CEM3389 LPF and PAN

The CEM3389 chip consists of a 4 pole lowpass filter, a VCA, and a V.C. pan circuit. The PCB I built around it supplies the circuitry for three of the modules on the front panel: Input Mixer, VCF1, and Output Mixer and Pan. The internal VCA is used as a final VCA whose CV comes from the midi Velocity.

• Input mixer mixes inputs from the standard signal sources: VCOs 1-3, Noise, and Ring Modulator. Any of these can be overridden at their respective input jacks.
• Input mixer has master gain (LEVEL) and output is connected to VCF1 input
• Four pole lowpass filter with resonance adjustable into oscillation
• Output mixer has two inputs dedicated to VCF1 and VCF2 outputs, and two free inputs
• Output mixer is fed into VCA which derives its CV from midi Velocity
• VCA feeds VC Pan with internatal LFO for rate
• Left and Right line outputs and headphone jack with bass boost

Status

PCB built and tested Input mixer and LPF wired to front panel and tested

I had to modify my original circuit with an extra inverter and the "Q enhancement" switch to feed back output signal into the input. The voltage controlled Q alone was not enough to send the filter into self oscillation, but coupled with the additional positive feedback it works.

Input Mixer schematic
LPF schematic
Output Mixer and VCA schematic
VC Pan schematic

CEM3389 Bill of Materials

CEM3389 module 2x PCB layout