AM1023 Filter Amplifier

Overview The ARP 1023 module consists of a 2-pole state variable filter followed by a voltage controlled amplifier. The filter is based on the ARP 4023 module used in the ARP Odyssey, and is a virtual 2500 module that was never announced but “could have been”. The module gave me an opportunity to layout the ARP circuit with the original resistors and semiconductors, for a vintage look and maybe sound!

AM1023 Module The circuit uses the same schematic as the original and is built as a 5U high MOTM compatible design. The PCB is 150 x 150 mm with 40 mil (fat) traces. The original component choices have been retained; LM301 Op Amps, carbon composition resistors in most locations, ceramic capacitors in the filter and a 680nF  high quality polyester film capacitor between the VCF and VCA.

Modernisation has been limited to a 1uF BP input capacitor (to raise the bandwidth), modern trimmers and a 10V precision power rail for CV control (removing some resistor selection at setup).

The module has the following front panel controls:

  • Filter Cutoff
  • Resonance
  • Amplifier Gain
  • VCF CV1 and CV2 Level
  • VCA CV1 and CV2 Level
  • Slide switch for VCA response – exponential or linear

There are 10x 3.5mm jack sockets mounted at the base of the front panel:

  • Audio Inputs (x4)
  • VCF CV Inputs (x2)
  • VCA CV Inputs (x2)
  • Signal Outputs (x2)

Build History I designed and built a 4023 copy PCB way back in 2002 as my first attempt using Eagle CAD. 13 years later I have laid out the filter on a large 150 x 150 mm PCB on 8 August 2015. The PCB was populated in early September and it will be fitted to my second 2500 Rack. The panel was ordered on 03 April, after making progress on the 1005, and I wired the 1023 up on 17 April 2016.

2019 Update The lettering on the 1023 panel quickly rubbed off due to manufacturing defects, and the module was stored. I plan to buy a new panel in 2020 and fit the module into my Tolex cabinet which holds ten modules. The filter works well but could do with zener diodes in the feedback loop to reduce the peak size of the self oscillation. I may also renumber it, so I use 1023 for a dual VCO module

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