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AM 4023 - ARP 12dB Low Pass VCF
 Overview
This filter dates back to late 1972 when ARP were designding the ARP Odyssey. They put a 2-pole Low Pass Filter into the new synthesizer, rather than the "moog" filters that they had originally used in
the ARP 2500 and 2600. Why they did this remains unknown, its more likely because they wanted a cheaper design which sounded different, rather than for copyright reasons (copyright problems seem to have happened in
the mid 70's). Whatever the background , this filter is powerful and raw. It has lots of sonic potential, giving a real good electronic sound and a very powerful self-oscillation at high Q settings - smooth it is not.
Original Circuit The original ARP design from 1972 is a 12dB voltage controlled low pass filter, and it was used in the original "white faced" ARP Odyssey. It is a 2-pole OTA design based around
transistors, the CA3080 (OTA) and the LM301 Op Amp. The exponential converters use matched and thermally coupled PNP/NPN pairs. The filter will self-oscillate at higher Q settings and it is
temperature compensated with a 1K87 3500ppm Tempco resistor.
AM Circuit The AM module contains the original ARP module circuit as well as the additional
circuitry of CV and signal summing which is held on the main ARP PCB's. This provides a self contained module for use within any modular system, rather than a replacement module for ARP synths.
The AM circuit is an exact replica. The CA3080 OTA's are retained, 2N3904 and 2N3906 transistors are used instead of the original TZ581 and 2N5172's. The CV summer can be the original LM301 Op
Amp or a modern OP177 Op Amp. The audio path Op Amps can be the original LM301's or improved modern OPA134.
The Tempco resistor can be omitted, and a standard metal film resistor used, if you are not
concerned about the filter oscillation tracking properly. I have had 1K87 3500ppm tempco resistors specially manufactured, and I recommend fitting one of them.
The frequency control circuitry of the original has been expanded to include a dedicated keyboard CV input, as well as a FINE front panel control and an internal frequency trimmer for setting up the filters response.
Front Panel
The front panel is an AM High Density design, 90mm wide, 3mm thick aluminium, 4U high, with black lettering. Controls pot spindles are 3.18mm in diameter, knobs are 19mm and
13mm in diameter - black and chrome, switches are sub-minature slide models.
Module Status The prototype passed all tests successfully in April 2005. Project Notes and production files are now available.
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