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The little amplifier that can. (And does.)

One of the original pillars of car audio worldwide, rising predominantly to fame during the 1980s and 1990s, American-born Cerwin Vega is a vastly experienced company with an extensive heritage spanning well over half a century. The company has grown synonymous with good quality but affordable equipment, and although its amplifier stable includes some of the mightiest components the car audio arena has ever seen, the Company also produces a range of smaller amplifiers,which cater for those of us not wanting or able to set either the world or their bank account on fire.

Monoblocks don’t always need to resemble office blocks in order to achieve their goal. Of course there will be reaers keen to retort that smaller monoblocks inevitably yield smaller power output and hence “less control”. 
But consider this. Just as a Hayabusa doesn’t require the 7000hp motor of an open-cut mining truck to get moving, if you inject a little forethought into your subwoofer selection you won’t require quite so many thousands of watts in order to have its motor structure moving fluidly. I’ve learnt this from experience and, clearly, so has Cerwin Vega. 
And of course the flipside of the equation is space. Not everyone wants to sacrifice half their boot in order to garner decent sub-bass. And with the rise of Class D amplification, this mindset has borne witness to the rise of a new genre of compact digital amplifiers, a genre that’s recently taken off like a veritable rocket.
No heat overload
So, this B51 is the monoblock from the esteemed new Stealth Bomber series from our American friends. It’s a high current digital class-D design, with the output channel being rated to produce over 175 watts continuously when presented with a four-ohm load, or 300 watts when faced with a two-ohm load. Loading it down further doesn’t compromise operations either; it just ups its power output to 500 watts continuous to face the challenge and continues right on its merry way. Compared with some, that might not sound too stratospheric, however just look at its merits — an insignificant footprint of 195mm x 100mm, height of just 35mm, and weight of a mere couple of kilograms. So given its size its power output is quite simply astounding.
The secret lies within the topology. Class-D amplifiers utilise very powerful transistors that are controlled digitally, switched between either fully on or fully off, as opposed to an analogue voltage tracking the input, as would be the case with a class AB design. This system yields high efficiency — leagues ahead of analogue counterparts. When the switches are open, all of the power supplied to them is delivered to the load and none converted into heat. On the other hand when they’re off they have full supply voltage standing across them but no current flowing, so again zero heat is dissipated. The darned laws of physics do get involved, sadly, so perfection remains an unattainable state, but the Class-D design can return an efficiency hovering around the 90% region, with the remaining energy converted to heat (energy cannot be eradicated but only converted). Distortion levels also remain inherently a little higher within this class of amplifiers because the output waveform is built using a square wave blocks. That said, the E51 does return an impressive total harmonic distortion of 0.5%. 
Frequency response is a suitable 10Hz–350Hz, and the signal-to-noise ratio is 85dBA at 1 watt, or 101.1 dBA at full tilt. Alongside the distortion, of course, is the other spec critical to subwoofers: damping facto — in plain English the amplifier’s ability to control the subwoofer’s motor movement, both acceleration and the perhaps more crucial deceleration to the zero point before it begins the opposing side of the sinusoidal wave. This is of utmost importance if you desire sharp and accurate sub-bass reproduction. Calculated nominally by dividing the subwoofer’s impedance by the output impedance of the amplifier, the B51 boasts a damping factor of over 250. 
Bold, not bulky
Appearance wise it’s fair to say Cerwin Vega amplifiers have always looked, well, typically 
American. They’re often large, bulky and covered in heat-sinks (a look which I’ll confess I actually have no bones with). The Stealth Bomber units, though, are the antonym of this trend. They’re small, anything but bulky, and the designers have spruced up the outward appearance a little by combining the overall black case with some very stylish looking silver end caps and side bars. This gives them the ability to look right at home virtually anywhere, although given their size they’re likely to be unseen anyway! 
The outer case is primarily aluminium extrusion anodised in black, while the bottom plate is stamped steel. The shock-resistant plastic end caps serve to protect the controls and connection blocks. Along one end reside all the physical power connections, including 
8AWG power and earth input terminals, and dual speaker output terminals so you can run multiple subwoofers if you so choose. 
Along the other end are the remainding connections, starting with high- or low-level inputs. You simply plumb either your RCA or speaker level into the plug provided. Also here is a remote port for a bass level controller, while located adjacent are the aural controls. Aside from 10V–0.2V sensitivity these include a 10Hz–55Hz subsonic filter, a 40Hz–400Hz low pass crossover and a 0dB–12dB bass boost.
There’s no phase controller included. Initially this appears to be quite an oversight because the ability to change phase can be critical to getting the subwoofer blending neatly with your front stage. However Cerwin Vega has method in its madness here, as swapping phase is as simple as switching the wires around — you’ll only do it once, during set-up. So to omit this is actually fiscally wise; it allows for the manufacturing costs to come down just that little bit more.
Dismantling the unit for inspection is as straightforward as removing four cap screws and sliding the bottom plate off. Revealed thereby is a busy electronic landscape, intelligently laid out, and quality components too, appropriate for handling high current demands, all robustly mounted upon a solid red glass circuit board. A quadruplet of 1000uF /25V Samwha stiffening capacitors smooth the current before it’s transferred across to a large air core transformer for voltage ascension. From here the higher voltage is transported to twelve 470uF/50V Samwha power capacitors for storage until needed, at which point it’s then distributed across to ten audiophile-grade MOSFETS. These transistors deliver power aplenty when called upon, and are lined strategically along either side of the heatsink, carefully placed to allow for maximum thermal dissipation when things start getting a little warm. Cerwin Vega has paid careful attention to where the signal handling components reside, for heat aside they must also be kept well clear of the current handling areas lest they induce whine. Not as easy task with a physical design this small.
Fits just about anywhere
Included in the box are mounting screws, tools, multilingual instructions, a bass-level remote and an input plug with RCAs plumbed in. 
Finding real estate for the unit is, of course, a non-event, with it fitting just about anywhere larger than an ash tray. I had it installed within 15 minutes and began by setting the gains against my oscilloscope. Despite its diminutive proportions the output waveform doesn’t reveal significant signs of deterioration until well into the output range, which is a good sign for those a little concerned that the small unit might struggle. 
As aforementioned, subwoofer choice is an important decision when using an amplifier of this ilk; you’re don’t want to be hooking up a row of Cerwin Vega Strokers here... I settled on testing with a single highly efficient 12-inch subwoofer running in a sealed 35-litre test enclosure. Starting out with softer genres such as acoustic, jazz and classical the amplifier proved clean and smooth, reproducing any and all low-end notes, with consistency to boot. Taking it from there I stepped up the genres to the more dynamic electronic music before finally landing at the hardest of genres for amplifiers to reproduce: rock and well-engineered heavy metal. When handling these demands the B51 remained in impressively complete control, even during the fast double-bass blast-beats of thrash and industrial-style metal, and to a surprisingly frightening volume! That said I must also add the disclaimer that you shouldn’t go harbouring any unrealistic expectations either. The B51 is not a titanic-sized amplifier, ergo there comes a point beyond which you cannot push it. Nonetheless it’ll provide more than enough bass to satisfy those not aiming to bounce along the road in a wild SPL display.
In conclusion, then, the B51 represents the epitome of what new-age mini power amplifiers should encompass. Its smart-looking, smart-sounding and given its meagre price tag of $449, it’s beyond a bargain. 
Cerwin Vega B51 amplifier
Cost: $449
+ Bold but not bulky, Smart and solid for its price
- Big sub arrays will need more
Type: Class D one-channel amplifier
Features: Adjustable crossover and subsonic filter, bass boost, subsonic filter, high and low level inputs
Continuous power rating: 1 x 175 / 300 / 500 @ 4 / 2 / 1 ohm