Luxman 5M21

Name: Luxman 5M21
Brand: Luxman

A 100-watt DC-coupled amplifier from Luxman's Laboratory Reference Series. It amplifies anything, including the DC you didn't mean to send it.

Overview

The 5M21 is a stereo power amplifier rated at 100 watts minimum continuous per channel into 8-ohm loads, both channels driven, across the full 10 Hz to 20,000 Hz band, with total harmonic distortion no higher than 0.008%. Those are the manufacturer's numbers. The amp came out when Luxman launched the Laboratory Reference Series, which the company advertised heavily in the American hi-fi press during 1977. Audio (June 1977, p.71) ran the announcement of the new LRS line, and High Fidelity (May 1977, p.125) carried the same campaign, positioning the series around "resolving elusive problems of distortion with innovative new techniques." The 5M21 was one of the amplifiers that campaign was selling.

The 5M21 is a DC amplifier, and that sets it apart. Luxman's own service documentation is explicit that this is not a conventional AC amplifier, and that the usual alignment procedures do not apply. The frequency response is quoted as DC to 100,000 Hz at -1dB, which means the amp passes direct current, not just audio. Most consumer power amps of the period were not built this way, and it has real consequences for anyone who owns one. The amp will amplify a DC offset riding on an input signal, and it will push that offset into the output stage until something gives. Luxman built a rear-panel Input Capacitor switch for this reason, and the manual tells technicians to set it to the "IN" position before powering up with unfamiliar sources or test equipment.

The rest of the measured performance is just as serious. Signal-to-noise is quoted at 120dB on the IHF A-curve with the input short-circuited. Residual noise is no more than 0.1mV. Crosstalk is no more than -100dB at 100 Hz and no more than -70dB across 20 Hz to 20 kHz. Damping factor is 80 into 8 ohms at 1 kHz. Input sensitivity is 890mV into a 50k-ohm load. Power consumption is 450W at 8 ohms at maximum output, and the amplifier weighs 19 kg net, about 41.8 lbs. Dimensions are 442mm wide, 400mm deep, and 146mm tall.

Key Features

DC-Coupled Output Stage with Switchable Input Capacitor

The defining feature of the 5M21 is that it amplifies DC. Luxman's service manual warns that even slight DC ingredients from old test equipment or other DC-coupled sources will be amplified enough to damage the amplifier, which is why the rear panel carries an Input Capacitor IN/OUT switch. Leave it "IN" and the amp behaves like a conventional AC-coupled design, blocking DC at the input. Switch it "OUT" and the amp passes everything, DC included — useful for certain lab applications, dangerous for casual use. The manual also notes that the final stage runs in class AB, not the pure class B of Luxman's earlier designs, and that idling current should be checked immediately after power-on to protect the output transistors from excessive current.

DML-IC Pre-Driver Stage

The pre-driver uses Luxman's own FET hybrid integrated circuit, which the company called a Dual Monolithic Linear IC. It packages a differential stage, a constant-current circuit, and a cascode circuit into one device. The service manual lists two DML-ICs in the 5M21, along with 49 transistors and 43 diodes. Anyone servicing one should check that the IC pins are correctly seated before applying mains power. A bad connection there is a fast way to a dead amplifier.

Protection Circuitry

Four separate protection systems are documented: a DC offset sensor, a DC-drift sensing circuit, an abnormal temperature sensing circuit, and excessive current protection. The DC-drift circuit is designed to trip the speaker relay when output drifts into dangerous territory — the manual describes it operating in the vicinity of 22 to 29V output with a 5 Hz sine wave applied. The DC offset test is even more specific: the pilot lamp turns from blue to red when a DC voltage of 20mV plus or minus 5mV is applied to the input, or when 500mV plus or minus 200mV appears at the output. The muting relay is specified to break when DC of 3 to 5V appears at the output. That is a lot of layered protection for a 1970s power amp, and it reflects just how much the DC-coupled design needed it.

VU Meters with Sensitivity Selector

The 5M21 carries VU meters for the left and right channels, with a Meter Sensitivity Selector switch offering 0dB and -10dB positions. Calibration is done with a 1 kHz sine wave, and the meter reads 0dB at 28.3V output. For the -10dB sensitivity setting, the reference voltage drops to 8.9V. Frequency response of the meter circuit itself is specified within -1dB from 10 Hz to 10 kHz, with mutual channel deviation within 0.5dB. There are also output terminals for an external Peak Indicator, taken from the speaker terminals through a 1k-ohm resistor. Luxman's manual warns bluntly about those terminals: they are not inputs, and shorting them can burn out the resistor or damage the amplifier.

Attenuator and Headphone Switching

Both channels have an audio attenuator, and the amp has a headphone jack that automatically switches signal away from the speaker terminals when a plug is inserted. The rear panel also carries an unswitched AC outlet, common for the era, handy for powering a tuner or preamp without tying up another wall socket.

Historical Context

The 5M21 belongs to Luxman's Laboratory Reference Series, which the company was promoting in the American press by mid-1977. The LRS launch campaign ran in Audio, High Fidelity, and HiFi-Stereo that spring and summer. The 5T50 frequency-synthesized digital FM tuner was featured alongside the amplifiers in the series advertising. Luxman's U.S. distribution was handled by Lux Audio of America, Ltd., based in Plainview, New York. The address and phone number appear on the service documentation.

The 5M20 is the companion model. It shares the same 100-watt rating, the same 0.008% THD figure, the same DC-to-100,000 Hz response, and the same input sensitivity and impedance. The differences are in the details. The 5M20 omits the VU meters and the meter sensitivity selector, and its protection circuit layout differs. The service manual notes that on the 5M20, the DC Offset Relay stands in for the meter relay, and only the pilot lamp lights up rather than the full stand-by blink sequence. The 5M20 also lists 45 transistors and 31 diodes versus the 5M21's 49 and 43, and weighs slightly less at 18.8 kg net.

Collectibility & Value

For collectors, the 5M21 is a genuine Laboratory Reference Series component with real documentation behind it. It is also a power amplifier rather than an integrated or a receiver, which narrows the buyer pool. The DC-coupled design is both the selling point and the risk. Anyone buying one should confirm the Input Capacitor switch is present and functional, and should test the protection circuits before connecting speakers worth caring about. The service manual's warning about the output transistors is not theoretical. Applying mains power with imperfect solder joints or wiring will blow them, and the giant electrolytic capacitors in the "B" power supply hold a charge long enough to be dangerous during service.

Values for LRS amplifiers have firmed up as the series has gained recognition, but the 5M21 is not the most sought-after piece in the lineup. That distinction tends to fall to the tuners and preamps. Expect to pay a premium for units with original documentation and confirmed-working protection circuits. Budget for a technician who understands DC-coupled designs, because a generalist shop may not.