Shure M-688 Stereo Mixer
Four mics, a pan pot, and a mono feed for the house. Shure's little stereo problem-solver that working soundmen actually kept.
Overview
The M688 is a stereo microphone mixer. Shure's ad copy called it "made to order for stereo recording and audio-visual work," and that's about right. It does one job, with four inputs, a stereo auxiliary high-level input, and individual volume controls on everything. There is no EQ section, no reverb, no metering to speak of. What it has is a routing trick: a left-plus-right mono microphone-level output that runs alongside the stereo high-level output. The same mixer can feed a tape deck and a sound reinforcement system for the presentation at the same time.
Amateur and professional users were both expected to find it ideal for mixing sound-on-sound tape recordings. Three of the four mic inputs carry front-panel switches that assign them hard left or hard right. The fourth gets a pan control instead, so that input can be parked anywhere between the channels. A stereo master volume rides all inputs at once. If four isn't enough, the unit parallels with a Shure M67 Mixer or a second M688 through mix bus jacks. That made it a common sight in small production rooms where the console was assembled from whatever Shure boxes were on hand.
The electrical specs are serious for a box this size. Input is transformer-coupled at 600 ohms terminating, balanced or unbalanced. Output is a transformer, also rated 600 ohms balanced or unbalanced. Frequency response in the flat position is plus or minus 1 dB from 20 to 20,000 Hz. Operating level spans minus 25 dBm to plus 14 dBm, with a maximum output level of plus 28 dBm into 600 ohms. Distortion is specified at less than 0.15% THD from 20 to 20,000 Hz at plus 24 dBm, and the design uses 40 dB of negative feedback to get there. Those are not consumer numbers. That maximum output is enough headroom to drive a professional line input without straining, and the transformer balancing keeps long cable runs in a studio or a church basement from turning into a hum problem.
The M688 shows up in Shure's own "Magnificent Seven" advertising alongside the M68, M68-RM, M67, M63, M62V, and M675. That was the company's lineup of small problem-solving control components, pitched as a family rather than as individual products. Shure's advertising said the group had drawn unsolicited rave reviews from soundmen around the country, which is marketing. The M688's place in the family still shows what it was for: not a flagship, not a console, but the piece you bought when you needed stereo and didn't have a stereo board.
The period press took the same view. Recording-Engineer (April 1972, p.48) ran the Magnificent Seven copy and singled out the M688 as the stereo mixer in the group, made for stereo recording and audio-visual work. DB-Magazine (August 1972, p.21) carried the same lineup, and DB-Magazine (September 1973, p.5) was still running it the following year. Studio-Sound (October 1975, p.68) pictured a Shure model M688-E stereo microphone mixer in a roundup of studio gear, next to the Raindirk and Schlumberger modules.
The chunks reference both the M688 and an M688-E variant, and the suffix is not explained in the available documentation.
Pricing needs care. The user net price for the M688 was $114. Shure positioned it as a low-cost professional tool within a family that included the M67, which the same advertising called the trail-blazing low-cost professional mixer. The M688 sat above that in capability, not in price bracket.
For anyone buying one today, the practical appeal is the routing. It gives you a stereo pair for the recorder and a mono mic-level feed for the PA at the same time, which modern small mixers mostly don't bother with. The transformer I/O plays nicely with older gear, and the 600-ohm terminating input is the correct match for a lot of period microphones. The pan-on-channel-four arrangement is limited. One pannable input out of four is a stereo mix with one movable element. For the audio-visual and two-track work it was sold for, that's sufficient.
Maintenance concerns are the usual ones for transformer-coupled gear of this vintage. The transformers are the parts you can't easily replace, and any unit that has been stored damp is a gamble. The switches and pots are the other wear point, particularly the left/right assign switches on channels one through three, which see every session.
What the M688 is not is a console. It's a problem-solver, and Shure sold it as exactly that. The Magnificent Seven framing is the honest one: seven small boxes, each solving a specific problem, and the M688's problem was getting a stereo recording and a mono PA feed out of the same four microphones without buying two mixers. For the money and the rack space, that was a good trade then, and it remains a reasonable one for anyone running a small archival or AV setup now.
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