Luxman SQ-507X (1971)
The SQ-507X came out in August 1971 at ¥86,500. It was Luxman's high-power integrated amplifier of the early transistor era, described in the period literature as the "pre-main amplifier corresponding to the high power model of SQ505X." The 505X was the lower-powered sibling. The 507X delivered 50W + 50W into 8Ω with both channels driven, and 60W per channel with one channel driven. The 505X made 30W, so this was a serious step up.
The circuitry is the interesting part. It was among the first Japanese transistor amplifiers of the new audio era, built with first-generation audio-grade transistors from NEC and Toshiba. The power amp section is a dual-supply, full-stage direct-coupled design with no output capacitors. It has a complementary PNP/NPN output stage intended to eliminate crossover distortion, and a two-stage differential input for stability. The preamp uses a three-stage DC-coupled equalizer (PNP-NPN-PNP) with an emitter-follower buffer to keep Aux input impedance high. The driver and power transistors are soldered directly to the power-amp PCB, so there is no extra wiring. Hilberink's technical retrospective calls it "compact and slick electronic design," and notes the build quality is very high and reliability excellent. It also warns that there is no serious speaker protection in the circuit.
The front panel is Luxman's house style of the period: white-gold dyed panel and rosewood case. The tone control system has switchable bending points (bass at 150/300/600 Hz, treble at 1.5k/3k/6 kHz) plus defeat. There is also a low-boost switch, high and low filters, and an attenuator. Phono 1 input impedance switches between 30k, 50k, and 100k Ω, and the phono stage accepts up to 300mV at 1 kHz. A and B speaker selectors, a headphone jack, and two tape monitor loops finish off the connections. The main amp is rated at 0.04% THD at 50W into 8Ω, with a damping factor of 40 at 8Ω.
Past the specs, the record gets thin. Sources disagree on production years, with claims ranging from 1969-1974 to 1975-1978, though the August 1971 release date is firm. One seller lists the original price at $900. Known issues come mostly from hobbyist teardowns. The tantalum coupling capacitors in the control amp section can develop DC leakage after decades, and polypropylene units of equal value fix that easily. The low-cut filter can be modified into a subsonic filter, and the main power supply capacitors can be upgraded to 10,000 µF for sturdier low-frequency performance.
For a 1971 Japanese integrated, the SQ-507X is fast and powerful, with good speaker control and low distortion. One restorer called it "A pleasure to listen to."
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