GOODSOUND!GoodSound! "Equipment" Archives

Published July 1, 2001


Hsu Research VTF-2 Powered Subwoofer

Presidential subwoofer candidate

Dr. Poh Ser Hsu, president of Hsu Research, "thinks, talks, eats, drinks and dreams subwoofers," in the words of a colleague. In fact, subwoofers are all Hsu does -- he has been selling his specialized, high-value products direct from his Southern California factory for about nine years.

The Hsu Research VTF-2 powered subwoofer recently underwent substantial improvement, but Dr. Hsu refuses to charge more for it. The price is still only $499. Bravo! The upgrades add greater amplifier output (160W), continuously variable low-pass cut-off from 30-90Hz (instead of 60-125Hz as before), a new low-pass filter (24dB/octave instead of 12dB/octave), and a crossover-bypass function. We'll touch on each of these attributes later, but first, let's first look at the unit's most unusual feature, for which it is named: VTF stands for Variable Tuning Frequency.

The cabinet sports two 3" side-firing ports, one left, one right. A snug-fitting foam plug inserts into either of the ports. This plug tunes the cabinet to 25Hz +/-1dB, giving the woofer its flattest and most extended response, the company claims. That is some very serious bass extension. With the plug removed, the cut-off frequency is raised to 32Hz, but a 4dB boost in the 40Hz band is added electronically by flipping the switch on the back panel from the "one-port-open" to the "two-ports-open" position. That boost enhances the impact of movie sound tracks, whose heavy artillery effects usually occur right around 35Hz or higher. Hsu's variable tuning feature is very trick -- he accomplishes it both in the acoustical domain (by using the plug to alter the system's "Q" or damping) and electronically through the crossover. The flexibility provided by variable tuning means the VTF-2 is a music/movie machine that's equally optimized for either purpose. And it takes all of five seconds to change.

Further specs: The VTF-2 is a down-firing design with a magnetically shielded 10" long-throw woofer. It offers rear-mounted 0/180-phase and continuously variable output controls. For connectivity, choose between line-level (preamp or receiver) RCA inputs and speaker-level ins and outs through spring clip speaker connectors.

The VTF-2's cabinet presents a distraction-free 16" square face with no switches, LEDs or grille, just a low-key logo decal. It is finished in pebbled black paint, and it has radiused corners and edges and its depth is 19.5". It weighs 57 pounds. Hsu offers a 30-day money-back guarantee, and the VTF-2 carries a one-year warranty on its electronics and five years on the speaker. As an added bonus, a "deep bass" test CD is included.

Democratic Party options

The VTF-2 offers multiple connection options: bass-fill (or augmentation); high-level filter mode; and home theater mode. (If you're already familiar with all the subwoofer in-and-outs, you may want to skip directly to the listening impressions.)

Bass fill, or augmentation mode, adds subwoofer response below that of the main speakers. With speakers that extend to 60Hz or lower, this is the preferred mode. It avoids putting two crossovers in series (the sub's plus the speakers'), and with speakers having bass output of their own, the subwoofer operates predominantly (sometimes exclusively) in omni-directional dispersion mode. That means your ears can't pinpoint its location -- which is just the sort of sonic invisibility that's a requirement for high-performance music systems. In addition, the VTF-2's new, lower cut-off settings -- down to 30Hz -- mean that even reasonably full-range tower speakers can benefit from bass fill.

Bass fill runs the loudspeakers full-range -- so you can dial in the subwoofer's low-pass cut-off to match the loudspeaker's high-pass roll-off (the point at which the loudspeakers cease putting out an audible amount of bass). If your main speakers' -3dB bass spec reads 48Hz, for example, dial the sub's low pass to around 50Hz as a starting point. Fine-tune the perfect low-pass setting by ear or with a test CD and a Sound Pressure Level (SPL) meter (an inexpensive -- although not terribly accurate model -- is available at Radio Shack for around $20).

You can hook up bass fill mode two ways. You can feed the sub's low-level circuit from a preamp/integrated amplifier/receiver's pre- or sub-out using a long interconnect. Or you can take a high-level signal from your amp's speaker binding posts and route it to the subwoofer's speaker terminals labeled "from amp." This way, your amplifier will have two sets of speaker cable connected to its speaker terminals: one pair going to the speakers; the other going to the subwoofer.

While bass fill bypasses the VTF-2's built-in filter, high-level filter mode uses the subwoofer's crossover to subtract bass information from the signal that's seen by your main speakers. This is recommended if your speakers are miniature satellites with little or no bass output -- or even if they are larger monitors that sound compressed, or as though struggling when playing loud and bass-heavy music. In high-level filter mode, the speaker leads from your amplifier/receiver/integrated amp don't connect directly to the speakers. Instead, they connect to the "from amp" spring clip terminals of the sub. The adjacent "to speakers" terminals loop back to the speakers with a second pair of cables. This "in and out" scheme inserts the VTF-2's 6dB high-pass filter into the signal path and splits the amplifier signal into two parts: 90Hz and higher passes to the speakers, while everything below 90Hz remains with the subwoofer. This filtering action protects your satellites. They no longer attempt to reproduce low or mid bass but concentrate on what they're really designed for -- upper bass and higher. There's a second benefit: Bass-shy satellites will sound cleaner and more open and play louder without distortion if they don't "see" deep bass information.

The VTF-2 can also be used in a home theater setting if you have a multi-channel receiver with built-in bass management. From the receiver's sub-out jack/s, connect to the VTF-2's RCA inputs and then engage its crossover-bypass function. From the receiver's on-screen menu (or panel-mounted switches), choose the appropriate bass management mode for the size of your main speakers (large or small). If you experience mid-bass congestion in this set-up, Hsu recommends activating the VTF-2's crossover to its upper 90Hz limit.

One, two, three, testing

For sonic evaluations, I partnered the Hsu Research VTF-2 with our in-house Axiom M3Ti reference monitors. In my room they don't require filtering, so the sub was connected in low-level augmentation mode from the pre-out jacks of the Cambridge Audio A500 (some may wish to filter the M3Ti through the VTF-2 since that will relieve them in producing the deeper bass frequencies and in turn perhaps allow for higher output levels). Placed between the Axioms, optimal low-pass frequency turned out to be 60Hz in my 13'W x 19'D x 10'H room.

Beauty and the Beast

To dial in the correct subwoofer level and evaluate pitch definition and agility, I cued up "Our Spanish Love Song" on Beyond the Missouri Sky [Verve 314537130-2] by Pat Metheny and Charlie Haden. It's a common mistake to set subwoofer levels too high. This lyrical guitar and bass duet quickly differentiated between settings that were heavy-handed and those that were spot-on. Except when Haden solos, you should hear him playing second bass-fiddle to Metheny's lead guitar, never overpowering him. At the same time, his bass lines should be clearly intelligible up and down the scales; the system should neither add emphasis to nor obscure individual notes. With my eyes closed, I listened for a seamless blend. The sound shouldn't scream "sub in the system," but rather make you wonder whether the sub even works. If 500 bucks seems a high price to pay for nothing, just ask a friend to turn down the sub's level control while you listen. Now you'll know what the sub did before -- not only will the bass disappear, but so will most of the subtle spatial cues that tell you so much about the specific room in which the recording was made. Perfect bass level in my listening room turned out to be a 9:30 setting on the subwoofer volume control.

On "Our Spanish Love Song," the VTF-2's steadfast refusal to do the one-note-boom really stood out. The subwoofer simply added weight, scale and warmth to the double bass, fleshing it out and making it more robust. It also made it clear as daylight exactly which notes Charlie Haden was playing way down low. There were other more subtle benefits. With the sub in the system, spaciousness increased. Very fine low-level data recreated the ambient environment in which both artists performed. I enjoyed more soundstage depth, more thereness, more realism. This spoke louder about the VTF-2's bass quality and inherently low distortion than thunderous SPL output levels (of which, incidentally, it is also quite capable).

To verify integration, I now grabbed Leonard Pennario's Romantic Piano Music [EMI CDM-7 69117 2] and cued up "La Valse" by Ravel. This is a fiendishly difficult orchestral work that Ravel later transcribed for solo piano (pianists refer to it as La Valse for Three Hands). The player must navigate extremes from the lowest to the highest notes in rapid succession, grabbing fistfuls of keys at a time. If there was any discontinuity between VTF-2 and Axioms, it would show up in the transition between the low bass and mid/upper bass. But there was nary a seam showing -- I hadn't the faintest idea where the sub stopped and the monitors began. Score another one for the Hsu, yes!

The unit having passed my musical invisibility tests with flying colors, ultimate bass extension was next. I dusted off Peter Hurford's Organ Spectacular [London 430710-2]. Shy of synthesized soundtrack effects or humongous Japanese Kodo drums, there's nothing like a good organ recording to showcase how a true subwoofer will energize a room with sound pressure that's more felt than heard. The VTF-2 uncovered the very lowest organ notes a full octave below those the Axioms managed on their own. Speakers can contribute flawed bass information called doubling. Instead of locking in the actual bass note (the fundamental), they emphasize its first harmonic, an octave higher. (The difference between one octave and the next higher works out to twice the frequency, hence the term "doubling".) Being a true subwoofer, the VTF-2 reproduced the organ's actual fundamentals in the 20Hz range, not merely their first 40 to 60Hz overtones. Day and night difference. The ominous pressure waves of super-low pedal notes pervaded the room without audible distortion, port noise or any other signs of misbehavior or distress. They also didn't muddy the overall sonic picture nor did the Axioms' last octave below 100Hz become congested. This precision of hand-over between sub and satellites is a testament to the well-implemented steep fourth-order 24dB/octave low-pass filter. It prevents the VTF-2 from spilling into the monitors' range above its own low-pass cut-off point.

Musical subs shouldn't be thought of as sonic weapons

A quick aside on a subwoofer's ultimate output capabilities at or below 20Hz: As independent measurements on the Hsu Research website show, the VTF-2 is capable of very high levels of low-distortion output down to its specified 25Hz -- and its output is still respectable at 20Hz. Still, such concerns are of little relevance to music lovers. Very little music occurs below 30Hz, and even when it does, it is never at pant-flapping, sonic weapon, structural damage type levels. In other words, don't get caught up in "mine plays louder at 18Hz" subwoofer war games.

More Beauty than Beast

Hsu Research VTF-2
Powered Subwoofer

Instead, play Angélique Kidjo's new Keep Moving, The Best of CD [Wrasse Records 85758 (reviewed this issue)] for some funky phat beats and deep grooves. Her "Voodoo Child" tribute to Jimi Hendrix is a blast, propelled forward by a tightly percolating synth & drum foundation. Without the VTF-2, the Axioms sounded predictably tame, not what I had in mind when thinking Hendrix. With the sub added, the entire gestalt changed, but the song didn't lose its bouncy precision. Rather, the well-damped articulation of the bass lines enhanced its energetic drive. Gimme more, voodoo child! Fast forward to "Open Your Eyes" with Kelly Price singing opposite Kidjo. Again, house and club beats commingle with synth effects to create a tightly woven groove. It's a kick in the pants. Makes you wanna dance. If Techno or Underground beats have you thinking low-rider boom-trucks, a sub like the VTF-2 might strike you at first as too civilized, too precise, too transparent, too clean. Simply remove the port plug and hit the rear-panel EQ switch. Presto, 4dB more oomph in the sock'em range. Works exactly as advertised. Now go back to extended flat mode. Doesn't that simply sound better? Don't agree? No need to argue -- the VTF-2 lets you have it both ways.

Marriage made in Heaven?

The VTF-2 is the best-looking sub yet from Hsu's stable. It's a true subwoofer, which operates below your main speakers' bass drivers, unlike the majority of affordable so-called subwoofers that really are just outboard woofers, not subwoofers. My only minor gripe is the use of spring clip connectors. I'd prefer to see quality five-way binding posts instead. Still, the overall performance of the VTF-2 is so awe-inspiring, and the variable tuning feature so well thought out, that this complaint nearly seems like a nitpick. Most $500 subwoofers are fortunate to hit 35Hz at minus 3dB and even then they suffer high distortion levels.

The Hsu Research VTF-2 delivers the real goods. Integration and connection is easy. It's surprisingly compact for what it does. It's attractive. Best of all, it's a true music sub. One that's as refined, sophisticated and accomplished at its job as the Axioms are at theirs. A marriage made in heaven then between these two? You bet. I wish I had known about the VTF-2 when I was still in retail. I was always at a loss to recommend a true performance sub to folks on a budget. Boy, I could have sold tons of these. It's a sure-fire recommendation for anyone who has superior monitors or even tower speakers and wants to add quality and quantity bass. And should you want to do Home Theater and goose things just for the heck of it, the VTF-2 will happily oblige and rattle your china to smithereens. How slick is that?

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