Audio Research D 76 (1971–1976; some sources say 1972)
A few things set this design apart. It was Audio Research's first amplifier to use the 6550 output tube. It had 4Ω, 8Ω, and 16Ω output taps, which is a big help when driving classic speakers like the Quad ESL. The original used all-tube voltage regulation and point-to-point wiring, with a brushed stainless steel faceplate and an optional walnut cabinet. A 1983 classified in Audio (June 1983, p.102) lists a D79 alongside a Linn Sondek and Koetsu Black for $2,150, which gives some idea of the resale market a few years on. The D76 itself shows up in period classifieds at $750-$625 through 1979-1981, per Audio listings (January 1979, p.133; July 1980, p.84; September 1981, p.89).
The Absolute Sound has been the most vocal champion of this amp. Reviewer Art Dudley bought one in 1980 and kept it in his system for 30 years, describing its sound as having "the remarkably open, transparent, 'airy' quality of the best tube equipment" — quoting J. Gordon Holt's original 1973 Stereophile review — and praising its "effortlessness and ease." Bass is tuneful and well-defined but not overly weighty; treble is smooth and extends well. The 6DJ8s are directly coupled to the output stage, which gives the D76 its particular transparency but puts severe strain on the drivers — they degrade over time and should be replaced.
As with any 50-year-old tube amp, restoration is not optional. Audio Classics reports the most common problems are worn-out tube sockets and failing capacitors, and notes that original-spec 6550s are no longer made. Budget $600-$900 for a full restoration - capacitors, tube sockets, and bias supply. The D76 is still one of the most collectible Audio Research amplifiers, with restored units selling in the $2,500-$4,000 range.
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