Luxman L-550 (1981)
The L-550 is the integrated amplifier that started one of Luxman's longest-running lines. Introduced in 1981 at ¥250,000, it was a pure Class A design at a time when most solid-state amps ran Class AB. Luxman billed it as the first pure Class A integrated amplifier. By the mid-1980s, period classifieds were already catering to U.S. audiophiles seeking out top-rated Japanese audio equipment, a sign of the growing overseas demand for components like this one (Audio, March 1984, p.102). It delivered 50W per channel into 8Ω, with total harmonic distortion rated at no more than 0.005% and a frequency characteristic of 10 Hz to 100 kHz within −1 dB on the line inputs. The phono stage covered both moving magnet (1.6 mV) and moving coil (80 µV) cartridges, with selectable input impedances for each - three settings on the MM side and three on the MC side.
Most of Luxman's engineering here went into keeping a Class A amp quiet and stable. The circuit is built around Luxman's DUO β Circuit / S, which pairs a controlled amount of negative feedback with a DC servo and a single-stage voltage amplifier. The second stage is eliminated entirely, and the output stage is fully independent so the first stage isn't loaded down. The MC head amplifier uses a high-gm, low-noise FET biased at 7 mA per device. Continuous Class A operation generates serious heat, and Luxman handled it with two large symmetrical heat pipes to minimize thermal radiation noise. The power supply is a 400 VA EI-core transformer feeding two 30,000 µF electrolytic capacitors made with 99.99% pure aluminum. There's a pre-heating function that warms the circuit without switching on the output transistors, a warm-up indicator, and a line phase sensor that detects AC polarity.
The front panel is unusually flexible for an integrated of this era. It has nine turnover frequencies for both bass and treble (±9 dB maximum), a linear equalizer with up-tilt and down-tilt in 1 dB steps, and a phono-straight switch that bypasses the tone controls and filters. There are also subsonic and high-cut filters, three tape loops with four-way dubbing, and a headphone jack. Dimensions are 453 × 174 × 460 mm. Weight is reported as either 18 kg or 21.4 kg depending on the source. The manual and most references lean toward 18 kg, so treat the higher figure with caution. Power consumption is 310W.
Class A amps run hot, and the power supply capacitors in these are now over forty years old. Units that have had a proper overhaul are the ones to want.
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