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Electro Harmonix Mono Synth

Why should keyboardists be the only ones who can play synth sounds? 



by Phil O'Keefe




Guitarists have always been a little envious of the wide variety of different types of sounds that synth players can make, but most of us don't want to have to learn how to play a keyboard, resort to using a dedicated "guitar synth", or modify our existing guitar with special pickups and MIDI interfaces in order to obtain them. Fortunately there are other options available to guitarists today that allow us to get some of those desirable sounds without having to deal with any of that. One of the leaders in this area has been Electro Harmonix. Their line of keyboard-inspired pedals have allowed countless guitarists to play realistic sounding organs, pianos, and other keyboard-type sounds directly from any electric guitar (or bass), without any modifications or add-ons to the instrument - just plug in and go. Their latest addition to the lineup is the EHX Mono Synth pedal. This one is a little different than most of the previous pedals in the line. Let's see what makes it distinctive.





What You Need To Know

  • The EHX Mono Synth pedal gives you the ability to play eleven different types of mono synths without any modifications to your existing guitar.
  • Housed in a metal case that measures 4" W x 4.75" L x 2.25" H, the Electro Harmonix Mono Synth looks like a typical guitar pedal much more than a synth. It is finished in a deep gray color, with yellow accents and with yellow and white colored labels.
  • Inputs and outputs are side-mounted. On the input side, there's also a EXP (expression) pedal input jack. In addition to being used for TRS-wired expression pedals (I tested the Mono Synth using my trusty Roland EV-5, which worked great), you can also feed the Mono Synth CV using a TS cable plugged into the  EXP jack.



  • On the output side there are two jacks - one for the Dry, unprocessed guitar signal, and a second one for the Synth output, which can also contain dry / unprocessed signal too, depending on how you have the control knobs set.



  • Power is supplied to the pedal with an included 9.6V DC 200mA power adapter. The power input jack is located at the top of the pedal, and uses an industry-standard 5.5mm x 2.1mm jack. It's wired center-negative, so you can power the pedal with a good multi-outlet "brick" style pedal power supply instead if you prefer. 
  • Current draw is 125mA @ 9V DC. Battery powering is not an option. There are also no internal trim pots or switches to worry about, so you don't need to remove the bottom plate from the pedal for any reason. I'm including a "gut shot" for those who are curious.



  • The bottom plate comes with four rubber feet pre-installed. I prefer it when they're included but not pre-installed, since it makes it easier for those who want to use Velcro to mount the pedal to their pedalboard, but to be fair, it's not too hard to remove them if you want to use that mounting method.
  • There are five knobs on the Mono Synth. The first two are Volume level controls for the Dry (guitar) and Synth sounds. They allow you to use either sound alone, or to blend them in any ratio that you prefer.



  • The third knob is a SENS (sensitivity) control. This determines how the pedal responds to your playing dynamics. Set it lower and you'll need to play harder in order to trigger the synth sounds, while higher settings lower the threshold and make it easier for even softer-played notes to trigger the synth.
  • The SENS control can be adjusted with the aid of the Mono Synth's multi-colored LED. The LED will light orange when the signal exceeds -4.6dB; setting the SENS knob to where the LED turns orange on only your loudest, hardest-played notes is the recommended starting position for this knob. But this is only a guide - if your playing fails to trigger notes, turn the SENS knob up. If you're getting too much false triggering, then turn it down a bit.
  • SENS can also have a slight impact on both the synth's volume, as well as a more noticeable effect on the filter sweep of some of the synth types. In those cases, higher SENS settings will result in larger, wider filter sweeps. 
  • The fourth knob is labeled CTRL. This Control knob is assigned to adjust different parameters, depending on the synth type selected. This knob, along with the EXP (expression) pedal, if one is connected, give you the ability to make changes to the sound, with each sound or synth type having pre-determined control assignments for the expression pedal and CTRL knob, such as filter cutoff, resonance, decay time, or frequency.
  • The 11 different synth types are selected with the fifth knob. Rather than try to come up with alternative descriptions of each synth type or sound, I've copied and pasted the list from the Mono Synth's manual, since it does a good job of describing them, along with listing the CTRL and EXP (or CV) functions assigned to each one:


1. NU WAVE – pulsating string-like synth.

CTRL: the CTRL knob sets the cutoff frequency of the filter.

EXP: the expression pedal sets pitch modulation depth. As the pedal is swept from heel to toe, modulation depth increases.


2. UNISON – the huge sound of stacking voices on a polyphonic synth.

SENS: higher settings make for wider filter sweeps.

CTRL: controls the volume of a separate oscillator tuned a Perfect 5th higher than the incoming pitch.

EXP: controls the cutoff frequency of the filter.


3. BLAST – aggressive and resonant synth with deep filter modulation.

SENS: higher settings make for wider filter sweeps.

CTRL: controls the resonance or frequency peak width of the filter.

EXP: controls the frequency range of the filter envelope. The filter’s frequency range increases as the pedal is swept from heel to toe.


4. TWIN – a throaty, dual filter synth sound.

SENS: higher settings make for wider filter sweeps.

CTRL: sets the decay time of the filter envelopes.

EXP: controls the filter envelope’s sweep depth or frequency range.


5. BASS – octave-down synth for adding low end.

CTRL: adjusts both the decay time and sweep depth of the filter.

EXP: controls the filter’s cutoff frequency. As the pedal is swept from heel to toe, the cutoff frequency increases.


6. XOX – fast decaying synth reminiscent of the TB.

SENS: higher settings make for wider filter sweeps.

CTRL: sets the decay time of the filter envelope.

EXP: controls the filter’s cutoff frequency.


7. WUB – pulsating synth with a modulating filter.

SENS: higher settings make for wider filter sweeps.

CTRL: sets the speed of modulation.

EXP: controls the filter’s center frequency.


8. TINKER – quirky synth great for bell-like tones.

CTRL: controls the filter’s cutoff frequency.

EXP: controls the pitch of the added note in half-step increments. The pitch ranges from -1 octave—at the heel—to +4 octaves—at the toe.


9. LAIR – a dramatic, expressive synth with expression filter control.

SENS: higher settings make for wider filter sweeps.

CTRL: adjusts both the filter’s decay time and frequency range.

EXP: controls the filter’s cutoff frequency.


10. GHOST – haunting synth tone with a warbling modulation effect.

CTRL: this knob sets the depth of VCO pitch modulation.

EXP: sets the rate of modulation which increases as the pedals is swept from heel to toe.


11. BLISTER – piercing synth with an added adjustable pitched note.

CTRL: detunes the oscillator. The noon position yields no detune. Turn clockwise from noon to detune sharp or counter-clockwise from noon for flat detune. The further you turn from noon, the more detune.

EXP: controls the pitch of the added note in half-step increments. The pitch ranges from -1 octave—at the heel—to +4 octaves—at the toe.



  • You might be temped to think of the eleven different synth types as patches, tones, or even presets. I was too at first, but after spending some time with the pedal I realized that the description EHX uses makes more sense. Why? Because each of the eleven synth types has a different foundation - not all of them use the same configuration of oscillators and filters. In that respect, they are more like different synths rather than different patches made on the same synth.
  • There are two footswitches on the EHX Mono Synth. The on / off switching is buffered Bypass. A second footswitch is provided for storing and recalling Presets, and 11 different presets can be saved, edited and recalled - one for each of the 11 positions on the Synth Type knob.



  • The LED indicates not only levels (as previously described) and bypass status (it turns off completely when the pedal is bypassed) but also the operating mode the Mono Synth is currently operating in. When the LED is green, you're in what Electro Harmonix calls WYSIWYG (what you see is what you get) mode, where the physical knob and expression pedal positions are relevant to the sound you hear. Hitting and releasing the Preset footswitch turns the LED red and recalls the preset for whichever one of the 11 synth type settings you currently have selected. 
  • Presets store all four of the top row knob settings, as well as the expression pedal position, if you have one connected when storing or editing the preset. Storing presets is easy. Once you have the knobs and expression pedal set where you want them, hold the Preset footswitch down for about two seconds - until the red LED starts blinking. Once it does, you can release the footswitch, and the settings are saved. Editing and recalling settings are also equally simple procedures.
  • Are you interested in bass synth sounds? Then be sure to check out Harmony Central's review of the EHX Bass Mono Synth pedal - you can find it right here.    




  • The Mono Synth usually functions best when it is the first pedal in your effect chain. If you want to use distortion, delay or any other effects with it (which can further expand your sonic options), EHX recommends putting them after the Mono Synth instead of in front of it.
  • It's monophonic only. Yes, it says so right on the case, but some people will try to play chords, double-stops and other multiple-note parts into it anyway. It responds unpredictably if you do, and it's not the unit's fault. You'll even need to watch out for previously played notes that are still sustaining, since they can cause issues if they're still ringing out too strongly when you play a subsequent note. So while the tracking is very good, you will still get the best results from clean single-note playing.
  • You may need to adjust the SENS knob differently for some of the 11 different synth types. The Presets make this easier to manage, since the SENS setting is one of the parameters that is saved with each preset.



While I can see some similarities to other EHX "keyboard" pedals, there are some notable differences with the EHX Mono Synth. While the tracking is every bit as good as on the other EHX "keyboard / synth" type pedals I've previously reviewed (which is to say, excellent), because of the monophonic limitation, the Mono Synth is a bit trickier to use. This really isn't the pedal's fault - what it does is based on what you feed into it. As long as you play cleanly, and avoid multiple notes and try not to let previous notes ring out as you're playing a new one, you'll be fine. But you do have to pay a little closer attention to playing cleanly with this pedal, and it's important to get the SENS setting in the general ballpark too - although you have a bit of leeway there, and it's well worth exploring various settings of that knob since it can also influence the way the pedal sounds too.


There's a nice collection of classic mono synth sounds here, and it's great to be able to play them from your own, unmodified guitar. And the EHX Mono Synth really does deliver synth sounds from your guitar, without the hassles. The Presets feature is really very cool too. Instead of just one preset, you can store a different one for each of the 11 synth types, and they can be recalled quickly and easily. The multi-color LED helps keep you informed about which of the two modes you're currently in, and what you're doing insofar as editing, recalling and storing presets.


The range of sounds available from the EHX Mono Synth is pretty broad, and the sounds can work well in a variety of musical contexts. Playing a strictly-monophonic instrument forces you to think a bit differently, which can actually be a good thing, especially if you're trying to recreate the vintage sounds and character of the earliest synths. There's no question that it significantly broadens your tonal palette as a guitarist, letting you create sounds that typically you'd expect to get only from a keyboard rig, such as a vintage mono synth or modern eurorack instrument. And believe me, that can be not only quite musically useful, but also a lot of fun.            -HC-



Want to discuss the Electro Harmonix Mono Synth pedal or have questions or comments about this review? Then head over to this thread in the Effects forum right here on Harmony Central and join the discussion!




Electro Harmonix Mono Synth pedal ($164.70 MSRP, $123.50 "street")

Electro Harmonix's product web page     


You can purchase the EHX Mono Synth pedal from:

Full Compass     


Guitar Center     

B&H Photo Video   

Musician's Friend     







Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Senior Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.  

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