AM8023 ARP 2-pole VCF

Overview  This was the first analog synthesizer module I designed back in the Autumn of 2002, after buying a copy of Eagle CAD to design the circuit. It was a prototype for later designs and many lessons were learned… PCB holes too small, no mounting system, ahhgh!! After a few weeks of rework, a production PCB was ready, with all issues corrected. The AM8023 provides a high quality 2-pole filter with strong resonance which does not lower the audio signal when turned up. The original PCB design has been upgraded over the last 10 years and there is a version licensed to SynthCube here in MOTM format.

The filter produces a sharp and aggressive sound, similar to other 2-pole OTA designs from the early 70’s included the SEM.  The ARP 4023 filter was designed way back in 1972 as a 12dB (2-pole) voltage controlled Low Pass Filter, and it was used in the original “white faced” ARP Odyssey. It is a 2-pole OTA design with the CA3080 (as the OTA), FET buffer transistors and the LM301 Op Amp. The filter will self-oscillate at higher Q settings and it is temperature compensated with a 1K87 Tempco resistor. The filter does have a Band Pass output but this is not used in the ARP synthesizers or in my AM8023 versions.

Build Notes  My 2002 design follows the original very closely. I have kept the CA3080 OTA’s and used unmatched but thermally coupled 2N3904/2N3906 transistors. I have upgraded the Op Amps with high quality Burr Brown OP177’s for CV summing and OPA134’s for the audio path. All capacitors are high quality WIMA versions or 1% polystyrene.

The module has the following front panel controls:

  • Signal Input Level (x3)
  • Frequency, coarse and fine
  • Resonance
  • CV Level (x2)

There are six 3.5mm jack sockets mounted on the left hand side of the panel, these are:

  • Signal Inputs (x3)
  • CV Inputs (x2)
  • Signal Output

The filter can be built without a 1K87 3600ppm Tempco resistor, but the oscillation at high Q settings will not track accurately.

Front Panel The front panel was originally FracRac and made from PCB material, but in 2010 the Filter Bank modules were re-housed in professionally manufactured EuroRack panels. The module uses Alpha potentiometers and push fit Rean knobs.

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