August 14, 2009
By John Krutke of Zaph Audio
The ZA14W08 driver is something I brought to market because I believe it fills a need. If it's not obvious from some of the specs, I was not after creating a 5" bass monster. For woofers of this size, these days it seems like the world is full of low sensitivity woofers with high Xmax and low Fs. I went in a direction that I consider opposite, creating a woofer that is as much of a dedicated midrange driver as it is a midwoofer. While 87 dB may not seem that high, there are far too many 5" woofers around the 85 dB mark or lower. 87 dB isn't so high that serious compromises or expensive design choices had to be made in other areas, but not so low that it couldn't be used as a midrange in a 3-way system. Low Fs, bass extension and excursion were not the leading requirements. The goal was only to have enough there to be able to reach a usable output level in a system with a typical subwoofer and crossover. Aside from what I considered enough extension and excursion to get that done, all other design choices were made in favor of low distortion and smooth in-band response. Midrange performance is always the most important to me. Music lives in the midrange and if that's done well, everything else falls into place easier.
I've created a driver that can be used up to 3kHz, though 2kHz is a bit closer to optimal depending on the system tradeoffs required. I've seen a few too many problematic woofers that require 1500 Hz crossovers, and the value of such a woofer suffers because it has to be used with an expensive tweeter. Part of a driver's value is in it's usability with other high value system elements.
My two favorite 5" woofers remain the Seas W15CY001 and the Scan Speak 15W8530. I believe these woofers still have their place in the market but they certainly don't cater to the high value segment. The ZA14W08 does not have the in-band smoothness of the Seas or the low non-linear distortion of the Scan Speak, but it does do well for a fraction of the cost. Honestly, if either of those woofers were half their normal selling price, I would not be here right now chatting about my own woofer.
Let's look deeper into some of the listed Design Features:
"Strategically placed Faraday rings in the motor allow for low non-linear distortion and limited inductance change through the voice coil's range of travel." This is of course a big part of the driver's low distortion. The motor was designed by myself and assembled by the build house according to my drawings and specifications. I won't be getting into too much detail as this is now a design secret. I'm not here to teach folks how to design a motor. But, I can say this: there's nothing too special in there. Many of the parts are right off the shelf. I could have done better, but not for the price I was shooting for. Custom creation of several new motor parts would have caused the price to skyrocket. It's easy to understand how the sell price of a driver can quickly pass $200 without a dedicated eye on costs. The secret to high performance with simple parts can be summarized in one word: balance. The motor is simple, but well balanced particularly in regards to the Faraday shielding. It's not about how much copper is in there, but about where it is placed.
Some build house customers have insufficient input on driver design. There is certainly not the level of design input that would be seen in-house at a Danish speaker manufacturer. Build house customers might say "I want low distortion, I want smooth response and I want this color." They might even go so far as to say "I want a copper ring and an undercut pole piece." But still, without any design specifics, all they get is a driver that visually looks good with performance based on whatever the build house happens to throw together. I don't consider myself a full fledged driver designer, but rest assured that I had input on all elements of this design. The only real part of this driver where the design was "all mine" is the motor. For everything else, a lot of experimentation was done on several elements to prove that the choices I made were good ones.
"A formed aluminum alloy cone provides very flat response within its passband and a breakup node that is high in frequency for extended usable bandwidth." This cone was selected because it is a proven performer. It is not a new cone but an off the shelf item. No additional work was done by me aside from selecting it for this product. I will say this: aluminum is a lot easier to get right in a low cost design than a fiber based cone. Smooth in-band performance was more important to me than a controlled out of band rolloff. These days most crossover designers know how to properly deal with a breakup node.
"Machined aluminum phase plug eliminates compression and reflection that would normally occur under a dustcap and promotes cooling of the voice coil." The cooling of the coil is a minor element while compression and reflection are the major elements. The breathability of the area under a dustcap is very important. There are some great drivers available that use dustcaps. They use design tradeoffs such as perforated cones, formers and/or a large hole through the center of the pole piece. With the use of a phase plug, I've bypassed these tradeoffs. I do have a hole through the pole piece but it's small and only there to help alignment and mounting of the phase plug. The removal of material for that hole did not substantially lower the flux field, BL or driver's overall sensitivity. I know some people worry about air leakage through the coil gap. No worries, it's not even close to being an issue. That goes for all phase plug drivers.
"Tinsel leads are mounted to opposite sides of the voice coil former to promote even cone loading." My original design specs wanted these mounted at a perfect 180 degrees on the coil former. But the manufacturing setup only allowed for about 120 degrees. Not perfect but good enough. The main point being that we avoided having tinsel leads mounted to the cone, or worse, both on one side of the cone. It varies per design, but I have proven through experimentation and testing that connecting the leads to the cone adds elements of both linear and non-linear distortion. It takes several forms, in a minor case it's a tiny bit cone flexing under load, up to a major case of lead slap. In an optimal driver, the only things that should touch a cone are a former and the surround.
"Rigid cast metal frame of an open design includes ventilation under the raised spider." The frame is a known nice unit used by many build houses. It can be seen on many drivers, and with good reason: it's excellent. I have proven with a tight focus IR temperature gun that the coil and former in a closed rear spider operate up to 30 degrees F hotter than an open spider. It was tricky to prove, since I can't get a temperature reading while a coil and former is inside the motor. I had to do a quick release magnet clamp system. Alignment was a pain but I did get the readings I wanted though some coils and formers were killed in the process. Additionally, there are some cavity effects from the cup shape of a closed spider system. These cavity effects primarily affect the lower treble frequency response. Knowing all that, it almost seems to be a no-brainer to use a frame with an open raised spider, but it's not. A serious negative side effect of an open spider is a lack of protection from ferrous garbage getting caught in the gap. This can cause distortion or buzzing. On the back side a concern is magnet chips. On the front side, for drivers with phase plugs a concern is screw shavings. Philips head screws are more notorious for this than socket head cap screws. Ultimately, I side with performance and hope that end users handle an "unprotected" woofer with care.
"Flat linear spider provides uniform restoring force throughout the voice coil's range of travel." The alternative is a "cup" spider. It's named as such because the sides are extended down to help a driver reach higher excursion for a given spider mounting location. This cup can cause suspension nonlinearity because it tends to flex easier in one direction than the other. The end result is higher harmonic distortion, particularly in the bass and midbass. I have been lucky enough to test a pair of drivers where the only difference was the spider, and it was pretty easy to see and hear the benefit. Suspension linearity overall is an often overlooked design element, with most eyes on other issues that are easier to solve.
"Integrated gasket for airtight seal and vibration resistance." It does help to seal the driver to the baffle and prevent rattles, but now we're getting into the minor details of design elements that might not even be worth mentioning, like gold plated terminals or ROHS compliance. Rest assured that eyes are on details. One detail has nothing to do with the driver itself, but the box it comes in. The box is very damage resistant and has a foam spacer on the cone side. The woofer sits in a folded and stamped insert that keeps the edges from touching the sides and keeps the driver centered in the box. UPS survivability was the idea here, because we all know that they love to play football with the packages and deliver them to your porch with an overhand through, Brett Favre style.
© Zaph Audio LLC, 08/14/2009