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AM 8023 Module - ARP 12dB Low Pass Filter
Overview This
was the first module I designed back in the Autumn of 2002. It was a prototype for later designs and many lessons were learned... PCB holes too small, no mounting system, ahhgh!! The later AM8023 module corrects these problems, and
provides a high quality 2-pole filter in a FracRac design. The filter produces a sharp and aggressive sound.
Original Circuit The ARP 4023 filter was designed way back in 1972 as a 12dB (2-pole) voltage controlled Low
Pass Filter, and it was used in the original "white faced" Odyssey. It is a 2-pole OTA design with a bunch of transistors, the CA3080 (as the OTA) and the LM301 Op Amp. The filter will self-oscillate at higher Q settings and it is
temperture compensated with a 1K87 tempco resistor.
ARP designed the 4023 as a potted sub module to fit on the synthesizers main PCB. There is a picture of the PCB in this sub module opposite.
AM8023 Circuit
My design follows the original very closely, I have kept the CA3080 OTA's and used unmatched but thermally coupled 2N3904/2N3906 transistors. I have upgraded the Op Amps with high quality Burr Brown OP177's for CV summing and OPA134's for the audio path. All capacitors are high quality WIMA versions or 1% polystyrene.
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
The filter can be built without a 1K 3600ppm Tempco resistor, but the oscillation at high Q settings will not track accurately.
Front Panel
The front panel is a 3" wide FracRac with Alpha potentiometers and push fit mixer style knobs.
Specifications The AM8023 filter has been analysed for frequency response, dymanic range and noise. The filter has been
compared with the AM8075 which has a lower frequency range.
The AM8023 filter has a frequency response of 120Hz - 11kHz at -3dB.
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