ARP 2500 Rack Modular

Overview The ARP 2500 modular synthesizer was developed as a direct competitor to Bob’s Moog Modular, which had made a massive impact in 1968-69 after “Switched on Bach” launched the analog synthesizer into the publics imagination. Alan Pearlman founded the Tonus company in 1968 with $200,000 and in 1970 launched the ARP 2500. The Massachusetts company implemented three innovations over the existing Moog and Buchla designs:

  • Stable oscillators with a wide frequency range
  • Matrix Patching System with no need for patch cords
  • Multi-voice keyboards

David Friend presented an overview of the ARP 2500 to the AES Convention in October 1970 and ARP took immediate market leadership from Moog by implementing an improved VCO design. This was made possible by implementing a stable and accurate exponential converter, which is at the heart of the VCO and VCF cores. These converters were encapsulated into small sub modules to improve temperature stability, and to stop other manufacturers copying the idea. The converters contain five transistors and a tempco resistor (100 ohms).

Pearlman also introduced Op Amps into his designs and used the Teledyne 1339 Hi-Fi chip, as well as the newly available LM301. The 2500 modules make extensive use of matched transistor pairs, and Op Amps are used in CV summing and final audio stages. The ARP 2500 made no use of the recently available OTA CA3080 chip (1969), but uses transistors to build VCA components.

Digital Technology The 1027 and 1050 modules make use of early digital technology. They are based around some early TTL logic chips, and the ARP design was probably the first sequencer to use a decade counter chip. This gave a 10 step sequence rather than the more usual 8 or 16 steps. The clocks and control circuits are based on a mix of early TTL chips and transistors and LM301 Op Amps. The audio paths have Teledyne 1339 Op Amps, which are a bit unreliable and were replaced with slower LM301 Op Amps in the ARP 2600.

Technology Cascade The ARP 2500 pioneered new analog synthesizer designs but it was a high price tag/low volume product aimed at Universities and the “Moog” record market. Pearlman spent the next 3 years reworking the technology into a complete range of synthesizers at lower prices that appealed to a much wider range of musicians.The ARP 2600 (1971) and Odyssey(1972) can trace their circuits back to 2500 modules, such as the Moog copy filter (4012 module) and the internal transistor clocks.

The 2500 Rack  The idea of building replica ARP 2500 Modules became possible in the spring of 2004 when MusicParts in the USA published the original schematics, complete with the original construction and part listings. I bought the full set and over 10 years gradually designed the PCB’s and sourced the parts needed, but with a lot of effort made in Summer 2015 to complete the project. I have also added some of the announced modules that ARP never manufactured, filling in the history.

The AMSynths modules are designed with these characteristics:

  • 5U high panels, MOTM style, 3.5″ wide.
  • +/-15V power supply with 12V incandescent bulb power on each PCB as required.
  • 2mm Aluminium front panel designs that are similar to the original.
  • 3.5mm jacks mounted to the front panel, no Cherry matrix switches.
  • Original push buttons with incandescent bulbs.
  • Original high quality 1/8″ shaft potentiometers albeit a tad expensive.
  • Large format 150 x 150mm PCB’s with the original component layout.
  • Wide PCB traces – 50 mil power and 40 mil signal.
  • 0.25W 5% carbon composition resistors as per the original.
  • 1% Metal film resistors where specified by ARP.
  • Ceramic disk capacitors.
  • TO99 “can” Op Amps such as LM301, LF356.
  • Some audio Op Amp upgrades – but still To99.
  • Sub modules are not potted but laid out on the main PCB.
  • 10V Precision Voltage rail on PCB for Control Voltage setting.

The “Lost” Modules ARP announced a further 13 module but never launched them, only the 1035 has ever been seen.
I have selected some of the more interesting to replicate., click on the links.

Copyright AMSynths 2017