The MXR M81 Bass Preamp is a bass preamp pedal from MXR, the effects brand owned by Dunlop Manufacturing, Inc. It gives a bass player tone shaping and level control in a compact pedal: a 3-band EQ with a sweepable midrange, an input control that sets pre-EQ gain, and an output control that sets post-EQ level, plus a Direct Out for feeding a mixing desk or recording rig. It sits in MXR's bass line alongside pedals like the MXR Bass D.I.+ and the MXR Bass Compressor, and it belongs to the wider family of preamp pedals built to shape a bass signal before it reaches an amp or interface.1
The EQ and gain controls
The M81's front panel is built around two gain stages and three EQ bands. The INPUT knob controls pre-EQ gain: turning it clockwise increases the signal going into the EQ section, and turning it counterclockwise decreases it. The manual rates this input control at plus or minus 10 dB, applied before the EQ. The OUTPUT knob works the other side of the circuit, post EQ, with plus or minus 20 dB of control over the pedal's output level.1
The EQ itself is a 3-band design with a sweepable midrange, meaning the mid control lets you choose where in the midrange it acts rather than locking you to one frequency. The manual gives these ranges1:
| Control | Range | Cut/Boost |
|---|---|---|
| Bass | 40 Hz | ±16 dB |
| Mid | 250 Hz to 1 kHz (sweepable) | ±17 dB |
| Treble | 4 kHz | ±18 dB |
The manual also gives a mid figure of plus or minus 17 dB across 250 Hz to 1 kHz in its specifications table, consistent with the control listing above.1
Bypass and the Direct Out
The M81 offers both true bypass and Hardwire bypass, with the 1/4-inch output internally selectable between them. True bypass removes the pedal's circuit from the signal path when it is off; Hardwire bypass keeps a hardwired connection through the pedal instead. Which mode the 1/4-inch output uses is set inside the pedal, not from the top panel.1
The pedal's internal settings also govern the Direct Out. As the manual puts it:
"The Bass Preamp's default setting is Direct Out 'ON' and 1/4" OUT bypass 'ON.'"1
So out of the box, the default setting is Direct Out "ON" and the 1/4" OUT bypass "ON". A player who wants the 1/4-inch output to pass the preamp's tone when engaged will need to look at those internal settings1.
Specifications
The maker's manual lists the following figures for the M81:
| Specification | Value |
|---|---|
| Input impedance | 2.2 MΩ |
| 1/4" output impedance | 300 Ω at 40 Hz |
| Maximum 1/4" output level | +13 dBV |
| THD | 0.03% at 1 kHz |
| Noise floor | -98 dBV (A-weighted, all controls at mid position) |
| Power supply | 9 volts DC |
| Current draw | 8.5 mA |
The manual lists the 1/4" output impedance as 300 Ω at 40 Hz, the input impedance as 2.2 MΩ, and the current draw as 8.5 mA.1
The manual's conflicting output level, output impedance, and current draw figures mean the table above shows the values from the specifications listing; the manual itself gives a second set of numbers for those three items.
Power
The M81 runs on a 9 volts DC power supply. The manual's current draw figure of 8.5 mA is low by pedal standards, so the pedal sits comfortably within the reach of a standard 9V DC supply on a shared board.1
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Questions
Is the MXR M81 Bass Preamp true bypass?
Yes. According to the maker's manual, the M81 offers true bypass and Hardwire bypass, with the 1/4-inch output internally selectable between the two. That means you can choose whether the pedal's output jack passes signal through the switching circuit (true bypass) or keeps a hardwired connection when the pedal is off, which some players prefer for long cable runs or always-on setups.
What power does the MXR M81 need?
The M81 runs on a 9 volts DC power supply. The maker's manual lists a current draw of 8.5 mA, though the manual also gives a 5 mA figure elsewhere, so the two documents disagree on that number. Either figure is low enough that the pedal is easy on any standard 9V DC supply.
What frequencies does the M81 EQ cover?
The M81's 3-band EQ covers the bass at 40 Hz with plus or minus 16 dB of cut or boost, the midrange from 250 Hz to 1 kHz with plus or minus 17 dB, and the treble at 4 kHz with plus or minus 18 dB. The mid control is sweepable, so you can pick the center frequency within that range rather than working with a fixed one.
What is the default Direct Out setting on the M81?
The maker's manual states that the Bass Preamp's default setting is Direct Out 'ON' and 1/4-inch OUT bypass 'ON.' In other words, out of the box the Direct Out is active and the 1/4-inch output sits in bypass mode until you change the internal settings.
Where can I find the MXR M81 manual?
MXR publishes the M81 Bass Preamp directions as a PDF on the Jim Dunlop website. The manual covers the pedal's controls, internal settings, and full specifications, and it is the source for the figures given on this page.
Sources (1)
- M81.pdf, Jim Dunlop, 2019
