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AM 4075 Module - ARP 24dB Low Pass VCF
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Overview The AM4075 is a clone of the ARP 4075 4-pole 24dB low pass filter in
the ARP Odyssey Mark III. It is very similar to the ARP 4072 filter, the only difference being the signal levels used.
The AM4075 is temperature compensated and it appeared in later ARP 2600
models as well as the Odyssey and Quadra. Its cut-off 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 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.
T
he 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 cut-off frequency.
The filter has a Q control (RESO) to adjust the resonance of the filter. Higher settings of the Q control will take the filter into sine wave oscillation.
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 LM1458's.
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 synthesizers.
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 Panasonic Audio capacitors in the audio signal
paths and Panasonic FC capacitors in the power supply. The trimmers are all cermet 20 turn. These upgrades pay off, the filter sounds fantastic!
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.
Specifications The AM4075 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 AM4075 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 setting the filter has a frequency response of 30 - 11kHz at -3dB.
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