This is my take on the 12dB Multimode VCF. This filter type is widely used in many synthesizers. You can find it in Electronotes or in the SEM modules and in many others as well. Because this one is for my Nest Generation Formant project i started with the original Elektor Formant schematic and added my changes to the design. All parts are updated to today (2017/4) available parts. The connections are the same as in the original to keep the possibility for internal wiring. If you don’t need those features just leave them out. This PCB provides all basic functions as in the original Elektor Formant. The additional functionality is put on an add-on board. The The CA3080 are replaced with LM13700. The JFET transistors are replaced with OpAmps. The signal level is raised to 10Vpp for a better signal to noise ratio. The exponentiator for generating Iabc for the OTA’s is temperature compensated. The only additional function on this PCB is the linear TM input. All other additional functions are on the add-on board.
The add-on board provides voltage control for Q, a sign changer for the ENV input for easier use when the filter is switched to high pass mode and volume indicator. This will be covered in another post.
Specs and features
- 12dB highpass filter, 12dB lowpass filter, 6dB bandpass filter, notch filter
- Temperature compensated exponentiator
- 10Vpp signal level
- TM log input
- TM lin input
- Positive and negative ENV control (with AddOn PCB)
- Volume display (with AddOn PCB)
- Voltage controlled Q (with AddOn PCB)
- Runs on +/-15V and +/-12V (with minor resistor value changes)