MXR Turbo Tuner

At the center of the pedal is a big, bright strobe display that tracks how close you are to pitch. The display is driven by the analog signal straight from your instrument, so it tracks with zero latency and no simulated display¹. The manual lists accuracy of ± .02 cents from a calibrated crystal oscillator, with a tuning range of C0 (16.35 Hz) to C8 (4,186.0 Hz)².

A true strobe tuner

Most pedal tuners use a needle or LED arc that averages your pitch and reports a result. A true strobe works differently: the display is driven directly by the analog signal from your instrument, so it tracks with zero latency and no simulated display¹. MXR says Sonic Research engineered the M400 to deliver the same tour-grade precision and speed as the original Turbo Tuner, accurate to a fraction of a cent¹.

The manual's figure is ± .02 cents, which is far tighter than most players can hear, and the tuning range runs from C0 (16.35 Hz) up to C8 (4,186.0 Hz), well beyond the needs of guitar or bass². The reference pitch is adjustable from A4 = 300.0 Hz to 599.9 Hz in .1 Hz increments, and an A4 LED on the face illuminates whenever the reference is set to anything other than 440.0 Hz, a useful reminder on a dark stage².

Guitar Pedal X's Stefan Karlsson called it "a really attractively priced high-spec compact tuner" at $150³.

How the strobe display reads

The strobe display is a ring of LEDs. The pattern in the ring rotates left if the note is flat, and rotates right if the note is sharp. The closer you get to pitch, the slower the rotation, until the pattern appears frozen, which means you are in tune¹.

Around the ring, the manual describes these display elements²:

The display has four brightness levels, so it stays readable in any light¹. Guitar Pedal X noted the LED display is readable in direct sunlight and dimmable for indoor use³.

Modes and presets

The pedal has a Chromatic mode that tracks all twelve notes, plus five customizable presets¹. The manual breaks these into six tuning presets: CHR (Chromatic), G (Guitar), B (Bass), and three Custom slots, C1, C2, and C3².

• Chromatic: responds to all 12 notes of the chromatic scale²
• Guitar and Bass modes: track your open strings and show the string number instead of the note name; the Bass preset supports standard 4-string and 5-string tunings (G2, D2, A1, E1, B0)¹²
• Custom presets (C1, C2, C3): each can be configured for open tunings with up to 12 strings, or custom chromatic temperaments²
• Offsets: each of the five presets can be reprogrammed with offsets of ±50 cents in 0.1-cent increments¹

Three of the presets come pre-loaded with standard guitar, drop D, and 6-string bass tunings¹. The pedal can also be loaded with open tunings for True Temperament necks, including Formula 1, D.W.G., and TT Low Tune presets². A reset option returns everything to the defaults: Chromatic mode, A4 at 440 Hz, flats cleared, and buffered bypass mode set to mode 1².

The left arrow button changes the tuning preset in normal operation; hold it down during power on to change the A4 reference².

Bypass options: true bypass or buffered

The M400 gives you both bypass types, selected by an internal switch².

True Bypass is the factory default, just like the original Turbo Tuner, and switching is achieved by relay². In this mode the tuner mutes your output while it is on, for silent tuning, and drops completely out of the signal path when off¹.

Buffered Bypass is selected by flipping the internal toggle switch. MXR describes it as transparent buffered bypass that keeps your high end intact across long cable runs¹. The buffered mode has three selectable submodes²:

Guitar Pedal X summarized the same three options as "Muted, Tuner always on with foot switch mute, output always on and foot switch turns tuner on and off"³.

Built on the Sonic Research engine

The M400 is a collaboration between MXR and Sonic Research, and it is built around the core engine of the original Sonic Research Turbo Tuner¹³. The heavy-duty footswitch and button design is exclusive to Sonic Research, which MXR says is "built to keep you in tune for decades"¹.

The original Sonic Research tuners have a track record in the press. That Pedal Show has used a Sonic Research Turbo Tuner 200 in its own rig⁴.

USB and connections

The pedal carries an onboard USB-C port for firmware updates and editor software, which MXR says makes all the preset tweaks even easier to make¹. Guitar Pedal X likewise noted the USB port is for ease of customizing³.

The full connector set is: Input and Output on 1/4" jacks, 9V DC In, 9V DC Out, and USB-C². The dual power jacks mean you can daisy-chain power to another pedal from the tuner itself³. The IN jack receives your instrument signal and the OUT jack sends it on².

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