audiolab

assets for the IDM / BxmC audio lab (2MTC, rooms 823/824)

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IDM ARP Synthesizers

ARP 2600 Overview

ARP 2600 in use

Much of what follows is adapted from the manual I wrote for the Frank-Ratchye STUDIO for Creative Inquiry at Carnegie Mellon University, where I spent part of 2016 helping to restore and document their vintage ARP 2600, pictured above. - Luke

The ARP 2600 is a semi-modular analog synthesizer designed by Dennis Colin and Alan Robert Pearlman - A.R.P. himself - and commercially released in 1971. It was one of the first synthesizers extensively marketed for education, with a retail price of below $3,500. It is considered one of the canonical synthesizers of its day, so a “mint” (or fully-restored) ARP 2600 can easily fetch $10,000 or more today.

The ARP 2600 shows up in many genres of music in the 1970s and 1980s - the synth was integral to the sound of artists ranging from Jean Michel Jarre to Joy Division. It’s also well-known in the sound effects community because of its built-in envelope follower; sound designer Ben Burtt used this to great effect in making the “voice” of R2-D2 in the Star Wars movies.

The ARP 2600 went through multiple changes during its manufacturing lifetime, ranging from several different cosmetic styles to a radical redesign of the filter in 1976 in response to a threatened lawsuit from Moog. The 2600 was manufactured by ARP until the bankruptcy of the company in 1981. Since then, the ARP 2600 has been resurrected as a software plug-in by Arturia Instruments and an open-source hardware kit developed by the Human Comparator called the TTSH (Two Thousand Six Hundred). The commercial license for the ARP line of synthesizers is currently held by Korg, which announced a limited-edition replica of the ARP 2600 in 2020.

A scan of the original ARP 2600 owner’s manual can be found here.

ARP also published a “Patch Book” of 100 setups for users to experiment with their ARP 2600. A scan can be found here.

A scan of the complete ARP 2600 Service Manual (119 pgs) can be found here.

A number of textbooks exist around electronic music / music technology that use the ARP 2600 as a teaching synthesizer. Samuel Ecoff’s Fundamentals of Music Technology: The ARP 2600 Synthesizer is a great one. A PDF can be found here. The CD examples for the book can be found on Ecoff’s home page.

What makes an ARP an ARP?

The ARP 2600 is:

Interface standards

The ARP 2600 has 14 or so modules, along with built-in speakers, an optional keyboard, and some other bits and pieces to make it easier to wire into an external sound system. Larger modules are arrayed along a top row on the main panel, with some smaller modules fitted into a bottom row between the speakers. The keyboard has some controls as well. Controls on the ARP consist mostly of slide potentiometers (sliders), along with a couple of knobs and a toggle switch or two. Small trim pots are arrayed under rubber protective caps; these are used to fine tune the synthesizer.

The most important thing to know about the ARP out is the default wiring between the modules. These are indicated by friendly diagrams under the patch points and sliders, e.g.:

TTSH slider

This slider lets you mix in a signal to a module (this particular slider is from the TTSH filter, but this user interface is consistent across the synth). Where it gets this signal from depends on whether or not a cable is patched into the jack below the slider. If no cable exists, the synth’s default wiring is used, and the symbol below the jack tells you the default source. In this case, it’s the square wave output from VCO 1 (the first oscillator on the synth). If you patch a cable into the jack, that will break the default connection and override it, so that you can wire in any signal you like from elsewhere on the synthesizer.

Nearly all modules have one or more sliders that allow you to control the mix of one or more inputs. If these sliders are down, the module will receive no input, from its default source or elsewhere, and will rely entirely on its internal circuitry to function. When a module has multiple inputs, their voltages are summed; in addition, these voltages will also be mixed with any control voltage being generated directly by controls on the module, such as a frequency control on the VCO modules. This elegant system allows you to offset and scale input voltages using controls mounted directly on the module itself rather than having to resort to separate voltage processing modules (although the ARP has one of those as well). Generally-speaking, the vertical sliders will provide a scalar to the incoming voltage (a multiplication of voltage), while horizontal sliders will provide an offset to the incoming voltage (an addition of voltage).

The default wiring allows most of the modules on the ARP to interact with no patch cords; without any cables inserted, the ARP wires its oscillators (labeled VCO 1, 2, and 3) through the filter (VCF), into an amplifier (VCA), and from there to a mixer that is wired to the built-in speakers. By looking at the symbols and adjusting the mix sliders, you can follow the signal path of the modules through the synthesizer.

To override the default wiring, modules can connected by patching within and between module jacks using audio * cables:

TTSH with patch cables

* More audio connector trivia: original ARP equipment used Switchcraft “Tini-Jax” connectors for all its synthesizers’ front-panel connections. Somewhat rare today, in the 1960s and 70s this connector was a competitor to the 3.5mm / 1/8” mini-phone jacks considered standard today after being popularized by the Sony Walkman. Tini-Jax were used by ARP (and Buchla) as an alternative to the larger 1/4” phono jacks used by Moog. 3.5mm / 1/8” cables will work just fine in a Tini-Jax socket, though they may feel a little loose; however, a Tini-Jax plug can damage or break a normal 3.5mm jack. The TTSH in the IDM analog studio uses standard 3.5mm jacks. Because the TTSH uses two-conductor (+ and -) cables, it’s somewhat easier to integrate with other A/V equipment than the Serge system.

The 3.5mm cables in the IDM Audio Lab are stackable by inserting one cable into the back of another. The are stored against the wall in the analog studio area, and are color-coded by length.

3.5 mm patch cables

IMPORTANT: unlike the banana connectors on the Serge system, the 3.5mm patch points on the TTSH (or any synth that uses phone connectors) can easily be damaged by forcing a plug into a jack, or by pulling a cable out of a socket with force. Please make connections on the TTSH with care.

ARP synthesizers use a 10V AC standard for both audio and control voltage, which is signifantly higher than the lower-voltage Serge system. As a result, Serge “Pulse” voltages (and Arduino pins) will not trigger the envelope generator on an ARP 2600 without being amplified (the CV.OCD “Gate Boost” module is available to help with this). The ARP 2600 uses a standard scaling of 1 volt per octave to represent musical pitch.

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TTSH

TTSH powered off TTSH powered on

The TTSH (the “Mini Meanie”) was custom-built for Luke in 2019 by Darrin Wiener and Flavia Ferreira at Patch Point in Berlin. Designed to be as faithful as possible to the original ARP 2600 design, there are a number of differences between the TTSH and a vintage ARP:

The Modules

TTSH

Below is a list of modules (left-to-right, top row then bottom row), an explanation of their capabilities, and their wiring (default input and output).

Pre-Amplifier (PRE AMP) / Envelope Follower (ENV FOLL) / Ring Modulator (RING MOD)

This module consists of three-submodules:

Pre-Amp/Envelope/RingMod

  1. PRE AMP input
  2. PRE AMP gain knob
  3. PRE AMP gain multiplier switch
  4. PRE AMP output (default destination: input of ENV FOLL)
  5. ENV FOLL input (default source: PRE AMP output)
  6. ENV FOLL mix amount
  7. ENV FOLL output (default destination: “lag” input of the VOLTAGE PROCESSORS)
  8. RING MOD carrier input (default source: sawtooth output of VCO 1)
  9. RING MOD carrier mix amount
  10. RING MOD AUDIO/DC switch
  11. RING MOD modulator input (default source: sine output of VCO 2)
  12. RING MOD modulator mix amount
  13. RING MOD output (default destination: VCF)

Voltage Controlled Oscillator (VCO 1)

This module is the first oscillator for the TTSH. Sliders at the top provide a base frequency (in two ranges) and the ability to fine tune the oscillator. A toggle switch labeled AUDIO/LF provides two functions - (1) it wires in control voltage from the KBD CV “keyboard” input so that you can play the oscillator with a 1 volt-per-octave signal and (2) it switches the oscillator scaling between a low-frequency oscillator (LFO) mode where its range is between 0.3 and 30Hz (LF or KBD OFF) and a high frequency mode where it outputs waveforms between 10Hz and 10kHz (AUDIO or KBD ON). There are patch point output jacks for a sawtooth wave and a square wave from the VCO. The mixer section at the bottom allows for frequency modulation of the oscilator. These inputs add to the voltage set by the sliders at the top; this allows you to control the range of FM through a combination of the frequency control sliders and the mixer.

VCO 1

  1. Coarse frequency slider
  2. Fine frequency slider
  3. AUDIO/LF switch
  4. Frequency Modulation (FM) Input 1 - only engaged in AUDIO mode (default source: KBD CV)
  5. FM Input 2 (default source: S/H output)
  6. FM Input 3 (default source: ADSR output)
  7. FM Input 4 (default source: sine output of VCO 2)
  8. FM Input 2 mix amount
  9. FM Input 3 mix amount
  10. FM Input 4 mix amount
  11. VCO 1 Sawtooth output
  12. VCO 1 Pulse (square wave) output (default destinations: VCO 2 and VCF)

Voltage Controlled Oscillator (VCO 2)

This module is the second oscillator for the TTSH, and has more features than VCO 1. As with the first oscillator, sliders at the top provide a base frequency and fine tune control. A third slider allows you to set the pulse width for the pulse/pwm wave output. The AUDIO/LF switch provides the same function as on VCO 1, allowing ‘keyboard’ (KBD CV input) control of the oscillator and switching between LFO and audible frequency mode. There are four output jacks on VCO 2 - a triangle wave, a sawtooth wave, a sine wave, and a pulse width modulation generator that can vary continuously between a square wave and a short pulse wave. The mixer section at the bottom allows for frequency modulation of the oscilator, as well as modulation of the pulse width for the pulse/pwm wave output.

VCO 2

  1. Coarse frequency slider
  2. Fine frequency slider
  3. Pulse width slider
  4. AUDIO/LF switch
  5. Frequency Modulation (FM) Input 1 - only engaged in AUDIO mode (default source: KBD CV)
  6. FM Input 2 (default source: S/H output)
  7. FM Input 3 (default source: ADSR output)
  8. FM Input 4 (default source: pulse output of VCO 1)
  9. PWM Input (default source: NOISE GEN output)
  10. FM Input 2 mix amount
  11. FM Input 3 mix amount
  12. FM Input 4 mix amount
  13. PWM Input mix amount
  14. VCO 2 Triangle output
  15. VCO 2 Sawtooth output
  16. VCO 2 Sine output (default destinations: VCO 1, VCO 3, VCF)
  17. VCO 2 Pulse output (default destination: VCF)

Voltage Controlled Oscillator (VCO 3)

VCO 3 is the TTSH synthesizer’s third oscillator, intermediate in complexity between VCO 1 and VCO 2. As with VCO 2, there are sliders for a base frequency, fine tune, and pulse width. The AUDIO/LF switch toggles KBD CV input control of the oscillator and switches between LFO and audible frequency mode. The oscillator has output jacks for a sawtooth and a pulse/pwm wave output. The mixer section at the bottom allows for frequency modulation of the oscilator.

VCO 3

  1. Coarse frequency slider
  2. Fine frequency slider
  3. Pulse width slider
  4. AUDIO/LF switch
  5. Frequency Modulation (FM) Input 1 - only engaged in AUDIO mode (default source: KBD CV)
  6. FM Input 2 (default source: NOISE GEN output)
  7. FM Input 3 (default source: ADSR output)
  8. FM Input 4 (default source: sine output of VCO 2)
  9. FM Input 2 mix amount
  10. FM Input 3 mix amount
  11. PWM Input mix amount
  12. VCO 3 Sawtooth output (default destination: VCF)
  13. VCO 3 Pulse output

Voltage Controlled Filter / Resonator (VCF)

The VCF is the ARP 2600’s filter. Early versions of the ARP 2600 used a reverse-engineered clone of a ladder filter design used in Moog synthesizers; as a result of legal action, these were replaced with ARP’s own design after 1976. The TTSH uses the pre-lawsuit (“Model 4012”) filter. In either case, the VCF consists of a resonant 24dB/octave low-pass filter with sliders controlling the cut-off frequency (coarse and fine) and a resonance amount. The mixer at the bottom allows you to control both audio inputs to the filter as well as control voltage inputs that vary the cutoff frequency. As with the VCOs, the control voltage adds to the cutoff frequency set by the sliders, so that you can set the range of CV control with a combination of the frequency controls at the top and the mixer gain for the CV input at the bottom of the module. There is an output jack for custom wiring of the output of the VCF.

VCF

  1. Coarse frequency slider
  2. Fine frequency slider
  3. Resonance slider
  4. Audio Input 1 (default source: RING MOD output)
  5. Audio Input 2 (default source: pulse output of VCO 1)
  6. Audio Input 3 (default source: pulse output of VCO 2)
  7. Audio Input 4 (default source: sawtooth output of VCO 3)
  8. Audio Input 5 (default source: NOISE GEN output)
  9. Control Input 1 (default source: KBD CV)
  10. Control Input 2 (default source: ADSR output)
  11. Control Input 3 (default source: sine output of VCO 2)
  12. Audio Input 1 mix amount
  13. Audio Input 2 mix amount
  14. Audio Input 3 mix amount
  15. Audio Input 4 mix amount
  16. Audio Input 5 mix amount
  17. Control Input 2 mix amount
  18. Control Input 3 mix amount
  19. VCF output (default destinations: VCA, MIXER)

Notes:

Envelope Generator (ADSR/AR)

The envelope generator on the TTSH creates control voltage signals that rise and fall in response to a trigger. These are used for amplitude curves, filter curves, etc. This module generates no audio, but can be used with other modules to process audio (e.g. through the FM inputs on the VCOs or the CV input on the VCA). The top half of the module controls an ADSR (attack/decay/sustain/release) envelope with sliders to control each stage of the envelope. The bottom half controls a simple AR (attack/release) envelope with two sliders instead of four. Both envelopes are “fired” through the same trigger mechanism and cannot be controlled independently: there is a button for manual firing, as well as a jack for an external trigger. There are output jacks for each envelope, as well as utility jacks that on an ARP 2600 output voltage from the keyboard, but on the TTSH allow you to wire in 10V signals to fire the envelopes - a GATE input (voltage high on key down, voltage low on key up) and a TRIGGER input (a voltage pulse on key down).

ADSR/AR

  1. ADSR Attack time slider
  2. ADSR Decay time slider
  3. ADSR Sustain level slider
  4. ADSR Release time slider
  5. ADSR output (default destinations: VCO 1, VCO 2, VCO 3, VCF, VCA)
  6. AR Attack time slider
  7. AR Release time slider
  8. AR output (default destination: VCA)
  9. Manual trigger button
  10. Gate input switch
  11. External gate input; active when switch 10 is down (default source: S/H gate output)
  12. External gate input; active when switch 10 is up
  13. External trigger input; active when switch 10 is up

Notes:

Voltage Controlled Amplifier (VCA)

The VCA of the TTSH modulates the amplitude of audio inputs based on control voltage sources. This allows you to shape the volume of a synthesizer “note”, allowing it to fade in and out. The VCA has a slider to set an initial gain - as with other horizontal sliders on the ARP, these voltages are added to external control voltages, allowing you to offset and scale them when used together with the vertical mixer sliders on the same module. The VCA has two input sources and two control sources, but only one amplifier circuit, which will sum the audio signals and amplify them based on the control voltages to generate one shaped output. There is a jack for the VCA output.

VCA

  1. Initial gain slider
  2. Audio Input 1 mix amount
  3. Audio Input 2 mix amount
  4. Control Input 1 mix amount
  5. Control Input 2 mix amount
  6. Audio Input 1 (default source: VCF output)
  7. Audio Input 2 (default source: RING MOD output)
  8. Linear Control Input (default source: AR output)
  9. Exponential Control Input (default source: ADSR output)
  10. VCA output (default destination: MIXER)

MIXER / REVERBERATOR / Output

The right-hand module on the top row of the TTSH consists of a final stage mixer, a reverberation unit, and jacks for audio output from the synthesizer:

Mixer

  1. TTSH left output
  2. TTSH right output
  3. MIXER output pan slider
  4. TTSH Left input; sums to MIXER and REVERBERATOR
  5. MIXER output tap
  6. TTSH Right input; sums to MIXER and REVERBERATOR
  7. MIXER Input 1 tap
  8. MIXER Input 2 tap
  9. MIXER Input 1 mix amount
  10. MIXER Input 2 mix amount
  11. MIXER Input 1 (default source: VCF output)
  12. MIXER Input 2 (default source: VCA output)
  13. REVERBERATOR Input (default source: MIXER output)
  14. REVERBERATOR output tap
  15. REVERBERATOR left mix amount
  16. REVERBERATOR right mix amount

Keyboard CV (KBD CV) / 4-in-1 / Left Speaker

On the lower left of the TTSH, next to the left speaker, there is a jack that allows external voltage to be multiplexed to all the KBD CV inputs on the synthesizer; on the original ARP 2600, this would be used to tap the control voltage output of the keyboard; on the TTSH, this is an easy way to have a single voltage input control, e.g. all three oscillators. In addition, there is a passive 4-in-1 jack that allows you to take any voltage and split it into three outputs using patch cables - the Tiny-Jax cables used on the original ARP 2600 couldn’t be stacked. On the right of the speaker is a vertical slider for its volume.

Keyboard/Mult

  1. KBD CV jack
  2. 4-in-1 jacks
  3. TTSH left speaker volume

Notes:

Noise Generator (NOISE GEN)

The NOISE GEN for the TTSH provides a random audio source that can be used for a variety of interesting things. The two vertical sliders allow you to adjust between white noise (constant power), pink noise (1/f power), and red noise (1/f2), as well as control the output gain of the module.

Noise

  1. Noise quality slider
  2. Noise gain slider
  3. NOISE GEN output (default destinations: VCO 2, VCO 3, VCF, S/H)

VOLTAGE PROCESSORS

The VOLTAGE PROCESSORS module allows for four different control voltage sources to be modified in a variety of ways, including scaling, inverting, summing, offseting, and smoothing (the “lag” amount on the fourth control). None of the outputs are hard-wired, but can be used to create more complex control voltage sources.

CV Processor

  1. Processor input 1
  2. Processor input 2 (default source: TTSH -10V DC)
  3. Processor input 3
  4. Processor input 4 (default source: KBD CV)
  5. Processor input 5
  6. Processor input 6 (default source: TTSH +10V DC)
  7. Processor input 7 (default source: ENV FOLL output)
  8. Inverter output 1
  9. Inverter output 2
  10. Lag output
  11. Offset slider 1
  12. Offset slider 2
  13. Offset slider 3
  14. Lag slider

Sample and Hold (S/H)

The sample and hold (S/H) module on the TTSH can be used to generate clocks and random audio sources by “sampling” an input voltage based on an internal or external clock (which does not have to be a clock at all). The internal controls allow for the generation of S/H control voltages with an amplitude and rate controlled by the vertical sliders. Input and output jacks for the switches and clocks of the module allow you to create more complex sample-and-hold systems for the TTSH which can simulate sequences, arpeggiators, and rhythmic voltage curves.

S&H

  1. S/H input (default source: NOISE GEN output)
  2. Internal clock output (default destination: ADSR/AR)
  3. S/H output (default destinations: VCO 1*, VCO 2)
  4. External clock input
  5. S/H level slider
  6. Internal clock rate slider
  7. Electronic Switch A
  8. Electronic Switch B
  9. Electronic Switch C

Notes:

Right Speaker / Power Switch / Headphone Output

On the lower right of the TTSH is the power switch for the synthesizer, next to the right speaker. The headphone jack for the synthesizer is there as well, just below the power switch. On the left of the speaker is a vertical slider for its volume. Plugging headphones into the TTSH will disable the internal speakers - the left and right volume sliders will then control the volume in the headphones.

Phones/Power

  1. TTSH left speaker volume
  2. Headphone jack
  3. TTSH power switch

Please leave the TTSH power switch in the “on” position and use the Furman power conditioner to turn on and off the synthesizer.

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