Strymon Tech Corner #1 – Anatomy of an expression pedal

Posted by Terry

Welcome to the first post of our new Strymon Tech Corner series! I will be posting technical articles on music electronics as part of our blog at least once a month. Pete, Dave and Gregg from our team may also write an article here and there when they can get time away from their PCB layout programs and DSP emulators. Hopefully you’ll find these posts helpful and informative.

In this first edition I’ll be going through the inner workings of the common expression pedal. Once we know how one works, then comes the fun stuff … tearing them apart, modding, etc, etc. But that will be left to next month’s article :)

expression pedal from moog

We knew from day 1 that we wanted some of our pedals to feature expression pedal inputs. So, the question was “what’s the standard?” That is, do all manufacturers make their expression pedals the same way? Luckily the answer is yes … mostly.

Expression pedals work by feeding a control voltage to a device, such as a guitar pedal or synthesizer. The voltage is read by the device and then used to change some type of parameter. The voltage range depends on the design of the pedal or synth. Our Strymon pedals, for example, read control voltages from 0 to 5 volts DC. Turns out that this is a fairly common voltage range, especially in music electronics where MIDI (a 5V system) is still popular and widely used after over 25 years. The expression pedal itself, however has nothing to do with the voltage range. It’s only function is to manipulate that range and control the control voltage. The way almost every expression pedal out there works is that it takes a reference voltage from the device it’s connected to, divides that voltage down by a certain amount and then feeds it back to the device. In electronic terms, this is most commonly accomplished with a TRS (tip / ring / sleeve) 1/4″ cable where the reference voltage is on the “ring,” the control voltage is fed back to the device on the “tip” and the “sleeve” is ground.

Here  is what a standard 1/4″ TRS plug looks like:

As you can see from this 1907 diagram, TRS has been around for a long long time ;)

Here is the schematic for a typical expression pedal:

As you can see, the simplest and most common method is to use a passive potentiometer. A reference voltage from the device would enter the expression pedal jack on the ring. Then that voltage gets connected across a 10k load which is the resistive element of the potentiometer. When you move the expression treadle up and down there is a mechanical mechanism that physically turns the treadle potentiometer or “pot” as it’s commonly known. You can visualize the arrow at pin 1 of the treadle pot moving from pin 3 to pin 2 as one moves his/her foot back and forth on the pedal. This is what varies the voltage at pin 1. This is the control voltage which then travels out of the pedal on the tip of the jack. R2 is only present as a current limiter and not applicable to this discussion.

The Moog EP-2, Roland EV-5, and M-Audio EX-P all work in this manner, and therefore, work with our pedals. The nice thing about this standard design is that the control voltage is very stable and the value of the potentiometer in the expression pedal doesn’t matter so much. The Line6 EX1 is the only one we’ve see that works differently, with only a simple resistor divider and a mono cable. The nice thing about their solution is that it uses a mono cable. Two disadvantages are: 1. The expression pedal input circuit is highly dependent on the value of the potentiometer in the expression pedal.  2. Their products won’t work with other manufacturer’s expression pedals and vice versa.

Watch our video for more info and audio demos with our Brigadier delay and Orbit flanger.

I hope you’ve enjoyed this first edition of the Strymon Tech Corner. Tune in next time where we’ll make our own D.I.Y. expression pedal from a broken crybaby wah!

Happy shredding!

*All product names used in this article are trademarks of their respective owners, which are in no way associated or affiliated with Strymon.

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33 Responses to “Strymon Tech Corner #1 – Anatomy of an expression pedal”

  1. [...] your treadle pot to a standard 1/4″ TRS (tip/ring/sleeve) jack according to the schematic in tech corner #1. Desolder all wires from the pot and switch and set [...]

  2. David says:

    Thanks,

    I read part 2 as well

    Peace

    David

  3. Spud says:

    I would love to build an expression pedal instead of buying a Line 6 ex 1.

    Have you any tips on building one of these please?

    Cheers

  4. Spud says:

    I just managed to build one – it was real easy.

  5. [...] just a potentiometer, sometimes with level adjustment, connected to a stereo plug. As described here. If you connect a voltage across the poles you get a CV signal from the tap. Couldn’t be [...]

  6. Pedro says:

    Terry!

    The line 6 ex 1 uses a mono or stereo jack?

    thanx

  7. Terry says:

    It uses a mono jack. The guys at Mission engineering make a compatible pedal if you want an alternative to the EX-1.

  8. Pedro says:

    thank you terry!

  9. [...] If you’ve got a volume pedal hanging around that doesn’t get much use or better yet one already on your board, you can use it as an expression pedal with one special cable. It’s called a TRS (tip/ring/sleeve) insert cable and the purpose of this cable is normally to break out a TRS insert jack (commonly found in mixers) to the separate send and return signals. Luckily we can take advantage of this wiring to convert our volume pedal into a standard 1/4″ TRS equipped expression pedal. [...]

  10. Roberto says:

    Hi, i have a doubt ’cause i see some schematics of expression pedal and appear two pot, one of 10k ohms and other of 50k ohms, what’s the function for pot of 50k ohms? thanks for reading. Greeting from Puebla, México.

  11. Jacob says:

    I wired a 25 Ohm pot with a 1k resister to a stereo jack and its not working, Is a 25 Ohm pot just not going to work?

  12. Terry says:

    25 ohm is much too low. Try a 10k or 25k instead.

  13. Zach D says:

    That’s a cool function for the delay time. To do this with a TimeLine would you program the delay time to HALF the original delay time for an octave down? I think that’s right…

  14. Terry says:

    What I did was use the minimum volume pot on the side of the expression pedal and adjust it so that the delay time correlates to an octave in pitch change. The important thing is that you need a variable clock delay which is available with the Brigadier or the TimeLine’s dBucket delay type.

  15. Freddyt says:

    I have wah pedal ( active) if I just disconnect the battery and connect special cable (trs) will this work as expression ped …or will I definitely have to drop the circuit board it is a
    DanWah pedal ( danolectro)

  16. Nacho says:

    hey, this is great. I’m about to do it right now. do you know how to turn a hammond pedal into a volumen guitar pedal? thanks!

  17. Nacho says:

    I mean im doing the wah-expression conversion haha

  18. Stef says:

    Hi Terry, could you tell me how the Line6 EX! is made?
    It should be a 10k linear pot soldered with a mono TS jack.
    Is there a resistance somewhere?
    Thanks, Stef

  19. Dario says:

    hi , i have a roland ev5 expression pedal , i plug into the strymon timeline , but doesn t work . you can help me?

  20. Jeremy says:

    Hi everyone, I´ve been thinking about how to use my old Zoom FP01 expression pedal with my two Strymon pedals.

    So, I´ve bought a Stereo 1/4 inch “Y” splitter adapter to two 1/4 stereo jacks, this:

    http://www.amazon.com/Monoprice-6-35mm-Stereo-Splitter-Adaptor/dp/B0069M5U6C/ref=pd_sim_sbs_e_1

    Then, I connected the stereo plug into the expresion pedal input, and use two stereo (or trs cables) to connect my pedals into the two stereo jacks of the adapter.

    I´m not sure if I may damage my Flint and El Capistan on the long run, but they´re working fine by now, can someone tell me if i´m doing something very stupid or kind of clever?

    Cheers!

  21. Nicolas says:

    Hi,

    I have a vintage synth guitar (Ibanez IMG2010) which I converted the signal to fit a new Roland VG-99, passing from 24 to 13 pins cable.

    But the BALANCE (synth/guitar) button on the guitar remains unused and I would like to get it’s signal to work as an expression pedal that I would be able to plug into the VG-99.

    Since this signal is ACTIVE, how could I do that?
    It should work as a regular expression pedal like the Roland EV-5.
    Somebody has a clue?

  22. rick says:

    Hi Nicolas, I just replied to the email you sent us at our support address. Take a look when you get a chance and get back to me if you have any other questions. Thanks!

  23. Ruslan says:

    Hi, I’m trying to build an expression pedal from broken Vox wah pedal (v847) and so far I have a question – do I need to use stereo cable (with stereo TRS jack) and a stereo jack socket?
    I used a mono cable and no luck at all)

  24. rick says:

    @Ruslan, yes. Use a stereo TRS jack and that should work.

  25. Juan says:

    Hi,

    I just have bought a Line 6 EX-1 to use it with my Moog MF-Drive but I noticed it didn’t work. Then I found in this website that you explain that it won’t work with products from different brands other than Line 6. So, I wonder, is there any way to make it work or Am I simply forced to look for another expression pedal?

    Thank for your time and for your answer, I’ll be waiting for it :).
    Best regards

  26. rick says:

    @Juan, you could try switching the jack out of the EX-1 to be a TRS jack and see if that would work. I’ve never tried it so I can’t guarantee it though.

  27. Dave says:

    Wondering if I can use my expression pedal (same as Roland) to A/B between two signals? I have an A/B footswitch and want to replace the hard switch with a TRS jack, and plug the expression pedal into that so I can “fade” between two line signals. Is this as easy as I am imagining?

  28. Hugo says:

    @Dave – I’m not too clear on what you are trying to set up here from your description. Can you send an email to support@strymon.net with further details on exactly what you are trying to control?

  29. Martin says:

    Hi, I’m about to build my expression pedals for my Timeline and BigSky, but I want to make sure what pot value to use.
    I’ve seen in t1m website http://this1smyne.com/shop/me-mini-expression-pedal/
    that he uses different values for every brand, and for Strymon he uses 100k to 250k
    Thanks.

  30. Daniel says:

    I can not for the life of me get my strymon to operate with my M-Audio EX-P. I have read the manual, I understand the instructions.

    Somehow it is not saving the heel/toe parameters, I believe.

    When I edit the heel and toe parameters, the indicator light does not yellow. Has it not registered the change?

    I’m probably missing something silly, but I am very troubled about this.

  31. Hugo says:

    Sorry for the trouble with using that expression pedal. Try the suggestion I provided you through our support email by turning down the knob on the left side of the EX-P all the way down (counter-clockwise). Turn this knob down before using EP SET and saving the preset and let me know if you still have trouble through our support@strymon.net email.

  32. Vav says:

    So it sounds like most devices use the tip to ground voltage, i.e. approx 0V with pedal at heel and sleeve voltage when pedal’s fully depressed? And don’t care about the ring to tip voltage?

    Got one device that adjusts slowly at first, then bunches up near the expression pedal toe, so I’m trying to picture what pot/voltage divider curve would work better.

  33. Vav says:

    Oops, uh make that 0 to ring not sleeve voltage.

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