ARP 2500 Analog Modular Synthesizer Replica

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 imposed 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 AM 2500 Rack I am replicating some of the ARP 2500 modules as a long term project (LTP) over many years (it has been 5 years already!), partly due to the expense but also the time taken to design and source the components. The idea of building an ARP 2500 Modular clone became possible in the spring of 2004 when MusicParts in the USA published the original schematics, complete with the original construction and part listings. The synthesizer is going to be like this:

• Single height rack and cabinet with just 5 modules.
• 5U high panels , MOTM style.
• The usual +/-15V power supply
• 2mm Aluminum front panel designs are similar to the original,
• Push buttons will be as the original, but the coloured knobs and 1/8" shaft pots are really expensive, so I may consider other options.