A semi-modular synth produced by ARP Instruments. It was ARP's second product to reach the market, after the 2500; it was first produced in 1970 and remained in production until ARP went under in 1981.
The 2600 was an analog synthesizer with three VCOs, a dedicated LFO, a lowpass VCF, a VCA, and two envelope generators -- an ADSR and an AR. It also contained a ring modulator, a spring reverb, and a noise generator with a built-in filter for varying responses. All signals were brought out to jacks on the panel, but a number of normalled connections reduced the need for patch cords. Oddly, the 2600 also contained a low-power amplifier and a pair of small built-in speakers. Techs and DIY'ers who add features often remove these and use the panel space for additional controls. Some 2600s have their electronics potted -- encapsulated in a resin-like material that makes servicing difficult. Performers and collectors interested in purchasing a 2600 are advised to find out if it has potted electronics before purchasing.
The 2600 underwent numerous design and engineering changes during its production run. Notably, the original monophonic keyboard was replaced with a duophonic keyboard having its own built-in LFO, a useful addition. The original VCF design (known by its part number, the 4012) was a clone of the Moog transistor ladder design; when Moog threatened a lawsuit, ARP replaced that with a different design (the 4072). However, a design error in the 4072 limited its cutoff frequency to about 2.5 KHz, resulting a dull sound. Techs have since discovered that changing a few resistors corrects the design error, restoring full spectrum response to the filter. Early production 2600s are known as "gray meanies" because they have the potted electronics, which makes them very difficult to work on.
The 2600 sold surprisingly well, and is still highly regarded, so much so that several soft synth emulations of it have been created. Several well-known techs have developed modification packages which improve the sound quality (usually by replacing the low-performing 741 op amps used in most versions) and add new features such as a second VCA or a multimode filter.