Q ACOUSTICS QB12 SUBWOOFER

What can we say? We’re in awe of its performance, particularly given its relatively compact dimensions and its modest retail price.

QAcoustics’ newest powered subwoofer, the QB12, is the first to use a 305mm diameter bass driver, and was specifically designed to integrate with Q Acoustics’ 3000i series speakers, but its design and performance are such that you’ll be able to use it in partnership with speakers from any other manufacturer… and to great advantage.

THE EQUIPMENT

When I first unpacked the Q Acoustics QB12, I’d done so by upending the carton on a soft rug, so that when I removed the cloth that protects the finish (four finishes are available, about which more later) I was more than a little taken aback to see that there was a fairly rough-cut hole in the bottom of the cabinet around the size of a 20-cent piece. I could not imagine what this hole might be for, but it seemed mighty strange to find on a subwoofer that retails for $1,449.

Since I was looking at the base of the cabinet I thought I would check out the feet while I was there, and discovered that Q Acoustics has been very clever, because what I initially thought were ordinary rubber feet turned out to be rubber ‘cups’ that fit snugly over four chromium plated spiked feet, so you could use the spikes on carpeted floor and fit the rubber cups to use the QB12 on polished wooden floors.

Once I’d placed the QB12 on those feet, I looked at the back of it and was again a bit surprised to see just two rotary controls, one for volume and the other for crossover frequency selection, both of which were almost completely recessed into a metal plate. Slightly above and midway between these two controls was a chameleon power LED. It was only when I removed this plate (easily done, because it’s held in place only by magnets), that I was able to see what that hole in the bottom of the subwoofer was for, because removing the plate revealed a rather large rectangularly-shaped recess behind it, inside which was a two-pin socket for a 240V mains power lead fitted with a

Figure 8 mains power connector.

So what you do is feed the 240V mains power lead up through the hole in the bottom of the subwoofer—along with the audio signal cables—and because that means there are no wires coming out of the back of the cabinet, you will be able to push it right back against a rear wall. And, if the cabinet is sited somewhere in your room that the back of it can be seen, it will be nice and tidy, with no visible wiring. If you’d prefer not to run your signal wires alongside your mains cable, there’s a small cut-out in the bottom of the plate covering the recess that is large enough for two signal wires.

In addition to the mains power socket, the rectangular recess also has a pair of RCA sockets for signal input (left and right, but the left input can be used to feed in an LFE signal), a mains power rocker switch, a two-way switch to choose whether you’d prefer the subwoofer to be permanently ‘on’ or switch on when it detects an audio signal and switch itself off after a period during which no audio signals had been detected, and a two-way phase switch (0/180°).

Q Acoustics rates the diameter of the QB12’s front-facing bass driver at 305mm but this is overall diameter of the basket. The diameter that really matters when it comes to evaluating the driver’s ability to deliver deep bass is the Thiele/Small diameter, which is what Q Acoustics’ engineers would have used to derive the piston area of the driver, and is the distance across the driver from the midpoint on the roll surround to the same point on the opposite side of the driver, and in this case I measured it as 245mm, which gives a piston area (Sd) of 472cm². As you might have guessed from the substantial difference between the overall diameter and the T/S diameter, the driver’s roll surround (which is made of rubber, rather than the more perishable foam) is quite large, presumably to enable the greatest driver excursion. The driver itself is made from paper, but the dustcap, which is made from plastic, is so large (140mm in diameter) that its surface area (at 154cm²) is almost one quarter of the total area. The diameter of the dustcap does not reflect that of the voice coil, which is much smaller, at just 50mm. Although it is possible to remove the QB12’s grille, I would not recommend you try, because Q Acoustics has made it extremely difficult to do by way of fixing it to the front baffle using 12 steel pegs, which is around eight more than would usually be used to attach a grille. It can be done, but you run the risk of damaging either the cabinet or the grille, or both.

The amplifier powering the QB12’s bass driver is a Texas Instruments TPA3255

Class-D monolithic device that’s actually a stereo amplifier that can be bridged for mono operation. Texas Instruments’ own specifications put its mono power output at 480-watts into 2Ω, so since Q Acoustics is claiming an output of 220-watts we can deduce that the impedance of the QB12’s driver is 4Ω.

Like most monolithic devices, the TPA3255 has the full gamut of self-protection circuits built in to guard against under-voltage, over-temperature, clipping, and short circuits, amongst others, plus it also has d.c. protection to protect the driver. Unlike most monolithic devices, the TPA3255 has extremely low distortion, with Texas Instruments rating it as being less than 0.006% at low power output levels and still less than 0.01% just below clipping. It’s also a very low-noise device, with an A-weighted signal-to-noise ratio ratio of 111 dB. These outstanding performance figures would suggest that it’s probably a little over-specified for use as a subwoofer amplifier, but Q Acoustics is obviously going all-out for best performance.

Although the TPA3255 is a very efficient Class-D device, and thus generates nowhere near the same amount of heat as a Class-AB amplifier would, it and the fairly complex power supplies required to operate it do create a certain amount of heat and, since the QB12’s cabinet is completely sealed, Q Acoustics has fitted them to a very nice custom die-cast aluminium plate on the rear panel that acts as a heat sink to dissipate this.

As you should be able to see from our photographs—and you will see for yourself in the flesh—it’s a far better-looking plate than most such rear-panel amplifier plates.

The QB12’s cabinet is beautifully built, with curved edges and a total weight of 21.5kg.

As I said previously, the QB12 is available in four different finishes: black or white vinyl finishes or black or white gloss painted versions. The vinyl editions cost $1,499 (a price recently reduced as a result of a change in the Australian distribution for Q Acoustics).

I was amazed to learn that there’s no price premium for either of the gloss painted finishes—they’re exactly the same price as the vinyl versions. I have seen the white gloss version of the QB12, and it’s certainly a premium finish, beautifully done. What may surprise you is that the vinyl versions also look great… the standard of the finish is much higher than you’d expect for vinyl. My review sample was finished in black vinyl, and I was impressed that the vinyl is stippled in such a way that it has very low reflectivity, and also by the fact that it’s very easy to clean (don’t ask!) and doesn’t show marks (again don’t ask!). The fact that the corners are rounded means that unlike rectangular-edged vinyl cabinets, the vinyl will never ‘peel’ or fracture at the corners.

As well as looking beautiful, the QB12’s cabinet is also not particularly large—particularly when you consider that it’s a sealed enclosure, rather than the more usual bass-reflex design. The fact that at 400×400×446mm (HWD) it’s also almost a cube makes it a most attractive proposition for unobtrusive placement in even the smallest room.

The advantages of not having a bass reflex port are many, and include greater flexibility for room positioning (particularly since the cabinet can be pushed back against a wall) and also that it makes the subwoofer easier

to build into a cabinet, recess into a soffit, or simply hide underneath or behind an item of furniture. There’s also the not-inconsiderable advantage that whereas small creatures have been known to make their home inside bass reflex subwoofer cabinets (using the port as a means of ingress and egress), this is clearly impossible with a sealed enclosure such as that of the QB12.

IN USE AND PERFORMANCE

You can own the best and most powerful subwoofer in the world, but if you put it in the wrong place in your room, it’s not going to work well at all. It’s like that mantra that real estate salespeople are fond of quoting: “It’s all about position, position, position”.

As a very first step, this means what you will have to do is place the subwoofer where your head would normally be when you’re listening to music (and/or watching the screen). I appreciate this may mean some creative use of stacked milk crates and/or furniture items, and the temporary moving-aside of the seat or couch you would normally be sitting on, but it’s worth it. Once the subwoofer is in place in this exact position, connect it to your system (you will need a long RCA–RCA lead, which you can either buy or borrow from your friendly local audio store), after which you should start playing a music track with plenty of low bass. Turn up the volume of the subwoofer a little higher than normal, so you’ll be ready for the next step.

The next step is to start crawling around the floor (and no, I am not joking) with some white electrical tape in your hand. As you crawl around, you’ll hear the bass alternately get stronger and weaker… in fact at some points in the room the bass might be so weak that you will barely be able to hear it at all. Mark all the spots where the sound is strong by sticking some white tape to the carpet. Once you have covered the entire floor area, your carpet should be littered with bits of white tape. For best sound, you then place the subwoofer on the bit of tape that’s roughly equidistant from your main speakers and you’re done. If this position is aesthetically unappealing to you or your better half, choose the next closest position, and if this isn’t OK, then the next, and so on. Ultimately, so long as the subwoofer is positioned over a piece of tape you’ll get good sound from it, but the further it is from the main speakers the less well-integrated its sound will be in relation to those speakers.

Once you have positioned the subwoofer, it’s then time to tune the sound using the volume and crossover controls. I’ve never seen anyone do this successfully by ear, so it’s best to use test equipment, though in this case that test equipment will be nothing more complex than a special audio test track (from a CD or a digital copy of that track) and an ordinary mobile phone onto which you’ve loaded a low-cost spectrum analyser app. The calibration process is quick and easy, even if you’re non-technical, but it’s a little too long-winded to go into in this review, so you’ll find all the information about how to go about it here: www.tinyurl.com/sub-cal

Having already established the best place for a subwoofer in the three differently-sized rooms I use for reviewing I didn’t have to do the crawl-around, but I did have to do the volume/crossover calibration and during the calibration process I realised the performance of the Q Acoustics SB12 was pretty special because it went so smoothly, was so easy to do, and returned absolutely fabulous measured results in all three rooms, so I was expecting great things even before I’d played my first bar of music.

However, as fate would have it, my first experience of the sound of the Q Acoustics QB12 was with a movie, and I have to report that the QB12 smashed through the action like a steam train, hitting me right in the stomach and taking my whole body along with it. What we have here is a subwoofer that can realistically deliver the deep bass component of the explosions, the high-speed crashes, the intergalactic space battles (even those these would actually be completely silent due to taking place in a vacuum, but let’s not bring reality into this) and all the deep bass sounds that make action movies so thrilling to watch… in other words, the Q Acoustics

It’s been too recently released to have won any awards yet, but I have absolutely no doubt it will win a great many in the very near future