ARP 2600: the semi-modular synth, plus the Solina and Pro Soloist

Semi-modular: patched inside, re-patchable outside

ARP Instruments of Lexington, Massachusetts, was founded in 1969 and introduced the 2600 in 1971 as the successor to its first instrument, the large modular ARP 2500. The retail price was US$2,600. Through the 1970s ARP was Moog's main competitor and eventually the world's leading maker of electronic instruments; performers found its oscillators stayed in tune better.

Unlike a fully modular synthesizer, whose modules are bought separately and wired by the user, the 2600 is semi-modular: a fixed set of components is connected inside, so it makes a sound with no cords at all. Plugging a patch cord into a jack overrides the internal connection, so routing can be changed one link at a time. Printed text and graphics on the panel show each section and the signal flow between them, which is why ARP first marketed it heavily to high schools and universities.

What is on the panel

Because the microphone preamp and filter can process outside sound, the 2600 is often used as much for effects and sound design as for notes. A sound designer combined a 2600 with a recorded human voice to create the voice of R2-D2 in the Star Wars films, and used it again for effects in Raiders of the Lost Ark.

Versions and the filter dispute

Every 2600 built through 1976 used ARP's 4012 filter submodule, an imitation of the Moog transistor ladder filter. That led to a patent dispute, settled out of court, after which ARP fitted its own design, the 4072. The two filters sound different, and reissues now let you choose between them.

The keyboard changed too. In 1973 an ARP dealer sold a kit that made the keyboard duophonic, able to trigger two oscillators at once; the following year ARP adopted the idea in its Model 3620 keyboard, which also added an LFO for vibrato and single or multiple triggering. ARP itself declared bankruptcy in 1981, which ended production.

Reissues and recreations

• Korg ARP 2600 FS (2020). Announced on 10 January 2020 as a faithful reproduction of the 1972 grey version, with switchable 4012 and 4072 filters, balanced XLR outputs, MIDI, a basic sequencer and arpeggiator, a flight case and a 3620-style keyboard with aftertouch.
• Korg ARP 2600 M (2022). The same design about 40% smaller.
• Behringer 2600 (2020). An updated variant in an 8U 19-inch rack chassis, without speakers, with a digital reverb simulation and both filter types on a switch.
• Kits and software. A 3/4-scale DIY circuit-board kit has been sold since 2013, and several software emulations exist, including one from Korg (2024).

The ARP String Ensemble (Solina)

The Solina String Ensemble was made by the Dutch company Eminent BV and distributed in the United States by ARP from 1974 to 1981 as the ARP String Ensemble. It grew out of the string section of Eminent's 310 Unique electronic organ of 1972. Twelve tone generators feed an organ-style divide-down network, so every one of its 49 keys can sound at once; its six voices are violin, viola, trumpet, horn, cello and contrabass.

Its famous lush sound comes from a built-in triple chorus that uses bucket-brigade delay chips swept by two LFOs. Four versions were made: the SE-I (mono, chorus always on), SE-II (mono, chorus switch), SE-III (stereo outputs and a redesigned chorus) and SE-IV (stereo, with LEDs on the panel). It has no filter, only a simple attack/release envelope and a gate output.

The ARP Soloist and Pro Soloist

Where the 2600 asked players to patch, ARP's Soloist (1970–1971) and Pro Soloist (1972–1977) asked them only to choose. These light preset synthesizers were designed to sit on top of an electric piano or organ. The Soloist offered 18 fixed monophonic presets on toggle switches but suffered from tuning problems; the Pro Soloist raised that to 30 presets, used read-only memory (ROM) chips to set every internal signal path, and a digitally divided tone generator that cured the tuning drift.

Working with ARP instruments today

• 2600: start from the internal connections and add one cord at a time; note which jacks are patched, because a cord silently breaks the internal link behind it.
• Ask which filter it has. A pre-1977 2600 has the Moog-style 4012; a later 2601 the ARP 4072. A reissue may offer both.
• Solina: the output is mono on the first two versions and stereo on the last two; the chorus is part of the sound, not an extra.
• Pro Soloist: sounds cannot be edited, so pick by preset name and shape the performance with aftertouch and the four sliders for volume, touch sensitivity, brilliance and portamento.