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AM8075 Module - ARP 24dB Low Pass VCF
Overview The
AM8075 is a clone of the ARP 4075 4-pole 24dB low pass filter in the ARP Odyssey Mark 3, Omni and Quadra. It is very similar to the ARP 4072 filter in the ARP 2600, the only difference being the signal levels used.
Original Circuit
The original design uses 2N5087 matched and thermally coupled transistor pairs, as well as the LM3900 Norton amplifier chip. These components are still available and ARP sensibly ditched the old LM301 Op Amp's, and replaced them with the LM1458. The original ARP filter is flawed with a cut-off range limited to just 12 kHz by a design error, which can be easily corrected. This is an excellent "smooth" low pass filter, with a clean low distortion output that will oscillate at higher resonance settings. The control inputs are accurately calibrated to 1V/octave, and there are two CV inputs which have front panel attenuators (CV1 and CV2). There is also an un-attenuated CV in for direct connection to a keyboard CV.
AM8075 Module 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 synthesizers.
Hand matched transistor pairs can be 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; Originally I used 1% polystyrene capacitors in the main filter core, but I
switched in 2010 to Polypropylene capacitors, WIMA and Panasonic Audio capacitors in the audio signal paths and Panasonic FC capacitors in the power supply. The trimmers are all cermet 20 turn. All these upgrades pay off, the filter sounds fantastic!
The filter frequency has two front panel controls; FREQ for coarse adjustment and FINE for fine adjustment, there is also an on-board trimmer to set the initial
cutoff frequency. The filter has a Q control to adjust the resonance of the filter. Higher settings of the Q control will take the filter into sine wave oscillation.The module has the following front panel controls:
There are six 3.5 mm jack sockets mounted on the left hand side of the panel, these are:
Signal Inputs (x3) CV Inputs (x2) Signal Output
Front Panel The front panel is a 3" wide FracRac with Alpha potentiometers and push fit mixer style knobs.
Specifications The
AM8075 filter has been analysed for its frequency response, dymanic range and noise. The filter can be set up in two levels of trim;
The original ARP setting which enables the filter to resonate as low as 16Hz, this setting is best for bass sounds. A High Frequency trim setting, which increases the HF response but also increases the lowest resonance frequency. This setting is better for lead
sounds.
See the plot below of an AM8075 filter with ARP "design flaw" correction. Notice the frequency range cuts off at 10-12KHz so the correction is not as
important as you may think. In HF trim the frequency response is 30 - 11kHz at -3dB.
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