AM1004-R Single VCO (2500)

Overview This is a replica of the ARP 2500 VCO that was developed in 1970 and featured in 3 modules; 1004 with various waveform switch combinations, the 1023 Dual VCO and the 1045  Synthesizer Voice. I originally used proven 2600 VCO’s in my 1004 modules, as in 2015 I had not worked out how to build the original 1004. In March 2019 I successfully completed the 1045 VCO card and worked out how to configure the 1004 VCO and set it up accurately. I have also located 100R SMD Tempco resistors which are essential to a stable VCO.

AM1004 Module The AMSynths design uses the same schematic and same PCB layout as the original ARP 1004 VCO and is built as a 5U high MOTM compatible design. The PCB is 150 x 150 mm with 40 mil (fat) traces. Some of the original component choices have been retained, but most have been slightly upgraded. Metal 1% and 0.1% film resistors have been used through out, the CV summers are low offset LT1012 Op Amps, and the core Op Amps remain as LM301. A TL071 is used in the sine wave converter and a TL072 to mix the waveforms.

Modernisation is added via LT1013 Op Amps and a precision 10V chip at the front end of the VCO, this provides precision 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 using a rotary potentiometer. The fine tune frequency pot has been retained (one octave range) along with the ability to switch the VCO into low frequency mode using a slide switch. A second slide switch switches the VCO on or off using JFET’s, and the status is indicated by a LED behind a red lens.

Controls The module has the following front panel controls:

  • Coarse Frequency (octaves)
  • Fine Frequency
  • FM1 & FM2 Level
  • Pulse Width
  • PW Modulation Level
  • 5x rocker switches to select waveform
  • Slide switch for Low Frequency selection
  • Slide switch for VCO ON/OFF
  • Red illuminated bulb for VCO status

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 March 2019 and a successfully prototype built and tested. I ordered 3x front panels and built and wired up 3x VCO’s. These went together well and they were mounted into my first ARP 2500 Rack, replacing the 2600 versions that are now in Rack 4.

How the 1004 VCO works is explained in detail here.


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