CEM3320 LP


More DSP hungry than your average BC filter module, but worth it. Emulates the chip of the legendary early 80's synths.


It can be a lot of fun when used polyphonically though ...especially when playing around with time and level modulation in 'retrig' repeat mode.

jhulk: The cem3320 is just a 4 times ota design with a log input and tempco on board it can use its 4 ota in different filter types as it could be lowpass hipass bandpass or svf all 3 types.
It had a resonance compensation circuit in that if the resonance was high the filter input would be cross faded with the negative feedback via an ota cell t5his would stop the -6db attenuation you got with other designs like the moog.

Q: sharc if your core design is on the gain cells then electric druid did a a page on all the circuit designs for the cem3320
A: This module's just using the PRO1FB1 atom so it is more limited to the Pro-One design.
I'm sure it would be feasible to recreate those CEM circuits you linked to using BC Modular or the SDK. However, unlike this module they would probably have limited polyphony
Q: So will you be doing the ladder atom also ...what about the proddysee oscillator atom.
A: IIRC those atoms are key protected which would go against the standard for BC Modules.
I could possibly do them and release them in a 'SCOPE 5.1 required' version. We'll see.
Q: Are there any poly limitations?
A: There shouldn't be any poly limitations in normal use, but when loaded in a 1DSP CLM it'll only manage a maximum of 8 voices.










Various synths implemented the CEM3320 filter design as a reference standard, for example:
  • Sequential Pro-One (LP)
  • Oberheim OB-Xa (LP)
  • Elka Synthex Multimode Filter (LP/BP/HP)

The Sequential Pro-One uses the filter in a circuit which is quite close to the datasheet. They’ve added a buffer op-amp on the output, which also affects the resonance path. They also use the virtual ground node at Pin 1 to sum signals from the two oscillators and the noise source. The filter capacitors have also been halved in value, which is going to raise the basic cutoff frequency of the filter by an octave.


The Elka Synthex has probably the most sophisticated filter circuit built on the CEM3320 to ever have been used commercially. It uses analogue switches to reconfigure the circuit and provide a variety of responses. The Oberheim OB-8 that followed the OB-Xa did something similar, but only provided the 12dB and 24dB lowpass reponses that the OB-Xa had.

Elka Synthex - VoiceBoard