Designed and produced before the Alpine "takeover / collaboration" and just before the DAD revolution, the L-550 did receive a "State of the Art" award in 1982 besides being selected as "Best Product" by Stereo Sound.
Most of the direct followers also became Stereo Sound "Best Product" : L-550X, L-560 and the successful L-540 and L-570.
All are versions of the same basic circuit : Single-Stage amplification, Duo-ßeta, plus-X, Circuit /S and Class A all the way, real hot Class A.
All did also sport something which was not to be discarded once the fad faded : double Heat-Pipes !
Feeding all is a big 400VA EI transformer and two low-ESR 50V / 30,000µF 99,99% pure aluminium chemical caps.
Six windings feed separately the pre-driver, main Duo-ß, phono MM EQ amp, phono MM EQ Duo-ß, phono MC and protection / regulation / muting / pre-heating stages.
The power amp is fed separately from the secondary.
The necessary Heat-Pipes have vents top and bottom of the enclosure : 2x 50W of pure Class A produces heat, especially when idling current is to be set at 160mV for each channel !
The pre-Heating switch is used to get the L-550 reach its cruising speed while the power-on button is off ; current is cut from the final transistors but heats-up the rest of the circuit.
As in all contemporary Luxmans, there is no second stage so the 1st stage uses a high Gm low-noise FET (for the MC input) with a 7mA current for the necessary gain.
If Duo-ßeta is used as usual in the power-supply, in the L-550, it also was implemented in the EQ phono section - that's Duo-ßeta Circuit /S.
Line level input reach the circuit where the "phono straight" mode starts, the latter bypassing the subsonic, high-cut, monitor and source selectors.
Unlike other contemporary LUX amps, the 9-turnover NF tone controls are not placed withing the power amp's main Duo-ßeta loop but the Linear Equalizer which can subtly tilt RIAA frequency response is.
The attenuator is a 2-gang 50kOhm pot sourced from, as always and for nearly everybody, ALPS, aka Alpine :)
Other features are... plenty :
Line-Phase sensor, front supplementray phono input, tape i/o and headphones output, three-way tape monitoring, 4-way tape dubbing, 6-position phono setting and warm-up indicator.
Plus pre/main i/o, all engraved front scripting and rosewood enclosure.
As for all of the L-550 series, the smaller models are, sonically, energetic bolids and the bigger models more relaxed providers : L-530 vs. L-550, L-530X vs. L-550X, L-535 vs. L-560 and L-540 vs. L-570.