M101 Phase 90

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MXR Phase 90 Phaser. This is the one-knob phaser that has, for all practical purposes, dominated most other phasers that have come before, or since. The only other one that even comes close is the Electro Harmonix Small Stone.

Thermionic Studios owns four (4) Phase 90 pedals available for rental:

  • Two (2) CSP101SL "Script" Phase 90s,
  • One (1) EVH-90 Edition Phase 90, and,
  • One (1) EVH-90 SE 35th Anniversary Phase 90

IRONY: Despite making the article about the standard "block" Phase 90 phaser, we don't have any standard MXR Block Phase 90 phaser pedals. We have nothing against the standard block phasers, we just don't think they sound AS GOOD as the older script-logo architecture model; we don't think they offer the best sounds - we believe musicians are more partial to the sounds of the script logo pedal.

If you're attached to the block pedal sound, it's possible to use an EVH-Phase 90 with the "script" button not pressed in.

If you're interested in renting one of our Phase 90s, please don't hesitate to contact us.


M101 Image from Jim Dunlop
  • Knob 1 - "Speed": Turning the knob clockwise increases the speed of sweep across the phasing effect
  • Footswitch 1 - "On/Off": This switch toggles the pedal between Active and Bypass modes.

Bypass: Buffered

All versions of the Phase 90, whether by the original MXR company, or from later Dunlop/MXR, have a buffered signal bypass. There is no exception.

General Information

Unlike the other versions of the M101, the "Block" Phase 90 has a tough textured coating. It maintains the historic orange color that MXR used when they first released (their first product!) the original bud-box script-Phase 90.

Of all the versions of the Phase 90 that are available, the block Phase 90 has the most pronounced phasing effect present in the affected signal. The script-logo Phase 90 has been re-issued by Dunlop, and we believe it's more subtle phase-shifting effect sounds better, or perhaps could better be described as "more musical".

Most "gearheads" tend to like effects or amplifiers with lots of knobs or other controls. The irony is the perennial popularity of the Phase 90 and its single knob control. We're of the opinion that the Phase 90 remains popular for a few reasons:

  1. It sounds really good
  2. It's tough
  3. A single knob makes it easy to control while playing

Script Phase 90

CSP101SL from Jim Dunlop

This version of the Phase 90 is easily distinguishable from the "block" version in that it has a smooth orange coating on the enclosure and displays the classic script logo.

There are a couple of different versions of the Dunlop/MXR "Custom Shop Script" Phase 90:

  1. A handwired version of the original "script" logo Phase 90 (CSP-026). This version is true to the original with:
    • No external AC power supply jack
    • No LED indicator to indicate active/bypass status
  2. A standard-built "script" logo Phase 90 (CSP101SL) that includes AC power jack and LED indicator.

The handwired (CSP-026) version is harder to get a hold of, and isn't displayed on the Dunlop website, making it more "apocryphal". However, there are music stores that have it available for sale.

We own two (2) copies of the "standard-built" (CSP101SL) pedal. We think the AC power jack and LED indicator are helpful to musicians.

EVH Phase 90

EVH-90 from Jim Dunlop

Dunlop started making this version of the Phase 90 several years ago, specifically for Eddie Van Halen, as a custom edition of the pedal. It is easily identified as it carries the trademark red-with-white-and-black stripes of EVH.

The LED differs on this version. It is blue instead of red.

We have one (1) copy of this Phase 90. We were able to get a good deal on it and thought it would be cool to pair with our EVH117 EVH Flanger 35th Anniversary version since they would be matchy-matchy.

If you're not already in the know, there are two things that are necessary in setting up your Phase 90 to get you the "Eddie Van Halen" phase effect:

  • Set the Speed knob to a slow setting. Adjust the knob indicator to around "9:00 o'clock"
  • Place the Phase 90 effect in the signal chain PRIOR to distortion, or into your amp. This "gooses" the distortion effect. Setting the phase effect after distortion, or using it in the effects loop doesn't recreate the effect.

((*note to wiki staff - place a small sound clip of the very beginning of "House of Pain" so people can hear this effect))

The Script Button

This version of the Phase 90 is different from the standard MXR Phase 90 because it comes with a "script" button. The script button is in the upper-left corner of the pedal. This button toggles between the original "block" version phaser effect, and, when depressed, the subtler "script" version phase effect.

EVH SE Phase 90

"EVH-90 SE" from Stratcat.biz

The EVH90SE is exactly the same pedal as the EVH90 version of the Phase 90. It's only real difference is the black-on-white motif of early Eddie Van Halen on the pedal enclosure. It is functionally no different. Despite our protestations of "don't get signature gear", we got this because we were able to get a good deal on it, and because we thought it would be cool and fun to pair with our EVH117 EVH Flanger that also shares the black-on-white style motif.

  • Being exactly the same model, except for the different paint job, the Eddie Van Halen 35th Anniversary version also has the "script" button. It functions in exactly the same manner.

Pedal Manuals

Phase Inversion: No

Per our inspection of the schematic, the MXR Phase 90 does not invert phase.

Schematic ID Electronic Part Action Phase State
IC1 RC4558 Does Not Invert Original Phase
Q1 2N5952 Does Not Invert Not Inverted
IC2 RC4558 Does Not Invert Not Inverted
Q2 2N5952 Does Not Invert Not Inverted
IC3 RC4558 Does Not Invert Not Inverted
Q3 2N5952 Does Not Invert Not Inverted
IC4 RC4558 Does Not Invert Not Inverted
Q4 2N5952 Does Not Invert Not Inverted
IC5 RC4558 Does Not Invert Not Inverted
Q5 2N4125 Does Not Invert Not Inverted
IC6 RC4558 Does Not Invert Not Inverted


We happened to find this schematic for the Phase 90. It's interesting in that it offers some alterations that, if you were to build your own Phase 90 from scratch, that you could implement for additional versatility:

  • A Phase 90 / Phase 45 toggle switch
  • Another toggle switch to enable an unusual "univibe" effect
  • An additional "Depth" knob to set the depth or shallowness of the phase swell

We can't speak as to how well these modifications would work if you were implement them. We're currently not interested in attempting them on our own pedals since doing so would require us to make changes to our original enclosures.

Phase 90 Schematic by Nero Rox, unaware of any Copyright claims.


Additional Sources