ARP 2600 - VCA

 At the very top of the module is a slider called initial gain.

Gain is another word for volume.

It's really important  control as it sets the volume of the signals which pass through the 2600.

It opens the amp up... as you move the slider to the right.

Normally, the INITIAL GAIN slider will be set to closed (full left).

This "gates the sound".

The envelopes can now open and close the VCA, 

allowing silence in between the times when 

notes are being played.

The VCA has four inputs, two of which are audio, 

two of which are control inputs.

One of the two audio inputs is normalled to the filter’s output.

The other is normalised to the ring mod.

The control inputs are hard wired to the AR & ADSR envelope generators.

The AR generator, is also labeled LINEAR,
while the the ADSR generator, is labeled EXPONENTIAL.

These are response curves.

Remember, that you can use both envelopes simultaneously.

That is, the AR + ADSR can create really complex envelopes

when used together.

+ ARP 2600 - Envelope Generators

+ ARP 2600 - Sample and hold (& electronic switch)

+ ARP 2600 - Ring Modulator

+ Arp 2600 - Envelope Follower & pre-amp

+ ARP 2600 - Voltage processor

+ ARP 2600 - Filter

+ ARP-2600 - Trigger and gate inputs

+ ARP 2600 - VCA

ARP-2600 - AD & ADSR Trigger and gate inputs

The vast majority of synths made today have two primary types of control voltages : 

CVs and gates. 

The CV is for pitch & gates are used to fire envelopes.

Most Eurorack modules output a 1V/oct pitch voltage, and a +5V gate. 

Korg and old Yamahas use Hz but that's another story.

When I first bought my 2600 , I exclusively used the S/H gate

jack as my gate input.

This mostly worked as long as I had a +10V gate.

And if you had installed a gate booster you could boost those

pesky Eurorack +5v voltages to keep the ARP happy.

In most cases the AR & ADSR envelope generators are used to control the filter & VCA.

They can also be used to modulate other sections such as the VCOs.

The AR has a constant sustain (it's open as long as you press a key).

The AR envelope is not prewired to the VCOs so we need to use a patch cable to use it.

It is however prewired to the VCA.

The AR has a linear shape, vs the ADSR which is exponential.

The human body naturally perceives changes (such as in audio volume or changes in light) exponentially rather than linearly.

So if you want the volume to change "naturally" use the ADSR 

This is the basic setting for connecting a sequencer/keyboard to the 2600.

Note that the switches above the CV inputs must be UP and the switch above the S/H gate must be down.

For about 10 years I mostly ignored those two jacks next to the S/H Gate thinking they were just 

gate & trigger outputs from the ARP 2600 keyboard (which they are) since the arrows pointed out.

Actually, they are also inputs !!  ... that is they are bi-directional.

After a bit of research I learnt that the old ARP 2600 (& ARP2500) use an almost forgotten system of triggering envelopes.

They use both triggers & gates.

How does this work?

The ADSR is as you'd expect.

Here is a common envelope shape when using a a trigger & gate  

The "trigger" is the envelope's cue to start the ball rolling but it won't do anything

unless it also sees a gate (in the above drawing its the spike at zero time).

ATTACK
When a gate signal is applied, the EG's output will rise exponentially to 10V
(the rate is determined by the "attack" setting).

DECAY
When 10V is reached, the attack phase is ended, and the voltage will decay exponentially to the Sustain level. (the rate is determined by the "initial decay time" setting). In modern synths, we just call this the "decay".

SUSTAIN
The sustain level is adjustable from zero to 10V
The output remains at the sustain level until the gate is removed.

RELEASE
When the gate is removed, the voltage drops to zero volts.
This is called the "Final Decay Time".
The rate of the drop is exponential and is determined by the "Final Decay Time" setting.

If the gate signal is suddenly removed, the envelope will immediately start to decay to zero unless
another gate is inputed. In this case the envelope will then restart the attack phase.

This system of gates & triggers makes very complex envelopes possible 

If you don't have a sequencer with both triggers and gates

you can "trick" the 2600 by plugging into both the trigger & gate jacks the same gate signal.

I've tried this with a Arturia keystep and it works fine.

We are multiplying the gate signal and sending this to both GATE & TRIGGER inputs. Make sure the switch above the S/H Gate is up.

Another option is to set two VCOs into LFO mode.

I like to use a pulse for the gate, and a saw for the trigger.

It works too !

This is from the ARP sequencer manual:

Notice the triggers and gates.

The 1601 & 1613 sequencer has a unique gate bus that allows mixing of Gate and "triggers".

Another sequencer  to use is the Klee.

It also uses a bus that mixes gates & triggers.

I have a Analogue Solutions French connection.

https://www.analoguesystems.co.uk/index.php/keyboards/the-french-connection

This unusual keyboard apart from the Ondes Martenot ring controller has CV, Gate and trigger outputs. 

Other possibilites in the world of eurorack include 

ALM Busy Circuits ASQ1

Or just combine a CV/Gate sequencer with a percussion sequencer.

(plenty out there)

Links:

+ Arp 2500 - 1003, 1033, 1046 envelope generators

+ ARP 2600 - Envelope Generators

+ ARP 2600 - Sample and hold (& electronic switch)

+ ARP 2600 - Ring Modulator

+ Arp 2600 - Envelope Follower & pre-amp

+ ARP 2600 - Voltage processor

+ ARP 2600 - Filter

ARP 2600 - different versions - Blue Marvin, Grey Meanies, etc

The ARP 2600 is a legendary analog synthesizer that was first introduced in 1971 by the American company ARP Instruments, Inc. 

Over the years, several versions and variations of the ARP 2600 have been produced. 

Here are some of the notable versions:

ARP 2600 (Original Model):  "Blue Marvin"

This is the first version of the ARP 2600 released in early 1971. 

They were made in a garage. The filter used was the 4012.

VCO: 4011 . The 4011 are 1st generation VCOs

VCA: Teledyne 4010 output chips.

The synth had a bright blue aluminum case with a attached 3604 keyboard and walnut handles.

It was nicknamed the "Blue Marvin" (after Marvin Cohen, ARP’s CFO at the time) but these days it's referred to as the "Blue Meanie.” 

Only about 25 of these were made. Apparently, the voice path uses microchips.

ARP 2600C: "Grey Meanie".

this version was produced in mid 1971 and featured a gray color scheme with a wood handle and sides. 

These were factory produced and the new model introduced a few circuitry changes and improvements over the original model. 

They used first generation 4011 & 4017 oscillator sub modules

The filter was a Tonus 4012

4010 VCA sub modules (used Teledyne 1339 op-amps).

The Teledynes had a much higher slew rate than the later LM301s (National Semiconductor).

They were replaced with the LM301s  in  later ARPs as the Teledynes weren't as reliable... they sometimes latched up. 

About 100 were made. They use the Tonus logo.

It had an updated and attached keyboard (model 3604C). The case like the earlier Blue Meanie was aluminium. 

ARP 2600P

There were 3 versions of this.

Probably 1700 units were made in total.

ARP 2600P v1: 

This version was released in late 1971 and featured a rugged Tolex covered

roadcase  and separate 3604P keyboard. 

VCO: 4017. (1st gen)

VCF: 4012

VCA: 4010 with Teledyne output chips

ARP 2600P V2: 

Late 1971 - 1972

V2.0 of the 2600P replaced the 4017 VCO chips with more reliable 4027 chips (First generation).

VCF: 4012

VCA: 4010 with National Semicnductor LM301s

** note that in mid 1972 the VCA was changed to a 4019 

ARP 2600P V3: 

The 2600P v3.0 was produced from 1972 to 74 and is the most common model.

VCO: 4027-1 2nd generation

VCF: 4012

VCA: 4019

The rectangular Tonus ARP logo was replaced by a new G-clef ARP logo which is often used to distinguish these newer 2600's from the previous models version of the 2600P.

The V3 version had a different filter design and added an output level control for the preamp.

ARP 2601 V1

Produced between 1975 & 76

Looks like the earlier 2600Ps but has a 3620 duophonic keyboard.

Check the serial numbers : starts with 2602-XXX.

VCO: 4027-1 (2nd generation)

VCF: 4012

VCA: 4019

** Note that in early 1976, the filter was changed to a 4072.

ARP 2601 V2

Produced between 1977 & 79

These had the new designed orange and white front panel

All the modules were now un-encapulated

3620 duophonic keyboard.

VCO: 4027-1 (2nd generation)

VCF: 4072

VCA: 4019

ARP 2601 V3

Produced 1980

3620 duophonic keyboard.

Avatar power-supply

VCO: 4027-1 (2nd generation)

VCF: 4072

VCA: 4019

ARP 2600 TTSH: 

The TTSH (Two Thousand Six Hundred) is not an official ARP product but a DIY clone created by The Human Comparator. It is a faithful replica of the ARP 2600 and was released as a kit in the 2010s.

The kit comes with a detailed assembly manual to guide users through the construction process.

There are 4 versions of the TTSH.

TTSH V1: 

Released 2014.

This was the original version of the TTSH, which closely replicated the functionality and design of the ARP 2600. It featured a four-octave keyboard, three VCOs, a resonant low-pass filter, a VCA, a ring modulator, a sample & hold circuit, an envelope generator, and a spring reverb tank.

I built one of these Kits in 2014

VCO: 4027-1

VCF: 4012 & LM301 op-amps

VCA : 4010 & 4019 (& LM301 op-amps at the output stage)

Ring Modulator : 4014

ADSR: 4020

TTSH V2: 

Released Jan 2016.

This version introduced some improvements and enhancements over the V1. It featured a redesigned power supply section, refined circuit boards, and some component substitutions to improve reliability and performance.

TTSH V3: 

Released Dec 2016.

The V3 version brought further refinements and updates. It incorporated changes based on user feedback and introduced additional enhancements to the build process, layout, and overall functionality.

TTSH V4: 

Released July 2019.

This version was a major update that included several new features and improvements. It adds a MIDI-to-CV converter. The V4 also featured a redesigned keyboard section and added a glide/portamento circuit.

Korg ARP 2600 FS: In 2020, Korg announced a reissue of the ARP 2600 under their own brand. The Korg ARP 2600 FS (Full Size) is a faithful reproduction of the original ARP 2600, featuring the same circuitry, semi-modular architecture, and a full-sized keyboard.

I understand this is made using SMD technology, so repairs may be more difficult.

The last major manufacturer of 2600 clones is of course Behringer.

They use a 8U rack mount format.

They have 3 versions:

Standard "Christmas Tree".

Black/orange 

Digital reverb - Coolaudio V1000 Multi-FX DSP chip

Multi coloured LEDs

The "GRAY MEANIE" 

features a mechanical spring reverb.

Uni coloured LEDS

Filter section uses some polyester through hole caps instead of the SMD caps used in the 

black/orange version.

"Blue Marvin"

features a mechanical spring reverb.

Uni coloured LEDS

Filter section uses some polyester through hole caps instead of the SMD caps used in the 

black/orange version.

Apart from the new paint jobs I wonder if there is much similarity between the Behringer Blue/Grey models and the original 1971 ARP Blue Marvin & Grey Meanie. And maybe the search for "original"

is a never-ending quest best left alone.

Behringer say these models contain "hand picked components" and are "Authentic reproductions of the ultra-rare ARP* Grey Meanie circuitry from the ‘70s with specially selected op-amps and transistors".

They provide no further details.

I've read that the filter section of the Behringer Grey/Blue use through hole capacitors instead of the SMD variety in the Orange/black type and that all 3 use LM4580 op-amps (in the filter) that replace LM301s. In the 4012 amp section, a TL072 and/or LM4580 is used instead of a LM301 ??

I don't have a Behringer 2600 so can't open one up to see if they are using 1st generation 4011 VCOs

and Teledyne 1339 op-amps.... probably not. The Telydyne 1339 & later National Semiconductor LM301s were a bit sluggish , and though this added to their sound it could be argued that using these would be less than ideal from a technician's view point.

I would love to see some schematics too.

It's important to note that the availability and production of different versions may vary over time, and some versions, like the TTSH and Behringer 2600 are not officially produced by ARP or its successors.

Do let me know if I've made any mistakes or omissions.

ARP 2600 links

+ ARP 2600 - Envelope Generators

+ ARP 2600 - Sample and hold (& electronic switch)

+ ARP 2600 - Ring Modulator

+ Arp 2600 - Envelope Follower & pre-amp

+ ARP 2600 - Voltage processor

+ ARP 2600 - Filter

ARP 2600 - Filter

 A bit about the ARP 2600 filter.

The 2600 VCF is a low pass filter.

The top slider is labelled "initial filter frequency".

This is more commonly calked the "frequency cutoff. 

(Range is between 10hz & 10kHz)

You can fine tune this frequency with the next slider : Fine Tune.

The next slider is Resonance.

When the slider is fully to the right, it will self oscillate. ... this usually creates a sine wave.

There one filter output and 8 inputs.

The inputs are divided into

5 audio & 3 CV

The audio inputs are:

+ Ring mod

+ VCO 1, 2, 3

+ Noise

The Control section:

keyboard, ADSR, VCO2.

These inputs control the filter frequency (or cutoff)

To test the keyboard input, put the filter into self oscillation mode ( resonance slider to the right)

and play your keyboard.

Re the VCO 2 input: remember, this VCO can be audio or Low Freq.

When you first use the filter its important to make a few more settings:

There are 4 basic settings you'll need to make to the VCA & mixer

The audio from the filter needs to pass into the VCA.

Thus, on the VCA section

1. fully open the VCF & AR inputs.

    (all other inputs are closed)

2. close the initial gain (slider to left).

The audio then passes from the filter to the mixer.

Thus on the Mixer:

1. Open the VCA input.

     Note: the VCF input should be fully closed.

------------------------------------------------------------------------------------------------

There are two types of filter circuits available on the ARP 2600 synthesizer: 

the 4012 filter and the 4072 filter.

The very early 2600s use the 4012 filter

(up to around 1975)

This early filter circuit was used in the metal cased Blue Marvin (2600) & Grey meanie (2600C), and the grey tolex covered 2600P & the 2601v1.0 versions of the ARP 2600.

The filter has a distinctive sound that is often described as slightly brighter, more aggressive when compared to the 4072 filter.

The 4012 filter is a 4-pole low-pass transistor ladder, very similar to a Moog ladder filter.

They were encapsulated to conceal their inner workings.

The 4072 filter, on the other hand, was introduced later in the production run of the ARP 2600, and it has a slightly different tone compared to the 4012 filter. The 4072 filter is also a 4-pole low-pass filter, but it has a maximum cutoff frequency of around 7 kHz. The 4072 filter has a slightly darker and more mellow sound compared to the 4012 filter, and it is often favored by synth enthusiasts for its ability to create warm and rich bass sounds.

Both filter circuits have their own unique characteristics and can be used to create a wide range of sounds on the ARP 2600 synthesizer. Some users prefer the 4012 filter for its brightness and aggression, while others prefer the 4072 filter for its warmth and richness. Ultimately, the choice of filter circuit depends on the sound you are trying to achieve and your personal preferences.

So both versions are 4 pole.

The "4-pole" designation refers to the number of filter stages, which affects the slope of the filter curve. A 4-pole filter has a steep curve, which means that it attenuates frequencies above the cutoff more quickly than a 2-pole filter would.

The filter on the ARP 2600 has controls for cutoff frequency, resonance, and keyboard tracking. The cutoff frequency determines the point at which the filter starts to attenuate frequencies, while resonance boosts the frequencies near the cutoff point, creating a characteristic "squishy" or "squelchy" sound. Keyboard tracking adjusts the cutoff frequency in relation to the pitch of the notes being played, which can be used to create interesting tonal variations across the keyboard.

-----------------

Even though its just a LP filter

You can configure it to be high pass & bandpass-ish

https://www.youtube.com/watch?v=kw65oO_nlQ8

https://www.youtube.com/watch?v=Nm7t3jQaOBc

--------------

filter feedback

Patch 1 filter feedback -- patch the output of the filter back into itself via the noise input.

patch 2 - also feed the VCA output back into the VCF

-------------

Some eurorack modules based on these filters which you make like to try:

Steffcorp VCF-12 --- based on the 4012 filter  G-Storm Electro Tonus --- based on the 4012 filter  Studio Electronics 4075 STG Soundlabs Post-Lawsuit Filter -- " not a clone of the ARP 4072 submodule. 

                                                                     It does, however, have an identical topology with the same                                                                                 transistors and operational amplifier array."

More links

+ ARP 2600 - Envelope Generators

+ ARP 2600 - Sample and hold (& electronic switch)

+ ARP 2600 - Ring Modulator

+ Arp 2600 - Envelope Follower & pre-amp

+ ARP 2600 - Voltage processor

ARP 2600 - Voltage processor

Talking about the ARP 2600 voltage processor.

It looks like a really complex module but once you understand the fundamentals

 its actually quite simple.

Starting at the top. The 1st voltage processor.

Lets call it VP 1

This processor acts like a mixer.

Both control signals and audio signals work equally well. 

It has 4 inputs (labeled 1 to 4) and a single un named output.

There are 2 sliders which attenuate whatever is entering their jacks (2 & 4).

Jack 2 is hardwired to a -10V source.

Jack 4 is hardwired to the keyboard CV

Jack 1 & 3 don't have any attenuation sliders.

The final out-put jack is an inverter.

Thus any input signal will be inverted when it leaves VP1.

An inverter is an electronic circuit which essentially flips a waveform or 

incoming voltage upside down. 

Another way to describe this is that the

signal is moved 180 degrees out of phase 

with the original signal.

eg: 

If nothing is plugged into jack 2 & the slider is fully to the right , the voltage at 

the final inverted output will be +10V

As the slider is moved to the left, the voltage at the inverted output will drop to eventually reach 0V.

This is a useful offset voltage.

You can use this as a voltage adder. 

EG: If you have a LFO who's voltage moves from the positive to negative ranges and you wish to move the entire LFO into the positive range, plug the LFO into jack 1 and move slider 1 to the right.  The new LFO (with voltage added) is from the inverted out.

The beauty of this device is that it allows you to add  & mix up to 4 voltages.

These can be inputted from external modules or with what is normalled to jacks 2 & 4.

VP2

This is similar to VP 1.

Its simplier... has just 2 inputs labelled 5 & 6.

Its single output is also inverted.

Input 6 is connected to a +10V source.

Thus if the slider is fully open, the voltage measured at the output will be -10V

--------------------------

If you are using VP1 to mix 4 audio inputs & you wanted a non-inverted output,

just take the inverted out of VP 1 and plug into an  input of VP2 like so...

------------------------------------------------

The Voltage Processor can also be useful for boosting external trigger/gates from eurorack sequencers. 

These eurorack modules often spit out +5V and are not sufficient for triggering 

the 2600's envelopes which need +10V.

This is the patch:

VP2 does most of the work... adding voltage to the original trigger.

Use it's slider to add voltage to taste..

VP 1 is just an inverter.

In this short video the Turing Machine is providing +5V pulses which are being boosted

to +10V by the Voltage Processors

-------------------------------------------------------------------------------------

The Lag Processor

Just 1 input & one output.

The input is normalised to the Envelope follower.

The lag processor has a parameter called "Lag time".

Lag is the act of slowing things down, or more specifically, 

the lag processor increases the amount of time it takes for change to occur. 

It's commonly used to process control signals from LFOs, envelope generators,

and the envelope follower.

Another common application is portamento. 

-----------------------

Some modern Eurorack takes of the classic 2600 VP module.

+ Befaco

The Dual Atenuverter is a dual attenuverter from Barcelona-based manufacturer Befaco that will attenuate and invert the input signal and, if necessary, can also shift it (+/-10V).

Doepfer A 138j Inverting/Interrupting Mixer 

This is a nice 4channel utility for all kinds of signals.

It will Mix, Attenuate, Invert, Offset, Mute 

Channel 1 can also deliver an Offset-voltage of +/-5V when 

nothing is plugged into the input. 

Manhattan Analog

CVP

Level – Attenuverter, positive attenuator or inverted attenuator.

Offset – Add a stable DC offset (+/-5 V) to make 

        sure the modulation is in exactly the range you need it.

Glide – Slew your signal to smooth it out for filter response or portamento.

There is a two-color LED present to provide a visual display of the output signal.

Synovatron CV Tools

my favourite

Its great for  adding, subtracting, attenuating, amplifying and offsetting.

It has a cascade architecture. 

It is a polarizing mixer which allows an input signal’s level to be adjusted or inverted.

It also has a lag processor at the bottom.

sadly, the module is out of production.

More Links

+ ARP 2600 - Envelope Generators

+ ARP 2600 - Sample and hold (& electronic switch)

+ ARP 2600 - Ring Modulator

+ Arp 2600 - Envelope Follower & pre-amp