MXR Micro Amp

The MXR Micro Amp is a clean boost pedal made by MXR that raises your guitar signal by up to +26dB without adding distortion or changing your tone. It is about as simple as a pedal gets: one knob, one footswitch, one op-amp. MXR describes it as a way to "boost your signal with a preset amount of gain up to +26dB," and notes it can also serve as a permanent boost in a long effects chain where signal drop off is a problem¹.

The design goal, according to GuitarPCB's technical analysis, is transparency: "The MXR Micro Amp is designed to be a transparent clean volume booster; it does not color or modify the guitar tone." The whole circuit uses only one op-amp and one potentiometer². MXR says it has been a staple among top artists for more than 20 years³.

What it does to your sound

The Micro Amp is a flat, clean volume boost. Its circuit is, as Guitar.com's boost pedal guide puts it, "a simple op-amp, dialled in with enough headroom to give a big boost of signal without any extra clipping"¹⁰. That makes it useful in two very different ways, and Premier Guitar's tone advice lays out both:

So into a clean amp it is a solo or volume switch; into an amp already breaking up it pushes the front end harder for more saturation. The M133 manual adds a practical use: it is "perfect for switching between guitars with unmatched output," letting you level up a quieter instrument⁶.

Controls, switching and jacks

The control surface is minimal:

The manual suggests starting with the control at 12 o'clock and adjusting from there⁶. Guitar World's buying guide describes the same behavior in plainer terms: "To increase the gain, turn clockwise (with up to 26dB of gain on tap); to reduce it, turn counterclockwise"⁵.

Note that the original 1970s release had neither the LED nor a power adapter jack. MusicRadar reports it "was originally issued without an on/off LED, or the means to accept an external power supply," and that its TL061 op-amp was chosen specifically for its low current draw, which let it run a long time on a battery⁸.

Power

The Micro Amp runs on 9 volts DC and draws only 2.5 mA⁶. Power options, per the product page and manual¹⁶:

• One 9-volt battery (remove the bottom plate to install)
• A Dunlop ECB003 or ECB003E 9-volt adapter
• Dunlop DC Brick, Iso-Brick and Mini Iso-Brick power supplies (not included)

That 2.5 mA draw is low enough that the pedal is easy on a shared power supply, and the battery option means it works on a board with no power distribution at all⁶.

History: the M133 and its Distortion+ cousin

The Micro Amp belongs to MXR's first Reference Series, released between 1973 and 1984². MusicRadar dates it to the early 70s as part of that series⁸, while Guitar World's buying guide gives 1978 for the first M133 Micro Amp pedal⁵. TONEHOME's page for the vintage unit records it as made in Rochester, New York, in 1980, with a 1982 German list price of 142,50 DM¹². The dates differ across accounts, so the mid-to-late 1970s is the window to keep in mind.

Its family relationship is with MXR's Distortion+. MusicRadar calls the Micro Amp "a close non-distorting cousin, of MXR's Distortion+"⁸, and Delicious Audio's review coverage notes that these classic 70s and 80s pedals, along with the DOD 250, "have the same basic circuit"¹³. The difference is intent: the Micro Amp's op-amp is set up for headroom and clean boost, not clipping.

The circuit has stayed in production and in circulation. Gearnews notes "the original Micro Amp became famous for its simple one-knob boost sound" as a studio and live favorite¹⁴, and the circuit was reused in 2022 in the Third Man Hardware Double Down, a stereo two-channel take on the Micro Amp built with MXR for Jack White's Third Man Records¹⁴¹⁵.

The Micro Amp+ difference

The MXR Micro Amp + (M233) builds on the original with EQ and other upgrades⁴. Side by side:

Guitar.com describes the Micro Amp+ variant as adding "a clean, active two-band EQ for extra shaping"¹⁰, and MusicRadar notes it also brings a lower-noise circuit⁸. Gearnews reports MXR upgraded the circuit with low-noise op-amps and true-bypass switching¹⁴. The Plus was shown at the Jim Dunlop booth at NAMM 2014¹⁸.

What reviewers said

Published reviewers have been consistent about what the Micro Amp does well, with some reservations about price. Guitar World's 2026 boost pedal buying guide found it "great for coaxing extra saturation from a Big Muff or getting your leads to jump out in a band mix," while also noting it is no longer the cheapest option in its class⁵. Premier Guitar has referred to the Micro Amp as "its legendary Micro Amp" circuit in review coverage¹⁹.

On bass, the evidence is direct: Flea of the Red Hot Chili Peppers uses an MXR Micro Amp as a clean boost for slapping and leads, per Premier Guitar's rig rundown of his setup²⁰.

Jack White and the Micro Amp

Jack White is a long-time user of the pedal. Guitar World reports his "own personal affinity for the original Micro Amp, which he estimates he switched to in 1997," after moving away from the DigiTech Whammy, and describes the original as "featuring a sole output, gain level control and single bypass footswitch"¹⁶. That affinity led to the Third Man Hardware Double Down with MXR, which Premier Guitar describes as "a stereo take on MXR's classic Micro Amp preamp pedal"¹⁵.

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