AM8075 Odyssey III VCF

Overview  This module is a replica of the ARP 4075 24dB low pass filter in the ARP Odyssey Mk3, and the 4072 VCF used in later models of the ARP 2600. The module has been re-engineered to improve and simplify the original design, with upgraded high quality internal components such as Linear Systems matched transistor arrays, and precision polypropylene capacitors.  The external design replicates the “look and feel” of the ARP Odyssey Mk1 with slide potentiometers and coloured caps, and captures the essence and sound of the ARP Odyssey Mk3.

EuroRack Module: 16HP wide
User Manual:  Download PDF

Detailed Description This 16 HP wide module has at its heart the LM3900 Norton Amplifier chip which delivers the distinctive “smooth” 4-pole low pass filter, with a clean low distortion output that will easily oscillate at higher resonance settings. The filter response at high resonance remains faithful to the original with a slight loss of bass at higher resonance settings.

The two uppermost jack sockets INA and INB are for connecting audio signals into the input side of the filter, these signals are mixed together. The level of each audio signal can be varied from nil to maximum using the front panel slide potentiometers InA and InB. The middle jack sockets CV1, CV2 and CV3 are for connecting modulation control voltages into the filter. These signals vary the cut off frequency of the filter, with the front panel slide potentiometers adjusting the amount of modulation, except for CV3 which is directly connected to the VCF. The lowest jack socket marked Out is the audio signal output of the filter and it is typically connected to a VCA.

The VCF Cutoff slide potentiometer varies the cut off frequency of the low pass filter. At the minimum setting the filter will cut off all frequencies with no audio output and at the maximum setting the filter will pass all frequencies. A Fine slider enables small adjustments to be made when the filter is in self oscillation and a Keyboard slider enable Cutoff to be controlled by the pitch signal in the Doepfer Bus.

The VCF Resonance slide potentiometer adjusts the resonant peak of the filter from a minimum setting of no resonance, through subtle resonance enhancement to a maximum setting when the filter will break into self oscillation. The HPF Cutoff slide potentiometer varies the cutoff frequency of the high pass filter, from zero with all frequencies passed to a maximum where the bass frequencies are cut out of the final output signal.

Outcomes  I originally built a version in 2007 with rotary pots for my ARP 2600 replica. Subsequently the module was re-designed into a slider version which was then released in 2016.  I have no plans to manufacture the module again as there are already so many clones on the market.

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