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Last modified
 08 February, 2006
 

ARP2500 Analog Modular Synthesizer

Overview The ARP 2500 modular synthesizer was developed as direct competitor to Bob's Moog Modular, which had made such 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.

The ARP 2500 Today These synthesizers are quite rare and expensive, modules fetch over US $500 each, and complete synthesizers are over $10k. Phil Cirocco in the USA provides replacement sub-modules and upgrades for the ARP range, his web site is here: more »

The Analog Metropolis is cloning a number of ARP 2500 modules and you can purchase a PCB/kit: more »

Analog Technology ARP took immediate market leadership from Moog by implementing stable and wide frequency range VCO's. This was made possible by implementing a temperature 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 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 makes no use of the recently available OTA CA3080 chip (1969).

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 synthesizer design but it was a 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.