VCA. CV1 acts as the control input for this VCA. The result is combined with the OFFSET pot to control the cutoff frequency of the VCF. Great for dynamic control.

Example: patch an envelope to CV1 and patch velocity to the CV2 input. Now the depth of modulation can be dynamically controlled by how hard you hit the keyboard (or other controller). Another option is to patch the pulse output from an LFO to CV2. Now the envelope will fade the LFO in and out. Simply put, CV1(CV2) mode is the same as placing a VCA module before one of the CV inputs.

It simplifies patching a bit by eliminating a module and saving a patchcord or two. It also encourages a bit more depth and/or detail in patches by simply flipping a switch.
CV1+CV2 mode sums/mixes the two control signals together.

CV1 pot is also a push/pull switch. The pulled position inverts CV1 when in CV1+CV2 mode. It inverts the internal control voltage VCA signal when the mode switch is set to CV1(CV2).

This filter topology combined with the characteristic smooth response of the vactrol-control element result in a unique sound and a very natural response to control signals.

Click here to download the PDF user guide.

Price: $240.00 + shipping
Email me with your address for a total with shipping.

Sound Samples

All samples are recorded straight to the soundcard. No effects, no EQ, no processing, nothing.

Return to the top.