AM1004 Single VCO (2600)

AM1004 Module

Overview This is a replica of the ARP 2600 VCO that I fitted into a 1004-R style panels in 2015. rather than using the original 1004 VCO which was difficult to setup and is alleged to only have a few octaves in-tune compared with modern VCO’s. I also couldn’t locate 100R Tempco resistors until 2018, so the original 1004 was impossible to build.

AM1004 Module The AMSynths design uses the same schematic and similar PCB layout as the original ARP 2600 VCO and is built as a 5U high MOTM compatible design. The PCB is 150 x 150 mm with 40 mil (fat) traces. The module uses a potted AMSynths 4027-1 sub module and the waveform converters use LM301 Op Amps and carbon composite resistors.

The front end of the VCO has the standard AMSynths designed stable CV summer with precision voltage rails. Large format 1% and 0.1% resistors have been used here. So the design is very close to the original ARP in 1971, but with some improvements to achieve better frequency stability. White rocker switches are used to select and mix the waveforms, so this is a 1004-R module.

Square Wave at 4kHz

Modernisation is achieved via OP2277 Op Amps and a precision 10V chip at the front end of the VCO, this provides stable voltages at 1.00V steps. A 12-way rotary switch selects 9 octave ranges from these voltages, making it much easier to set a coarse frequency than a rotary potentiometer. A Fine tune frequency pot has been retained along with the ability to switch the VCO into low frequency mode using a slide switch, which also illuminates a LED behind a red lens and detaches the Keyboard CV from the VCO. A high quality LF412 dual Op Amp is used to mix the waveforms.

Controls The module has the following front panel controls:

  • Coarse Frequency
  • Fine Frequency
  • FM1 & FM2 Level
  • Pulse Width
  • PW Modulation Level
  • 5x rocker switches to select waveform
  • Slide switch for Low Frequency selection
  • Red illuminated bulb for low mode

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

  • FM Input (x2)
  • Pulse Width Modulation Input (x1)
  • Individual Waveform Outputs (x5)
  • Waveform Mix Output (x2)

Module Build & Outcome The 150 x 150 mm PCB was laid out in early July 2015 and a successfully prototype built and tested on 25 July. I then ordered 3 panels and built and wired up 3 VCO’s. These went together well, only one minor kludge, I forgot the bypass capacitor on the Sine Op Amp. By January 2016 they were finished and mounted in the initial Tolex Rack. They were moved into an Oak cabinet in March of 2019 when the complete 2500 systems was been commissioned

VCO Accuracy I tested the VCO’s after they had warmed up for 30 minutes and they are accurate to within 1% (or 1 cent) from 64 Hz to 1kHz (C2 to C6), outside this range they go to 4% sharp at 16 Hz and 2.5% sharp at 4kHz. The waveforms also deteriorate from perfect shapes as they get above 1kHz, but most musicians don’t play much above C6.


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