AM8035 Module - ARP "Moog" Low Pass Filter
Overview This is
a classic "moog" transistor ladder low pass filter with the usual 4-poles and 24dB cutoff. In 1974 ARP went back to their "moog" filter design (the 4012 module used in the 2600) and updated it to use transistor array
chips and put it into the Odyssey as the 4035, and the Pro-Soloist and Explorer 1 as the 4034. This two modules are virtually identical and both are temperature compensated.
The Odyssey was soon updated to use the 4075 filter,
but the "moog" 4034 filter remained in the Pro-Soloist and Explorer for the rest of their production life.
The original design is very similar to the ARP 4012 filter, with the same ladder design, but using 4 transistor array chips (CA3086). A dual FET buffers the ladder into the usual LM301 Op Amp. Another LM301 is used to sum the control voltages and provide CV scaling. The CA3086 were selected by ARP, presumably to match the transistors.
AM8035 Module The replica circuit is exactly the same as the original, as all the components are easy to locate. The AM8035 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 Op Amps have been updated to modern versions (OPA134
for audio and OP177 for CV's) and CA3046's are used as at least 2 of these transistors are matched at manufacture.
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 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)
The filter can be built without a 1K 3600ppm Tempco resistor, but the oscillation at high Q settings will not track accurately.
The front panel is a 3" wide FracRac with Alpha potentiometers and push fit mixer style knobs.