Boss KM-04
A battery-powered four-into-one micro mixer that Roland aimed squarely at the small-mixer market, and priced to match.
Overview
The KM-04 is Boss's compact four-into-one mixer. It runs on batteries, and it was pitched as a preamp as much as a mixer. Boss designed it for multiple-instrument setups. The same input stage that sums four sources also amplifies the pickups of an acoustic guitar or a bass. That made it a natural fit for players who wanted a small, self-contained front end instead of a rack piece.
Contemporary coverage confirms the intent. Modern Recording (October 1982, p.28) reported that Roland had "gone for the small mixer market" with the introduction of the Boss KM-04 Micro Mixer, and described it as carrying four high-impedance inputs. DB Magazine (November 1986, p.59) later called it a 4 x 1 instrument mixer for multiple-instrument setups, and noted the battery-powered operation and its use as a preamp for acoustic guitar or bass pickups. In the same roundup, the magazine listed the KM-04 at $75.00. That is well below the BX-400 keyboard mixer ($175.00) and the BX-600 stereo mixer ($240.00) that shared the page.
The price tells you most of what you need to know. The KM-04 sat at the bottom of Boss's mixer range, and the design shows it: four high-impedance inputs feeding a single output, battery power instead of a mains supply, and a form factor built for a pedalboard or a small tabletop rig. You can sum a few sources without a wall wart or a rack space. High-impedance inputs suit instrument-level pickups and line sources directly, which is why the acoustic guitar preamp use comes up in period write-ups.
The surviving documentation does not describe a full-featured mixer. The period sources cover the input and output topology, the battery operation and the preamp application, and that is about all that's firmly documented. Treat claims about channel EQ, effect sends or metering on this model with suspicion unless a specific unit's manual turns up. Those features are described on the BX-series mixers in the same period roundups, not on the KM-04.
The model turns up in dealer listings alongside the broader Boss pedal and mixer catalog of the early-to-mid 1980s. It sat on the same price sheet as the DS-1, the CE-2, and the TU-12 tuner, not in the recording console aisle. It was a utility box, sold to the same customer who was already buying Boss pedals, and priced to be an easy add-on rather than a considered purchase.
The practical questions for anyone hunting one now are the usual ones for battery-powered Boss gear of the era. Are the input jacks and the output clean? Is the battery compartment intact? Has the unit been stored somewhere that didn't turn the enclosure into a science experiment? Documentation on the KM-04 is thin compared to the pedals that shared its price sheet, so expect to verify condition from photos rather than a spec sheet. The $75 original price makes it an affordable tool, and that should temper any expectation of collector-grade pricing on the secondary market.
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