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AM 4012 Module - 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. It appeared in early ARP 2600 synthesizers, before Moog took legal
action. The circuit is a refinement of the ARP 1006 filter in the 2500 Modular synthesizer.
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 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. 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.
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