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AM 8012 Module - ARP 24dB Low Pass VCF
A You Tube demo of the filter in an ARP 2600 Synthesizer is here>>
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.
AM8012 Module I originally designed this way back in 2004 as a larger 4U fomat PCB, but the prototype didn't work. I discovered the fault 5 years later in 2009 when I relaided the design onto a FacRac format PCB. It
woprks perfectly now. My design uses standard 2N3904/3906's and a dual FET (2N3958) and an original LM301 Op Amp.
The AM module contains the original ARP 4012 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 final stage Op Amp can be updated to modern versions (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
use newly manufactured 1K87 3500ppm tempco resistors..
Front Panel The front panel is a 3" wide FracRac with 16mm Alpha potentiometers and push fit mixer style knobs.The module has the following front panel controls:
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
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