Altec 771-B (early 1970s)
A rack-mount biamplifier that split the work between a 60-watt low section and a 30-watt high section - and, if the scuttlebutt is right, had a habit of letting the smoke out.
Overview
The 771-B lived its whole documented life in the back of a trade magazine. It's a biamplifier - one chassis with an active crossover and two power amplifiers, so a studio monitor's low and high sections can be driven separately. Altec pitched it as the companion amplifier for its monitoring systems. Recording-Engineer (October 1972, p.20) described it in those terms: a unit that could drive both the low- and high-frequency sections independently, rather than asking one amp to cover the whole band.
The layout is simple. The signal is split at line level and sent to dedicated amplifiers, so the low section never shares its power supply with the high section. The crossover sits ahead of the amps, where it doesn't interact with driver impedance. Altec rated the low section at 60 watts continuous sine wave power and the high section at 30 watts. That's a sensible ratio for a horn-loaded monitor, since the compression driver on top needs a fraction of what the woofer does.
The crossover is switchable at 12 dB/octave across three frequencies: 500, 800, and 1500 Hz. Recording-Engineer (February 1973, p.2) noted that the switchable crossover let the unit be adapted to most studio monitoring systems and coaxial speakers. That suits a house amplifier in a facility with a mixed inventory of Altec monitors. Set it at 500 Hz for one cabinet, flip it to 800 or 1500 for another, and the same amp stays in service.
Input sensitivity is documented at 0.5 VRMS into a 60 kΩ unbalanced input, or 0.1 VRMS into a 600 Ω balanced input, for full output. The balanced figure is the one that counts in a studio. 600-ohm balanced lines were the standard interconnect of the period, and a tenth of a volt for full output means the 771-B will reach full power from a console output without straining. Noise is quoted at 80 dB below rated output on both sections. Distortion on the high section is 0.3% THD. The low section is listed at 0.3% in most appearances, though at least one early listing shows 0.5%. The 0.3% figure is the later and more commonly reproduced one, and the discrepancy is likely a typo that propagated through the ad copy.
Key Features
60W LF / 30W HF Biamplifier Sections
The 771-B's two amplifier sections are rated at 60 watts and 30 watts continuous sine wave power respectively, both at 0.3% THD. That's not a lot of power by later standards, but it's the right amount for the horn-loaded monitors Altec was building in the early 1970s — high-efficiency cabinets that don't need hundreds of watts to reach working levels. The 2:1 ratio between low and high sections reflects the reality of compression-driver monitors, where the top end is doing a small fraction of the acoustic work. Both sections are described as running continuous sine wave power ratings, which is a conservative way to spec an amp — it means the number is meant to be sustainable, not a peak figure pulled off a marketing sheet.
Switchable 12 dB/octave Crossover at 500, 800, or 1500 Hz
The crossover is the feature that makes the 771-B useful beyond a single monitor model. It's a 12 dB/octave network with three selectable turnover frequencies — 500, 800, and 1500 Hz — and Recording-Engineer (February 1973, p.2) specifically called out its adaptability to various studio monitors and coaxial speakers. Twelve dB per octave is a shallow-ish slope by modern active-crossover standards, but it was a common choice for monitors of the era and it keeps the phase behavior relatively benign around the crossover point. The three-frequency switch covers the practical range of Altec's monitor line: 500 Hz for the big two-way systems, 800 and 1500 Hz for the coaxial and smaller-format cabinets. An owner running a mixed room could standardize on one amplifier and set each unit to suit its cabinet.
Balanced and Unbalanced Inputs with High Sensitivity
Input sensitivity is quoted at 0.5 VRMS for full output on the 60 kΩ unbalanced input, and 0.1 VRMS for full output on the 600 Ω balanced input. A tenth of a volt is a low threshold, so the 771-B will reach full output from a standard console feed without an outboard line amp. The 600-ohm balanced input is period-correct for studio interconnection. The 60 kΩ unbalanced input lets the unit sit in a smaller setup where a console or monitor controller feeds it single-ended. The 80 dB signal-to-noise figure, quoted for both sections, is respectable for the era and suggests Altec paid attention to the input stage rather than just bolting power amps behind a crossover board.
Historical Context
The 771-B is documented in the trade press from the early 1970s. Recording-Engineer ran it in both editorial listings and Altec's own advertising through 1972 and 1973. What survives in the written record suggests a short-lived or low-volume product aimed at the professional market rather than the home hi-fi buyer. The Internet Archive catalogs it under the title "Altec 771B 771BX Electronic Crossover Biamplifiers," which hints at a related 771BX variant without documenting what the difference was. A predecessor, the 771A, is mentioned by at least one forum poster with the verdict that it "sounds better, but blows up worse." The relationship between the A and B is not spelled out anywhere in the available record, so treat any claimed lineage with suspicion. No production years or original price have surfaced, but manufacturer documents list dimensions of 26.5" wide, 31" high, and 23.5" deep, with a weight of 112 lbs. Beyond that the unit exists as a spec sheet and a handful of magazine appearances.
Collectibility & Value
Now the bad news. A collector who's been watching the secondary market reports that of roughly twenty 771-B units that turned up on eBay over a couple of years, eighteen were non-functional. That's a 90% failure rate among survivors. The 771-B is not a piece to purchase untested and hope for the best. Buy it cheap, assume it's broken, and plan to service it. The good news is that a commercial service manual is available for the model, which is more than can be said for a lot of Altec's professional line. The manual provides the servicing information and operating instructions needed to diagnose and repair a failing unit. No reliable current market price could be established. The eBay listings that surface under a "771-B" search are overwhelmingly unrelated Sony and TV parts that happen to share the number, so anyone shopping for one needs to filter hard and read every listing carefully. There is no documented spare-parts availability, no published repair procedure, and no consensus on what a working example should cost.
eBay Listings
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