Luxman 5M20 (1977)

A hundred watts of DC-coupled Luxman power in a chassis that weighs as much as a small child, and it doesn't even give you the VU meters.

Overview

The 5M20 is a stereo power amplifier, released in March 1977 as part of Luxman's Laboratory Reference Series. The original Japanese price was ¥210,000. That tells you where it sat in the line - this was not an entry-level piece. Contemporary hi-fi press such as HiFi-Stereo (September 1977, p.57) was covering the integrated amplifier market, where separate power supplies for output and preamp sections were becoming a selling point. The 5M20's channel-independent supply arrangement shares that philosophy. Luxman's own literature describes the 5M20 as constituting the core of the Laboratory Reference Series, and the model is essentially the 5M21 with the VU meters deleted. Same amplifier, cleaner faceplate, no meters. Collectors have opinions about which looks better. The plainer front is either the purist's choice or the one that looks like something is missing.

The headline number is 100 watts per channel into 8 ohms, both channels driven, 20 Hz to 20 kHz. Luxman rated it to FTC rules, which is the honest way to state output power, unlike the peak-music numbers that cluttered so many seventies spec sheets. Distortion is 0.005% or less at full rated output across the full audio band, with cross-modulation distortion quoted at the same 0.005%. Frequency response runs from DC to 100 kHz at −1 dB, and output bandwidth is listed as 5 Hz to 100 kHz at −3 dB. Those are not numbers you see on a typical 1977 power amp.

The topology behind those figures is the interesting part. The 5M20 runs a DC amplifier configuration, which Luxman says eliminates phase distortion in the low range and transient distortion in the high range. That is the usual argument for DC coupling. The output stage is a parallel push-pull arrangement using four large output transistors, operating in class AB with collector output capacitance minimized as far as the design allowed. The driver stage uses high-switching-speed transistors alongside Schottky diodes in a cascode connection, and an emitter-follower circuit with constant-current drive feeds the output. The pre-driver stage is built around Luxman's DML-01, a "Dual Monolithical Linear-Integrated Circuit". It is Luxman's own hybrid IC, containing a FET differential amplifier, a supplementary cascode circuit, a current mirror, and a constant-current circuit, all enclosed in blocks. The DML-IC exists to deal with DC drift from temperature changes, which is the classic problem with DC-coupled designs and the reason so many of them misbehave when they get warm.

The power supply is where the weight comes from. Two large toroidal transformers, described by Luxman as high-thermal-efficiency with minimal leakage, feed four custom-made ultra-low-impedance large-capacity electrolytic capacitors, with the left and right channels on independent supplies. The toroidal choice is about minimizing flux leakage and mechanical hum, so the amp doesn't buzz and doesn't spray hum into sensitive nearby gear. The service manual is blunt about the consequences of this design. The 5M20 amplifies DC, so any DC offset fed to the input gets amplified along with the music. The manual warns technicians to set the rear-panel input capacitor switch to "IN" before doing anything, because old test equipment with a bit of DC on its output can damage the amplifier. It also notes that the giant electrolytics in the "B" supply take a long time to discharge, and that anyone disconnecting a board should do it after a proper discharge through a resistor or bulb. This gear does not forgive carelessness on the bench.

The protection circuitry is more thorough than most. A DC drift detection circuit trips the speaker relay when roughly ±3V DC appears at the output. A heat-sink over-temperature circuit protects when the sink exceeds 90°C. An eddy-current prevention circuit short-circuits the output terminal if excess current flows into the power transistors. There's also a DC offset sensor, and the front panel carries a headphone jack. The rear panel offers a peak indicator connection terminal, an unswitched AC outlet rated at 500W, and inputs with an input capacitor IN/OUT switch. An independent left/right input attenuator lets you trim each channel separately, a useful feature on a power amp and one that most of the competition didn't bother with.

The measured performance is where the 5M20 earns its keep. The service manual's alignment procedure calls for distortion no higher than 0.005% at 1 kHz with 28.3V into an 8-ohm load, and no more than 0.008% at 20 Hz and 20 kHz at the same level. Crosstalk is specified at −70 dB or better at 10 kHz. The signal-to-noise ratio is quoted at 120 dB or more on the IHF-A curve with the input shorted, and residual noise at 0.1 mV or less — numbers that put it in genuinely quiet territory. Damping factor is 80 or more at 8 ohms and 1 kHz, which is respectable rather than spectacular; it will control most speakers without drama but isn't the last word in grip on difficult loads.

The heat sink is horizontally stacked, and the speaker terminals are designed to minimize contact resistance. The chassis uses an interlocking leg-and-bracket arrangement with metal supports so the units stack securely without sliding around. The metal-film semi-fixed trim pots are chosen for temperature stability and low noise, and the capacitors include silver mica types. Dimensions are 442 × 146 × 400 mm, and the weight is 18.8 kg. That is a lot of metal and copper for a hundred-watt amp, and a good indication of how much of the cost went into the power supply and heatsinking rather than the front panel.

Some third-party spec aggregators list different numbers for this amp: 13.5 kg and 110 dB S/N on one site, 9.8 kg on another. Those figures conflict with Luxman's own documentation and with the service manual, and the 9.8 kg figure in particular looks like a transcription error rather than a real measurement. The service manual gives 18.8 kg net weight, and that is the one to trust.

Collectibility & Value

The 5M20 sits in an interesting spot in the used market. It's a Laboratory Reference Series piece, which gives it cachet among Luxman collectors, but it's also the meterless sibling, so it doesn't get the same attention as the 5M21. Recent asking prices have ranged from around $750 for a unit described as the same as the 5M21, to US$929 including tax from a Japanese seller. Those are asking prices, not sold prices, and the spread suggests the market hasn't settled on a firm number. HifiShark tracks used listings, but no reliable average has emerged.

The good news for buyers is that the 5M20 appears to be a sturdy design with no documented common failure modes. The bad news is that it's now approaching fifty years old, and the four large electrolytic capacitors in the power supply, however custom and however low-impedance they were in 1977, are well past their expected service life. Capacitor restoration kits specifically for the 5M20 turn up on eBay, which tells you two things: recap work is a known requirement on these amps, and the parts are findable. Anyone buying one should assume a recap is either due or already done, and should ask which.

The DC-coupled design is the other thing to keep in mind. A DC amplifier with a fault can put DC on your speakers, which is why the protection circuits exist and why they should be tested rather than assumed. The service manual's alignment procedure is detailed and specific: idling current set to 150 mA, then re-checked at 140 mA five minutes later, driver board output trimmed to 0V DC, protection circuits verified. Anyone servicing one should follow it rather than improvising. The manual is available, which is more than can be said for a lot of seventies gear.

Owner reports are thin but positive. One Audiokarma member described the 5M20 as sounding "very good" and "well-built," adding that "these little Luxmans are very underrated." Another owner noted a single scratch on top but otherwise mint condition. No negative owner opinions have surfaced in the documented record.

For anyone assembling a Laboratory Reference Series stack, the 5M20 is the natural power amp choice. It pairs with the 5C50 preamp, and the series also includes the 5M21, 5F70, 5T10, 5T50, 5G12, and the integrated 5L15. The 5C50, 5M20/5M21, and 5L15 all have special outputs for connecting a peak level indicator device. Whether the 5M20 or the 5M21 is the better buy comes down to whether you want meters on the front panel. Electrically they're the same amplifier.