Shure SR106 Electronic Crossover
A rack-mountable, unity-gain crossover that split the signal so the amps didn't have to. Three frequencies, one switch.
Overview
The SR106 is a rack-mountable, unity-gain, selectable-frequency dividing network - Shure's own words. It sits between a console or mixer-preamplifier and a pair of power amplifiers. This is biamplification in hardware. Instead of asking one amp to reproduce the whole spectrum, the SR106 splits the signal into two bands and hands each to its own amplifier. Per the manual, the payoff is lower distortion, increased high-frequency power, wider dynamic range, and maximum efficiency. The low-frequency amp isn't fighting to reproduce cymbals, and the high-frequency amp isn't wasting excursion on kick drum, so both work less and sound better.
The crossover has a three-position switch with crossover points at 500 Hz, 800 Hz, and 2600 Hz. The 500 Hz point suits a two-way system with a substantial horn or cone midbass. The 800 Hz point is the classic mid/high split for smaller two-ways. The 2600 Hz point is a high-frequency crossover for systems with dedicated midrange drivers. One SR106 gives you two output bands. Add a second unit and you get three bands, each routed to its own amplifier, so two of these in a rack make a triamped system.
Physically, it's a standard 19-inch rack unit: 44.5 mm (1¾") high, 483 mm (19") wide, 216 mm (8½") deep, weighing 3 kg (6 lb, 8.8 oz). It takes a single rack space, and the shallow depth means it fits in cases where deeper processors won't. Shure supplied it with a protective power-switch cover, which makes sense for units that lived in mobile racks and got knocked around. Rack-mounting screws came with it too, for standard 19-inch racks or optional carrying cases.
The front panel is minimal: a power On-Off switch and indicator lamp, and the three-position Crossover Frequency selector. No level controls, no phase switches, no filters. Unity gain means the box doesn't add or subtract level. What goes in comes out at the same amplitude, just split.
Shure positioned the SR106 as part of its SR sound-reinforcement line, and the contemporary trade press treated it that way. Recording-Engineer (April 1975, p.20) covered the SR line's introduction. DB-Magazine (October 1975, p.24) described the SR106 in its new-products column as an electronic crossover designed for use with two- or three-way systems, the same framing the manual uses. The SR106 slotted into a system alongside the SR101 mixing console, SR105 power amplifier, and SR108 two-way horn-loaded speaker.
For owners today, the appeal is a purpose-built analog crossover with fixed frequencies, unity gain, and rack-mount construction. It's not a flexible digital processor, so you can't dial in arbitrary crossover points or slopes. For a two- or three-way biamp setup built around 500, 800, or 2600 Hz, it does the one job without menus. The three-position switch is the entire interface. Original carrying cases (the A30A and A105A) turn up occasionally and are worth grabbing if the unit is going on the road.
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