MXR Bass Preamp

The MXR Bass Preamp, also known as the M81, combines a clean bass preamp with a studio-quality Direct Out in a Phase 90-sized box. Dunlop describes it as a tool to "add a new voice to an old passive bass, sweeten up any amp, or put some special sauce on a particular song"¹². In practice that means a pedal that shapes your core bass tone with a 3-band EQ and level controls, and at the same time feeds a balanced signal to the mixing desk, with or without your EQ applied.

It sits in MXR's bass line alongside pedals like the MXR Bass D.I.+ and the MXR Bass Compressor, and it lists at $189.99².

The controls: Input, Output and a 3-band EQ with sweepable mids

The front panel is built around separate Input and Output level controls and a 3-band EQ section with a sweepable midrange, from 250 Hz to 1 kHz¹. The Input knob controls pre-EQ gain with a range of ±10 dB, and the Output control sits post-EQ with up to +20 dB of boost³. That split matters: you can push the preamp harder or lighter before the EQ, then set the final level leaving the pedal independently.

The sweepable mid is the shaping tool that separates this from a fixed three-knob EQ. Boosting or cutting anywhere between 250 Hz and 1 kHz covers the region where a bass sits in a mix, from low-mid thickness through to the growl area around 1 kHz¹.

The manual lists a noise floor of -98 dBV, A-weighted, with all controls at their mid positions³.

Direct Out, pre/post EQ and ground lift

The Direct Out is the pedal's second job. Dunlop's pitch is to "dial it in just the way you want it and send it straight to the house, pre or post-EQ, with the built-in Direct Out"¹. The PRE/POST EQ switch decides whether the engineer at front-of-house receives your EQ moves or a flat version of your signal, which is useful when you want to sculpt your on-stage amp sound without changing what the desk gets¹.

A GROUND LIFT switch is included for the times a ground loop between the pedal and the desk introduces hum¹.

This is the same feature set other bass preamp/DI pedals carry. The Walrus Audio Badwater Bass Pre-amp and D.I. routes its XLR out directly to front-of-house while its 1/4 inch out runs to an amp⁴, and the Gallien-Krueger Plex Preamp pairs its direct XLR output with a ground lift and a pre/post selector⁵. The Source Audio ZIO Analog Bass Preamp + DI likewise works in front of an amplifier or as a preamp/direct box through its balanced XLR output⁶.

Bypass modes and internal switches

Out of the box, the Bass Preamp is factory preset to operate in 1/4" out buffered bypass with the Direct Out active¹. The manual's default setting is Direct Out "ON" and 1/4" OUT bypass "ON"³.

For players who only want to use the 1/4" out, two internal switches disable the Direct Out and the 1/4" out's buffered bypass, giving a true bypass mode on the 1/4" output¹. The manual lists the bypass options as true bypass and hardwire bypass, with the 1/4" output internally selectable³.

For comparison, some competing preamps are true bypass only: the Xotic Bass BB Preamp uses true bypass switching⁷⁸, and the Chase Bliss Preamp MKII, now discontinued, was also true bypass⁸.

What happened to the MXR Thump Bass Preamp?

MXR also made a second, smaller bass preamp, the Thump (M281). Dunlop lists it as a legacy product that is no longer in production². It was announced in January 2020, when Guitar World described it as a mini pedal that "revisits the highly renowned tone of the Echoplex pre-amp"⁹.

The Thump came out of the MXR design team working with the company's in-house bass players, who found the Echoplex Preamp's circuitry made a useful foundation for a bass tool². Using that circuit as a base, the team added a tone-shaping interface intended to punch up a solo, boost signal in a live mix, or sculpt more edge into a passive bass². It packed its controls into an MXR mini housing¹⁰:

• OUTPUT knob sets the effect's output level¹⁰
• BASS knob boosts or cuts bass frequencies at 40 Hz¹⁰
• MID knob boosts or cuts midrange as set by the FREQUENCY knob¹⁰
• FOOTSWITCH toggles the effect on and bypass, with a blue LED indicating on¹⁰

Its manual lists a noise floor of -95 dBV, A-weighted, with all controls at their mid positions³. Unlike the M81, the Thump cannot be powered by a battery and runs only on the Dunlop ECB003 9-volt adapter¹⁰².

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