Listening #189: Luxman MQ-88uSE

For this month’s column, I did something I’ve occasionally set out to do but never quite managed: I lived with a new power amplifier for nearly two months, used it to enjoy a variety of records, made scads of listening notes, and wrote most of the subjective portion of my review—all without knowing what was inside it.

From the time the Luxman MQ-88uSE arrived at my house until I sat down to write these opening paragraphs, I knew only the following: It uses tubes for voltage and current gain—the latter accomplished with two KT88 pentodes per channel—but not for power-supply rectification; it’s built on a spartan and decidedly old-school chassis, its vitals protected from people, and vice versa, by a removable tube cage; it was made in a limited edition of 100 units (the SE in the model designation stands for Special Edition; a similar amp, the MQ-88uC, will take its place in the line when all of these are sold); and its retail price is $5995 (USD). Also, because the shoebox-sized MQ-88uSE doesn’t have room for output transformers of greater-than-moderate size, and because single-ended amps usually require much-larger-than-average transformers, it was a safe bet that it was a push-pull design.

I learned most of the above when I first saw the MQ-88uSE at the 2018 Montreal Audio Fest; not until today did I get around to inquiring about its US retail price, which is also $5995 (footnote 1).

Neither the design specifics that I ignored nor my reasons for doing so were trivial. In 33¹/SUB>3 years (footnote 2) of writing about domestic audio, I’ve gathered a great many opinions—some informed, others grounded in nothing more than whim, hunch, or emotion—on the musical and sonic consequences of the various choices one makes in the design and construction of a tubed amplifier: Is it hardwired, or made using circuit boards? What’s the output stage’s class of operation? Is it auto-bias or fixed bias? Does the circuit include feedback, either local or global? Are the tubes run as triodes or pentodes—and if as pentodes, what’s the ratio between the amount of DC on the screen grid and the power-supply rail? Is the power supply built like a fretting hammer or a jackhammer?

An even bigger question, at least for me: Is the circuit designed along the lines of classic audio amplifiers, or does it involve such microprocessor-fueled refinements as sliding bias and switchable power-supply rails?

It was with open-eared ignorance of all of the above that I approached the Luxman MQ-88uSE—which I describe, in the last page of my listening notes, as “one of the nicest-sounding and most engaging amps I’ve ever heard, bar none.” Now for a look inside . . .

Historically grounded
Is it hardwired, or does it have circuit boards? The answer is a resounding Yes: All of the components in the MQ-88uSE’s signal path, and the larger components in its power supply, are hand-wired, point to point—yet the MQ-88uSE’s two solid-state full-wave rectifiers, plus a scattering of connectors and two relatively small capacitors, are mounted on a slender circuit board tucked away near the front of the chassis. Also mounted on small boards are the amp’s left- and right-channel output-bias supplies, each fed by its own 60V secondary from the mains transformer. Both of those boards also contain the coupling caps—labeled “LUXMAN OIL CAPACITOR”—for their respective channels, plus a pair of miniature trim pots for adjusting bias current; the pots are tucked away on the hidden sides of the boards, accessible via small holes in the top of the chassis, just behind the output tubes themselves (footnote 3). From that, and the fact that the tubes’ cathodes are at ground, we know that the MQ-88uSE is a fixed-bias design.

While poking around inside the MQ-88uSE, I took a few simple measurements with my Fluke multimeter. With the amp powered on and warmed up, and with shorting plugs on the inputs and loudspeakers connected to the outputs, I measured 485V DC on the plates of the output tubes and on the tubes’ screen grids; indeed, on all four KT88 output tubes, those two elements were connected together through a 100 ohm resistor. With the same charge on their plate and screen grids, and with the output signal present on the former also appearing on the latter, we know the output tubes are run as triodes.

To know the class of operation of any fixed-bias tube amp requires a reliable means of detecting when that amp goes into clipping, usually determined with the help of a signal generator and an oscilloscope, both of which I lack. That said, given the amount of bias voltage used (footnote 4)—I measured between –55.4 and –56.7V DC on the signal grids of the four KT88 tubes—as well as the MQ-88uSE’s highish plate voltage, I’m comfortable saying that the Luxman runs in class-AB. Connections between the output transformers’ secondary windings and the cathodes of the ECC803 dual-triode input tube that comprises the amp’s first stage—each channel has its own ECC803, the two halves of which are in parallel—indicate the presence of global feedback.

Each channel also has its own ECC802 dual-triode tube, used as a cathode-coupled phase splitter, a means of doing the job that also adds voltage gain—and so the MQ-88uSE is regarded as a true three-stage amplifier. If I understand correctly, this type of phase splitter was first described by the British tube manufacturer Mullard, and used in their famous “5-10” amplifier circuit of 1954, so named because it used five tubes to produce 10W of output power.

As for the power supply, it’s as simple as can be: the aforementioned solid-state rectifiers, three big 600V filter caps aboveboard, one or two smaller ones belowdecks, and a hefty choke—which, like the EI-style mains transformer and Luxman’s limited-edition OY-series push-pull output transformers, is designed and wound in-house. Other parts are equally well pedigreed, including small-signal and output tubes from JJ Electronic, of Slovakia. As Jeff Sigmund, president of Luxman America, told me via e-mail, the tube-selection regimen for the limited-edition MQ-88uSE is referred to by Luxman as their “high confidence” process, meaning that the tubes undergo a more exacting than usual battery of tests and more precise matching to ensure the closest match and the best ultimate specs.

The construction quality is excellent throughout. The chassis is steel painted semigloss black, with a brushed-aluminum top plate and a painted steel bottom cover fitted with four cushioned feet. Inside, the wiring layout was sensible and clean, with no stray strands or shreds of insulation, and the soldering was neatly done. The inside of the chassis is painted, and I saw no stray droplets of solder or resin.

The MQ-88uSE’s only controls are a pushbutton power switch and a level control on the front and, around back, a rotary switch that allows the user to remove that level control from the circuit. Also in back are two pairs of RCA input jacks: one pair affected by the level control, and a Direct pair, which is not. I had to strain to hear the difference between the two: a dulling of the trebles with the control in the circuit, so minuscule I might have imagined it. Specs include a power output of 25Wpc into all loads, 4–16 ohms; an input impedance of 32k ohms; and a signal/noise ratio of 105dB.

Swinging volts
Experience suggests that my DeVore Fidelity Orangutan O/93 loudspeakers love low-powered, push-pull amplifiers—and the Luxman MQ-88uSE seemed awfully fond of the DeVores. Indeed, the 25Wpc Luxman amp had much in common with my 20Wpc true-pentode, zero-global feedback, fixed-bias, class-AB Shindo Laboratory Haut-Brion amp ($12,500). Each had its own distinct voice—put me in the dark and I don’t think I’d have much trouble telling one from the other—yet both coaxed from the DeVores a much higher-than-average level of musical involvement.

Footnote 1: Luxman Corporation, 1-3-1 Shinyokohama, Kouhoku-ku, Yokohama-shi, Kanagawa 222-0033. Japan. Tel: (81) (0)45-470-6980. Web: www.luxman.com. US distributor: Luxman America Inc., 27 Kent Street, Unit 122, Ballston Spa, NY 12020. Tel: (518) 261-6464. Web: www.luxman-global.com.

Footnote 2: My first day on the job at The Abso!ute Sound was Monday, January 7, 1985. I began writing about the Luxman MQ-88uSE in early May 2018.

Footnote 3: The owner’s manual doesn’t mention those pots, let alone instructions on how to adjust the bias of the output tubes. The Luxman amp is shipped—in a rigid double carton with well-designed packing—with its tubes preinstalled, presumably with their bias set at the factory.

Footnote 4: Not to be confused with bias current, of course.

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