March 2009

ASL HB1 Hybrid Tube Headphone Amplifier
Large miniature with big sound.
Review By A. Colin Flood
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  I am a big fan of Antique Sound Labs (ASL) since their $99 Wave 8 Monoblock Amplifiers (now $375 Wave AV-25 DTs)blew me away in March 2002. Their Remarkable $1,095 AQ1003 DT Integrated Tube Engine left me no less impressed. Ever since I got my winged Audio-Technica ATH-A700 cans, I have been wondering about using a digital-to-analog converter (DAC) and/or headphone amplifier at work. This is my third review in a series of four, maybe more. Only the new $200 Trend Audio PA-10 hybrid amplifier remains. For $299, the ASL HB1 headphone amplifier is a hybrid design with an EL84 single-end triode tube input/gain stage and a solid-state output stage. It consists of two pieces: a black front-end unit and matching power supply (PS).

On tube amplifiers, I like to see transformers as big, or bigger, than the tubes themselves. I think this indicates good bass control. See the majestic Delta beast for an example of this. Tash Goka of ASL says tubes generally run on much higher voltage, regardless of the power the unit is designed to produce, than solid-state devices. He says the transformers have to be large to supply high voltage levels. This is the main reason for all tube amplifiers to be heavier. The two pieces of the HB1 amplifier weigh six pounds. The PS consists of the solid-state power transformer, filters, regulators and rectifiers. There are no output transformers. Here in the land of headphones, everything is smaller. Yet stacking the HB1 front-end unit on its PS makes the pair look like a regular amplifier, except half the size. If the DAC1 USB is a sleek, powerful Vette and the HeadRoom Micro combo are chunky and punchy Mustangs, then the HB1 is a semi-truck.

Bigger than the wall warts of printers and computers, the black PS fills the hand of Goliath. The PS weighs half a brick and is as large as two of the Headroom power supplies combined.

The power switch is on the backside of the PS, which in my case, means it was located inconveniently down under my desk. When switched on, there is a bright blue LED. The tubes take a few minutes to warm up to their best sound, so power up the HB1 first, then the PC, before going for your coffee. The warm-up period makes the amplifier slow for quick Windows use. The HB1 has no auto-stand by feature, so Windows use and the awkward switch makes it easy to leave the unit on not just all day, but all the time.

The front unit fits smartly on the PS. But at almost 9-inches tall, the units together look like overkill for the minuscule 15mW output required to drive my headphone's 2-inch drivers. Like swatting a ping ball with a tennis racquet, the combo seems like driving horns that need three watts with a monster Pass amplifier. The front unit is as tall and as wide as my palm. It is twice as large as the stack of Headroom Micros. It could not snuggle in nicely beside my monitor. A square black cage, big enough to carry a mouse, covers the tubes. The two tubes stick out the top of the front-end unit. The tube cage makes a perfect resting place for the tufted black earmuffs of my ATH-A700 cans. The cage comes stuffed with Styrofoam to protect the tubes inside. Tubes and their matching sockets are marked. The cage requires a Philips driver to unscrew, but ASL does not include a screwdriver. Shipping takes two to three days.

Inside the cage, thumb-size tubes come double-packed in Styrofoam and bubble wrap. The tape requires scissors. ASL labels both the Sovtek EL84 tubes and the HB1 sockets. There is no bias adjustment; nothing to reset aging tubes that drift off setting. There are no other controls except a smooth black knob the size of a thumb tip. There are only two RCA inputs on the rear and only one output, the standard size headphone jack, on the front.

Input impedance, he says, does not make a large difference "due to the use of solid-state" stage. "Any line level components," Goka says, "such as CD players, D/A converters, preamplifiers and computers with sound card that has RCA outputs" can plug directly into the HB1. No wires are included, but HeadRoom sent a passel of connections, so I plugged the HB1 directly into the back of my Dell PC; not to the USB port, but to the mini headphone jack.

Quality-wise, the only clue that the HB1 is an Asian product is its low price and awkward manual. As a professional Technical Writer, I wish all importers would rewrite their manuals for the U.S. market. The ASL manual is eight half-size pages with specs, graphics and schematics. It does the job. The manual is not as charming as the HeadRoom, nor as thorough as Benchmark ones, but then I didn't need to read it either. Otherwise nothing indicates that this isn't another quality Asian car…made in America .

To break-in the HB1 for best sound, Director Tash Goka recommends "about 100 hours." He says the SOVTEK EL84s are "good, easily available and reasonably priced" tubes. In the HB1, they "operate at a much lower power range, since they are normally used as power tubes in power amplifiers. The useful life of the tubes therefore, should be "much longer in this design as they operate at very little power than if they were used as power tubes in other amplifiers." Goka says they should last "several years, but tubes can fail for reasons other than regular wear and tear."

Jimmie Two
I listened to Robert Plant's Raising Sand and my Reference disks on all the amplifiers. First impression of the HB1, on Diana Krall's wonderful Stepping Out, is that even on headphones, tubes have the rich smooth flavor of Hagen-Daz ice cream. Not so with the bass. With the HeadRoom Micro combo, the slow, growling draw of the bow across the cello strings on "Jimmie" is deliciously sensuous and decadently enticing. Not so with the HB1. Bass was warm and a mite bloomy, as might be expected with tubes. Yet it didn't trick me into thinking there was a subwoofer under the desk.

The all important mid-range vocals were instantly delectable. There is a "naturalness" to the midrange that is convincing, detailed, coherent and physically present. Voices appear with tumbrel and textural reality. As wonderful as the mid-range was, the bass simply did not have the energy and punch to make the deepest notes come alive. It does not have the awesome depth of the HeadRoom Micros and Benchmark DAC1 USB. The HB1 knob has no indentation or red dot to indicate position, but somewhere about noon on the dial, it is just beginning to open up. Only at 3:00, ¾ power or more, did it actually begin to be as LOUD as the other two amplifiers.

Jimmie One
"Whole Lotta Love" on the classic Led Zeppelin II did not have the immediate grab that the HeadRoom Micros did, but a little more twist on the dial and it really rocked. I often cranked the dial halfway to get more punch and definition out of the music. Then the textures of the music seem to stand out. The guitars rasp like a heavy smoker. The treble does not have that hard solid-state edge to it. On the high-end, the silk of sensuality replaces the clinical accuracy of the Benchmark DAC1 USB.

Like most tube amplifiers, the HB1 capabilities are more like a smooth Omega single driver, than the wide competency of a three-way speaker. Meaning that there is a lushness - usually in the middle range, but often in the treble too — with a textural quality that very few solid-state amplifiers can reproduce without ending up sounding flat and dead. The DAC1 USB has this lack of emotional involvement. While Dick Olsher found it "free from tonal balance distortions" and "digital heart burn" in April 2005, I thought its "neutrality" can be mistaken for lack of emotional drive.

Little Boxes
This is the third headphone DAC or amplifier I reviewed for Enjoy the Music.com. I am a low grader, awarding four Blue Notes only for categories where the component shines with silver medal performance, better than the other components reviewed. Gold medals (five Blue Notes) go for excellence that is the best it could possibly be.

None of the headphone amplifiers reviewed so far however are clearly standout bests in my own humble opinion. This review was not like the Silver Shoot-Out interconnect cables, for example, where blind testing spotted the gold DACT cables every time. Instead, it is more like a Greek tragedy. Choosing a headphone amplifier can be a matter of difficult choices and compromises; with money, size and sound all playing equal parts. With a hybrid tube amplifier, you get an awful lot for a whole lot less, but you don't get everything right.

Each amplifier has its own sound. I could award each of them four Blue Notes for their enticing Tonality. Despite its obvious charm in the mid-range, the ASL HB1 is not highly accurate. It is colored, but that grabs your attention. Yet there are more things right with this unit than wrong. I give the HB1 four Blue Notes for Tonality, Mid-range and my own personal category, Enjoyment. That doesn't mean that is quite as capable as the DAC1 USB, or that it is in the same league as the imminently listenable HeadRoom combo either.

Specifications
Type: Vacuum tube headphone amplifier
Output Power (feedback): 15mW on 32 Ohm Headphone
Frequency Response: 15 Hz to 100 kHz ( -1dB)
Distortion: <1%
S/N Ratio: 78dB
Input Impedance: 100K
Input sensitive: 1V
Warranty: Limited five-year parts and one-year parts and labor warranty 
               90-day replacement warranty on tubes
No warranties extended to modified, mishandled or misapplied products

Company Information
Divergent Technologies Inc. 
342 Frederick Street
Kitchener, Ontario
N2H 2N9
Canada

Voice: (519) 749-1565
Fax: (519) 749-2863
E-mail: divergent@divertech.com
Website: www.divertech.com