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AM 4075 - ARP 24dB Low Pass VCF
 Overview
This is a 4-pole low pass filter based around a cascade of integrators. It is temperature compensated and it appeared in later ARP 2600 models. Its cutoff range was severely limited to just
12kHz by a design error, which can be easily corrected. This is an excellent filter, with a clean low distortion ouput that will oscillate at higher resonance settings.
Original Circuit
The original design uses 2N5087 matched and thermally coupled transistor pairs, as well as the LM3900 Norton amplifier chip. Both of these components are still available. ARP
sensibly ditched the old LM301 Op Amp used in previous filters, and replaced it with a LM1458.
AM Circuit The cloned 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.
Hand matched transistor pairs have been used, rather than expensive pre-matched transistor
pairs (e.g. SSM2210). However the filter works perfectly well without hand matching the transistors. The CV summing Op Amp is a high quality (low offset) OP177 rather than the original
LM1458. All capacitors have been upgraded as well; 1% polystyrene capacitors are used in the main filter core, WIMA and Pansonic Audio capacitors in the audio signal paths and Pansonic FC
capacitors in the power supply. The trimmers are all cermet 20 turn. These upgrades pay off, the filter sounds fantastic!
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.
The frequency control circuitry of the original has been expanded to include a dedicated keyboard CV input, as well as a FINE front panel control and an internal frequency trimmer for setting up the filters response.
Front Panel
Our module is fitted to a front panel using pot brackets. It uses the 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 toggle. The PCB can be fitted to AM Low Density panels, or MOTM format front panels - the pot bracket spacing is 40mm.
Build History The prototype PCB was successfully tested in early April 2005 and production boards are now available (from March 2006), either bare or populated and tested.
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