Hsu Research VTF-2
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
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
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
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
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
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?
Prices of Equipment reviewed: