The MXR M77 Modified O.D. is an overdrive pedal from MXR, part of the company's Custom Badass line. MXR describes it as a classic overdrive circuit hot-rodded for added versatility: it adds sustain and gain without coloring your guitar's tone1. What sets it apart from a basic three-knob overdrive is its EQ section, a dedicated 100Hz knob and a Bump switch that changes the pedal's voicing at the press of a button.
The controls: gain, output, tone, 100Hz and Bump
The M77 has three knobs, a 100Hz EQ knob, a Bump switch and a footswitch. Here is what each one does, per MXR's manual1:
| Control | What it does |
|---|---|
| GAIN | Controls the amount of overdrive; clockwise increases, counterclockwise decreases |
| OUTPUT | Controls overall effect volume; clockwise increases, counterclockwise decreases |
| TONE | Shapes overall EQ, ±8 dB at 2.7 KHz |
| 100HZ | Cuts or boosts at 100 Hz; clockwise increases the low end for a thicker tone, counterclockwise gives a more focused sound; at 12 o'clock the frequency is flat |
| BUMP | Engages an alternate EQ voicing that boosts lows and mids, indicated by a blue LED |
| FOOTSWITCH | Toggles the effect on/bypass; a red LED indicates on |
The 100Hz knob is the pedal's signature control. RockBoard's PedalPedia says it allows for a more focused EQ when cut, or a beefier tone when boosted2. In practice it works like a bass cut or boost: roll it back for a tighter, more focused sound, or push it forward to thicken the low end. The manual rates the 100Hz control at ±24 dB at 100 Hz, while the gain control itself is rated at ±25 dB range at 100 Hz1.
The Bump switch is the second voicing option. RockBoard's PedalPedia describes it as engaging an alternate EQ voicing that boosts the lows and mids2, rated in the manual as a +6 dB low to mid boost1. A blue LED lights when it is active, so you can see the pedal's state on a dark stage.
"The Bump switch engages an alternate EQ voicing that boosts the lows and mids."
Powering the M77
The pedal runs on 9-volt DC. You can power it three ways1:
- One 9-volt battery, installed by removing the bottom plate
- A 9-volt AC adapter such as the Dunlop ECB003 or ECB003E
- A DC Brick power supply
Current draw is under 4 mA1, which is low enough that the pedal is easy on a shared power supply.
The M77 takes a standard 9-volt battery, but you have to remove the bottom plate to install it, so plan ahead if you run it on battery power.
Hooking it up
Setup is the standard two-cable arrangement: run a cable from your guitar to the M77's INPUT jack, then another from its OUTPUT jack to your amplifier1. With a 1 MΩ input impedance1, the pedal presents a normal high input impedance to the guitar, and its True Hardwire bypass1 leaves the signal alone when the effect is off.
Videos
Prymaxe
N Stuff Music
Dorin Ardelean
Sources (2)
- M77.pdf, Dunlop Manufacturing, Inc., 2019
- M77 Custom Badass Modified O.D. by MXR, RockBoard PedalPedia, 2026
