AM 4035 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.

Original Circuit The original design is very similar to the 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.

AM Circuit The replica circuit is exactly the same as the original, as all the components are easy to locate. 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 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. We have had 1K87 3500ppm tempco resistors specially manufactured, and we recommend fitting one of them.

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 27mm, 19mm and 13mm in diameter - black and chrome, the switch is a sub-minature slide model.

Build History The prototype PCB is under test.

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This design is not available as a PCB.
 

Sound Clip

The Original