Control Voltage for Dummies

Frequent asked about building blocks: gain stages, buffers, clipping configurations, ...
Post Reply
User avatar
drbob1
Cap Cooler
Information
Posts: 452
Joined: 21 Dec 2010, 20:30
Has thanked: 7 times
Been thanked: 40 times

Post by drbob1 »

The quest to understand the ability of some effects to be controlled by external pedals/devices started with a Mooger Fooger LPF and got truly out of control with a delay pedal. Here's what I've learned, and here's hoping it might be helpful.

The earliest use of CV was in modular synthesizers. I believe Bob Moog thought of it, although I could be wrong. His idea was that both note pitch and all the parameters available in a modular synth could be controlled by the same set of voltages (0-5v in his implementation, we'll run into this again). For notes 0v was the lowest and 5v the highest, with one octave per volt (and offsets to reach higher or lower frequencies). Other parameters that could be controlled included LFO rates and depths, envelope generation, sample and hold, ring modulators, noise injection, volume (VCA) and filters (VCF).

In the late 70s, there was a desire to be able to adjust effects without having to bend over and move knobs, and many started to use simple CV control for delay time, primarily. These devices seldom followed anything resembling a standard and are pretty hard to integrate into any modern rig using CV because of that.

The next push towards CV use came from MIDI. The protocol allowed extensive control of parameters using MIDI CC messages, and it wasn't long before the thought filtered back to analog devices. Again the use was primarily to allow a pedal to control one parameter, such as delay time. It wasn't until the resurgence of analog synths in the early 90s that Moog again got the idea of turning their modular synths into guitar effects, and the Big Briar series of modules (including LPF, Ring Modulator, Phaser and eventually Delay, Chorus/Flange and sequenced filter banks) were born. This started a boom in CV implementation and now many companies have delays, tremolos, chorus/flangers, filters, distortions, sample and hold circuits, and sequencing with extensive CV control. And it's only become more widespread as digital control over analog devices has blossomed.

So, how does it work? At it's heart, as we agreed above, a voltage is applied to a voltage sensitive component to change how it reacts. The problem comes in that no one agrees on what that voltage is, how it should be derived or what it should do...

The most common approach is to provide a source voltage (B+) to a potentiometer acting as a voltage divider: the B+ is attached to the hot lug, ground to ground, and the wiper provides the CV as some percentage of the total B+ available, controlled by the overall resistance and taper of the pot. I should mention that some manufacturers have chosen to put a voltage on the tip, and use a variable resistance to return the signal thru the sleeve. This isn't truly CV and is incompatible with any devices that output a control voltage, so it can't be used to integrate these devices with a CV system.

On the original Moog devices, the B+ was supplied on the ring of a TRS connector, with the CV returning on the tip. While this made conceptual sense, it did mean that anyone plugging a TS plug into a CV jack was shorting the B+ directly to ground, eventually burning out the power supply. Different manufacturers have addressed this fault in several ways such as a resistor or diode to prevent the full voltage from flowing thru a shorted ring or sourcing the B+ at the tip like Toneczar, others just strongly encourage you NEVER to use a TS plug in a CV jack. Moog used a -2.5 v to 2.5v B+, today there are devices that use0 to 5v, -5 to 5v and 0 to 10v as well. This requires an understanding of what's going on before trying to integrate them into a system.

So, what do I mean by “integrating them into a system”? Basically, any devices that share CV styles can be used together, and often one can control multiple parameters with a voltage applied to the necessary part of a TRS cable (Moog builds several control modules that output 0-5v, to allow a single source voltage to control multiple parameters on multiple devices simultaneously). The same voltage could control the rate of a VCA (acting as a tremolo) and the pitch of a Ring Modulator for example. There is one other drawback to this: because the curve of response to voltage differs from company to company, there is no current way to guarantee that the same change in voltage that will double, for example, delay time in company 1's product will result in doubling of the delay time in company 2's. Therefore, implementing “tap tempo” synchronization between devices is currently impossible.

The following I believe are accurate CV implementations:

Moog: Described in the manual as 0-5v, but I measure 2.5v, and several other sources quote this as -2.5 to 2.5v, B+ on the ring, return on the tip
ElectroHarmonix: same as Moog
Lightfoot Labs Goatkeeper: Same as Moog
Toneczar: 0 to 10v, B+ on tip, return on ring
SIB: pedal is used as a resistance, no ability to use external voltages
WMD: 0-5v, requires an adapter for full function with Moog voltages
Lexicon Vortex: 0 to 5v, return on the tip
DOD rackmount: Pedal is used as a resistance, no external voltage use possible
Pigtronix-same as Moog
CAE: voltage?, B+ on tip, return on ring
ADA: +3.75 to+11.25v, B+ on the tip, return on the ring

For anyone who wants to try integrating different effects into a system, there are some tools that might help. First off, find out with certainty how your effects are utilizing CV so you don't break anything! Moog makes a interfacing device called the CP251 that combines an LFO, a sample and hold circuit, a lag processor, various attenuators, B+ of -5 to 5v with an offset to allow 0 to -10v or 0 to 10v, and a mixer and multiplexer (multiple outputs of the same signal). As a swiss army knife it will allow almost any 2 devices that use CV to work together, although you will need to build cables to adapt the Moog type B+ scheme with the Toneczar or CAE versions. Moog used to build the CF201 which allowed the use of a CV pedal, tap tempo and both midi and analog signal outputs so one could integrate midi control messages with CV. They're stopped building it for the moment, but there are a few out there for sale.


I'd appreciate anyone who feels they know more about his adding to the list or correcting any errors. Special thanks to Ed Rembold from Toneczar for his VERY patient walking me thru how all this works and assistance in designing tests to figure out the details. Great customer service and great products. Also, I should mention that for practical applications, David Koltai of Pigtronix is using a rig integrating a Mooger Fooger controller and a number of these pedals.

User avatar
iznogoud
Breadboard Brother
Information
Posts: 61
Joined: 10 Mar 2012, 20:14
my favorite amplifier: Triaxis+Marshall 20/20+ Electrovoice
Location: Lothringen
Has thanked: 13 times
Been thanked: 34 times

Post by iznogoud »

As there are companies like Doepfer that sell nice midi-to-cv kits, we can think about building programmable analog multi effects, but, err, sorry for being such a noob in this domain, in fact, a few weeks ago, I even didn't knew this way of control existed before buying a moogerfooger, but, let's say I'd like to mod a fuzz factory or BigMuff Ge4 circuits (I use them as examples as they are perfect candidates : they can make so many different sounds that it would be too cool getting them becoming programmable), how can I do? Especially considering that if I can be very good to make effects or tube amp work, I'm totally dumb when it comes to talk bout electronics theory :oops:

Post Reply