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Equipment Review

August 2006

Hyperion Sound Design HT-88 Mono Amplifiers

by Vade Forrester

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Review Summary
Sound "It rocks. Bass extends quite deep, with strong dynamics supplying lots of low-frequency punch." "Music rolled out with strong forward momentum, what the Brits call PRAT." "But the HT-88 sounded less spacious, more like a push-pull amp," "lacking in the 'lit from within' sonics that constitute one of SET’s major attractions."
Features "The HT-88’s class-A design uses two KT88 output tubes per monoblock…. It uses not one but two 5Z4 rectifier tubes to supply a healthy amount of current to the output tubes, making it easier for the amp to drive real-world speakers."
Use "Input sensitivity for rated power output is 2 volts, which is fairly low; your preamp had better have a healthy output. Most do, but a passive preamp will likely not be a good match for the HT-88s." "The HT-88 is eerily quiet…. You have to put your ear almost directly on the speaker cone to even tell the HT-88 is turned on. That’s quiet."
Value "The HT-88s are hardly cheap, but I haven’t heard another amp in the same price and power ranges that sounds as good."

A series of rave reviews on the HPS-938 speakers first alerted me to the existence of Hyperion Sound Design. The HPS-938s look a little like Wilson Audio's WATT/Puppy speakers, but cost one-seventh as much as the current WATT/Puppies. If the Hyperion speakers aren’t in the same league as the Wilsons, their awards clearly show the designers were dedicated to providing good sound at a reasonable price. That’s my kind of company. Because it’s rated at a somewhat high 90dB sensitivity, the HPS-938 doesn’t require a high-power amplifier; Hyperion recommends a minimum of 10 watts. That’s where the subject of this review, Hyperion’s HT-88 mono amplifier, enters the picture.

The $2800-USD-per-pair HT-88 is rated at 18 watts. The HT-88’s class-A design uses two KT88 output tubes per monoblock. Now, you may wonder (as I did) why an amplifier using a pair of KT88s produces only 18 watts; after all, it’s common to find amps using this tube producing 40-50Wpc. But higher-power amps typically use push-pull circuits, while the HT-88 uses the KT88s in an unusual parallel single-ended circuit that, according to Hyperion, "avoids the recombinant errors that arise in push/pull designs."

Even so, other single-ended KT88-based amps produce nearly as much power as the HT-88 from a single KT88 tube. The expensive Audiopax Stereo 88 amplifier, which I reviewed last year, produces 15Wpc from a single KT88, as does the budget-priced Radii MMSKT88 mono amp, which I haven’t heard. But a look at the HT-88’s other tubes sheds some light on its dissimilar design goals: It uses not one but two 5Z4 rectifier tubes to supply a healthy amount of current to the output tubes, making it easier for the amp to drive real-world speakers.

Appearance-wise, the HT-88s reminded me a little of early McIntosh amplifiers, with two black potted transformers and a third silver-capped transformer on the 13 1/2"-square chassis. Actually, one of the potted transformers is really a huge choke, part of the power supply. Perhaps the choke is the reason the HT-88 is eerily quiet -- even on my 102dB-sensitive speakers, which gleefully expose any amplifier noise. You have to put your ear almost directly on the speaker cone to even tell the HT-88 is turned on. That’s quiet.

The all-black amplifier chassis is made of aluminum, even the half-inch side pieces. Although there’s no conventional faceplate, the thick base and side pieces give the impression of a wraparound faceplate. Weighing a substantial 42 pounds each, the HT-88 monoblocks are mirror-imaged. The left channel amp has its gold-plated RCA input jack on the right side of the chassis, while the right-channel amp has its RCA jack on the left side. That means the jacks point toward a centrally located preamp. That’s clever and useful; it means the interconnect cables from the preamp can be a few inches shorter, often a consideration with monoblock amps situated on the floor or on amp stands.

Around back there are binding posts for ground, 4-, 8-, and 16-ohm output. The posts look sturdy but not fancy, with plastic-covered WBT-style knurled knobs. On one of the review samples, the plastic cover of one post cracked and came loose from the metallic section inside, but because I could remove the cover and still achieve a solid connection, I regarded this as a minor cosmetic failure. On the front of the amp -- where it belongs -- is a gold on/off button below a bright-blue LED that tells you when the amp is powered up. A tasteful gold Hyperion badge adorns the center of the amp on the front.

A further indication of the consideration that went into the design can be found underneath the HT-88. It rests on three feet that appear thoughtfully designed to isolate the amp from vibration. How many high-priced components have you seen that use cheap rubber feet like the ones you can buy at Home Depot? The HT-88 isn’t one of them.

Electrically, each HT-88 monoblock uses two 12AU7 input/driver tubes along with the aforementioned two KT88 output tubes and two 5Z4 rectifiers. The 12AU7s are configured in the popular series-regulated push-pull (SRPP) configuration, while the KT88s are wired as triodes instead of tetrodes, which further explains the low power output. Input impedance is 50k ohms, a comfortable load for most preamps. Input sensitivity for rated power output is 2 volts, which is fairly low; your preamp had better have a healthy output. Most do, but a passive preamp will likely not be a good match for the HT-88s.

The HT-88 uses a relatively low plate voltage of 390 volts, and an 80-milliamp bias current; you should expect long tube life. Bias is automatically set for KT88 tubes, so you won’t have to adjust it when you change tubes. Although it’s possible to use other types of tubes, like EL34s or 6550s, you’ll be running them with KT88 bias settings. The stock tubes are Chinese, Shuguangs in the case of the KT88s. It should be possible to replace all of the tubes for one amp for under $100 -- unless you have a thirst for exotic NOS tubes. Your local guitar store probably stocks replacements. Just be sure you use a matched pair of KT88s.

Setting up

I placed the HT-88s on homemade butcher-block amp stands spiked through the carpet to the concrete slab beneath. Each amp was beside and behind my main equipment rack. I used the nondescript power cords that came with the amps for most of my listening, but I also tried some aftermarket power cords to hear if they made a difference. They did, but the stock power cords sounded surprisingly good. There’s no compelling reason to rush out and buy new power cords to get good performance from the HT-88s.

I used my reference Second ReTHM speakers with the HT-88s, although their 102dB sensitivity made most of the HT-88s’ power superfluous. The ReTHMs do just fine with the 6-watt amp I normally use. Fortunately, in house for a review was a pair of Opera Audio’s Consonance Barque M-12 speakers, a two-way design with a lower (but still quite high) 97dB sensitivity rating. These proved to be a superb match for the HT-88s; their wider frequency response (they have 12" woofers) provided a better demonstration of the HT-88’s excellent bass drive. Most of my comments are based on using the HT-88s with the M-12s.

I tried several interconnects between the deHavilland Mercury 2 preamp and the HT-88s. My usual interconnect selections made the HT-88s’ highs and midrange sound closed in, but a set of Silver Circle Audio TimeWise interconnects opened up the sound quite nicely. This provided yet more confirmation that individual pieces of audio equipment operate as a system, not in isolation.

Encouraged by the sound from the TimeWise interconnects, I tried Silver Circle Audio’s DreamCatcher speaker cables, which sonically resemble the TimeWise interconnects. The combination was just slightly on the bright side of neutral, but this was the best balance I achieved. The bass response of the Silver Circle Audio cables was also quite powerful and detailed. Although I no longer had them on hand, I would expect the TARA Labs RSC Vector 1 interconnects and speaker cables that I reviewed late last year, which have a frequency balance similar to the Silver Circle Audio cables, to match well with the HT-88s.

Hyperion sells only through dealers, and expects them to provide all the technical information a customer requires. Thus, there is no manual for the HT-88. Although lots of users won’t read a manual even if provided, it can be very useful if written well. I certainly would have read the HT-88's manual if it had existed.

The sound

It’s easy to describe the HT-88’s distinct sonic signature: It rocks. Bass extends quite deep, with strong dynamics supplying lots of low-frequency punch. These characteristics were evident with both the ReTHM and Opera speakers, although the latter’s deeper bass response better showed off the HT-88’s prowess. Deep bass not only had plenty of power, but also pitch accuracy and detail. Bass spectaculars like the Verdi Requiem (Robert Shaw conducting the Atlanta Symphony Orchestra and Chorus [Telarc CD-80152]) had plenty of sock when the percussionist whacked the legendary Telarc bass drum. And this sock was not just an indistinct thump; I could easily distinguish the drum’s pitch.

Associated Equipment

Loudspeakers –  Second ReTHM, Opera Consonance M-12 Barque.

Power amplifier –  Art Audio PX 25 stereo amp, Kailin AM34-845-QAM mono amps.

Preamplifier – deHavilland Mercury 2.

Digital – Meridian 508.24.

Analog – Linn LP-12 turntable, Graham 2.2 tonearm, Dynavector DRT XV-1 cartridge, Audio Research PH5 phono stage.

Interconnects – Crystal Cable CrystalConnect Piccolo, Purist Audio Design Venustas, Silver Circle Audio TimeWise.

Speaker cables – Crystal Cable CrystalSpeak Micro, Purist Audio Design Venustas, Silver Circle Audio DreamCatcher.

Power cords – Purist Audio Design Venustas, Silver Circle Audio.

Through the HT-88s, music rolled out with strong forward momentum, what the Brits call PRAT (pace, rhythm, and timing). PRAT is often associated with rock, due to this type of music's strong rhythmic component, but I submit that PRAT is not just a rock phenomenon. With the HT-88s, I could hear it in orchestral music, which made performances more coherent and interesting.

For an amplifier using a single-ended-triode output, the HT-88 seemed to be rather lacking in the "lit from within" sonics that constitute one of SET’s major attractions. I have four SET amps in my collection, and all of them have that sonic characteristic in abundance. It’s responsible for the reach-out-and-touch-it sound that makes music through an SET amp sound so palpable. But the HT-88 sounded less spacious, more like a push-pull amp. That’s a description, not a slam. I’ve heard excellent push-pull amps in my system with which I could have happily lived. But for me, the extra palpability of a true SET amp is a significant attraction. It’s a matter of personal preference; you may not care at all about that sort of sound.

The numerous percussion instruments that open Jennifer Warnes’ "The Panther" on the CD layer of her SACD The Well [Cisco SCD 2034] were crystalline, unsmeared, and had very extended high frequencies that fully displayed the instrumental color that makes the song so interesting. The Consonance M-12 speakers produced excellent detail that didn’t sound edgy or overemphasized, just pure and direct.

Orchestral weight is a function of bass extension and bass dynamics. On Keri-Lyn Wilson and the Simón Bolívar Symphony Orchestra’s CD Danzon [Dorian xCD-90254], the orchestra’s performance of Arturo Márquez’s "Danzon No. 2" had a commanding, effortless sweep from very quiet levels to its noisy climaxes. The HT-88s produced no sense of strain in the loudest passages during the plentiful bass-drum parts, even at the elevated volumes I sometimes used in my evaluation. Again, these amps can rock -- even when playing music other than rock.

Single-ended tube amplifiers have a reputation for sounding gorgeous with voices, so I cued up my fave cut from Eva Cassidy’s Songbird [Blix Street Records G2-10045], "Autumn Leaves." The sound was detailed and finely nuanced, and, again, there was no strain at all when Cassidy cut loose on the climaxes. I thought I noted an occasional edge to her voice, which sounded like a narrow-band resonance, but that might have been in the recording. The original CD does sound a little raw.

The HT-88s' soundstaging was very good, but not exemplary. On the piece "Allegri: Miserere," from the Tallis Scholars’ Miserere [Gimmell 454 939-2], the singers were spread realistically across the stage, and the separate solo group in the background was definitely behind the main choir. Some amplifiers (mostly SETs) present more palpable images and greater depth, but the HT-88s are competent at the very least.

On the other hand…

Although it’s only rated at 6Wpc, the $6500 Art Audio PX 25 stereo amp is more dynamic through my ReTHM speakers, has bass and treble extension that equal those of the HT-88s, and offers loads of SET sound (it is an SET, with real triodes). It sounded equally good, if not better, driving the Consonance Barque M-12 speakers to enthusiastic though not head-banging levels. Bass was more highly resolved through the HT-88s, but for a six-watt SET amp, the Art Audio PX-25 does an amazing job driving the Opera speakers’ 12" woofers.

OK, for $6500 the Art Audio amp doggone should sound better. So to try something closer in price and power, I inserted my vintage Kailin AM34-845-QAM monoblocks, freshly tuned up and retubed by restoration specialist William Green at Circle Stereo. Selling for about $2000 per pair when manufactured, the Kailins produce about 23 watts per monoblock from their new Shuguang 845M tubes. And since the 845 tube is a real triode, it is replete with the spacious SET sound -- perhaps more so than any other SET tube.

In spite of its huge transformers, the Kailin amps are a bit shy in the low frequencies. The HT-88s were unquestionably superior to the Kailin amp in the lows, but in the midrange and highs, I preferred the Kailin amps. Hyperion makes a more expensive HT-845, which uses the 845 tube to produce 25 watts per monoblock. That amp would make for an interesting comparison.

When I played Eva Cassidy’s "Autumn Leaves" through the Kailin amps, the edginess noted with the HT-88s was absent. Which was more accurate? Without having the master tapes for reference, I can’t say. Several times during the Kailin amps’ residence in the reference system, instrumental details emerged from the mix that surprised me by how real they sounded. I can’t recall a similar reaction to the HT-88s. I suppose that’s another example of the SET magic.

"Allegri: Miserere" probably revealed the most pronounced difference between the two amps. While the HT-88s produced a tidy, well-organized soundfield that suggested a wide soundstage, the Kailin amps’ soundstage was huge, with both width and depth galore. Voices seemed to have more body. I had a much greater impression of choirs performing in a large church -- precisely how this recording was made.

Call me a sucker for SET sound, but it definitely provides more musical enjoyment -- for me, at least.


The Hyperion Sound Design HT-88 amplifiers offer deep, hard-hitting bass along with open, detailed midrange and high frequencies. Soundstaging is good, and the sound has terrific forward momentum, just like music does. Matched with the right speakers and cables, the HT-88s will produce a very dynamic sonic picture. Their build quality and fit'n'finish are worthy of any high-end system.

At $2800 per pair, the HT-88s are hardly cheap, but I haven’t heard another amp in the same price and power ranges that sounds as good. If your speakers are right for them (Hyperion's own speakers likely are), they rock out like few low-power amps can.

...Vade Forrester

Hyperion Sound Design HT-88 Mono Amplifiers
$2800 per pair.
Two years parts and labor, 90 days on tubes.

Hyperion Sound Design, Inc. (California office)
1305 John Reed Court
City of Industry, CA 91745
Telephone: (626) 968-1022
Fax: (626) 968-1136

E-mail: sales@hyperionsound.com
Website: www.hyperionsound.com

Hyperion Sound Design, Inc. (New Jersey office)
436 North Harrison Street
Princeton, NJ 08540
Telephone: (646) 262-7027

E-mail: awu@hyperionsound.com

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