Hyperion HT-88 monoblock power amplifier By Art Dudley   •   January, 2009 Remarkably, I set out to audition the Hyperion HT-88 amplifier ($2800/pair) over two years ago, only to be confounded by shipping errors, miscommunications, and, in the end, a stealthily defective tube. I almost gave up.

But in audio as in the I Ching, persistence pays: Happily, a pair of the Chinese-made Hyperion amplifiers has now performed glitchlessly in my home for weeks, where they've transcended their comparatively modest price in terms of not only sound but of musical character—a quality I associate with rare vintage amps and the best of today's artisanal products, and not the sorts of amplifiers that are built by the indifferent.

Chinese manufacturers of high-end audio gear are all too often cast in a light of opportunism, as if the ability to make things cheaply is their only reason for being. But I've come to realize that the people behind Hyperion—as those behind the Cayin line and any number of other very good Chinese brands—are, first and last, hi-fi enthusiasts. No matter where, no matter the cost, I believe that Hyperion's Albert Wu would have found a way to bring the HT-88 to market, and to an appreciative audience.

Description
The HT-88 is a monophonic amplifier, built and sold in mirror-imaged pairs. The left-channel amp's input socket is on its right-hand side, the right-channel amp's on its left; this permits shorter, simpler cable runs in systems where source and preamp are midway between the speakers. And while the symmetrical arrangement of the other parts serves no practical purpose, the cosmetic advantage is obvious. The Hyperion's low-slung chassis is fashioned from individual sheets of black-anodized aluminum, and its top surface plays host to six vacuum tubes: two 5Z4 half-wave rectifiers, two 12AU7 dual-triodes, and two KT-88 output pentodes. Completing the skyline are two reservoir capacitors and three transformers, the latter comprising a generously sized AC trannie with a chrome cap and two nicely painted "cans" for the power-supply choke and output transformer. The last has secondary taps for loads of 4, 8, and 16 ohms, with associated EU-spec output connectors on the rear panel, also symmetrically arranged.

The Hyperion HT-88 isn't at all big, but each one tips the scale at a whopping 42 lbs. Much of that weight is accounted for by the amp's massive output transformer: The HT-88 is, in fact, a single-ended amplifier that requires a good deal more iron than would a push-pull amp using the same output tubes. I admit to having been ignorant of the Hyperion's single-ended architecture when I agreed to write about it—and thus was surprised when I tried to lift one of the review samples out of its shipping carton.

Yet, strictly speaking, the HT-88 isn't a single-ended triode (SET) amp: The KT-88 is a thoroughly modern (defined in my worldview as post–Ottoman Empire) device, with a cathode and a heater that are electrically separate from each other, and a total of three grids. The adventurous designer is free to ignore the KT-88's two extra grids, however, in which case the tube can be operated as a triode, if not a directly heated one—which is precisely what Hyperion has done. In this circuit, the plates of two parallel KT-88s are tied to the rail through the primary winding of that weighty, custom-made output transformer, and their grids are fed the full-wave input signal from an SRPP voltage amp comprising the two 12AU7 tubes. Output power is specified as 18W (compare that with the 30W or so available from a pair of KT-88s in a class-A push-pull circuit), and negative feedback is applied to the first cathode of the input section from the 8-ohm output tap.

The HT-88's output section is an autobias design: Chunky 10W resistors lift the cathodes of the output tubes above ground, eliminating the need for a negative DC voltage supply for the signal grids. Also, by choosing indirectly heated pentodes as power tubes, Hyperion has sidestepped the need for an additional stage of regulation and rectification. (Although opinions vary among SET designers—especially those who work with such low-filament-voltage tubes as the 2A3 and 45 output triodes—unrectified filament voltage on directly heated output tubes can create unpleasant levels of 60Hz hum.) The result is an amplifier with a distinctly low parts count of only 13 capacitors and 18 resistors per channel, and a complete absence of solid-state parts.

Build quality is good overall, with some aspects faring better than others. Point-to-point wiring was neatly done, and the smaller parts were cleverly laid out and well secured. On the other hand, a couple of the heavier-duty solder joins looked gloppy and tentative, and despite the presence of lock washers, many of the largest bolts inside the amps were sufficiently loose that the transformers were free to shift around, howsoever slightly. And the simple act of shipping the HT-88s from here to Brooklyn so that John Atkinson could measure them and back caused some of the machine screws holding the bottom plates in place on both amps to come completely undone. (Those lacked lock washers entirely.) On the other hand, cartons and packing materials were exceptionally well thought out: Sturdy inner sleeves with built-in nylon straps made it easy to remove each amp from its outer carton, and the tubes were well protected with custom-made Plexiglas partitions. (On yet another hand, one of those partitions had some cracks after a number of trips, though it continued to serve its purpose.)

Installation and setup
As noted above, my first attempt to audition the Hyperion HT-88s ended in disappointment. I began by using them to drive my Quad ESL loudspeakers in my 12' by 19' listening room, in place of my then-reference Lamm ML 2.1 amplifiers. About 30 minutes after I'd switched on the Hyperions, an intensely loud 60Hz hum overpowered the music in one channel. Fearing the worst for my then-newly-restored Quads as much for my hearing, I all but dove to disconnect the speaker cable.

And that was a darn shame: Just before the hum forced me to shut things down, the music making was wonderful—so wonderful that I took the unusual step of writing notes based on my first few minutes of listening. (I scrawled an exclamation point after the word texture with sufficient force to leave a mark on the next sheet of paper in the pad.) While the amp cooled down, I schlepped into the room another pair of speakers—something ostensibly less fragile, I suppose, although I forget just what it was—and retraced the same steps. With the same results.

I draw Mark Twain's curtain of charity across the scenes that followed. The important bits: The offending part was discovered, the amp was replaced, apologies were made, and I eventually overcame my initial reluctance and invited the Hyperion amps back into my home.