A circuit to help experimenting with various VCF topologies
A voltage-controlled filter (VCF) is an electronic filter whose main characteristics (cutoff frequency, order and resonance) can be set by an input control voltage. VCF are widely used in analog music synthesizers, in our case the Themis.
CEM3320 is a commonly used VCF, it allows a large type of filter topology, including lowpass and high pass response, bandpass and notch are also available but more difficult to operate. With CEM3320 the filter type selection depends on input component either a resistor or a capacitor (cf.Illustration 1).
Illustration 1: CEM3320 pinout
For a first lowpass order the pin n°1 is input section and the pin n°7 the output, the first stage is then located between pin’s n°1 and n°7 and similar to a current source in pair with an amplifier. The transfer function calculation allows to have a preview of the importance of the component selection to fix the filter characteristics (cf.Equation 1).
The impedance Zf, Zc and Ze represent respectively Rf, C and Rc which are the same used on the first illustration. With the help of this equation the main constant that’s stand out are:
|𝜏 = 𝜏0/||A|
It works similar with the other topology, each topology has its own transfer function because of the difference between the component placed on input. At this time, it is quite complicated to switch between two types of topologies like from lowpass to high pass because changing component require time and as things progress the high temperature of welding damage the VCF. To overcome this problem a printed circuit board (PCB) has been design to take into account the different filter topology and realize experimental tests more easily.
The filter selection and the desire order are managed by the PCB by using a set of jumpers. Jumper is the solution that fit perfectly in the Themis spirit because easy to use, modular and kind intuitive. The actual architecture is based on the “Output_State.v2” available on github, this old version was functional but only for a lowpass topology.
On the actual version each filter topology needs to be easily activate (cf.Illustration2), the “MA04” connector is in charge to make the connection on every input and output with the component the user wants.
Illustration 2: VCF_PCB with two stage
“MA04” connector rename “STAGE1” and “STAGE2” as above is an optimise solution to select le connection between the amplifier output (which is on pin n°7) and the second stage, four possibilities are open to the user:
n°7 to n°4 (on the connector) // lowpass filter
n°7 to n°2 (on the connector) // hightpass filter
n°7 to n°1 (on the connector) // end of stage (last order)