Luxman SQ 507X (1971)
The Luxman SQ 507X is a solid-state stereo integrated amplifier released in August 1971 at ¥86,500. It is the higher-power companion to the SQ 505X, with 50 watts per channel into 8Ω versus the 505X's 30. It sits in a small family with the SQ 503X, SQ 505X, and SQ 700X. The surviving owner's manual is unusually explicit about what Luxman was after: a full-stage direct-coupled circuit with no output capacitors, fed by a dual power supply, and a pure complementary output stage meant to eliminate crossover distortion entirely. The preamp's equalizer stage runs a three-stage direct-coupled PNP-NPN-PNP arrangement. The phono inputs are specified with a 300 mV maximum allowable input at 1 kHz, a deliberate headroom allowance for the dynamic range of real program sources. Luxman's own literature calls the SQ 507X "the newest transistorized pre-main amplifier" built around "the very circuitry theoretically regarded as the best one under exhaustive study." That is house copy, but it shows how the model was framed at launch.
The numbers hold up reasonably well for 1971. RMS power is quoted at 50/50 watts into 8Ω with both channels driven, or 60 watts per channel driven singly, at under 0.04% distortion. Power bandwidth spans 5 Hz to 50 kHz at -3 dB, frequency response 10 Hz to 60 kHz within -1 dB, damping factor 40 into 8Ω and 80 into 16Ω, and residual noise under 1 mV. Signal-to-noise is better than 63 dB on phono and better than 80 dB on the line inputs. The tone controls use Luxman's own NF-type network with switchable turnover points - bass at 150/300/600 Hz, treble at 1.5/3/6 kHz, each defeatable. There is also a 70 Hz low-cut filter, a 6 kHz high-cut filter, and a 100 Hz bass boost. Phono 1 offers selectable input impedance of 30k, 50k, or 100k ohms. Aux 2 and 3 sit at 200k. There are two tape monitor loops, two recording outputs, a pre/main separator switch, a -18 dB attenuator, and a headphone jack. Semiconductors total 33 transistors, 2 diodes, and 4 varistors. Power consumption is 200 W at full output into 8Ω both channels, 15 VA at no signal. Dimensions are 450 × 160 × 268 mm.
Two caveats. Weight is contested: one dealer lists 9,800 g, another 11 kg. Treat it as roughly 10 kg and verify against a specific unit. The release year also has at least one dissenting claim of 1969 from a Dutch repair shop, and a third listing suggests a 1970-1973 build window. August 1971 is the best-attested date and the one Luxman's own material supports. No independent lab measurements have surfaced, no original retail price outside yen, and no production end year. What does survive is service documentation: a 16-page owner's manual, a schematic, and enough of a repair trail that the known weak point is documented. Aging tantalum coupling capacitors in the preamp can leak DC, and the fix is replacing them with polypropylene types. The low-cut filter is sometimes modified into a subsonic filter, and the supply can be rebuilt with larger capacitors and split rectification. One eBay listing turned up a unit with a power amp fault and a working phono stage, which is the kind of failure pattern these amps show at fifty-plus years. Working, serviced examples have listed around US$969 and RM 1,200 in recent years, with the occasional higher ask.
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