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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:
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.
Analog Metropolis 2500
I am replicating most of the ARP 2500 modules as a long term project over many years, partly due to the expense but also the time taken to design and source the components. My ARP 2600 FracRac was a proving ground for ARP circuit topology, and paved the way for this new modular synthesizer. 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. Whilst the schematics were only copies, they did enable the original circuits to be recreated (with a bit of effort!). The availability of the 4014 and 4019 PCB's from Tony at Oakley Sound Systems in 2009 has also made the project more achievable. I also spent 2008 perfecting the ARP 2600 VCO which may be used instead of the ARP 2500 VCO design.
Designing the 2500 Replica
Confident that I could find the electronic and hardware components, as well as design and manufacture the PCB's, I have noew planned the physical format. The original design stands out through its use of contrasting lights, switches and knobs - and of course the awesome matrix switches, which I am afraid I cannot hope to replicate!
- The original modules were 202mm high and usually 75mm wide. My first replica 1016 Noise Source used a 4U format, but I am now planning on using the MOTM standard 5U
format - this means designs can be sold to MOTM users.
- The aluminium front panel designs are similar to the original, with no jacks sockets on the main panel. Whether I use screen printing or engraved is still undecided.
- One cabinet is planned with a 19" racking system.
- The matrix patching system is too expensive to clone, and it gave cross talk problems anyway. I am considering building an electronic patching system using 16x16
analog switches and a PIC and LCD.
- There will be a MIDI interface.
- 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.
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