Expression Pedal Fix

VRs have a bug on the pedal voltage processing for expression pedals:
Independent of the pedal model, the volume reaches maximum at 50% of pedal lift.
Especially ‘short hub’ pedals (including the original Roland EV5) degrade into “ON-OFF switches”

To regain the full travel of your pedal an easy mod can be applied to your pedal by adding (soldering) 2 resistors into the pedal (or into the cable-plug).

The bug

An expression pedal is a simple voltage divider using a potentiometer that is driven by the pedal.
Every keyboard uses a ‘reference voltage’ and a ‘working voltage’ set by the potentiometer. The ‘working voltage’ changes between zero and the reference voltage

On the VR, the reference voltage is 5 Volts (precicely: 4.85 Volt)

In an ideal case, when the working voltage approaches zero, the expression volume gets zero. When the working voltage reaches the ‘reference’ , the expression volume gets maximal.
The problem is that the VR has a faulty calibration:

  • maximum volume is already reached when the ‘working voltage’ is at ca. 50% of the reference (ca. 2.5 – 2.7V) which corresponds to the pedal at ‘mid position’. So you can use only the ‘upper half’ of the pedal travel.
  • minimum volume is already reached when the ‘working voltage’ is 0.3 – 0.7 Volt (depending on the pedal type. Practically every keyboard has that ‘voltage offset’ for zero volume but on the VR the value is quite high and we lose some millimetres of pedal travel on the low volume side
  • as a consequence, the ‘working travel’ of short hub pedals is reduced to ca. 1 cm!

The mod

The mod consists in adding some 10 cent resistors nearby the potentiometer of the pedal.
First let’s take a look the signal path of the pedal and its TRS plug.
A TRS plug has 3 sections: ‘tip’ is the – guess what – tip of the plug, ‘ring’ the middle section, ‘sleeve’ the base section
In the diagram, the ‘reference voltage’ is between sleeve and ring and the ‘working voltage’ is between sleeve and tip
As said, in the case of the VR, when the pedal potentiometer is in half position the working voltage is at ca. 2.5V and the VR interprets this as maximum volume.

The trick is to add a resistor (R1) into the ‘reference path’ that has approximately the same value as the full resistance of the pot (we call it R0).
Now when the pot is set to 100% (full pedal travel) the working voltage is 100% of R0 (pot) divided by 2 times R0 (pot + R1), resulting in 1/2 of the reference voltage = 2.5V – which the VR interprets as ‘full volume’ – but this time at ‘full pedal travel’. Problem fixed.

The above explains the ‘principle’ of the mod. In reality things get a bit more complicated because pedals have a small resistor (so called shunt resistor) in the Tip-path, usually 1 kOhm. Some pedals even have an additional ‘volume offset’ potentiometer (in the tip path) that allows to set the minimum volume to a higher level (this is useful for using the expression pedal like a switch between ‘accompaniment’ and ‘solo’). Modding this shunt resistor can optimise the pedal travel on the ‘zero volume’ side.

The wiring digram shows the two ‘additional’ resistors for the ‘maximum volume issue’ (R1) and the ‘minimum volume optimisation’ (R2).
The indicated values are best values for VR + Behringer FCV organ pedal, giving you the longest possible ‘working travel’ of the pedal.
Your pedal might be different and you have to figure out the values by experimentation: you can use little ‘trim pots’ for R1 and R2 that you can exactly calibrate for the best setting. Calibration can be done by ear, which is not very precise. The far better way is to attach the VR to a midi monitor e.g. Windows laptop + MidiOX and follow the midi values of the ‘expression’ pedal (0 = zero vol, 127 = max vol)

NOTE: the ‘volume offset pot’ of some pedals can be used instead of R2 but there’s a risk that you accidentally lose its correct position.

 

 

MODDING

 

What you need:

  • an expresson pedal
  • 1 or 2 stereo TRS (6.3mm) plugs and a ‘stereo cable’ depending if the pedal comes with a cable or not
  • resistors and or if possible 2 ‘trim potentiometers’
  • soldering iron
  • Volt/Ohmmeter
  • highly recommended: laptop/tablet/PC with midi monitor app + midi-5-pin-usb converter
  • basic skills in electrics or a friend with skills …

Preparation

On the VR-menu, set the expression pedal curve to ‘3’ (you can change this again later)
If possible connect the midi-monitor (via 5-pin-midi-to-usb converter) to the VR-5-pin-out (VR usb-out does not send midi expression !).
Solder the trim pots as resistors R1 and R2 into the tip and ring signal path between cable and pedal: you can do this a) in the pedal itself b) in the TRS jack that will be plugged into the VR or c) like shown on the photos in the TRS jack that is plugged into the pedal if the pedal has no fixed cable:

 
 

 

The plug used on the ‘pedal side’ is a flat HiCon TRS plug:

Dimension of trim pot R1 = 1.5 * R0 (if you have no pots in this size, use a smaller one + preresistors)
Dimension of trim pot R2 = 10 kOhm
Connect the cable to VR and expression pedal, test if the volume changes, eventually check the ‘midi expression control change (CC7)’ signal on the midi monitor
Connect the voltmeter to the ‘tip’ side of the VR: you must measure the ‘tip’ voltage that is at the VR
Set trim pot R1 to ca. R0 and trim pot R2 to 0.

Calibrating

  • Move the pedal to maximum and adjust trim port R1 until the sound volume just starts not to decrease (ca 2.5 – 2.7 Volt depending on the pedal type)
  • Move the pedal to minimum and adjust trim port R2 until the sound volume just starts not to increase (ca 0.3 – 0.7 Volt depending on the pedal type)
  • Readjust R1, R2

NOTE: to estimate the volume by ear is very difficult. It’s strongly recommended to use the midi monitor to verify the expression volume control changes

The pedal should now work on it’s full travel.
Next we add some ‘security margin’: resistances won’t be 100% exact every day because of mechanical tolerances, sweat on the plugs, constellation of planets…

  • Move the pedal to maximum volume and adjust trim port R1 until the tip voltage has increased by ca. 0.10 Volt (ca 2.6 – 2.8 Volt depending on the pedal type)
  • Move the pedal to minimum volume and adjust trim port R2 until the vip voltage has decreased by ca. 0.10 Volt (ca 0.1 – 0.6 Volt depending on the pedal type)

IF you have done the modd in the pedal itself and IF you decide to keep the trim pots: you have finished here
Or replace the trim pots by constant resistors (necessary if you did the mod in the TRS plug itself because of limited place): unsolder the pots, measure their resistance, try to find equivalent resistors.

Values for Expression foot pedals like Behringer FCV and Yamaha FC7 with 50kOhm potentiometer and 1kOhm R2 shunt resistance:

Volume reference tip voltage tip voltage ‘with margin’ trim pots (with ‘margin’) resistors used
max 2.70V 2.84V 59.7kOhm R1 = 55 kOhm
min 0.30V 0.20V 3.49 kOhm R2 = 3.3 kOhm

Values for Expression foot pedals like M-Audio (and eventually EV5) with with 12kOhm potentiometer and 1kOhm R2 shunt resistance:

Volume reference tip voltage tip voltage ‘with margin’ trim pots (with ‘margin’) resistors used
max 2.50V 2.6V 13kOhm R1 = 13 kOhm
min 0.70V 0.6V 2.0kOhm R2 = 2.0 kOhm
Mod done into the TRS plug (‘HiCon’ plug) of the expression pedal cable, using constant R1 and R2 (R2 is covered by isolation and not visible) instead of modifying the pedal itself.
Right side: plug closed and ready to rock