AM1006 Filter Amp


Overview The ARP 1006 module consists of a traditional 4-pole low pass filter followed by a voltage controlled amplifier. The filter is a “moog” transistor ladder with a transistor buffer amp, it uses ceramic 10nF capacitors in the ladder which give it a drier tone than contemporary Mylar or polypropylene capacitors. The design makes use of a lot of matched transistors, as does the VCA to create current mirrors. The VCA has both linear and exponential modes, selected by a slide switch.

AM1006 Module The replica uses the same schematic and PCB layout as the original and is built as a 5U high MOTM compatible design. The PCB is 150 x 150mm with 40mil (fat) traces. The original component choices have been retained; LM301 Op Amps, carbon composition resistors in most locations, ceramic capacitors in the VCF ladder and a 680nF  high quality polyester film capacitor between the VCF and VCA.

Modernisation has been limited to using matched transistors from Linear Systems, 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 bought the ARP detailed service notes back in 2004 and made a couple of attempts to design a PCB over the next few years, and even tried to fit the design into EuroRack. The MOTM format for my ARP 2500 Rack modules was finally decided upon in June 2015, and in early July the large 150 x 150 mm PCB was laid out and a prototype built on 11 July. The circuit worked well with only two minor changes. The range of the VCF CV signal was adjusted to provide full filter sweep and cut-off, the 100K resistor was increased to 150K, and this did the job. A frequency trimmer was also added.

The initial build used 2N3904/06 transistors, these were replaced with Linear matched transistor arrays. and the PCB fitted to the panel at the end of July 2015.

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