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