Shure SR-107
A ten-band octave equalizer that Shure pitched at the working sound engineer who didn't have a big-budget analyzer to go with it.
Overview
The SR-107 is a single-channel, ten-band octave equalizer - one rotary control per octave, from 31 Hz to 16,000 Hz, with roughly 15 dB of boost or cut on each band. It's a compact, line-level tone-shaping box, simple enough to run from the front panel without a manual. Shure built it with balanced inputs and outputs. It sits between an audio console or mixer and the power amplifier in a sound reinforcement rig, or between the preamp and power amp in a home hi-fi system.
The band count and the frequency spread are the numbers to look at. Ten octaves from 31 to 16,000 Hz covers essentially everything a working system needs to correct: room modes down low, horn honk in the mids, and the top-end glare that shows up when a speaker and a room disagree. The controls are octave-spaced rather than third-octave, so the SR-107 is a broad-stroke tool. It shapes the overall balance of a system and doesn't notch out a single problem frequency. Shure's own documentation frames it that way, describing it as providing "tonal balance within or between vocal and instrumental elements of a performance for overall improvement of sound quality." That's what an octave equalizer does best, and it was a deliberate design choice.
The rest of the feature set is minimal. There's a master level control, a bypass switch, and an overload indicator. The equalized output is adjustable over a ±15 dB range, and the unit can add up to 20 dB of overall gain to make up for a low-level input signal. So the SR-107 can sit in a signal chain and make up level as well as shape tone. The bypass switch is the feature that gets used most. A/B the equalized and unequalized signal and you can tell whether you improved the sound or just moved the problem around.
Shure sold the SR-107 alongside a companion analyzer rather than as a standalone product. The M615AS Equalization Analyzer System was designed to work with the SR-107 directly. It included the M615 analyzer with its built-in pink-noise generator, the ES615 omnidirectional measurement microphone, a tilt bracket, and a carrying case. Shure's advertising described the pairing as a way to "balance a system" and "balance a budget," and positioned the M615AS as a breakthrough in analyzer cost. The workflow was simple. The M615 puts an equal-energy-per-octave pink-noise test signal into the system, the engineer places the microphone in the listening area, and then adjusts the SR-107's filters until the analyzer's LEDs show each of the ten octaves balanced. Shure claimed accuracy within ±1 dB this way, without having to "play it by ear." That's the sales pitch, so read it as marketing. The idea behind it was that an affordable analyzer plus an affordable octave equalizer could get a small operator into territory previously reserved for bigger budgets.
Audio (July 1978, p.86) listed the SR-107's manufacturer specifications in its equipment directory: frequency response of 30 Hz to 20 kHz within ±2 dB, a signal-to-noise ratio of 99 dB, output noise of −84 dBV from 300 Hz to 20 kHz, a clipping level of +18 dBm, input impedance of 70 kilohms balanced bridging, a line output impedance of 115 ohms actual balanced, harmonic distortion of 1 percent maximum, filter boost/cut of ±15.5, ±2 dB maximum at the center frequencies, and physical dimensions of 19 inches wide by 1¾ inches high by 8⅙ inches deep. That 19-inch width and 1¾-inch height is a single rack unit — the SR-107 was built to bolt into a rack alongside the rest of a system, not to sit on a shelf.
The signal-to-noise figure is worth a second look. A 99 dB SNR on a line-level equalizer is a quiet number, and it's the spec that separates a box you can leave in the chain permanently from one you have to bypass when you don't need it. DB Magazine (June 1977, p.46) reviewed the M615/SR107 combination as a system, and described both components as designed "for the sound engineer who lacks a big budget for sophisticated analysis gear." The review noted that the analyzer's input level handling was a consideration, with a separate front-panel control for cases where the incoming signal was too great. The SR-107 was the simpler half of the pair: ten knobs, a master level, a bypass, and an overload light.
The harmonic distortion spec of 1 percent maximum is the number that dates the SR-107 most clearly. By modern standards that's high for a line-level device, and it says the unit was built to a price point and a purpose. In live sound, where the room contributes far more distortion than any single box in the chain, 1 percent THD hardly matters. In a critical hi-fi playback system, a picky owner would notice it on paper even if he struggled to hear it. Shure's own literature describes the SR-107 as equally adaptable to feedback control in a house sound system or a stage monitor system, and to eliminating equipment response problems like transducer incompatibility, tape hiss, and disc surface noise in a playback system. That's a wide brief for a ten-knob box.
One thing to know before buying: the SR-107 is a mono device. Shure's documentation is explicit that two are required for stereo. That's not a flaw. It comes from the unit's origin in sound reinforcement, where a single mono house system was the norm and stereo playback was the secondary application. But a hi-fi owner hunting for one SR-107 for a stereo system is actually hunting for two, and the second one is the harder find. The 2E variant, with switch-selectable operating voltages of 105-125 or 210-250 Vac, was made for international use. The standard SR-107 operates from 108-132 Vac.
Shure's original user net price for the SR-107 was $250.00, with the SR-107-2E at $297.50. The companion M615AS analyzer system was $429, or $470.25 for the M615AS-2E. That meant a working sound engineer in the late 1970s could put together a complete analyze-and-correct package of analyzer, microphone, case, and equalizer for under $700 at list. The SR-107 also appears in Shure's own advertising as a suggested companion to the M610 equalizer, which suggests Shure saw the analyzer as the centerpiece and the equalizers as the things you drove with it.
Collectors and working engineers today tend to run into the SR-107 in two ways: as a curiosity pulled from an old install rack, or as a useful utility equalizer for a small PA or a home system that needs broad tonal correction. The rotary controls and the single-rack-unit format make it easy to integrate, and the balanced input and output work with professional-level gear instead of being limited to consumer line levels. Parts availability is the usual concern with any 1970s Shure product. The rotary pots and the power supply are the items most likely to need attention, and neither is a common off-the-shelf part today. The bypass switch and overload indicator are simple enough that they rarely fail. The pots are the wear item, and a unit that's sat in a rack for forty years may have scratchy bands that need cleaning or replacement.
The SR-107 isn't a precision instrument. The octave spacing, the 1 percent distortion figure, and the ±2 dB frequency response tolerance all say the same thing: it's a working tool built to a budget. Shure didn't market it as a mastering-grade equalizer or a studio centerpiece. The company sold it as the affordable half of an affordable measurement-and-correction system, and DB Magazine's review confirms that's how it was received, as gear for the engineer who needed to get a room under control without spending analyzer-system money.
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