ARP 2500

Modular Analogue Synthesis System (Retro)

Published in SOS August 1996
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Reviews : Keyboard

close encounters
of the ARP kind



The ARP 2500 is so rare that most of the existing examples are in private collections. Though few of us will ever get to play with one, the story of this modular monster remains fascinating. Lucky 2500 owner ROBERT ALEXANDER relates the tale...


Probably best known for its brief 1977 appearance calling to the alien spaceship in the Steven Spielberg sci-fi classic Close Encounters of the Third Kind, the ARP 2500 dates from a time when synthesis was an art form which nevertheless required you to have a degree in physics to stand any chance of comprehending what was going on! Because they are so rare, very few people have ever had the chance to see a 2500, so unfortunately it is not often listed among the 'classic' synthesizers -- that accolade usually goes to the smaller, much simpler ARP 2600. Total production numbers of the 2500 were probably as low as 100 worldwide, and maybe no more than 12 were sold in the UK, making it an exceptionally rare instrument. Rarest of all are examples with so-called 'wings', additional cabinets at the side of the main instrument which make the overall synthesizer truly enormous in size.


The 2500 was designed in 1969 by former NASA engineer Alan R Pearlman (hence ARP) who was responsible for amplifier design on the Gemini and Apollo space programmes. When the instrument was commercially launched in 1970, it was an enormously complex undertaking, combining all the features of the Moog modular series that was available at the time, plus a lot more. Where it differed was to allow the musician every possible combination of choice in sound creation within a matrix system, rather than using the more familiar patch cords (see the 'Matrix Switching' box for an explanation of this system).

Several variations of the 2500 were made, by far the most common of which was the single large cabinet configuration (model 2515) with a single keyboard. Additional, smaller 'wing' cabinets (model 2508) with a second keyboard were optional, with the 2500 able to continually add these on as required. An even rarer example of a triple cabinet (model 2524) exists, but very few were ever made. This would bring the full complement of modules in three cabinets to 31 'function modules' -- 15 in the centre cabinet, with eight in each of the wings (this includes power control modules) -- plus two 61-note, 5-octave keyboards.


ARP offered 2500 owners a combination of 1000-series function modules for the two cabinet types available, the choice obviously determined to some extent by synthesis basics -- VCOs, VCFs, VCAs, and so on. Beyond that, each instrument was custom-made for each owner, depending on their requirements and budget. The units available were:

• 1002/230: Power Control Module

• 1003a: Dual Envelope Generator

• 1004-T: Single Oscillator Module

• 1004r: Single Oscillator Module

• 1004p: Single Oscillator Module

• 1005: Modulator Amplifier

• 1006: Filter Amplifier

• 1016: Dual Noise/Random Voltage Generator

• 1023: Dual Oscillator Module

• 1026: Preset Voltage Module

• 1027: Clocked Sequential Control Module

• 1028: Sequencer Slave Module

• 1033: Dual Envelope Generator

• 1036: Sample & Hold/Random Voltage Circuit

• 1045: Voltage Controlled Voice

• 1046: Quad Envelope Generator

• 1047: Multi-mode Filter/Resonator

• 1050: Mix Sequencer

The 2500 was a natural rival to the Moog modular systems. These mostly used 921-series VCOs that tended to overheat, causing a drift in pitch. This necessitated constant re-tuning, one of the major setbacks when taking early synthesizers on stage. The ARP did not have this problem, and even Bob Moog himself had to admit that ARP oscillators were superior to those on the Moogs. This stability gives a clarity of sound that is different from any Moog: it is richer, warmer, fatter -- any number of adjectives -- but essentially different. The rivalry between the two manufacturers would eventually lead to the synthesizer itself being referred to as a 'Moog' or an 'ARP'.

ARP VCOs are quoted in the specification section of the manual as having an output frequency of .03Hz-16kHz, voltage-controlled over 10 octaves -- a rather optimistic set of figures. However, the 1004 variants and the 1023 do seem to respond over an enormous range, and each has five waveform characteristics -- sine, triangle, pulse, sawtooth and square. The 1004-T has a toggle switch to invert the waveforms.


The choice of 3000-series 5-octave keyboards for the ARP 2500 was equally complicated. Both black and white keys were used in various octave combinations (the colour of the keys of the lower octave or octaves were sometimes reversed, with black keys where you'd expect to see white, and vice versa), each of which could be assigned voices, dependent on which 1000-series function modules were installed. The 4-figure keyboard model number is in fact a code relating to the layout of the keyboard: the first digit, '3', means 'keyboard'; the second digit refers to the number of black octaves it has; the third digit refers to the number of black voices it has; and the fourth digit refers to its number of white voices. So a '3112' keyboard would have one black, single-voice octave, and a two-voice white section. In all, 3001, 3002, 3112, 3212, 3221, and 3222 models were made, and in the last four of these, the two sections of the keyboard were completely independent. The 3001 and 3002 keyboards, as the code suggests, had no black octaves.


Once all the module and keyboard choices were made, the musician was then faced with the somewhat daunting task of getting a sound out of the instrument. This can be complicated by the fact that the manual for the 2500, whilst explaining what synthesis actually is (in a rather quaint chapter with little diagram drawings that look like doodles at the side of the page) does not explain how to go about making a sound! It is usually at this point that musicians begin to appreciate just what the lesson in synthesis is all about.

Programming the ARP 2500 is not for the faint-hearted. In order to get anything (and I mean anything) out of this device, it is essential that you have a grasp of the basic concept of analogue synthesis and its structure. Sound creation with modular synthesizers is a time-consuming labour of love. This does, however, make it all the more rewarding when, after several hours of work, you end up with a sound that is literally unique -- nobody has a sample of it, because it has probably never been created before -- and you did it!


When the 2500 is powered up it looks magnificent (something like one of those control panels that you used to see in Thunderbirds or Captain Scarlet) with loads of flashing coloured lights. The sequencer is no exception, with a row of 10 green lights adjacent to three rows of coloured rotary potentiometers. Analogue sequencing was almost unheard of before Alan Pearlman came along, and today some of his later, dedicated ARP sequencers are among the most sought-after and prized of all analogue musical instruments.

Basically, the 1027 Clocked Sequential Control Module will allow a 10-step sequence to be programmed from three input signals and sent to three output signals. This means that up to 30 notes can be sequenced in three separate groups of 10 notes per 1027 function module. As the sequence is started, the steps are indicated by the adjacent green light flashing on.

Tempo is controlled in two ways: internally or externally. Internal clocking is via a black rotary knob labelled 'rate', and this can be set to steps of 20 'clocks' per minute at the slowest rate, up to 400 'clocks' per minute at the fastest. External clocking is split between trigger input mode and gate mode. Should you require a sequence of less than 10 steps, a sequence reset button is provided. This automatically cuts the sequence at the step required and loops it back to the beginning.

Today, most of these functions would be found on reasonably cheap synthesizers or workstations, but in 1970 they were a revelation that would have been considered electronically quite impossible just a few years before. The 1027 is a beautifully-designed piece of hardware, and would be copied many times in the years following its release.


There are many features on the ARP 2500 that are not possible to explore in any depth in an article of this size. Learning to programme an instrument such as this would require a book, many hours of time, and enormous patience. In many ways, despite its amazing sound possibilities, this was the biggest problem with the instrument -- it was too hard for most musicians to get to grips with, and eventually the demand for a synth that was easier to use forced Alan Pearlman to develop the ARP 2600 (1971-1978) and later the ARP Odyssey (1972-1981). This meant that most of the ARP 2500s that were sold ended up in university science departments (rather than music departments!) as examples of the potential of analogue synthesis.


The ARP 2500 is undoubtedly one of the most important electronic musical instruments ever made. It was also Alan Pearlman's folly, costing too much to build and buy, being far too complicated to programme (unless you just happen to have worked for NASA), and quite enormous in size. And yet, this instrument is accorded a place in synthesis history that is unparalleled. Its status is reflected by the fact that to buy a 2500 today (assuming you could find one) would set you back somewhere in the region of £15,000 -- perhaps even as high as £20,000 for a system with wings and a full complement of modules and keyboards.

The 2500 allows the creation of unique sounds with a power and vibrancy that no other synthesizer can get close to. Because so many variations are possible, you never run out of surprises when creating new sounds. Perhaps that's the fascination of an instrument that is fast approaching 30 years of age: the sounds it produces are as new and current now as the day it was first conceived.

It's rumoured that the Science Museum has a pristine-condition 2500 kept in the basement as an example of the genre 'electronic synthesis' -- not a Prophet 5, not a Jupiter 8, or even a Minimoog, but an ARP 2500.



On each of the 2500's cabinets, above and below the function modules, there are matrix switches of 10 x 20 switch position points (on very early models, the matrix switches were only above the modules). In total, this could give the musician with a full three-cabinet configuration a possible 9,600 matrix points for each sound! In practice, this meant that the ARP 2500 had potentially more sound variations than any musical instrument that came before or after it -- until Yamaha's DX7 came along in 1983.

In their simplest form, the matrix points allow patching from the output of one module to the input of another. The switches on each module are divided into groups of 10 sliders, which can be moved up or down vertically to line up with any one of the 20 horizontal lines. The spaces between switch points are 'off' positions. In order to connect all the modules, the 20 horizontal lines act as busses, making an electrical connection the entire length of the cabinet with any slider positioned on that line.



• PETE TOWNSEND: "Mine plays six notes at once, can have 20 sets of preset control voltages available, six audio signals mixed and instantly usable at various levels. The most incredibly subtle tone colours can be produced using the multimode resonator in conjunction with a normal filter, and the sequencer will provide exhilarating cascades of arpeggios."

• JIMMY PAGE: "It's a fabulous instrument capable of translating literally any sound you can hear in your mind into a reality. It is a joy to own."

• HUGO MONTENEGRO: "We used the ARP extensively on our first quadraphonic record for RCA. Whatever sound you want, you can get -- it's that simple." (Hugo Montenegro was responsible for RCA's first quadraphonic recordings.)

• GERALD SHAPIRO: "The ARP synthesizer is unquestionably the finest studio instrument that I have seen." (Gerald Shapiro of Brown University, USA, was one of the foremost contemporary lecturers on synthesis.)

Other famous 2500 users include Wendy Carlos, Vince Clarke, Tony Banks, and Jean-Michel Jarre.

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