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All about expression pedals

Expression pedal database and how they work

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What is an expression pedal?

An expression pedal is a device used to control various parameters of multi-effect units, stomp boxes, keyboards and MIDI controllers.

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What does an expression pedal connect to?

A multi-effect unit , stomp box, keyboard or MIDI controller requires a control port into which the expression pedal can be connected.

 

 

The expression jack is connected to the control port of an effects unit via a cable. The cable may be hardwired to the expression pedal, or it may plug in.

 

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The picture here shows the internals of the system

What does an expression pedal do?

An expression pedal can be used to control volume or other effect parameters such as Wah-Wah or pitch.

How does an expression pedal work?

There are a few different methods that expression pedal manufacturers use. However, most of them use an electrical resistance device known as a potentiometer, shown here

 

 

What is inside the potentiometer?

The inside of a potentiometer shows a moveable wiper arm between two end points. The inside of the potentiometer consists of the following parts.

A. Shaft
B. Resistive carbon track
C. Wiper that rotates around carbon track
D. Wiper attached to shaft here
E. Electrical terminal for one end of the resistive carbon track.
F. Electrical terminal for the wiper
G. Electrical terminal for the other end of the resistive carbon track
 

 

The electrical resistance of the potentiometer is measured from the wiper to one end of the resistive carbon track. As the shaft is turned, the wiper also turns and slides along the resistive carbon track. And so the electrical resistance between the wiper and the ends of the resistive carbon track changes as the shaft is turned.

In this picture the potentiometer in a Line 6 EX-1 expression pedal is measured from one end of the resistive track to the other end, and shows an electrical resistance of 11.25 Kohm

 

 

 

 

 

 

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How does moving the treadle, move the potentiometer?

Within an expression pedal the shaft of the potentiometer is connected to the treadle of the pedal via a lever, geared rod, belt or wire. The shaft can then be turned by rocking back and forth with the heel and toe on the treadle. This then changes the electrical resistance of the potentiometer because the wiper moves with the shaft

 

 

 

 

How is the potentiometer connected to the expression jack?

The electrical terminals of the potentiometer are connected to the expression socket via wires or a printed circuit board (PCB).

 

 

 

 

 

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What is Polarity?

Polarity refers to the wiring configuration of an expression pedal and the wiring configuration of an effects unit. The options are TRS (Tip, Ring, Sleeve), RTS and TS. For things to work correctly the polarity of the expression pedal should match the polarity of the device it is connected to.

A TRS plug is shown here. The tip, ring and sleeve on the right are electrically connected to terminals on the left, where a cable would terminate. A TRS plug may also be called a 6.35mm stereo plug.

A TRS plug may also be called a 6.35mm stereo plug and a TRS socket may be called a 6.35mm stereo socket.

The tip, ring and sleeve on the right are joined to the terminals on the left.

As the plug pushes into the socket you can see that the tip of the plug touches the rear terminal.

This is a TRS cable, outside and inside.

Inside the TRS cable, the tips join together, the rings join together, and the sleeves join together. Exchanging the tip and ring positions on one end only, changes the polarity between TRS and RTS.


TRS cable

Which expression pedals have which polarity?

Many expression pedals have a single polarity, usually TRS, though a few have a single polarity of RTS, or only TS. Many expression pedals have a polarity switch that enables changing between TRS and RTS . A few expression pedals enable all three; TRS, RTS and TS.

Although generally the polarity of the expression pedal and effects pedal should match each other, there are a few exceptions depending on manufacturers. For example Line 6 devices require a TS configuration, but of the Line 6 devices we have seen the ring connection inside the effects unit is not connected to anything, and so a TRS expression pedal should also work with Line 6 devices. This is not true for early Digitech effects units such as the RP50 that also specify a TS configuration. The ring connection inside the effect unit is connected to the shield and so using a TRS expression pedal with these early Digitech effects produces unusual results. To use a TRS expression pedal with one of these early Digitech effects unit, would require disconnecting the ring at one end of the cable.

Examples of re-configuring polarity

Mission engineering EP-1 expression pedal (TRS) to Zoom effects unit (RTS) – Swap the tip and ring wires at one end of the cable.

Bespeco VM12ARTE expression pedal (TRS) to Digitech RP-55 effects unit (TS) – disconnect the ring at one end of the cable. (note that theVM12ARTE has a 100K potentiometer, and so the Digitech effect will not quite be fully modulated)

Rocktron Hex expression pedal (RTS) to Digitech RP-90 effects unit (TS) – Disconnect the wire at the ring, and move the tip wire to where the ring terminal is, at one end of the cable.

Rocktron Hex expression pedal (RTS) to Boss PS-5 effects unit (TRS) – Switch the tip and ring wires at one end of the cable.

The database of expression pedals lists the polarity for most expression pedals. 

*note that re-configuring a cable is not necessarily the best way to go about things. Although it will work, having cables laying about with different configurations, could get confusing.

What is the difference between RTS and TRS polarity?

The TRS and RTS configurations accept a voltage from an effects unit and send back a proportion of that voltage dependent on the potentiometer wiper position. The TRS configuration sends the voltage back on the tip, whilst the RTS configuration sends it back on ring. The same TRS to TRS cable is used regardless of the polarity configuration of the expression pedal and effects unit.

Assuming the polarity of the expression pedal, and of the effects unit is the same, then a TRS to TRS cable is all that is required.

If however the polarity of the expression pedal is TRS (e.g. Dunlop DV3) and the polarity of the effects unit is RTS (e.g Zoom), then the tip and the ring of one end of the cable needs to be reversed. If you are handy with a soldering iron, you can reverse it, though as time goes by it can become confusing to have different configuration cables lying about. And, so there are adapters available to convert between TRS and RTS. Also, many expression pedals have a built in polarity switch so that adapters or cable reconfiguration are not required. (e.g. DOD mini expression pedal). Some have instead, separate polarity outputs ( e.g. AMT EX-50)

 


Polarity switch

 

 

 

 

 

What is the TS polarity configuration?

The TS configuration does not use one end of the potentiometer. It only uses the shield connected to one end of the potentiometer, and the tip connected to the wiper. The voltage reduces as the wiper (tip) moves toward the shield. A TS plug is sometimes called a 6.35mm mono plug. It is the same plug used for guitar to amplifier connections.


                                                 TS plug

What is the potentiometers resistance?

The resistance refers to the electrical resistance of the potentiometer inside the expression pedal. This can be anywhere from 10k ohm – 250k ohm, but the usual values are 10Kohm, 25Kohm, 50Kohm, 100Kohm and 250Kohm. For the TRS and RTS polarity configurations the resistance is not so important. The TRS and RTS configurations use the voltage divider principle where the voltage sent back to the effect unit is relative to the position of the wiper regardless of the resistance value. And so for example the half way position of a 10K ohm potentiometer will produce the same voltage as the half way position of a 100k ohm potentiometer.

There are two possible symbols used to represent a potentiometer in drawings.


Schematic symbol for a potentiometer


Alternate symbol for a potentiometer

 

You can see here how an actual potentiometer relates to its schematic symbol.

What about TS?

For TS configurations, the value specified by the manufacturer of the effect units, should be used. For example, early Digitech effects units like the RP-90 specified the use of a volume pedal. This means at least 250K with a TS configuration. Using too low a resistance value means that full modulation cannot be obtained. Tests show that at least a 120K ohm potentiometer is required for full modulation of the early Digitech RP series effects units. Later Digitech units use a TRS or RTS of at least 10kohm, but they also still work with a TS 250K volume pedal.

Rather than a potentiometer a TS configuration uses a variable resistor, which is very similar to a potentiometer.


Variable resistor

 

As a further TS example, Line 6 specifies the use of their EX-1 expression pedal for their effects units. The EX-1 has an 11K ohm potentiometer. Using a larger value means that the full modulation effect may still be obtained, but the full affect happens before the pedal completes it movement. For example, if a 100Kohm pot was used, for the last 90% of travel nothing would happen. After that there is no further change in modulation. We have had some unusual things happen with some Line 6 devices when too high a value is used. So, it is best to stay under 15Kohm

What is taper?

The taper of a potentiometer refers to how quickly the electrical resistance changes as the pot is turned. A linear taper has a linear change in resistance as the wiper is moved from one end to the other. For example, if it is a 10K ohm potentiometer, at ½ a turn the resistance is 5K ohms, and at a ¾ turn the resistance is 7.5K ohm.

A logarithmic taper changes in an exponential way as the pot is moved. There are different ratios of logarithmic taper in use, but the common one, known as the audio taper, has a small amount of resistance change up to about ½ a turn and after that the resistance changes much quicker.

 

 

The graph shows that with a logarithmic pot, the change is very small until the pot is rotated about half way, and then the change is quicker. This type of pot is used with audio applications where the human ear is involved, such as analog volume controls and volume pedals. It is done like this because the human ear works in this fashion. The ear is sensitive to quiet sounds but not sensitive to loud sounds, and so a logarithmic taper is required for sound applications.

Effect parameters however, respond best to a linear change and hence a linear pot is usually used in expression pedals. There are some exceptions though. Older RP series Digitech effects units asked for a volume pedal to be used as the expression pedal. A volume pedal uses a 250k – 500kohm logarithmic potentiometer. Newer Digitech units can use a 10Kohm linear expression pedal, but can still use a 250k logarithmic volume pedal.

Polarity switches

Some expression pedals have polarity switches making them somewhat universal in that the switch can be changed to suit the polarity configuration required for an effect/MIDI/keyboard unit. A RTS/TRS polarity switch, changes the tip and ring position before the wires meet the potentiometer. Depending on the manufacturer a TRS/TS switch, may disconnect the ring before it meets the potentiometer, or join the tip and ring together. Disconnecting the ring makes it suitable for Line 6 and older Digitech RP series effects units. Joining the tip and ring is suitable for Line 6 devices only.

In this picture, the M-Audio expression pedals has two polarity settings. M-
Audio is TRS, and other setting is for RTS. Pedals that also offer TS, are available, but not common.

 

 

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                        Polarity switch example
                  (M-AUDIO = TRS; OTHER = RTS)

What is a minimum depth control?

Many expression pedals have a minimum depth setting. This sets the quietest sound to be heard. It adds resistance in series with the potentiometer and so prevents the potentiometer from reaching a value of zero. Effectively it seems that the full heel down position cannot be reached. This can be used for example to start a Wah Wah effect above the muddy tone point, or to limit the lower range of a pitch or volume effect.

A very few expression pedals also have a maximum depth setting. This limits the maximum range of the pedal. Effectively it seems that the full toe down position cannot be fully reached. This can be used for example to limit the upper range of a pitch effect or to adjust for a resistance value mismatch between expression pedal and effect unit, where continued pedal movement has no further effect, or causes an effect parameter to malfunction.

The Alesis F2 uses a physically adjustable screw to prevent further pedal travel, and the Oz inventions expression device uses software control to limit the potentiometer value.

 

 

 


Minimum depth control

 

 


 

How does the minimum depth control work?

The picture here shows that when the minimum depth control is turned, there is some resistance in series with the expression potentiometer wiper, and this of course means that zero can never be reached. So for example if volume is chosen as the effect to modulate, a setting of 'no volume' is not possible because regardless of the pedals lowest heel down position, there will always be some resistance.

The further the minimum depth control is turned the higher will be the lowest volume level, until a point is reached where the pedal has no affect at all upon the chosen effect parameter.

A maximum depth control is not possible with passive components. Either a physical limit needs to be placed on the pedal such as something underneath the toe area to prevent it moving all the way down, or an active pedal uses software to set the limit on a digital potentiometer.

What voltage do effect units send out to an expression pedal?

The voltage used on most modern effects units/MIDI controllers/keyboards is 3.3 volts. Older units are more likely to use 5 volts, though a few modern units also use 5 volts. The voltage is not important because the voltage sent back to the effect unit is relative to the wiper position of the potentiometer. So for example, if a Strymon effect unit sends out 5 volts, when the expression pedal potentiometer is at ½ a turn, then 2.5 volts (half of 5) is sent back to the effect unit. And for example, if a Boss effect unit sends out 3.3 volts, when the expression pedal potentiometer is at ½ a turn, then 1.65 volts (half of 3.3) is send back to the effect unit.

In both cases the effect unit receives back half of what it sent out and things work as expected regardless of the potentiometer resistance value.

What is reverse sweep?

Some expression pedals have a reverse sweep switch. And so for example instead of the volume changing from low to high when the pedal is depressed, the volume will change from high to low.

For a TRS pedal, a reverse sweep switch reverses the ring and sleeve connections. For a RTS pedal, a reverse sweep switch reverses the tip and sleeve connections. Either way the outer connections on the potentiometer are swapped.

 


Revers sweep switch

 

 

Is an expression pedal the same as a volume pedal?

An expression pedal is not a volume pedal.

Although an expression pedal can be used to control the volume of a guitar. It does this by modulating the volume parameter of an effects unit. But it is not in itself what is called a volume pedal.  A volume pedal looks physically like an expression pedal but is designed to connect inline with an audio signal and is therefore able to attenuate that signal.

Here is a stereo volume pedal. It physically looks like an expression pedal with a treadle and a minimum depth control. But a closer look at the jacks shows it is a volume pedal. The jack on the side is a tuner out jack.



Volume pedal

 


Is an expression pedal  the same as a CV pedal?

An expression pedal is not a CV pedal.

Although an expression pedal looks physically like a CV pedal, it is not a CV pedal. A CV pedal generates a variable voltage. The voltage is often 0 - 5volts but there are other standards used. The voltage is fed to a CV socket usually on an analog synthesizer, to vary parameters such as filter frequency and pitch. Although CV sockets are usually found on analog synthesizers, they may also be found on some effect units. Some  of these analog synths and effects units have a jack that not only can accept a voltage from a CV pedal, but can also generate a voltage for use by an expression pedal. The Mooger Fooger series is an example of a device with a jack that functions as both a CV jack and an expression jack.

Some expression pedals are multi-function devices and include the functionality of a volume pedal or a CV pedal as well as an expression pedal. There are many brands of combined expression and volume pedals. Behringer FCV-100 is an example. Combined expression and CV pedals are less common. Examples are EHX next step and Source Audio Reflex. Oz inventions sell an adapter that goes inline between an expression pedal and a CV input, thereby converting the expression pedal to a CV pedal.

Although it is difficult to determine if a pedal 'is' a CV pedal without the manual or actually trying it, you can determine if a pedal 'is not' a CV pedal. If it is not a powered pedal such as via battery or AC/DC adapter, then it cannot be a CV pedal. This does not mean that all powered pedals are CV pedals though,  as some expression pedals use power for other functions, and combined expression/volume pedals use power for active attenuation of volume.

Using a volume pedal as an expression pedal

Although an expression pedal cannot be used as an inline volume pedal, a volume pedal can be used in place of a TRS expression pedal by using a specific type of cable known as a Y-cable, and running it from the input and output jacks of the volume pedal to the expression jack. The output of the volume pedal goes to the tip, and the input of the volume pedal goes to the ring connection.


Y-cable

Although this provides the correct wiring configuration, the potentiometer in a volume pedal has a logarithmic taper and so there will be very little effect for the first half of the pedal movement. The resistance value is not important because of the voltage dividing principle explained earlier.

A volume pedal can be used in place of a TS expression pedal by running a cable from the output jack only of the volume pedal to the expression jack. The value of the potentiometer however is important and will not suit most devices, but does suit early Digitech RP series effects.

What is a toe switch?

A toe switch is provided on some expression pedals and is connected to a separate jack on the expression pedal for switching things on and off, rather than having a separate foot switch.

The type of switch can be momentary or latching. A momentary switch makes contact when pressed and is automatically released when the foot is raised. A latching switch remains engaged until it is pressed again. So, a momentary switch might be used to switch between effect banks, whilst a latching switch might be used to turn a Wah-Wah effect on and then off.



Expression pedal with toe switch

What is a spring loaded expression pedal?

Some expression pedals are spring loaded, which means that they will automatically return to the heel down position when released. This can be used to good effect with Wah-Wah and can be used with Fractal Audio’s auto-engage feature on the AX8 where it removes the need for a toe switch. Mission engineering sell spring loaded expression pedals.

Effects with built in expression pedals

A number of effects units have built in expression pedals. These usually work by using a light dependent device rather than a potentiometer. As the pedal is pressed a light reflection stub is moved closer to a light emitting diode. The light from the diode reflects off the stub and is directed toward a light sensor. As the stub gets further away, the light intensity reduces, and additional electronics uses the changing light intensity to modulate an effect parameter.


Effect with built in expression pedal
 


 

Some effects units such as the Line 6 POD XT live, have a built in pedal, but also have a jack for an external expression pedal

What is an active expression pedal?

Most expression pedals do not require power and are called passive expression pedals. They contain nothing more than a passive potentiometer and perhaps a passive toe switch and/or a passive minimum depth control. However, a few expression pedals are active and require power to use them.  The functions of an active expression pedal vary between brands and models. The key point here is that active refers to a device that requires power from a battery or DC supply, to function.

You can see in the picture here that the Source Audio Reflex pedal has LEDs and a digital display, and hence it requires power, and is therefore an active pedal.


Source Audio, Reflex expression pedal

Active expression pedals may have additional expression features such as adjustable taper, maximum depth settings and low frequency oscillator settings. They may still use a passive potentiometer but use active electronics for additional expression pedal functions.

An active expression pedal may use passive components for the expression section, but use active electronics to generate a control voltage so as to act as a CV pedal, or to attenuate an input signal so as to act as a volume pedal.

Source Audio's Hot hand 3, and Oz inventions Black magic motion, are active expression devices that use a digital potentiometer as well as additional active electronics for added expression features.

What is a Low Frequency Oscillator (LFO)?

A LFO may be built into an active expression device providing a range of waveforms that will automatically modulate an effect parameter. For example the waveform might be a slow rising and rapid falling waveform, known as a sawtooth waveform, and hence the effect parameter is slowly modulated from minimum to maximum and then quickly drops to minimum before starting again. The frequency, or how often, the pattern repeats is adjustable. The pictures here show a sawtooth waveform at a particular rate, then again at a faster rather.

Sawtooth waveform

Sawtooth waveform modulating at twice the speed

What is an inertial expression pedal/device?

There are a few inertial expression pedals/devices on available. The EHX next expression pedal uses an accelerometer to sense acceleration and uses software calculations to then determine the angle that the pedal must be at to produce that acceleration. Further software control acts on an operational amplifier to provide a variable resistance proportional to the angle. Inertial expression devices contain active electronics so require power to operate





EHX inertial expression pedal

Though not technically a pedal there are a couple of inertial expression devices available. Source Audio’s Hot hand 3 uses an accelerometer to sense motion. There are two parts to the system; a transmitting sensor that sits on a user’s finger, and a receiving modulator that receives motion data from the sensor and connects to the expression jack on an effects unit. It uses a digital potentiometer under software control that changes proportionally relative to the angle.



Source Audio Hot hand 3, inertial wireless effects controller


Oz inventions, Black Magic Motion is also an expression device. It contains an accelerometer to sense angle, and a gyroscope to sense speed. It uses a digital potentiometer under software control that changes proportionally to the angle, speed or direction, as chosen by a user. It uses a digital display to show the current position of the sweep. It has a variable LFO feature where a user can make their own waveform based on the motion of the device over a period of time.


Oz inventions, Black Magic Motion inertial wireless effects controller


Variable LFO

What is an expression roller?

An expression roller performs the same function as an expression pedal but rather than a pedal, a user rolls their foot on a cylinder. This makes for a smaller unit, with more precise control, a longer sweep action and a quicker action. It is a belt drive system. The roller is also available as a volume pedal.  Some brands have an ‘E’ or ‘V’ on them to easily tell the difference, but as with expression pedals, the jack labels also let you know which is which.



A expression roller showing the graduated markings that identify the position of the sweep.

There are only a couple of brands on the market. One uses graduated markings on the roller so a user can determine where they are within the sweep. Another is powered and used an LED that gets brighter or dimmer relative to the wheels rotational position.

What is Calibration?

Some effect units, typically multi-effect units, but also some stomp boxes, require calibration for accurate use of an expression pedal. The routine for this varies between manufactures and so it requires the user manual or similar, but generally it requires choosing the calibration option or screen and then setting the heel down position and then the toe down position, and sometimes the toe switch if it is an option.

 


User manual example


The operation of an expression pedal that is not calibrated correctly varies but can be no function at all,  partial modulation or full modulation over a short sweep.

Calibration can be used to overcome the limitations of some pedals such as a potentiometer that does not travel over its full rotation due to enclosure or gearing design.

What effect units express, and which effects parameters are good with expression pedals?

Many multi-effects units provide expression control over most or all effect parameters. Though some such as the early Digitech RP series only provide expression control for a couple of effect parameters. Stomp boxes usually only provide expression control over a single parameter, though some stomp boxes with expression control, notably those made by Strymon, provide expression control over all knobs.  Here is a list of many effects units and which parameters can be modified with an expression pedal. http://www.expressionpedals.com/effects-that-express

 

The user manual for an effect unit is the best way to determine which parameters can be modified by an expression pedal. Which ones sound best is a matter of choice. Commonly used parameters are Wah-Wah, vibrato depth and rate, tremolo depth and rate, volume swell, delay rate, reverb, pitch changes and changes in distortion level.






user manual example

What to look for in an expression pedal

  • The metal ones are study and last for many many years. The plastic ones are cheap but break relatively soon.
  • Adjustable pedal tension is good to get it feeling the way you like.
  • A return spring is good if you want it to auto-return to the heel position.
  • Having a long throw suits some, but a short throw is good too. It depends how quickly you want things to change.
  • Size can be important if you want it to fit in your pedal board or other place.
  • Polarity reversal (TRS to RTS) is useful if you have equipment that requires the different polarity.
  • Being able to adjust the minimum depth is handy for things like volume and pitch and even Wah so you don't get the muffled end of things.

 

  • Being able to adjust the maximum depth also, is doubly handy. Particularly for pitch effects as when combined with the minimum depth you can set the exact pitch distance to move over (1st, 3rd, 5th, octave etc).
  • Expression pedals configured as RTS and TS are in the minority and so are effect units or MIDI controllers that use them. (Zoom®, line 6® and Rocktron®).
  • Be sure to match TRS expression pedals with TRS effects units. If for example A RTS expression pedal is used with a TRS effects unit, the results will vary depending on the chosen effect parameter. It may seem like it is working a bit, and if it is your first time you might think that is as good as it gets. However, modulation should sound smooth from one end of the pedals travel to the other.

 

  • Nearly all expression pedals have a linear pot which is generally what you want, but some may havea logarithmic pot which means the modulation effect occurs nearly all at once, at one end of motion.
  • Multiple expression jacks can be good if you want to control multiple effects at once.
  • Being able to reverse the sweep is handy if that's the way you want it. For example go from maximum volume to minimum volume rather than vice versa.
  • A toe switch can be useful for switching things on and off.
  • Whether it can also act as a volume pedal is useful
  • Inertial motion control is interesting as it gives a different type or an expanded range of expression.
  • Pedal action is often a direct gear drive from pedal to potentiometer. But you can get a smoother action from belt/wire driven models.