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Hsu Research ULS-15 mk2 Subwoofer Review

by August 31, 2016
  • Product Name: ULS-15 mk2 Subwoofer
  • Manufacturer: Hsu Research
  • Performance Rating: StarStarStarStarStar
  • Value Rating: StarStarStarStarStar
  • Review Date: August 31, 2016 00:00
  • MSRP: $ 769
  • Buy Now
  • 15” driver, sealed enclosure
  • 600w continuous amplifier
  • 30 Hz- 90 Hz crossover frequency range, by-passable
  • 24dB/octave crossover slope
  • Linkwitz-Riley crossover type
    0°/180° phase switch
  • L/R Unbalanced inputs
  • L/R Speaker-level inputs
  • L/R Balanced inputs
  • 0.7-0.3 Q control
  • 720-watt power outlet requirement
  • Weight: 65 lbs.
  • Enclosure Dimensions: 18”H x 18”W x 18”D
  • Dimensions w/ feet, grille, amp: 19”H x 18”W x 19.5”D
  • 7 years woofer warranty/ 2 years amplifier warranty

Pros

  • Excellent sound quality
  • Not huge or very heavy
  • High dynamic range
  • Well protected against bottoming out
  • Above average finish

Cons

  • Limited headroom in EQ1 mode

After feedback from customers who wanted a subwoofer that was friendlier to living room decor than the behemoth ported boxes and tall cylinder subs Hsu had been producing, Hsu Research released their first sealed subwoofer in 2008- The ULS-15. Hsu had already built a solid reputation for deep-digging, accurate, and affordable subwoofers. The caveat had always been size per Hoffman’s Iron Law which states that you can have two of the following, but never all three: sensitivity, small enclosure size, and deep bass. Low sensitivity necessitates a powerful amp and heavy-duty driver, which raises costs considerably. So, in an effort to make their subwoofers affordable, Hsu had traditionally decided to compromise on size instead. However, a strong demand for subwoofers with a more attractive size and shape prompted Hsu to break with their tradition in the ULS-15. While the ULS-15 went on to successfully fill its niche in the Hsu line-up, the years went by and new technologies made a compelling case for an overhaul, which brings us to the ULS-15 mk2.

The ULS-15 mk2 shares some of the same design features of the mk1, but also some important differences. However, before we get into design discussion, let’s begin where most users would with the ULS-15 mk2: unpacking.

Unpacking and Setup Guide

2_ULS_mk2_unopened_box.jpg   

The ULS-15 mk2 showed up in a large box proudly sporting the Hsu logo. The external packaging is serious, with both packing tape and heavy-duty staples holding the box shut. There is certainly no danger of the subwoofer falling out, as I had to use pliers to pull out the staples in order to open the box.

       4_ULS_mk2_inner_packing.jpg

Inside the box, removing a light foam cover reveals an inner box and some heavy cardboard corner protection pieces. Inside the inner box we see a large heavy-duty foam top and bottom piece neatly sandwiching the subwoofer. The subwoofer itself is wrapped in an internal layer of a soft foam-ish plastic cover and an external layer of a heavier plastic covering, presumably to protect it from liquids and moisture. The packaging overall consists of double-boxing, double foam insert shock-absorbtion, both tape and staple sealing, and double-bagging. This is the best packaging I can recall seen on a subwoofer. I guess Hsu really hates dealing with shipping damage claims.

The included Owner’s Guide is well done if not exhaustive. It has a lengthy trouble-shooting section and contains lots of tips and advice about optimizing subwoofer performance as well as basic instructions for subwoofer novices. It is also available on the ULS-15 mk2 website product page as PDF file. One thing that might be easy to miss for those who are unpacking the ULS-15 mk2 is the Hsu demo disc that is shipped in the packing list envelope. This CD contains a handful of tracks of classical music that makes for excellent demo material, and also a number of different subwoofer test tones. The demo disc is a nice touch, and the first track, the Saint-Saëns Organ Symphony excerpt which contains a powerful 16 Hz fundamental from a pipe organ, is a real corker for subwoofers.

Appearance 

5_ULS_mk2.jpg

6_ULS_mk2_w_grille.jpgThe ULS-15 mk2 does not look dramatically different from the original ULS-15,  which sported a conventional and tasteful appearance. The mk2 cabinet is the same, and the only obvious visual difference is the cone. With grille on, they look identical, except for markings on the plate amplifier. The ULS-15 mk2 comes in satin black finish and also a real rosewood veneer for an additional $150. The satin black of this review unit is a very smooth black that is not polished-level shiny but certainly not a matte-black light sink either. It reflects light but in a diffuse manner. The cabinet itself is essentially an 18” cube with rounded longitudinal edges; this symmetry lend it a stylish simplicity that would make for a great fit in modern decor, but its conservative demeanor lets it blend in with more traditional interiors, especially in the Rosewood veneer. For those who want minimalism, the grille makes the ULS-15 mk2 look nearly featureless. However I prefer it with the exposed cone; the glossy woofer center and beefy surround make the mk2 look poised to do some serious rocking.

 

Design Overview

The 18” cube cabinet is constructed with 3/4” MDF side-w7_ULS_mk2_interior.jpgalls and a 1.5” front baffle. A ¾” window brace helps support the driver while reinforcing the side-walls at the center length of the cabinet. The sidewalls are neatly lined with egg-crate type foam for stuffing, which is a departure from the mk1 which used fiberglass wall insulation for stuffing. The foam stuffing looks like it would work well, while staying well out of the way of the driver and amplifier. The feet are sturdy rubber rings that do well to protect the cabinet when setting it down, although they make it awkward to move the sub by sliding.  These feet look like they would make use of a subwoofer isolation pad a moot point, much like SVS’s SoundPath Isolation feet. Altogether the ULS-15 mk2 sports a simple and sensible enclosure for a sealed subwoofer of this size. A higher-end and more expensive subwoofer might sport more bracing and thicker walls, but that would only add weight and expense for negligible returns. The ULS-15 mk2 cabinet construction is orderly and very pragmatic.

8_ULS_mk2_driver.jpg 

The biggest point of departure of the mk2 from the original ULS-15 is the driver.  The first ULS-15 used an und9_ULS_mk2_motor.jpgerhung XBL^2 motor. A design goal of the XBL^2 design was greater linear excursion due to a more uniform magnetic field for the voice coil to travel in (for those who want to get into the details of the XBL^2 technology, here is a good starting point). Indeed, the ‘ULS’ stands for Ultra Linear Sealed, denoting the extraordinarily long linear stroke delivered by the XBL^2 design. Another characteristic of the XBL^2 design is a wider-band driver than conventional topologies, since not as much voice coil mass is needed, thereby greatly reducing induction. However, a disadvantage of the XBL^2 design was a hefty penalty in sensitivity. Hsu dispensed with the XBL^2 design in favor of a more traditional overhung design in the mk2. While the XBL^2 design was an interesting approach in the mk1 driver, the mk2 driver is no slouch. A stack of two hefty 1” x 6.75” magnets comprise the bulk of the motor.  The pole piece is undercut for a better magnetic field symmetry, and multiple shorting rings reduce induction effects which reduces even-order distortion. The mk2 uses a 12 spoke aluminum basket and a Butyl foam surround holds a fiberglass-impregnated cone in place.  

The amplifier is a 600-watt continuous BASH a10_ULS_mk2_amplifier.jpgmplifier. BASH amplifiers have been around for a while and combine the efficiency of Class D amps with the fidelity of class AB amplifiers (although that is somewhat of an over-simplification). The ULS-15 mk2 amplifier has a host of noteworthy features. Balanced XLR inputs make it a good choice for higher-end and pro-audio setups. Speaker-level inputs make it a good choice for setups with no line-level outputs such as older receivers and integrated amplifiers. An operating mode switch can set the frequency response flat down to 20 Hz in the ‘EQ1’ mode or a more typical roll-off to sealed subwoofers with a shallow slope starting at 50 hz in ‘EQ2’ mode. A ‘Q control’ knob changes the slope of the response; lower Q settings will make for a steeper rolloff and mid bass will be accentuated more. There are also the more traditional features on the amplifier such as a 0°-180° phase switch and a 30 Hz to 90 Hz variable low-pass filter. One nice touch is the 120V~60Hz/240V~50Hz voltage selector which makes for easy use in electrical standards around the world.

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About the author:

James Larson is Audioholics' primary loudspeaker and subwoofer reviewer on account of his deep knowledge of loudspeaker functioning and performance and also his overall enthusiasm toward moving the state of audio science forward.

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Recent Forum Posts:

Langston posts on October 31, 2018 14:49
Sorry to make another post for such a small bit of information. The input impedances of the (3) connector types should be included in the documentation, but aren't. This is helpful to know when paralleling multiple units. The DUT was a HSU ULS-15 MK2, but the results are likely consistent across their self-powered product line.

Impedances were consistent from test frequencies of 100Hz to 100kHz independent of gain control. Rounding is used below, measured impedances were ± 1% of these figures. The test instrument used was a Keysight U1733C LCR meter.

Balanced XLR inputs: 20k?
Unbalanced RCA inputs: 10k?
Speaker level Banana Jacks: 1k?
Langston posts on October 24, 2018 12:05
I love simplicity and I'm sure the long term cost will be less than adding a high quality active crossover in front of the subs. I also didn't want the potential degradation of an active high pass in front of the CLS amp, thus I'm using a simple passive high pass around 100Hz to yield a net acoustic high pass at 150Hz to integrate with the subs. I'm about to make outdoor ground plane measurements of the subs at the same distance as the CLS loudspeakers to phase align the passbands through the crossover region. Should be finished this week.
shadyJ posts on October 24, 2018 11:47
Langston, post: 1276838, member: 86611
Like several other reviews I've read on this site, the ULS-15 MK2 review was probably the best I've seen on the topic. Tons of work with excellent and sometimes unusual measurements. Partially based on this review I purchased a pair of these subs to go with the MartinLogan CLS loudspeakers I just rebuilt and modified. The support at HSU Research is as good as their products, when I sent an email asking how to increase the maximum low pass frequency from 90Hz to 150Hz, I got an immediate reply with instructions including the obligatory “your warranty will be toast” warning. This simply included changing (4) 10k? resistors to 6k?, which I did with parallel 15k? resistors to make it easy to clip them off and return the subs to OEM operation if I changed my mind someday. HSU treats their customers like adults. Amazing.

I've attached pictures of before and after near field measurements while keeping the high pass alignment constant (EQ1, Q=0.3).
Wow, very cool of Hsu to give you those instructions! To be honest, very few other manufacturers would even be knowledgeable enough to know what to do; they just buy the amps and assemble the products.

May I ask why you wanted to modify the low pass filter instead of just getting an external crossover? I suppose it's less expensive, but it does void the warranty.
Langston posts on October 24, 2018 02:35
Like several other reviews I've read on this site, the ULS-15 MK2 review was probably the best I've seen on the topic. Tons of work with excellent and sometimes unusual measurements. Partially based on this review I purchased a pair of these subs to go with the MartinLogan CLS loudspeakers I just rebuilt and modified. The support at HSU Research is as good as their products, when I sent an email asking how to increase the maximum low pass frequency from 90Hz to 150Hz, I got an immediate reply with instructions including the obligatory “your warranty will be toast” warning. This simply included changing (4) 10k? resistors to 6k?, which I did with parallel 15k? resistors to make it easy to clip them off and return the subs to OEM operation if I changed my mind someday. HSU treats their customers like adults. Amazing.

I've attached pictures of before and after near field measurements while keeping the high pass alignment constant (EQ1, Q=0.3).

26299

26300

26301
KEW posts on September 19, 2018 12:20
Good choice on the ULS-15. I think it is among the best values in a sub and not one you are likely to outgrow, ever (unless you hang out with some of the severely deranged bass-aholics here). Certainly a pair of them will want for nothing for music in any normal sized room!

Here is a great 5 minute video Marshall did so you can be ready to do the sub crawl when it arrives:
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