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  • Electro Harmonix Mod Rex Polyrhythmic Modulator

    By Phil O'Keefe |


    Mighty modulation monster





    Modulation is one of the most popular types of effects for use with guitar, but there's a problem of sorts - there are a lot of different types of modulation; with such a wide variety available, you may have difficulty finding room for everything you'd like to have on your space-starved pedalboard. Another potential issue is trying to get (and keep) everything synchronized. Tap tempo-equipped pedals can help here, but there are still serious challenges getting multiple effects in sync - especially at non-standard subdivisions of the beat. Well the good folks at EHX have once again come up with a really clever solution to both of these issues with their powerful Mod Rex Polyrhythmic Modulator pedal.




    What You Need To Know

    • The Mod Rex isn't just one type of modulation pedal, but offers several different types of modulation in a single pedal. Even more impressive? You can use up to four different types of modulation simultaneously.


    • There are four sections or effects "engines" on the Mod Rex. A dedicated Mod section offers up your choice of Vibrato, Flanger, Chorus or Phaser. You can only use one of these at a time… but you also have three other sections that are always available - a standalone Tremolo, Modulated Panning, and a Modulated Filter.


    • Tempo is a big deal with the Mod Rex, and you can define the musical tempo in a variety of ways - with a Tempo knob, with a Tap Footswitch, using an optional expression pedal, as well as by locking to incoming pulse clock, or by locking to MIDI clock from another MIDI device. Regardless of which method you use to set the tempo, all of the effects synchronize to the same master tempo.


    • There are nine tempo subdivisions to select from for each section of the Mod Rex, and each section can have its own separate tempo subdivision.


    • There are three different main display modes for the large four-digit LED display; it can show preset numbers - the Mod Rex comes with 20 factory presets pre-programmed, and can store / recall up to 100 presets using the dedicated Preset Select / Store knob / pushbutton. The other two display modes let you see the master tempo in BPM, while the third display mode gives you Divisions. A LED to the left of the display shows you which of the three display modes is currently active. Going between the different modes is super-easy; the pedal switches between them automatically, depending on what you're trying to do. 


    • In Divisions display mode (which is activated by turning or pressing / releasing any of the Divisions knobs on the Mod Rex), the four individual numbers in the display indicate the tempo subdivision assigned to each of the four modulation engines.


    0 = Off

    1 = Whole note

    2 = Half note

    3 = Dotted quarter note

    4 = Quarter note

    5 = Quarter note triplet

    6 = Dotted eighth note

    7 = Eighth note

    8 = Eighth note triplet

    9 = Sixteenth note


    • For example, if the display reads 2704, the Mod engine would be set to a half note subdivision, the Trem would be set to a eighth note subdivision, the Pan would be turned off, while the Filter would be set to a quarter note subdivision. Changing the tempo will change the rate of all the effects simultaneously, but all of them will maintain the same ratios and subdivisions, unless you change those separately and individually, or call up a different preset.




    • All of the subdivision values are fairly easy to remember, with the possible exception of the eighth note variants, but in case you forget, they're listed right on the front panel of the Mod Rex, immediately below the four digit display itself.


    • Turning one of the Division knobs will cause the corresponding LED digit to blink; once you have the value you want dialed up, pressing down on the Divisions knob sets and activates the new subdivision value.




    • Let's take a look at the controls for each effect engine. On the far left side of the pedal is the Mod engine. Like all four of the effects engines, it has a Division knob. This allows you to set the tempo subdivision. Remember: 0 = Off; it's the only way to bypass any of the effects individually, without bypassing the entire pedal.


    • Again, depressing the Division knob's built-in pushbutton sets the selected time division.


    • The Mod section has a Type pushbutton that lets you select between four different types of effects - VIB (vibrato), FLN (flanger), CHR (chorus) and PHS (phaser), and small Depth and Feedback knobs allow you to adjust the sound of the selected effect; the Feedback knob only works with the Phaser and Flanger options. Four LEDs shows which of the four modulation types is currently selected - green for vibrato, yellow for flange, red for chorus and white for the phaser.


    • Two more pushbuttons are also available in this section. The R INV button inverts the polarity of the LFO on the right output channel, which gives you a stereo effect when using a mono in / stereo out configuration.


    • When depressed, the Shape button will illuminate the currently active waveform for the section's LFO on of the four yellow waveform LEDs in the waveform shape section, which is located just below the large four digit LED numeric display. While holding Shape down, you can use the Select button in the waveform select section to pick which one of the four available LFO waveforms you want to use for that effect. This can be done separately for each of the four modulation sections on the Mod Rex, making it possible to have vibrato with a rising sawtooth LFO waveform, square wave LFO tremolo, triangle wave stereo panning, and a descending sawtooth waveform for the filter's LFO, or any combination you prefer.


    • The next effect engine is the Trem. A Depth control lets you adjust how deep the throbbing gets, from barely noticeable to deep on/off cycling when using the square wave LFO shape. As with the Mod engine, you can select from four different waveforms for the tremolo's LFO using the Trem section's Shape button, and the Division knob lets you select the tempo subdivision you want to use with the tremolo.


    • The third effect engine on the Mod Rex is the Pan. When using a stereo in / stereo out, or mono in / stereo out configuration the Pan functions as an auto panner, moving the sound back and forth across the stereo soundfield. As with the Trem section, you get Division, Depth and Shape controls, and they function in the same way.


    • The modulated Filter has three different modes, which can be selected with the filter section's Mode switch. A LED indicates which one is selected - green for lowpass, yellow for highpass, and red for bandpass. As with the other effects engines, you also have a Division control to set the tempo subdivision for the filter section.


    • The filter section's Shape button allows you to select which of the four different LFO waveforms you want to use for the filter section modulation, and a R INV switch allows you to invert the LFO polarity on the right channel output for wide stereo effects when using a mono in / stereo out setup.


    • You also get a Depth control as well as a Res (resonance) control for the filter, so you can set it for deep and very highly resonant filter sweeps if you need to.


    • When running all of the effects engines simultaneously, the signal path is Filter -> Mod -> Tremolo -> Pan.


    • One other control on the Mod Rex needs to be discussed, and that's the Lock button, which is located right below the Tempo knob. This works in partnership with the Mod Rex presets. When the Lock button is depressed, recalling a preset will not recall the tempo saved within the preset, but keeps the current;y active tempo value in place instead. This allows you to use the various presets while keeping the tempo consistent.


    • The Mod Rex is a true stereo in / out pedal. You can also use the left input alone for mono sources while still feeding two separate outputs, or even run it as a mono in / mono out pedal if you wish. The input and output jacks are mounted on the right and left sides of the pedal, respectively.






    • On the back of the pedal, you'll find the 9V DC power jack, which uses the industry-standard 2.1mm center-negative plug and wiring configuration. A 9.6V DC 200mA power supply is included with the pedal. The Mod Rex draws 100mA of current at 9V DC.


    • The Mod Rex is adapter-powered only, and can not run on batteries. There are no user-adjustable trim pots or switches inside the Mod Rex, so there's really no reason to ever open the case, but for the curious, here's a photo of the inside of the pedal.




    • Also on the back of the pedal is a 5-pin DIN style MIDI input jack.




    • The MIDI capabilities of the Mod Rex go beyond just locking the tempo to MIDI clock coming in from another device. You can also recall presets using MIDI Program Change, as well as as use MIDI Control Change (CC) messages to control any parameter on the pedal - all of which make it a powerful addition to both studio rigs and live EDM setups.


    • The Mod Rex also includes a Clock / Foot Controller input jack. This can be used to send a pulse clock signal to the Mod Rex to control the tempo, as well as to connect an expression pedal, which can also be used for on-the-fly tempo adjustment. And finally, an optional external footswitch controller can be used to quickly scroll through and recall presets.


    • The Mod Rex measures 4.75" D x 5.75" W x 2.5" H, so while it isn't a small pedal, it still takes considerably less space than having multiple individual modulation pedals on your board.


    • The Mod Rex features buffered master bypass switching. 




    • Some people may feel a bit overwhelmed and confused when they first see the Mod Rex. Don't let that turn you off - It's much less confusing in actual use that it may at first appear, and despite its complexity, it can be quickly learned and mastered. It's much more intuitive in actual use than you might think. 


    • Obviously stereo panning won't work if you have the Mod Rex connected as part of a single amp / mono rig; in those cases, the Pan section functions as a second Tremolo, with the LFO wave shapes inverted relative to the Trem section.


    • There is no MIDI out or Thru jacks on the Mod Rex, so it can not be used as the MIDI clock master to sync other devices - only as a MIDI slave.


    • The two large blue LEDs (rate / tempo and bypass status) are annoyingly bright.


    • The filter can not be driven to self-oscillate.


    • I know it's asking a lot, but it would have been really cool to have panning position and / or stereo width options for each of the four effects engines.




    I really like the Mod Rex a lot. The individual effects may not be quite as fully-featured as some of the ones found on other EHX modulation pedals, and they aren't going to sound quite as good as your favorite analog versions (the Mod Rex is digital) but they are still very good sounding and quite usable, and all of the features that you really need to adjust them are included. But if you're buying a Mod Rex just to add a bit of chorus to one song and some phase shifting to another one, you're kind of missing the point. Yes, it can be used like that (and it will take up a lot less pedalboard real estate than separate phaser, flanger, chorus, vibrato, auto panner, filter, and tremolo pedals would!) , but where it really comes into its own is when you're using multiple effects simultaneously, and running them at different subdivisions of the tempo. That's where the real fun kicks in, and where the Mod Rex shows off its true power. The Mod Rex makes it easy to get multiple effects to play in sync, and is capable of creating sounds that would be difficult or impossible to make, even using four high-end modulation pedals in its place. The Mod Rex will not only appeal to sonically adventurous guitarists, but also keyboardists, electronic musicians and recording aficionados too. Regardless of what electric or electronic instrument(s) you play, you'll find the Mod Rex to be a uniquely powerful addition to your effects toolkit. It's a modulation monster! -HC-



    Want to discuss the Electro Harmonix Mod Rex 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 Mod Rex Polyrhythmic Modulator ($333.40 MSRP, $249.00 "street")

    Electro Harmonix Mod Rex product web page     


    You can purchase the Electro Harmonix Mod Rex from:


    Full Compass     

    Guitar Center     

    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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