Luxman SQ503X (1971)
The Luxman SQ503X is a solid-state integrated amplifier released in October 1971 at ¥51,000. Luxman positioned it as the more economical sibling of the SQ505X - a model "with emphasis on practicability and without waste," as the period literature put it. The specs fit that. It is a straightforward, well-built 20W + 20W (8Ω, both channels driven) amplifier with a clean circuit topology, not a flagship loaded with extras.
The inside is where the SQ503X gets interesting for its era. The main amplifier uses a ±2 power supply system with all stages directly coupled and no output capacitor. A complementary PNP/NPN output stage eliminates crossover distortion. The first stage is a constant-current differential pair using low-noise PNP transistors for DC stability. The phono equalizer is a two-stage direct-coupled design. Luxman also used tantalum and Mylar coupling capacitors with low leakage current between stages, specifically to avoid the shock noise you'd otherwise hear through speakers when flipping switches. The tone control is Luxman's own NF-type circuit, with left/right channels independent and switchable turnover points (200/400 Hz low, 2.5/5 kHz high). A front-panel attenuator drops gain by 18 dB without touching the volume knob. There are two speaker outputs plus a tape monitor with DIN connector. The front panel is hairline-finished aluminum with white gold anodizing, and it wears well.
Full specifications, per the Japanese catalog data:
- Output: 20W + 20W (8Ω, both channels); 25W per channel single-channel
- THD: 0.1% or less (8Ω, 20W); IM distortion: 0.1% or less
- Frequency response: 10 Hz–80 kHz (−1 dB); output bandwidth 10 Hz–50 kHz (−3 dB)
- Damping factor: 38 (8Ω), 76 (16Ω)
- Phono inputs: 2 (MM), 2 mV sensitivity, 50kΩ impedance, S/N 60 dB
- Aux inputs: 3, 95 mV sensitivity, S/N 70 dB
- Tone control: NF type, bass turnover 200/400 Hz, treble 2.5/5 kHz
- Dimensions: 450 × 160 × 268 mm; weight: 10 kg (a used listing reports 9800g)
- Power consumption: 100W at maximum output
Documentation on this model is thin beyond the spec sheets. The hilberink.nl Luxman resource lists the SQ503X with "restore & repair info," and service manual PDFs circulate online. A diyAudio thread shows an owner working through DC offset and bias adjustment, so this is a serviceable amp for a competent tech. On the used market, asking prices have ranged from around ¥62,900 at a Japanese auction to roughly US$759 from a dealer (note: many Japanese-market units are 100V and need a step-down transformer in North America). One Audiogon seller called it a "jewel of an integrated," and a YouTube commenter reported "good matching" with NHT SuperOne speakers. That's anecdotal, but positive.
The specs conflict on power. HiFi Engine lists 30W per channel, while the Japanese catalog data says 20W + 20W both-channel operation (25W single-channel). The Japanese source is the manufacturer's own published spec, so the 20W figure is the more credible one.
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