In the 1970s a relatively new company called MXR introduced their now famous Phase 90 phase shifter. The world definitely took notice. Soon after it was introduced, guitar players around the world started using them because they loved the swirling, beautiful effect that it gave to their guitar. It is difficult to describe the sound that the phase shifter imparts except to say that it is beautiful. My personal opinion is that the MXR Phase 90 creates the best phase shift effect of any pedal that I have tried. I have had mine since the mid 70s and it has definitely held up over time both physically, and stylistically. By that I mean that the effect is built like a tank, and that the effect still sounds current, and useful.

Note that the MXR Phase 90 has gone through some evolutionary changes during its life. From the early "script logo" phase to the current day models, the features are similar with subtle variations, and the sound has changed somewhat. The pedal I am reviewing is from the mid 70s.

What Is A Phase Shifter?

In general, a phase shifter is a device that alters the sound of a signal by making a delayed copy of the sound and recombining that copy with the original signal. Because the two signals are shifted slightly in time, the signals combine with constructive and destructive interference leading to a change in the timbre of the signal. By doing this repeatedly and with differing delays, the signal changes over time. This leads to a modulation of the signal that has an interesting and desirable sound. There are different parameters that can be adjusted to tweak the sound, but on the MXR Phase 90 there is only one knob. That knob affects the speed of the phasing effect. But, it actually does more perceptually than just change the speed because at certain speeds there are emergent features that become apparent that make it sound like a fairly different effect. For example, at very high speeds it can make the sound warbly or bubbly. At medium speeds the effect can be described as whooshing or swirling. At slow speeds it is not always even noticed as an effect, but adds a richness and variation to the sound.

MXR Phase 90 Features

The MXR Phase 90 Effects Pedalis housed in a very sturdy metal box that measures about 3 1/2" x 1 1/2" x 1". It provides an input jack and an output jack. You plug the cable from your guitar into the input jack and then plug a cable between the output jack and your amplifier. The unit uses a normal 9 volt battery. There is no on-off switch. Rather, the input jack has a switch built into it such that if there is a cable plugged in, the phase shifter is turned on. If there is no cable attached, the unit is off. It is important to remember to disconnect your cable when not using it to preserve battery life.

There are only two controls. There is a speed control and a "Effects / Normal" footswitch. The speed control affects the speed of the effect, and the footswitch determines whether or not your signal is passed unaffected, or if the effects are applied. My unit is old enough that it does not have one, but the new units have an LED that is lit when the effect is enable. This is very handy because it allows you to determine what state the effects pedal is in before you make a sound.

Comparison To Other Phase Shifters

There are a wide variety of phase shifters available to guitar players. They are available as stomp boxes, rack effects, and as part of multi-effects units and floorboards. The overriding thing to consider when deciding what phase shifter to buy is the sound. I personally prefer the sound of the MXR Phase 90 to any other phase shifter I have tried. One common stomp box competitor to the Phase 90 is the Smallstone phase shifter. I personally prefer the sound of the MXR to the Smallstone. I have also compared the MXR Phase 90 to common multi-effects units and in my opionion, no multi-effects unit can compare to the sound quality of the Phase 90. I have spent considerable time with multi-effects boards like the Boss GT-10 Guitar Multi Effect Pedal , and in all cases I preferred the sound of the MXR Phase 90.

The other important consideration is durability. Effects units undergo a lot of physical abuse under hard touring conditions. The term "stomp-box" is very appropriate in that people literally stomp on them to turn the effect on and off. The MXR Phase 90 is a clear winner in the durability department. The heavy metal box, the high quality switch, and good input and output jacks stand up to heavy road work and concerts in not so perfect circumstances.

Other Applications

The MXR Phase 90 is not limited to use as a guitar pedal. Any instrument level signal that can be plugged into the 1/4" input jack can be processed by the Phase 90. One common application is to use it on a Fender Rhodes electric piano. (Or virtual model of a Rhodes) There was a time when it was popular to run a bass guitar through a phase shifter, especially on solos. This can definitely lead to cool effects. The MXR Phase 90 can also be used on vocals, but it is more common to use a rack or software phase shifter for vocals.

Advantages And Disadvantages Of The MXR Phase 90 Pedal

As in all things, guitar phase shifters have different advantages and disadvantages.


Great sound! (What more can I say?)

Rugged enclosure

High quality components

Small, portable

Simple to use (just one knob!)


Only one effect (but what a great effect!)

A bit noisy when the effect is turned on without a signal

No indication of whether the unit is in effects mode or if it is just passing the signal (though newer models have a LED to indicate this)

No connection for a power supply (newer models have this)

Battery is engaged whenever a cable is in the input jack

Somewhat pricey for single effect


In summary, almost every electric guitar player needs a phase shifter. Of course, it is always desirable to have as many options available as possible, but if you can only have one phase shifter, the MXR Phase 90 should definitely be on your short list to check out. If you decide to buy a MXR Phase 90 be sure to shop around, because prices vary from place to place.