MXR Flanger/Doubler rack unit

The MXR Flanger/Doubler is a two-space rackmount effects unit, the M-126, that MXR built in the 1970s to do two related jobs with one analog delay line: flanging and double tracking¹². It emerged in 1976 and lasted most of MXR's early period before the company's bankruptcy in 1984 ended production². In flanger mode it sweeps a very short delay to produce the jet-like whoosh players know from the era's records; in doubler mode it uses a longer delay to simulate two guitars playing the same part, thickening and widening a single track²¹.

The unit has kept a cult following, and in August 2026 MXR brought the circuit back as the MXR M226 Flanger/Doubler pedal, with PastFX releasing its own take, the doppelflänger, the following month³⁴⁵.

How the circuit works

The Flanger/Doubler is essentially a delay-line manipulator: the same basic topology is honed for two different jobs, and the two modes deliver different short delay times through two different bucket-brigade device (BBD) chips⁶. A BBD passes the signal down a chain of internal capacitors in steps, which is how the delay itself is made; modulating that delay with an LFO produces flanging, and holding it at a longer, fixed time produces doubling.

The two chips split the work:

The doubler's 14 to 66 ms range is what lets it sit far enough behind the dry signal to read as a second guitar rather than a comb filter⁵. The unit also used a Vactrol, a light-dependent resistor, in its modulation path, and reviewers note a slight noise floor at higher intensity settings, one of the quirks that comes with the vintage parts⁶. Those components combined to give what Guitar Pedal X describes as:

Controls and connections

The control set carried over from the rack unit into the modern pedal, which is how players today know it. The core of it is three knobs plus a set of switches⁷⁴⁸:

• Manual (Delay Time): sets where the delay sits, the center point of the sweep²
• Width: sets how far the LFO sweeps around that point⁸⁵
• Speed: sets the modulation rate⁸⁵
• Regen / Feedback / Intensity: controls how much of the delayed signal feeds back on itself, from subtle movement to the extreme jet sweep²
• Doubler button: switches the Delay Time to the more expansive range the doubler needs²
• Phase Invert button: flips the phase relationship of the modulated signal²

The rack unit also carried an input level arrangement that MXR kept in the reissue: an INPUT LEVEL switch adjusts input and output gain staging, with a Line Level setting for the highest headroom when connecting synths and sequencers, and an Instr. Level setting for the lowest possible noise with a guitar⁵⁸. The modern pedal adds a rear-panel line-level input switch for the same purpose, and a CV OUT jack that sends a 0 to 5V signal derived from the internal LFO to control modulation on other CV-compatible pedals, synths, or external devices⁵⁷.

Who played it

The rack unit was a guitar rig favorite for a documented list of players⁶³:

• Dimebag Darrell³⁴
• Zakk Wylde⁶
• Eric Johnson⁶
• Eddie Van Halen⁶
• Billy Corgan⁶

Dunlop's own history of the rackmount era notes that heavy players the world over flocked to the Flanger/Doubler for its massive tone thickening and widening, and Gearnews reports the original rack still has quite a cult following³⁴.

From the rack unit to the M226R pedal

The pedal recreates the rack unit's all-analog flanger and doubler effects with authentic bucket-brigade devices and the original control set, and takes a design cue from the black frame of the original rack unit with a black framing effect of its own⁵⁷². MXR's manual describes the relationship plainly:

PastFX followed with the doppelflänger, its own vintage-spec, high-headroom compact take on the same 1970s rack unit, so the circuit is now available in two pedal forms alongside the surviving racks⁶. Players who want the wider MXR flanger family can also look at the MXR Flanger and the MXR EVH Flanger, both descendants of the same era of analog delay-line design.

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