[Note: See the historical details of Cerwin-Vega founder Eugene “Gene” Czerwinski's life in the intro to our article on the CVM S1000.1 Class GH monoblock.]
An Amp With No Outputs?
This test originally seemed to be pretty standard. Another modestly powered four channel amp, OK, we can always use more of those if they’re good. But nothing seemed particularly interesting until Wayne Connolly at Cerwin-Vega Mobile said the amp did not have output devices. Huh?!?! Maybe the power flows to the speaker via some extra dimension? I had visions of needing to revisit a graduate physics class I took on black holes to be able to understand this product. A little digging revealed that actually there ARE output devices, but they are hidden. No, not inside a black hole; they rest inside a giant chip. Well, not so giant really at 12×16 millimeters (that’s not much more than a half inch square)! That’s pretty tiny considering that not only do the output transistors fit inside, but so does the amp drive circuitry and so on. So why isn’t the amp like a one inch cube? Well, you still need a power supply, connectors, and audio circuitry which all take up space. Plus, even Class D generates some waste heat, so you still need a heatsink to accommodate that.
Did you say Class D?
Yes, CVM has entered the full-range Class D amplifier fray. For a long time, Class D in audio existed only for subwoofers. Making the outputs clean enough to operate full range simply couldn’t be done in a mobile environment. Plus, the bass amps were the ones with humongous power, not the full-range amps. The lesser power of full-range amps made their waste heat and current draw less of an issue, so their efficiency didn’t matter as much. But nowadays, the power levels have gone up, and simultaneously the room inside vehicles has sometimes shrunk.
Ladies And Gentlemen, Introducing the Stealth 440.4
Which for some reason becomes SX440.4 in abbreviationese. I feel a “Stealth mode” joke coming on, but to keep my conscience from troubling me at night, I’ll spare you that pain and simply inform you this amp is a compact four-channel priced at 9 retail. We’ll see how efficient it is, but Cerwin-Vega Mobile must feel some confidence because plastic forms the ends and big logo of the amp.
And Now, The Feature Presentation
(Please imagine the drum roll of your choice). Ladies and Gentlemen, CAE presents for your entertainment and edification the features of the Stealth 440.4:
•The wire connections follow CVM’s S1000.1: handy screw-onto-wire terminals in 2.5 and 2 mm sizes of somewhat soft metal. Don’t try to use non-metric Allen wrenches, you will strip these puppies out.
•The RCAs have Red inserts for the Right channel, and white for the Left, a nice little touch for dim installation locations. Plus, their labels say left/right front/rear. A small thing, and not uncommon, but we hate testing amps that just say “Channel 1, 2, 3, 4” while mentally trying to keep track of everything.
•Lest the balance between Heaven and Earth be shattered, the SX440.1 features BASS BOOST! And not just bass boost, but VEGA BASS, which enables you to adjust the frequency as well as the boost. Oh, surely the Rapture shall be hastened!
•Size is quite compact at 7” deep, 1.85” tall, and 9.35” in length (178x47x238mm). That’s pretty compact for 260 rated watts into four ohms. A check of the reminisce-o-meter shows that not all that long ago, 4×30 watt products would have been literally twice as big, and run hotter to boot.
•The diminutive size yields 0.675 watts per square centimeter* of mounting surface. For comparison, our reference Phoenix Gold Ti800.4 showed 0.304 W/cm2, so the SX440.4 more than doubles that density. We shall see what penalty, if any, you pay for that performance.
•CVM paid a lot of attention to matching the gains between channels. We shall likewise see if this effort succeeded.
You can see the connections and controls in these photos:
A Tight Match
As usual we let the amp warm up a while and then checked the low-level performance. The residual (background) noise level measured way below one watt, so you should never hear any amp hiss even with very sensitive speakers. Impressively, the noise didn’t really vary even when swinging the gain from minimum to near-maximum (some amps turn up all the noise when you turn up the gain). Channel-to-channel gain balance proved truly impressive, with only tiny differences between lefts and rights at the different gain levels.
If you wonder why the chart notes 3.6V instead of the usual 4V, that’s what would theoretically clip the amp with the gains set all the way down. In this case, it’s not a big deal. CVM assured me the input circuitry itself doesn’t clip, and the amp has some power headroom as we’ll see later.
Separation between channels measured quite good for the rears, and significantly more than the fronts. I re-checked that unusual result on a different day only to get the same numbers. Probably the physical routing of sound inside this tightly compact chassis causes a little more bleed between the front channels. With music in a car this won’t be a problem.
Distortion measured on the high side compared to most amps (though within spec). We rechecked these values, and made some weighted measurements besides. It doesn’t seem to be Class D noise, since the amp switches way up around 350 kHz. That gets filtered out by a special Audio Precision filter in-line, and the level was not higher than other amps. There must be some other interaction going on. But to put this in perspective, it would take trained listeners in a quiet room to be able to hear this level of distortion, so for you in your car on the freeway it should be fine.
Smooth Response
Checking the frequency response of each channel yielded the following picture:
One nice feature: the tight grouping of the curves. When Cerwin-Vega Mobile says they focused on matching channel gains, they succeeded in spades. Differences between the channels amount to an inaudible couple of tenths of a decibel at most. The response shows a nice smoothness. Yes, you can see high frequency response peaking, but above the audible range, and less than a decibel. Typically this peaking results from the designer trying to make the internal Class D passive filter very steep above the audible range. For more details keep your browsers looking for a future article about Class D frequency response problems. At the other end of the spectrum, the bass shows no peaking as it rolls off slowly, less than one decibel at 20 Hz.
Crossin’ Over
A different bass story appears with the lowpass crossover switched in. Some kind of subsonic filter, not mentioned in the manual, appears to kick in. It’s a steep one, maybe 24 dB/octave, around 18 Hz or so. This won’t match a vented sub unless that sub is tuned very low, so what is it for? Well, an active lowpass filter indicates subwoofer use, a tough load on a compact amp. This filter probably acts as a kind of protection for the amp itself.
Meanwhile, the highpass curve shows a textbook smooth rolloff of the bass frequencies. I set the frequency pot at the lowest 50 Hz setting, corresponding to more like 70 Hz in real life if you look at the curves. Don’t believe potentionmeter markings too much, they often vary like this.
Unhappy Watts?
After all the low level stuff, we ran the power tests. One nice effect of the gains matching so tightly? The clipping points were very close to each other, as you can see in the chart:
However, all was not completely rosy. Like other amps we’ve measured, the SX440.4 did not like a steady two ohm load. The distortion readings climbed while we watched and after a bit some kind of rollback or protection kicked in, reducing the output a decibel or two until the amp cooled off. (Measurements were made quickly, with rest in between, but even so you can see how the power droops from 4 to 2 ohms.)
We’ve measured other amps with this behavior—it’s a consequence of trying to pack ever more power into ever smaller chassis. Even the major amp brands will be following this trend, according to our secret intelligence channels. Is it a problem? Good question, and nobody has done professional research in this area yet. My seat-of-the-pants theory is that folks blasting the heck out of the whole parking lot or river are probably too deaf and/or drunk to catch a one decibel rollback, whereas those folks listening at less painful levels will have all the power available for dynamic peaks. Stay tuned for further industry developments in this regard.
At four ohms, however, Cerwin-Vega Mobile’s “little amp that could” easily puffed its way past specified power, even with the tough 50 Hz tone we use. The measurements were fine and stable. That’s a great result, and a goodly chunk of watts for the money. If the watts could be split up individually, the 99 Cents Only store could sell them per each! Interestingly, power tests at 1 kHz resulted in about the same numbers, so the amp appears to have strong bass by that measure.
Conclusion
One problem with this review is that I want to use the phrase “eye-poppingly compact”, but can’t. You can’t call it a “tiny” car stereo amp. Which is too bad, because Cerwin-Vega Mobile has packed a lot of power into a chassis we can surely call “compact.” Amazingly, they shoehorn 500 watts into this same chassis in the form of the SX500.1, also 9 retail. A little Internet perusal reveals some really jaw-dropping prices (check if the dealers are authorized), so you could put together a powerful system with small impact to your vehicle space and your wallet. Cerwin-Vega Mobile’s Stealth 440.4 offers great value versus size and price. Yeeehaw!
But, you ask, what about those 2-ohm results? After all, even satellite speakers which generally have a 4-ohm rating can present a load equivalent to 2 ohms or less under some conditions. We’d advise as follows:
•SPL aficionados who never turn the volume down from MAX may not be happy, as the amp may roll back the power. But it’s also possible they may be so deaf and/or inebriated (not while driving, please!) they won’t notice the power rollback at all.
•Customers listening at moderate volumes should be perfectly happy.
As for me, I’m really curious to test other upcoming compact amps. Bring ‘em on!
Car Audio and Electronics