VELODYNE ULD-18 SERIES II THX--Powered Subwoofer

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[adapted from 1996 Stereophile Guide to Home Theater (SGHT) article]

First introduced in the late ‘80s, the Velodyne ULD-18 subwoofer was intended for music playback, not home theater. In fact, there was no well-defined home-theater market at the time, though certainly there were laserdiscs, video projectors, surround processors, and a few hearty individuals willing to put them all together.

Since that time, the subwoofer market has changed dramatically. Home theater—now a reality—is the primary reason. And Velodyne remains a name to reckon with in subwoofers. The ULD-18 remains at the top of their line, now in Series II form and now available in both THX and standard versions.

According to Velodyne, the only differences between THX and convention al ULD-18s are found in the electronic crossover network. In the standard model—which sells for about $250 less—the built-in high-pass and low- pass filters are set at 85 Hz. The nominal rolloff rate on both sides is 12 dB per octave. (Though Velodyne states that the rolloff characteristic is much more complex than that, starting at 6 dB per octave, then increasing gradually to 24 dB per octave as you get further away from the crossover point.)

In the THX version, the low-pass crossover point is set at 200 Hz so as not to interfere with the low-pass crossover built into THX and other high- quality surround processors. The high- pass filter in the ULD-18 generally is not used in a THX system—the THX processor’s high-pass filter is used instead. There fore, the ULD-18 THX’s own high-pass filter is also set at 200 Hz, simply as a matter of design convenience. The subwoofer level control is also deleted. The fact that Velodyne chose to include an internal crossover at all in the THX version is interesting and perhaps useful; with the simple change of a plug-in module, the ULD-18 THX can be reconverted to an 85 Hz crossover frequency, if desired.

(Presumably, the reverse is also true of the less expensive, standard version.)

Unlike most powered subwoofers on the market, the ULD-18 THX’s electronics are mounted in an external chassis. The latter contains the cross over circuits, amplifier, and servo controller. Velodyne makes extensive use of servo feedback in their best sub- woofers, including the ULD-18 series. (See the sidebar, “Big, Low, and Loud.”)

While not the largest subwoofer I have ever seen, the ULD-18 THX does take up space. With its downward-facing driver, it resembles a slightly chunky end table (the cabinet is avail able in oak, walnut, and black). How ever, I don’t recommend placing any thing on top that is prone to rattle. The 18-inch driver is not magnetically shielded, a likely heritage from its pre—home-theater basic design. Velodyne recommends that the ULD-18 THX be placed at least four feet away from a video monitor.

The only feature lacking on the ULD-18 THX, commonly found on many current subwoofers (including the B&W 800ASW and M&K MX 5000THX reviewed in this issue) is a phase reversal switch. You can’t reverse the phase by reversing the polarity of the external connection between the ULD-18 THX and its external amplifier. It is critical that the hookup here be plus to plus, minus to minus. Reverse the polarity, and the subwoofer will, in all likelihood, be seriously damaged. I highly recommend that color-coded cable be used for this connection to prevent such an occurrence.


I placed a single ULD-18 THX with its driver about 2 feet out from a front corner in my large home-theater room. I connected its controller directly to the subwoofer output of either the Pioneer SP-995D processor (for AC-3) or the Proceed PAV (for Pro Logic). The main (high-pass) outputs of the processor were sent directly to the main amplifier (a Carver AV-806x) without passing through the Velodyne controller. This is the same hookup arrangement I used with the other subwoofers reviewed for this issue, and the arrangement I generally recommend with any surround processor having its own high-pass filters.

The remainder of the system was as described in my review of the B&W 800 Active Subwoofer, also in this issue.

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Ser.#: 55294028 Price: $2995 (black ash)

Manufacturer: Velodyne Acoustics, Inc.

1070 Commercial St. #101 San Jose, CA 95112

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The Velodyne ULD-18 THX sounds different from most subwoofers in my experience. My first reaction to it was “Where’s the bass?” Believe mc, the bass is there. But the Velodyne is less inclined to add a general warmth to the sound than are most sub woofers. Velodyne would argue that this is because of the ULD 18’s low distortion, and I would not argue the point.

‘Whatever the reason, this is one clean, powerful-sounding subwoofer. I began my evaluation, as I usually do, by using CD-based, audio-only program material. Bass drum was big and impressive-sounding—but not oppressive or overdone. If the sound was a little less tight than from the most highly damped, audiophile-approved, full- range loudspeakers, few of those loud speakers can match the combination of weight, extension, and detail offered by the ULD-18 THX. This sort of bass power can actually be physically distressing with some program material, but don’t blame the messenger.

While the Velodyne was occasionally “caught out” by an overwrought bass transient, such occurrences were rare enough to attribute to an unfortunate combination of program material, inherent subwoofer capability, and room modes. Occasionally, I was also conscious of the bass appearing to come from the physical location of the sub, rather than the main loudspeakers. But this happened no more often than with other good subwoofers I have used, and less often than with most.

Moving on to video-based material, the good vibes (pun intended) continued. While the Velodyne was not, subjectively, the most extended subwoofer I have heard in my home theater (two B&W 800s, the M&K MX-5000, and a pair of Snell SUB 1800s seemed to extend subjectively deeper into the bottom octave), the more I listened to the Velodyne, the more impressed I was by its combination of extension and detail.

The synthesized drums in the sound track to 1492, Conquest of Paradise were crisp and strong. The rumble of the fires in Back Draft proved to be more than just a droning muddle. In Jurassic Park, I could hear every gurgle out of T-Rex’s throat. And the bass in Geronimo, from the opening drum roll to the tramp of the horses’ hooves, reminded me again of why this Pro Logic soundtrack is one of the very best I have heard—Pro Logic or AC-3.

The ULD-18 THX had no problem with any of my bass torture tracks, audio or video. One bass note in Searching for Bobby Fischer (chapter 13) gave it the slightest problem (overhang, not over loading), but this killer sequence gives most subs fits. There are also some ungodly bass transients during the final game sequence in Little Big League (a very good movie, by the way) that could well destroy lesser subwoofers. The ULD-18 THX did strain a little as Randy Johnson pitches to the final batter but it was not alarming.

On all my other bass-challenge discs— from Jurassic Park to Aladdin in Pro Logic; Outbreak to Generations in AC-3, the Velodyne was not fazed by any thing.

Compared with a pair of B&W 800 Active Subwoofers, the Velodyne appeared to go slightly less deep, though it was equally accomplished at shaking the house. The Velodyne was less prone to strain on the most demanding material, however, and was cleaner and more detailed in the mid and upper bass. With the B&Ws, I was more conscious of the fact that there were subwoofers playing, though very good ones. The Velodyne was less conspicuous until it really counted, then it was everything you could ask for. The single Velodyne was easier to position than a pair of B&Ws, but the latter was more flexible; if stereo subs are your thing, it will cost you a lot more to get a pair of ULD-18s (though Velodyne does, of course, offer less expensive models).


By the time you read this, Velodyne will have introduced an 18-inch, servo subwoofer in its F-series range. This model will likely be considerably less expensive than the ULD-18 THX. Might it be better because it’s a newer design? Or as good, for less money? Your guess is as good as mine at this point. The ULD-18 THX is unlikely to become second-rate in this century. Maybe even in the next. I cannot imagine that anyone using it in their home theater will feel bass deprived.

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Servo Control

One of the signs of a well-designed sub woofer is, paradoxically, not how loud and deep it goes, but how skillfully the designer has managed to keep its inevitable loudness and extension limitations from becoming audible.

One common example of engineering sleight of-hand often used to extend a subwoofer's deep bass is to boost the lower frequencies in the on-board amplifier; but limit that boost at higher output levels to prevent overload. If skillfully done, you will rarely notice it. If poorly executed, you will hear side effects, such as a feeling that the bass is being compressed or 'reined-in’ at higher levels, actual amplifier clipping, or the rattle of a driver hitting its physical limits.

Servo control-such as that used on the Velodyne ULD-18-is another clever design technique. Expensive to execute well, and thus relatively uncommon, a servo subwoofer typically uses a sensor to track the woofer cone's motion. The sensor compares the motion with the output of the subwoofer's amplifier and, if necessary, applies a correction to bring the cone into line.

Aside from the obvious ability of such a design to reduce distortion, it can also extend the response at the bottom end-given enough amplifier power and driver capability. Again, if poorly done, the audible result can be serious overloading from high-level, very low-frequency inputs.

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[adapted from 1996 Stereophile Guide to Home Theater (SGHT) article]


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Updated: Monday, 2016-04-11 1:06 PST