CGS twin CMOS filter

CGS49 the CGS twin CMOS filter board contains two independent VCFs based on the popular CMOS based Wasp VCF, In addition it also features adjustable post-filter distortion.

A little on how it works
Of note is that the filter core is running between 0V and -5V, allowing a simple exponential converter to be used to give an appropriate response to the control voltage. The distortion channel straight signal are mixed via a seque circuit (reverse panner). The final stage is just a regular, inverting amplifier, but with a voltage divider in the feedback loop to avoid the need for an excessively high value resistor to achieve the required gain.

Construction
Before you start assembly, check the board for etching faults. Look for any shorts between tracks, or open circuits due to over etching. Take this opportunity to sand the edges of the board if needed, removing any splinters or rough edges. (With the boards supplied by me, the edges are already milled, and etching faults are very rare.)

When you are happy with the printed circuit board, construction can proceed as normal, starting with the resistors first, followed by the IC socket if used, then moving onto the taller components.

Take particular care with the orientation of the polarized components such as electrolytics, diodes, transistors and ICs.

The CMOS chip MUST be of the unbuffered type. 4069 continued to be made in UB form long after most other UB 4000 series chips were stopped, so locating them should not be difficult. CD, HEF etc. are acceptable designators. Do not use chips with designators starting with "74".

It has been suggested that connecting the frequency pot between +VE and -VE instead of +VE and GND as shown will allow the filter to be used at lower frequencies. If doing this, you may find it beneficial to change the wiper resistor up to 270k, or add an extra resistor of 150k in series with the wire from the wiper to the PCB.

Tuning
Set the trimmers for a 1 volt per octave response. It will never be as accurate as a VCO as there is no thermal compensation, so "close enough" is good enough.

The photo shows two additional 100k trimmers I added to allow centering of the frequency pots. They are wired between +VE and -VE, with 100k wiper resistors fed to the CVSN pads on each filter. Alternatively, the CCW end of the frequency pots could be wired to -VE (via resistor if needed) instead of to 0V.



Parts list
This is a guide only. Parts needed will vary with individual constructor's needs.

CC-BY-NC
Readers are permitted to construct these circuits for their own personal use only. Ken Stone retains all rights to his work.

Suppliers

 * CGS49 Twin Wasp Filter, Elby Designs