Understanding Expression Pedals

Expression pedals are used to control variable parameters on electronic music equipment such as digital amplifiers, rack effects, stomp boxes, MIDI controllers, and keyboards. The pedals do not contribute to the sound themselves, but remotely control aspects of the device they are connected to. It might help to think of an expression pedal as a remote knob that can be controlled with your foot. Exactly what an expression pedal can control, will depend on the features of the device it is connected to.

 

Most digital amplifiers and multi-effects units support expression pedal control of basic functions such as volume, wah, and whammy which are traditionally controlled from this type of pedal. But often many more functions such as reverb trails, delay feedback, rotary speaker speed, mix, and so forth can also be controlled, giving the musician significant benefit in a live situation. It’s important to remember that expression pedals can only be used with devices that have a dedicated expression pedal input and/or have a MIDI control input.

Examples of digital amps and multi-effects units with expression pedal control include:

Line 6 Pod and M Series
Avid Eleven Rack
Fractal Audio Axe-Fx

Increasingly, stomp box manufacturers are also adding expression pedal control to many of their products. Examples include:

Strymon
Pigtronix
Eventide

How They Work
Expression Pedals comprise a pedal assembly that sits on the floor or can be mounted to a pedal board, with a rocker that can be moved up and down with the foot. Inside the pedal the rocker attaches to a potentiometer that moves proportionally to the pedal. The potentiometer is connected to an output jack or output cable that attaches to the expression pedal input of the device you are controlling. Unlike an in line volume pedal, there is no ‘input’ on an expression pedal as it does not connect to the instruments signal chain. There is just the single output that connects to the dedicated expression pedal input on the effect device. The device sends out a control voltage on one conductor of the cable which passes through the potentiometer and then is received back by the device on another conductor. As the pedal is moved up and down, the resistance within the potentiometer changes allowing more or less of the input control voltage to be returned. The amount of returned voltage is continually measured by the device allowing it to determine the position of the pedal and hence vary the effect. Expression pedals are typically passive devices that require no power of their own, with the control voltage being generated by the equipment they are connected to.

Expression pedals and CV pedals are not the same although they are often confused as they perform largely the same function but in a slightly different way. A passive expression pedal does not generate the control voltage itself; it receives it from the connected device, and returns it on a separate conductor along the connecting cable. A CV pedal generates the control voltage itself. A CV pedal requires a battery or external power supply. Some devices can use either, but if a device is not designed to be used with a CV pedal, then it could potentially be damaged by connecting one, so please read the manual before connecting a CV pedal to anything. CV pedals have their origins in analog synthesizers and are commonly, though certainly not exclusively, seen with synths and keyboard related devices. Built in expression pedals that are integrated into certain floor mounted effects units use different proprietary systems.

Compatibility
There is no recognized standard for expression pedal inputs. Effects and amp manufacturers use whatever variations are appropriate for their particular application. This can cause problems for the consumer needing to find an expression pedal that will work well with particular devices. The following describes the typical variations.

1. Potentiometer resistance.
Many devices require an expression pedal potentiometer resistance of somewhere between 5k Ohm and 50K Ohm. Some equipment is quite forgiving and will work well with anything in this range, others may require a very specific resistance to work properly. Still more equipment needs specific resistances outside this range including 100K, 250K, even 500K. Using a pedal with an incompatible resistance for the device can result in limited range, jumping or notch like response or in some cases, the pedal just won’t function at all.

2. Potentiometer taper.
The taper of a potentiometer describes how its electrical resistance changes proportional to its mechanical movement. In a linear potentiometer, the electrical resistance changes at the same rate throughout the mechanical range of the pot. In most cases, although not all, linear taper works best for expression pedals. In a logarithmic taper potentiometer, the resistance changes more slowly as you first move the pot, becoming increasingly faster as you get near the end. Log, or near log pots are commonly used in volume controls but don’t always work well for other effects. This is one of the reasons why trying to repurpose a volume pedal as an expression pedal often produces unsatisfactory results. Some wah pedals use pots approximating a reverse log taper. In most cases this will not work well for an expression pedal either, but there are at least two known cases where this is actually a requirement for an expression pedal.

3. Wiring.
Most potentiometers have three connectors; Clockwise, Counter-clockwise, and Wiper. Amazingly, there are multiple different ways these can be wired, all achieving largely the same result, which means yet more variations for expression pedals. The most common expression pedal wiring is to connect the pot to a 1/4″ stereo (TRS) instrument jack as follows:

CW —— Sleeve
Wiper — Tip
CCW —– Ring

An alternative is with the tip and the ring reversed as follows:

CW —— Sleeve
Wiper — Ring
CCW —– Tip

Yet a third way is as follows:

CW —— Sleeve
Wiper — Tip
CCW —– Tip

In this last one, the wiper and CCW are bonded together and connected to the tip and the ring is unused. This requires the use of a mono (TS) cable such as a regular guitar cable, in place of the stereo (TRS) cable used in the other two. These three are the most common that we see in expression pedals, but we keep finding more. Sometimes a dual gang potentiometer is wired in parallel to create a single pot with half the resistance. For example you may see a dual 20K Ohm potentiometer bridged into a single 10K. In this case the pedal will function the same as if it were a native 10K Ohm single pot.

The most unusual configuration we have seen appears to be the original Univibe speed control pedal which looks like it used a dual gang 100K Ohm log pot with the wipers bridged together and connected to the ground. It would be interesting to find out what the story is behind how that came about. There are some Univibe clones on the market that require a similar type of wiring.

An expression pedal with wiring polarity that matches the equipment specifications is required. Using a pedal with incompatible wiring can result in limited range, jumping or notch like response, or the pedal just won’t function at all, so make sure you check the requirements of your equipment. It’s often listed in the User Manual.

Mechanics
As previously mentioned, most expression pedals are passive devices and employ a mechanical means of converting the linear movement of the rocker to the rotational forces required to turn the potentiometer. Commonly these utilize a rack and gear mechanism or a system that uses a kevlar string or metal band. Most off the shelf potentiometers are designed for panel mounting and to be turned by hand, and this creates a couple of problems for expression pedals. Firstly the physical characteristics are not normally designed for foot operation. The forces involved in a foot pedal are often way beyond the mechanical specifications of a panel mount pot, causing them to break. Secondly, most of the shelf potentiometers with stops will rotate between 270 and 320 degrees. If this is not specifically matched to the movement of the rocker, the pot may not move through it’s full rotation, or may hit the stops. This can cause problems with dead spots and limited range or damage to the pedal. For best performance, it’s important to have a potentiometer that is purpose designed for foot pedal duty and with a rotation angle that matches the movement of the rocker mechanism. Most commercial expression pedals will have this, but if you are attempting to make your own, you will need to keep this in mind.

Calibration
Some of the more sophisticated effects and controllers, in particular MIDI devices, incorporate a calibration utility that can mitigate some of the issues with pot rotation. In these devices, there is normally a software option that allows the user to match the device to a specific expression pedal. In most cases this involves moving the pedal between it’s maximum and minimum settings and the device measuring the result. It then sets it’s internal parameters so that it recognizes where the maximum and minimum settings are of that particular pedal. This can often resolve problems of limited range. When using a device with this capability, it’s important to calibrate all expression pedals in accordance with the manufacturers instructions. If the pedal is ever replaced, even with the same model, calibration should be run again.

Calibration utilities sometimes cause confusion when using MIDI. This confusion is normally due the how the calibration is displayed on the device relative to MIDI values. In MIDI, continuous controller (CC) movement is described using a range 0-127 where 0 is normally minimum or off, and 127 maximum. For example if using a volume CC, 0 would be no volume, 63 would be about half volume, and 127 would be maximum volume. The calibration utility measures the characteristics of a particular pedal and then internally maps them to the correct MIDI values, so that when a pedal is pushed all the way down for example, a MIDI CC value of 127 is sent. The confusion arises because when calibrating, the device is often displaying to the user it’s own measurement scale, and not the MIDI values it is mapping to. For example, if a device displays 0-99 when calibrating a pedal, this does not mean that it will only go to 99 on the MIDI scale. The device is displaying it’s own reference numbers and NOT MIDI VALUES. Internally, the device maps 99 to 127. There is no standard. Different manufacturers use different scales such as 0-10, 0-99, 0-1024. Some manufacturers actually use the MIDI scale 0-127 which reduces the confusion. Either way, when using MIDI, internally all these will all be mapped to the correct MIDI range. Also, users should not be overly concerned about reaching the maximum and minimum calibration values. The purpose of calibration is to map the values of an individual pedal. For example, on a device with a scale 0-99, a pedal may calibrate at 3 and 96. Again, this simply means that on this pedal, 3 will be mapped to MIDI 0, and 96 to MIDI 127. If when calibrating a pedal, the numbers are significantly out of line, such as 59-69, or the numbers don’t change at all, it most likely means something is wrong in the setup such as the pedal is incompatible or is using the wrong cable. Users wishing to validate MIDI CC values can usually connect a computer running MIDI monitoring software such as MIDI-OX to the MIDI channel and monitor the MIDI traffic.

When using a MIDI configuration, remember that you should use an expression pedal that is compatible with the MIDI device you are connecting it to and not necessarily the MIDI controlled equipment at then end of the chain. For example, if you are using a MIDI floor controller from vendor A, and that is in turn connected to a digital amp from vendor B, and an effects unit from vendor C, the expression pedal would normally be connected to the MIDI controller and you should use a vendor A compatible pedal. The amp and effects unit will be receiving digital MIDI data, not analog expression pedal voltages. Choose an expression pedal that’s compatible with the device that you are plugging it’s cable into.

 

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38 thoughts on “Understanding Expression Pedals

  • I have a EP-11s, connected to a Ground Control Pro. If I may suggest. Clearly inform your customers that the configuration position of the “rocker” is “4”… and, that the configuration position of the SWITCH, under the rocker toe, is “43”. I was thinking that “4” was the configuration of the heel position, and “43” was the toe position.

    Also, the tiny switch under the rocker must be in the forward position. (This was mentioned in the instructions.)

      Reply
    • Thanks for the comment. In this article I tried to cover expression pedals in general, and your question is around a very specific combination of products. I’ll take a look and see where the Ep-11 config you mention is documented and see if it can be made clearer.

        Reply
  • EDIT FROM LAST VERSION (took out reference to other manufacturer and a pricing comment! 🙂

    Expression pedal questions:

    I am a bit confused about how to set up expression pedals with my new pedalboard. I have an RJM mastermind PBC controller which I will be hooking up to the first expression pedal which will look after the midi compatible pedals (Strymon timeline and an H9). I think I have that part OK.

    The question is do I need another “separate” expression pedal for those non-midi capable pedals on the board that have exp pedal “in”. I have three additional effects pedals (Red Panda “particle delay”, Montreal Assembly “count to five”, and MWFX “Judder”) all with exp in that I would like to also control and right now am thinking I might need to find an expression pedal with either three separate outs to control each of those or use a second device like the mission expressionater (3 outputs) that acts as the connector to allows the single output expression pedal to hook up to all 3. Do I have that right? Any advice?

    Here’s the specifics on the Exp. In for the pedals I am looking to control.

    MWFX Judder: exp input – Nom 25k ohm
    Red Panda Labs Particle Delay: 10k-50k ohm linear pot will work (Mission ep1)
    Montreal Assembly Count to Five: Most expression pedals work but it needs to be TRS setup with tip being the wiper. 10k/100k/1M pots all work.

    Many Thanks,
    Glenn

      Reply
    • Yes, you have this correct. You can connect an expression pedal to the RJM Mastermind and it will convert the expression pedal movement to MIDI CC messages that can be sent to any or all of the MIDI pedals depending on how you program the Mastermind. Mission has an expression pedal designed to work with the Mastermind controllers here: /?product=sp1-rjm-2

      For the non MIDI devices you would need another pedal. You can use Expressionator to connect a single pedal to three devices. A Mission EP-1, EP-25K or similar TRS tip to wiper expression pedal will work on the input. The outputs from Expressionator are 20K Ohm TRS tip to wiper. Although I have never tested the specific pedals that you mention, it would appear from the info that Expressionator looks to be compatible with them. There’s not really any other way to do this, other than having three separate expression pedals, or using one and just plugging it into each device as and when you need it.

      If you just want to connect two effects rather than three, you can use the Mission EP-25K or Pigtronix Dual pedals that have two outputs. They control both outputs simultaneously though, there is no way to switch the pedal between one and the other. If you really want to connect one expression pedal to three devices and be able to fully control how they operate, then you either have to use MIDI when the pedals support it, or Expressionator if they don’t.

        Reply
  • Dear Mission Team,

    I use an Expressionator with a Roland EV5 expression in order to control 2 Iron Ether effects (low pass, Bit Reducer) and a Diamond memory lane 2. Whilst for the first two i have no problem whatsoever, with the Diamond i am facing difficulties. Even when i plug directly the EV5 to the memory lane (on the delay time or feedback), the exp pedal starts to overheat and then the resistance is destroyed. I have already repaired the expression resistor twice. Also the range of the exp pedal do not match with the range of the potntiometer of the pedal. I contacted Diamond and they suggested to use another Exp. I am willing to buy a new Mission but I am not sure which. Can you please indicate which will be appropriate for my case?
    Have you tried your pedals with Diamond Memory lane 2?

    Thank you in advance for your help and support

    Regards

    George

      Reply
    • Hi, Passive expression pedals are generally low voltage, low current interfaces. I would be concerned if enough current is passing to a passive expression pedal that it is actually heating up and becoming damaged. Do you mean it is actually getting physically hot to touch? This suggests to me that there is something wrong. Did you describe the heating symptoms to Diamond? What did they say?

        Reply
      • Hi,
        Diamond said that it looks like a reverse polarity issue.
        When i plug the expression it starts to heat and it smells like sth is burned, also when the pedal is on the heel position is acting strange (like tremolo sound). In the manual they suggest”mono plug insert cable used with a standard volume pedal (minimum 100k)” but the 100k are for a volume pedal what would be the right value for an expression?

        Thanks a lot for the help

          Reply
        • We have not tested the Diamond Memory Lane 2, and I believe this product is now discontinued, so I cannot guarantee an expression pedal that will work with it. The only TRS expression pedal we have greater than 100K is the EP1-TC which is 250K Linear. This pedal is primarily designed for use with the TC Electronic G-System, and we have no way of testing it ourselves as we do not have a Diamon Memory Lane 2 in our collection. If Diamond are recommending you use a volume pedal, then you may be better off following their direction.

            Reply
  • Hi, a week ago I received a Keeley Monterrey, great pedal, inmediatly fall in love with it. I was playing with it for a couple of days without any problem. Tho days ago I purchased a Roland EV5 an plugged it into the EXP port on the Monterrey. The pedal let me take control of the RATE parameter with my fott, everything OK. BUT…when I unplugged the EV5, the RATE knob stop working. Rotating the knob makes nothing at all, it now stays at a fixed rate (the same in the three modes). Now the only way to take control of the rate is connecting the exp pedal.
    What could it be? Does the EV5 ruined something inside the Keeley? Is there a way to re-calibrate or fixing this rate knob problem?

      Reply
    • Hi. It’s very unlikely that connecting the expression pedal has damaged the Keeley. I would check with Keeley technical support and see if it’s necessary to reset the Monterrey settings after an expression pedal has been connected.

        Reply
      • Hi, I emailed Keeley and they say that they recommend the M-audio EXP because the Monterrey has a passive exp pedal…but I don’t get it. As far as I know, EV5 and MaudioEXP are similar passive pedals. My EV5 works ok in my Line 6 MM4 and Source Audio Nemesis delay.

          Reply
  • I have an EP-25 pro and a new pedal that wants a Reverse Log 100K S taper pot. Could I replace one of the pots on the EP-25 to work with this pedal? Where would I find a reverse log taper pot that would work for a pedal application like this?

      Reply
    • The EP25 uses a dual 25K linear pot, so that it can operate two devices simultaneously. If you are looking to keep one output the same 25K linear, and the other at something different, then finding an off the shelf dual pot with these two specs, suitable for use in a pedal is unlikely. Reverse log taper, and S taper are usually two different things. Is the requirement for reverse log taper OR S taper, or some custom mix of the two?

        Reply
  • I have a Line 6 Helix. I have an EP-1 pedal that I am able to use with the Helix by disconnecting the GREEN WIRE. I also have an SP1-R that I would like to use as an expression pedal and also use the switch on the SP1-R to turn my wah on and off.

    My question is, if I disconnect the green wire to the potentiometer will it work with my Helix? Also, do I have to disconnect any other leads to get the switch in the SP1-R to turn my wah for the Helix.

      Reply
    • We do not recommend making modifications to products as this invalidates the warranty, and may damage the expression pedal or the device it is connected to if connections are made incorrectly. The Mission EP1-L6 and SP1-L6 products are recommended for use with the Line 6 Helix.

        Reply
  • Very useful as I finalize SoulPedal(R). I’m putting the 2nd channel of a digital potentiometer out to a stereo jack (maybe a mini due to size), with 10K ohms, and use the first wiring diag you listed (as most common). I figure the hackers will re-configure anyway, but they can’t put more than 3.3V (which is fine for my Boss pedal exp jack). 5V on my input could fry my Pot.

    You also actually helped me realize that the taper is gradual to fast (not the other way around). I’ve got an inverse log output, so a correction factor is critical in my code. (I thought it was a bit touchy.)

    Did you notice that Source Audio (maybe others) made a universal controller with many pot values and jumper settings. I’m saving all those features for future versions (if this ever gets off the ground). The fact that it’s on the market tells me that people don’t really know what their equipment requires and just get the pedal that covers everything. Interesting…

      Reply
  • Hi there Missions! 🙂

    I wonder could Pigtronics Dual expression pedal be used with the Fractal AX8 or I need strictly Mission pedal that is dedicated to Fractal devices…?

    Thx in advance!

      Reply
  • The pigtronix dual expression pedal works with my mothership 2; however, I’ve yet to get it to control the rate on my Boss ph-3 phaser. Will it control the ph-3? I’d love to control both pedals with one expression pedal. How do I get it to work with the ph-3? Thanks for your reply

      Reply
    • Maybe, but BOSS pedals generally work better with a 10K expression pedal such as the Mission EP-1. The Pigtronix Dual is 25K.

        Reply
  • Will pigtronix dual expression pedal control a Boss ph-3 phaser? I’ve got it working with the pigtronix mothership 2, but can’t get it to control the phaser! Please Help!

      Reply
  • Interesting info’, but I wonder if you have come across this situation. I have a Line 6 Pod HD Pro connected to a Line 6 FBV Shortboard MkII. Decided to add the Line 6 EX-1 pedal, but when it arrived, not only was it a piece of plastic junk (IMHO), it was also faulty. So it was returned.

    I subsequently ended up with a Boss FV500H, which has expression pedal capability as well as being usable as just a volume pedal. As a standard volume pedal it is fine. But when connected to the Line 6 FBV expression pedal input it operates completely back to front (e.g. if controlling volume via the FBV volume is loudest when in the fully open heel position and silent when pushed down to the toe position).

    It is supposed to be compatible with the Line 6 gear, but no matter which combination of cables/adaptors/wiring I use (I have also switched wiring around in the cables I have in various configurations), it always ends up running the expression functions back to front (or not at all).

    Even tried the trick of using an insert cable from the FV500H’s volume input/output sockets and then partially inserting the TRS end into the FBV. Still ended up with sound back to front.

    Making me wonder if the FBV Shortboard itself is perhaps faulty.

    Anyway, be interested to know what you guys think of all this.

    Cheers.

      Reply
    • Line 6 devices have a different expression pedal interface and often do not work with TRS expression pedals such as the FV500. The Mission EP1-L6 is specifically designed for use with Line 6 devices and will work with the FBV.

        Reply
  • Magnatone recommends the ME EP1 to use for their modulation remote speed control input. However, I happen to own a MOOG EP-2; do you know if the EP1 is similar in functionality to the Moog EP-2?

      Reply
  • lebihanj,
    THANK YOU SO MUCH for sharing your expert knowledge on this subject!
    I just purchased a Mission EP1-L6 for my classic Line 6 DL4
    Gary

      Reply
  • What’s the actual component difference between the EP-11 and EP-1? I have an EP-11 that I would like to try and use with my Axe FX 3. I know several people use the EP-1. Is there any differences in components?

      Reply
    • The EP-11 has an internal polarity reversal switch. The EP-1 does not. We haven’t tested it but I suspect the EP-11 should work with an Axe-Fx III. You would need to make sure the polarity toggle is in the correct position.

        Reply

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