AM1004 Voltage Controlled Oscillator


Overview This is a replica of the ARP 2600 VCO, rather than the original 1004 VCO which has been found to be difficult to setup and has only a few octaves in-tune compared with modern VCO’s. My 2500 project is trying to keep to the original as much as possible, but I do need VCO’s that work well. The 1004 module consists of a potted 4027-1 sub module and the waveform converters from the ARP 2600 with 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.

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 150mm with 40mil (fat) traces. The original component choices have been retained; LM301 Op Amps, carbon composition and selected use of metal film resistors. White rocker switches are used to select and mix the waveforms, so this is a 1004-R module.

Modernisation is achieved via OP2277 Op Amps and a precision 10V chip at the front end of the VCO. A 12-way rotary switch selects 9 octave ranges, 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 dual Op Amp is used to mix the waveforms.

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

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)

Build History 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 forget the bypass cap on the Sine Op Amp. By January 2016 they were finished and mounted in the first ARP 2500 Rack.

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