When subwoofer engineer and expert Vance Dickason did the objective testing on the Stroker Pro, he said it was the most outrageous car subwoofer he’d reviewed to date. Like the RE Audio XXX subwoofer, this is another behemoth of a subwoofer, weighing in at 63 pounds. But as Vance explained, the product’s technology was a lot more remarkable than the weight.
The Stroker Pro cast aluminum frame acts as a giant heatsink (an old but good trick) and a highly effective Faraday shield or shorting ring (a new, patent-pending trick). The cutaway drawing shows that this woofer not only has two separate magnets (actually, the lower magnet is two magnets stacked together, so technically three magnets), but also two gaps, a technology known as Multiple Magnet Air Gap (MMAG). Because of the physical layout of the MMAG motor format, CV could mount the lower magnet system tot the back of the frame, like a normal woofer, but turn the fram’s inside diameter upward to forma shorting ring.
In addition to attenuating nasty eddy currents produced by the woofer motor and lowering distortion, a large shorting ring also reduces motor temperature by decreasing inductive heating. Figure 2 reveals what an outstanding thermal pathway this frame makes with the upturned section directly adjacent to the naked voice coil. The Stroker has four other thermal pathways: two in the spider-mounting area and two at the rear of the motor. There are two sets of vents integrated into the lower-spider mounting shelf (this woofer has two mounting shelves for three spiders). A 1/8″ gap goes around nearly the entire perimeter of the shelf’s 10.25″ diameter except for the six half-inch diameter mounting posts that it suspends from. THis gives substantial venting area for the airflow generated by the lower spider motion to move air past the exposed voice coil and top plate. For the air caught between the two spiders, there are an additional six quarter-inch by two inch vents. At the back of the side of the motor you have a series of six 3/32″ diameter peripheral vents that move air from between the pole piece and the voice coil out the back of the T-yoke. Lastly, an aluminum-sleeved 1-3/4″ diameter pole vent powered by the large, moving clear plastic dustcap supports the third spider at the top of the woofer. Because of this, the sleeve extends to about double the height of the pole and thus acts as another heatsink.
The real heart is the MMAG motor structure. Since the voice coil is shorter than the combined length of the two gaps, it appears similar to a conventional underhung voice coil motor. In the conventional motor, as the short coil rides out of a single large gap in either direction, the total number of voice coil turns in the gap decreases and so does the total Bl, or total horsepower, of the motor. However, with the Stroker Pro dual-gap motor, the situation is more like having two gaps working in unison. As the number of turns starts decreasing in one gap, the number of turns in the other gap increases so that the number of voice coil turns of wire in the gap stays constant. The cool part is that it operates in a way that the two gaps are always working in conjunction with each other such that the number of turns does not begin decreasing until the voice coil starts to leave just one gap. (See the sidebar at the bottom for more info on dual-gap technology.)
Other features for the Stroker Pro motor include a forged and CNC-milled single piece T-yoke that includes an extended pole piece with a pole vent. This vent gives access to a patented adjustable spider bias system for the top or third spider. All the original Stroker woofers had this adjustment system; however, it was set at the factory for maximum linearity. Stroker owners found that tweaking this setting enabled the woofers to play even louder. Biasing a spider toward the front side and causing the voice coil to be deliberately off-centered toward the front of the gap area creates higher amounts of even-ordered distortion (2nd and 4th harmonics, to be exact). Besides the warm tones caused by the biased spider, the increased even-order distortion also increases the bass due to a psychoacoustic phenomenon called the missing fundamental (which produces perception of low bass from notes an octave higher). So if you want to go from very clean linear bass to an altered warm bass sound quality, Stroker Pro woofers have a tool that allows the user to do that.
One of the key features to good subwoofer performance is a solidly stiff cone. The one in the Stroker is about as stiff as it gets. Built from layered carbon graphite and thick paper, the cone is further reinforced by the large 7-1/2″-diameter clear poly-carbonate dustcap. Long excursion in a subwoofer requires the suspension to move long distances. One solution is incorporating a wide surround that allows the cone to move further. However, this decreases the cone area, which in turn requires the cone to move further! CV’s patent-pending solution is SdMaxx (Sd is an engineering abbreviation for the area of a cone). Rather than being low and wide, the polyether foam surround is more of an ellipsoid shape, tall and narrower. The result is the same excursion ability, but a greater cone diameter and area. The other part of the SdMaxx system is an attachment design that allows the entire cone assembly to be quickly removed and replaced.
The rest of the assembly consists of the three-polycotton/conex blend spiders. The two lower spiders (mounted inverted to each other to cancel out some of the non-linear behavior) are 8-1/2″ in diameter while the top spider is about 7″ in diameter. All three are progressive, which means they increase in stiffness the further out they move in either direction. The dual four-layer voice coil is wound with high-temperature copper wire on a black anodized 100mm (3.9″) diameter aluminum former. Voice coil tinsel leads are connected to dual connecting blocks on opposite sides of the frame. Each terminal block has two sets of hex screw terminals that accept up to 14-gauge wire, allowing the Stroker Pro to be easily configured with the voice coils in series or in parallel. —Vance Dickason
Subjective
The Cerwin-Vega Stroker Pro 15 arrived on my doorstep in an Anvil-type case with steel flip-up handles, just like those used to carry concert gear. Opening the box I was blown away–the subwoofer itself looks fantastic. It’s tall, massive and damn near 70 lbs.! But it’s the clear dust cap that captures your attention, allowing you to view the front spider and the carbon-fiber cone structure beneath. Beautiful!
Once I got the sub out of its case (with a little help), I quickly pulled out the instruction manual to see what the power handling was. I damn near fell off my stool when I read the 2,500 watts RMS power figure and the mind-bending 5,000-watt music rating! But this technological wonder doesn’t come cheap. As a matter of fact, the Stroker Pro 15 is the most expensive subwoofer I have put through its paces on these pages with a staggering retail price of ,699.00.
Installation
A quick call to Cerwin-Vega got me to Bob Diamond, CV’s senior design engineer. He recommended I use a sealed enclosure with an internal volume of 2.7ft3 for the best overall sound quality. Jayson Olson, master installer at Speaker Works, was enlisted to build this special enclosure. The enclosure would be constructed out of two layers of 3/4″ MDF with internal bracing to add strength and minimize flexing of the enclosure walls.
Connecting the wires was fairly simple. The Pro 15 uses heavy-duty metal blocks with Allen head set screws to lock the wires in. The subwoofer’s dual 2-ohm voice coils were wired in parallel for a 1-ohm load.
Mounting the subwoofer into the enclosure was a bit tricky. First, you are dealing with a front panel on your enclosure that is twice your regular thickness and Cerwin-Vega supplies these threaded steel plates and cap screw bolts to hold the Stroker in. My guys predrilled the holes in the box’s face and then glued the threaded steel plates into position. Remember, this is a large and heavy enclosure at an estimated 70 lbs. Combine that with the Pro 15 and we’re dealing with approximately 140 lbs.
It was time to see if we could fit this behemoth into my Scorched-Earth Black Ford F-350 truck. There was no way to place it in my normal location on the floor, so we placed it carefully on the rear seat. We tried the Stroker facing up and forward, pointing left and right, but it did not sound correct. With the enclosure on the rear seat, my seating location was in a null point and the bass response was pitiful at best. If I put my head up by the steering wheel, bass level increased exponentially. I flipped the polarity of the Stroker, which did not help, so it was back to the drawing board. Our solution was to pull the rear seat out of the Ford. This would allow us to get the Stroker into the location that generally works best for most subwoofers in my truck. The acoustic match greatly improved and now we had a great-sounding subwoofer (what I won’t do for CA&E magazine, jeez!).
To power the Stroker Pro 15, I installed a Zapco C2K-9.0XD amplifier to run it. The 9.0XD features a 24dB-per-octave crossover. With the 1-ohm impedance of the driver, this Zapco will deliver 2,000 watts of power. The front half of my reference speaker system consists of a pair of USD Audio B-72Pro WaveGuide separates. I power these with a Zapco C2K-6.0X amplifier at 150 watts per channel. The built-in highpass crossover filter was used to block the bass to the component system. These amps are fed via Zapco’s Symbilink balanced line driver SLB-U. There are no other signal processors in the signal path.
Listening
After four hours of break-in time I inserted a favorite track–Tracy Chapman’s “Heaven’s Here on Earth.” This subwoofer goes from very low frequencies all the way up the scale smoothly without drawing attention to itself. The bass imaging was extremely good. The bass line always stayed up front and did not pull to the rear of the vehicle at any point. The bass drum was tight and the bass guitar smooth and proper. That’s what’s so intriguing about the Stroker Pro. There is nothing faint about the looks of this subwoofer, indicating that it could not be anything less than a brute, yet it had an articulate and delicate reproduction quality when the music called for it. And, it had no trouble replicating layered bass lines. Overall, each instrument stayed well defined and taut.
On Diana Krall’s Love Scenes album, the stand-up bass note changes were clear and well defined. The note-to-note levels were equal and at no time were they out of proportion to each other. The bass plucks were very punchy and taut. This subwoofer is no one-note wonder. It accurately reproduces the frequency changes up and down the scale from the string bass to the guitar and the drum kit. It was very snappy and tight with excellent control. The Pro 15 exhibited no blooming in the upper bass frequencies like many of the other subs I have tested.
A great benefit to using a large diameter subwoofer like this one is how well it energizes the interior of your vehicle at really, really low frequencies. Case in point, I had to remove my garage door opener and sunglasses from their usual perch overhead at far lower volume levels because they were rattling almost instantly.I played the 10,000 Maniacs remake of “Peace Train” next. It opened with a bass drum kick that was very low in frequency. Many subwoofers will do an adequate job on this passage, but the Stroker Pro 15 really got the job done. Each kick of the bass drum was sharp, solid and deep. This track illustrated the performance gains that large format subwoofers have over smaller subwoofers.
Wrapping up, I put in Usher’s latest album and cranked up the “Intro” track that leads into the track “Yeah!” “Intro” uses mostly acoustic instruments with super dynamics and at 0:23 secs into it, the Stroker punched out a super tight, loud bass line that was still very realistic sounding. On “Yeah!” it simply laid down powerful, pumping bass that went down to the lowest bass notes clearly. Even on this repeating boomfest, the bass image stayed up front, solid and fixed.
Subjective Score Chart
Overall Sound Quality 45 (of 50)
Tonal Balance 8 (of 10)
Low-Frequency Extension 9 (of 10)
Clarity at Low Volume 8 (of 10)
Clarity at High Volume 9 (of 10)
Impact 9 (of 10)
Total: 88
Conclusion
The more I listen to the Cerwin-Vega Stroker Pro 15, the more I like it. It is the best all-around large-format subwoofer that I have tested by a large percentage. It did not matter what type of music I played, the Stroker Pro 15 made it sound correct and good. It does its job at all volume levels and can handle fantastically high amounts of power. This thing is very expensive, but its performance is exceptional! Looks like I am going have to do some begging to get a couple for our VW Beetle show car. Hey, Mr. Diamond! We need to talk. —Eric Holdaway
SPL in-car measurement at 2.83 volts, 1 meter—with Bruel & Kjaer Type 2231 Level Meter set to Un-weighted, SPL and Fast.
20Hz > 90.6 dB
40Hz > 93.4 dB
80Hz > 94.6 dB
Max SPL > 136.3 dB
The Max SPL measurement was taken using only 2,000 watts. At 5,000 watts, a gain of 3-5 dB is possible.
[Sidebar]
Xmax and Dual-Gap Technology
If you were to look at the Stroker’s motor in conventional terms, and consider the distance from the outside of one gap to the outside of the other, a distance of 44mm, as one long gap and figured Xmax based on this and the voice coil length (remembering that Xmax is voice coil length minus gap height and that number divided by 2), which is 38mm for the Stroker Pro, then the Xmax would only be a silly 3mm. However, when you consider the dual 12mm gap aspect of this motor, the Xmax number is really 32mm. And if you look at the actual Bl curve of this monster, it’s really more like 36.8mm, which is the manufacturer’s Xmax spec for this woofer.
There are a number of positive aspects to this dual-gap technology, the biggest being the enormous excursion potential. For a given coil length, the dual gap motor will have 2.5 times more Xmax than a conventional single gap motor. For example, a 38mm voice-coil length with a single 12mm gap would have an Xmax of 14mm, compared to the 32mm of the CV dual-gap woofer. The other benefit has to do with how constant Bl is at high SPL levels. Many conventional woofers have a more or less “bell” shaped Bl curve, which means that as they start operating at maximum SPL levels, the Bl or motor drive level will begin decreasing rapidly. This coupled with the elevated voice coil temperatures mean that the effective box Q and F3 numbers are generally higher than at low SPL levels. Effectively, this suggests that the really low bass tends to go away somewhat when you get extremely loud with most subs. With the Stroker Pro, the Bl curve is very flat and extended and the result is that the motor does not exhibit as much box Q and F3 shift as is normally experienced at high SPL levels. While this extra high output stability is primarily due to the flat Bl characteristics of teh Stroker Pro, some of this stability can be attributed to its excellent thermal colling characteristics. —Vance Dickason
Car Audio and Electronics
Thanx for the info. This sub woofer is a great sub woofer and wold prefer it to every on. I just love tho low end of the speaker.
Rating:5/5