MXR M400 Turbo Tuner

The MXR M400 Turbo Tuner is a strobe tuner pedal built around the core engine of the original Sonic Research Turbo Tuner, bringing what the manual calls "modern versatility to the gold standard in strobe tuning"¹. MXR collaborated with Sonic Research on the pedal, which was announced on 13 October 2026 and is shipping from the Sonic Research Webstore at a MAP/RRP of $149.99². For a player, it is a compact, high-accuracy pedalboard tuner that can also store custom tunings and act as a buffer in your signal chain.

The strobe approach works differently from a needle-style chromatic tuner. A ring of LEDs displays a rotating pattern: the pattern rotates left if the note is flat and rotates right if the note is sharp, and it stops moving when the string is in tune¹. This gives a continuous read of which way you need to turn the tuning key and how far off you are, rather than a needle that jumps around near the target.

What it is and how it reads pitch

The M400 is a true strobe tuner rather than a simulation of one. The display combines a ring of LEDs for the strobe pattern with an LED note display that shows the note name in chromatic mode or the string number in Open Tuning mode¹. Sharp/Flat LEDs indicate when a note is more than 50 cents out of tune, and these are only active in Open Tuning mode¹.

The display is bright enough to be readable in direct sunlight and can be dimmed for indoor use, which matters both for outdoor stages and for dark rehearsal rooms². The pedal sits in a rugged die-cast aluminum case, consistent with MXR's standard enclosure construction².

Guitar Pedal X's Stefan Karlsson summed up the value proposition when the pedal was announced²:

Controls and tuning presets

The panel is built around a footswitch, two arrow buttons and the display. The footswitch toggles the effect on and bypass, and when held together with other controls it accesses certain advanced settings¹.

The preset indicator shows which of six tuning presets is active: CHR (Chromatic), G (Guitar), B (Bass), and three custom slots, C1, C2 and C3¹. The pedal operates in chromatic and guitar/bass modes².

The three custom slots ship with useful defaults and can be reconfigured:

• C1 is set to standard guitar tuning¹
• C2 is set to drop D tuning¹
• C3 is set to standard 6-string bass tuning¹
• B (Bass) default supports standard 4-string and 5-string bass tunings, covering G2, D2, A1, E1 and B0¹

Each of C1, C2 and C3 can be configured for open tunings with up to 12 strings, or for custom chromatic temperaments¹. When editing an open tuning you can select an octave from 0 to 7 for each string¹. That range of octave selection is what lets one custom preset serve instruments from an extended-range bass to a 12-string guitar.

The right arrow button has a second function aimed at drop tunings: holding it during power on lets you change the number of semitones to drop¹. This is a quick way to set up a pedal that tracks a drop-tuned guitar without reprogramming a whole custom preset.

Power and connections

The M400 runs on 9 volts DC, and the 9V DC IN jack connects to a power adapter or a pedalboard power supply¹. It also runs on one 9-volt battery, installed by removing the bottom plate¹. Current draw is 48 mA at minimum display brightness and 73 mA at maximum, typical¹. The brightness-dependent draw means a dimmed pedal is easier on a power supply budget than one left at full brightness.

There are dual power jacks: alongside the 9V DC input there is a 9V DC output, so the pedal can pass power along to another effect on the board². The full connector set is:

• Input (1/4") jack, which receives the instrument signal¹
• Output (1/4") jack, which sends the instrument signal¹
• 9V DC In¹
• 9V DC Out¹
• USB Type-C¹

The USB Type-C port is for accessing the software editor and firmware updates¹. Guitar Pedal X describes it as a USB port "for ease of customizing"², which is where the custom tunings and temperaments are set up before being loaded to the pedal.

True bypass and the three buffered modes

Bypass is selectable by an internal switch between true bypass and buffered bypass¹. True bypass is the factory default mode, and switching is achieved by relay¹. Relay-switched true bypass avoids the mechanical contact wear of a conventional footswitch contact arrangement while keeping the signal path out of the circuit when bypassed.

If you choose buffered bypass instead, there are three selectable modes¹:

Guitar Pedal X lists these as flexible buffer output options: muted, tuner always on with footswitch mute, and output always on with the footswitch turning the tuner on and off². Mode 3 is the one to look at if you want the tuner tracking pitch continuously while you play, muting the output only when you need silence between songs.

From the Sonic Research ST-300 to the MXR M400

Visually, the M400 cleans up the interface compared with the ST-300, with the white box outlines removed from the new variant². The collaboration puts Sonic Research's tuning engine in an MXR-branded die-cast enclosure with the preset system, USB editor and dual power jacks described above.

For players comparing strobe tuners, the M400 sits alongside options like the Peterson StroboStomp HD and the VOX VXT-1 Strobe Pedal Tuner in the tuner pedal category, with the Sonic Research engine as its distinguishing feature¹².

Resetting the pedal

The M400 provides two reset options. Reset option 2 is a full reset that restores all factory defaults, including the tunings and temperaments¹.

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