A modular synthesizer marketed by ARP Instruments beginning in 1969; it was ARP's first product. The 2500 used an unusual format and case design; each case contained two large pin matrices, at the top and bottom, with empty slots in between where the modules were installed. Modules connected to the pin matrices via connectors at the top and bottom; they had no patch cord jacks. (The cases contained a few jacks for patching to external equipment.) Each module had color-coded knobs and color graphics which guided the user as to which columns in the pin matrix connected to which signals in the module. Each system sold with a base case, and wing cases were available for expansion.
The 2500 hit the market at a good time, when the demand for modular synthesizers was high, Moog Music was having trouble keeping up with its orders, and other manufacturers had not gotten into the market yet. It became a hit with university music departments (with sales enabled by ARP founder Alan Pearlman's academic connections), who liked the clean design and the fact that they didn't have to keep an inventory of patch cords. This despite the fact that the variety of available modules was limited, and the pin matrices had some crosstalk problems. Among other things, 2500 users liked that the VCOs stayed in tune much better than the Moog ones. (Bob Moog later acknowledged the superiority of ARP VCOs in this regard.)
Professional musicians were a bit slower to catch on to the 2500, although The Who's Pete Townshend was quite fond of his. However, it was always a highly regarded synth, and surviving examples are among the highest-priced synths on the collector's market today. Probably the most famous use of a 2500 was not in a piece of music, though: ARP provided one to movie producer Steven Speilberg, who used it as a stage prop in Close Encounters of the Third Kind. ARP engineer Philip Dodds, who went along to install the 2500 in the prop console used in the "contact" scene, so impressed Speilberg that he was given a speaking role as a scientist, and played the famous five-note sequence.
The 2500 remained in regular production through 1971, and was produced to order through the mid '70s. Even after it went out of production, parts of it continued on; many of the 2500 module internals were re-packaged into circuit subassemblies which ARP used as building blocks in later products.