AM1006 Filter Amp

AM1006 Module

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 where they are used to create Wilson current mirrors. The VCA has both linear and exponential modes, selected by a slide switch.

The 1006 was one of the first modules to be designed in the Spring of 1970 and it had six engineering changes in its life; The removal of R13 and C5, changes in component suppliers and resistor values to accommodate a change in matched transistors, adding 27R decoupling resistors in the power rails in 1972. I have replicated the REV G version from 22 October 1974.

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 150 mm with 40 mil (fat) traces. The original component choices have been retained; LM301 Op Amps, carbon composition resistors in most locations, ceramic disk capacitors in the VCF ladder and a 680nF high quality polyester film capacitor inserted between the VCF and VCA. The original design used a fast 1339 Op Amp in the VCA final buffer, I have used a TL071 but I may replace this with a MC1439.

AM1006 PCB

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).

Controls 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

Connections 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)

    Filter Capacitors

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 laid out 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 AM1006 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, so that full cutoff could be easily adjusted at setup time, rather than selecting resistors.

The initial build used 2N3904/06 transistors to prove it worked, then Linear Systems matched transistor arrays were fitted to improve the sound quality. The PCB was fitted to the panel at the end of July 2015. I subsequently used THAT transistor arrays in the 1045 voice, but they did not work as well possibly due to the lower Hfe. I now use Linear Systems or Rpar AS194CH matched transistors in the ARP ladder filter.

2019 Update I conducted a few more tests in December 2019 and ensured the VCF and VCA were accurately setup. The filter will self oscillate from 500 Hz up to beyond 20 kHz. A minor change to the CV summer Op Amp gain was need to bring down the highest cutoff frequency to 20 kHz from 80 kHz.

AM1006F Module

2024 Update The work on the AMSynths 1006 was reused in the Eurorack Behringer 1006 module in 2019 (which was a big success), its uses all SMD components and RC4580 Op Amps. The AM1006 5U module was sold to a customer in 2021, as there was insufficient space for three large 5U cabinets in my studio.

In 2024 I revisited the 2500 project and designed some of the “lost modules” in Eurorack, to complement my favourite Behringer modules (1027, 1050). I listened to feedback on the Behringer 1006 module and responded by developing the AM1006F module which is as close as possible to the original.

  • It is just the filter section (1006F)
  • It has more inputs (3x audio, 4x CV), to mirror the matrix switch capability
  • Carbon resistors in the inputs, to mirror the variations in the original
  • Ceramic disk capacitors in the transistor ladder, same as original
  • 680nF polypropylene capacitor for DC coupling the filter output, same as the original
  • A fast slew rate BJT Op Amp on the output, to emulate the original MC1339 Op Amp
  • A V/OCT trimmer has been added for precise tracking
  • ARP 2500 control knobs, same as original
  • Self-oscillation down to 400Hz

The AM1006F was developed in the Spring of 2024, and is available from our webstore here.

Copyright AMSynths 2022