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AM 4012 - ARP 24dB Low Pass VCF
Overview This is a 4-pole 24dB low-pass filter constructed around a traditional "Moog" transistor ladder. It is temperature compensated and has a wide cutoff range between 0 -
35kHz. The filter was deigned in late 1971 for the new ARP 2600 semi-modular synthesizer. It remained in this model until Moog took legal action in the mid 70's for copying their ladder design. ARP then switched to the ARP 4072 filter. The
4012 circuit is a refinement of the ARP 1006 filter in the 2500 Modular synthesizer, making use of FET's at the ladder summing stage.
Original Circuit
The original design uses 8 pairs of matched and thermally coupled TZ-81 and TZ-581 transistor pairs, plus a dual FET. There is also a LM301 Op Amp in the final signal path and temperature compensation is achieved with a 1K87 tempco resistor, next to the exponential converter.
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 transistors have long been obsolete, so I have used standard 2N3904/3906's (which are equivalent). The dual FET (2N3958) is hard to locate (and expensive), so
single FET's (2N5459) can be used instead. Ideally the matched pairs should be just that.
Initially I have kept with the LM301, but it can easily be replaced a modern low noise Op Amp such as the 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. We have had 1K87 3500ppm tempco resistors specially manufactured, and we recommend fitting one of
them.
Panel The front panel is a standard AM design, 135mm wide and 4U high, with black lettering. Controls knobs are standard Eµ Modular designs.
Build History The schematic has been transferred into Eagle CAD, and a protype is being tested.
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