Audio Research D51 (1974–1976)
The Audio Research D51 is a solid-state stereo amplifier from the mid-1970s, back when the company was still best known for its tube designs. It was introduced around 1974, though the record gets tangled here. A 1976-era advertisement lists the D51 at $1,200 as its "current" price, while the original price appears to have been $750. One source suggests production was taken over by Audio Research itself in 1976, which implies earlier units may have been built by a subcontractor. There's also an observation, noted on vintageshifi.com, that the D51 appeared to be a modified version of a design by a small manufacturer named Janis. That's an observation, not a confirmed fact. Classified ads from the period show Audio Research gear on the used market, with a D350 amplifier listed at $1,875 in 1978 (Audio, November 1978, p.149). A used Tympani IA speaker was also advertised for $650 in Audio, January 1976, p.70.
The basics are reasonably well documented. The D51 was rated at 50 watts RMS per channel, or 80 watts into 4 ohms, with frequency response of 20 Hz to 20 kHz. A 1978 classified ad in Audio (November 1978, p.149) listed a used D350 amplifier at $1,875, which shows the brand's presence in the used market. The case had a black anodized aluminum front panel, and speaker connections were via 5-way binding posts. The 1976-era ad copy claimed "unmeasureable distortion" and "no negative feedback." Meanwhile hifi-wiki.com lists THD at less than 0.04%, which is a more believable engineering figure than "unmeasureable." The same ad positioned the D51 as Audio Research's demonstration that the high quality of their tube amplifiers could be achieved in solid-state form.
Beyond that, the record is thin. No dimensions, weight, damping factor, input sensitivity, or signal-to-noise figures appear in any source. The input configuration needs a caution flag. One source's description of a period ad mentions a phono input, but that page conflates the D51 with another model, so it shouldn't be treated as firm. No first-hand owner reviews or contemporary magazine tests have surfaced. The only "reception" material is the advertising copy itself. No market pricing data exists in the sources either.
So what we have is a mid-1970s solid-state amplifier from a company known for tubes, priced at $750 initially and $1,200 by 1976, rated at 50 watts per channel, and possibly built on a modified Janis design. If you own one, or have period documentation, the archive would welcome the details.
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