Audax Mini - AP100Z0 and TM025F1
Parts list
Drivers and crossover components as ordered from Madisound:
Don't forget your choice of terminals, port tube (if required), damping, wire, and other internals. Then there's your choice of cabinet building materials. I will not be getting into cabinet building and/or finishing techniques in this write-up, just a quick overview.
If Madisound is not your speaker parts vendor, these are all standard value parts which can be ordered anywhere. For the inductors: It's important to keep the DCR of L2 low so as to not kill an already inefficient speaker any further. That is why I recommend steel laminate core. For the other inductors you have some flexibility in the DCR values.
Design Details
The crossover is 4th order Linkwitz-Riley. The 6db down point is about 3300hz. The drivers themselves do not present very smooth response curves, particularly the tweeter. It's not like it's a bad tweeter, but in the $20 each range there are a few better options.
The TM025F1's response curve is like a frown upside down. Even once I got it flat, it didn't really sound good until I pushed the crossover point up to 3500. The TM025F1 suffers from the same thing that afflicts many cheap neo tweeters - high Qts resulting from too small an airspace behind the dome. Then you get the big honk that comes from the peak in much the same way a 15" woofer has a serious boom problem when you put it in a 1 cu ft box. For this system to sound any good at all, I had to stay far away from the tweeter's low end. It does sound pretty good though after a little work.
I see this tweeter misused in several designs. Because of it's low cost and small size, some designers are compelled to use simple (read: cheap) crossovers with it, usually 2nd order electrical consisting of a single cap and a single coil. This is a poor design, and the tweeter needs much more response shaping than a simple crossover like that can provide. The crossover I've designed represents the minimum number of components that I feel is required to make this tweeter usable. Joe D's Audax home theater design for the front channels looks like a somewhat correct use of this tweeter although he crosses over a bit too low. On the other hand, his rear channel is simplified to the point where I think he doesn't care how they sound, just as long as they don't blow up. Note that I'm not saying Joe's rear channel is a poor design, just that he limited the number of components to reduce cost, and it's better to do that on the rear channels than the critical fronts.
The woofer response curve was better and more easily controlled. There is a slight null in it's response at 2kHz but it's not enough to be audible. Overall this is a great sounding woofer, though it's power handling is on the low side. It would probably make a great midrange driver too. When used as a midbass, don't expect a whole lot of bass depth or output. (obviously- It's a 4" woofer) These mini-speakers will sound the best when actively crossed over to a subwoofer, and a stereo pair of smaller subwoofers with one on each side would be even better. Stereo, or dual mono subs will allow you to cross over higher without tilting the balance to one side.
Measured Results and Speaker Placement
The measured response came out pretty close to the modeled response, though it was off a little at the crossover point. This is likely because I didn't have a .55mH in my stock and just used a .50. Notice how that tweeter's low end peak is still kinda showing up at 1500hz. The tweeter really could have been a 4th order electrical but I didn't want to throw another inductor at it to get it under control.
This design has full baffle step compensation. As such, don't plan on putting it on a shelf or in a bookcase. It needs to be on stands, away from the walls. This arrangement will provide not only the smoothest bass level but also the best imaging and soundstage. With speakers of this size, a nice pair of "surround stands" should do. The listening axis is designed for the tweeters to be at ear level.
Being a very small design, there is some power handling concern. The chart above starts with one watt and doubles until 128 watts at the top. The AP100Z0 has 2.7mm Xmax, and as you can see, running full range this driver will not handle much more than 10 watts or so. This may be enough for many of you using these near field or for lower level listening. For those who like it loud, your only option is to add a sub and cross over high. Preferably, dual subs for each side so as to not tilt the tonal balance. If you use one sub, put it somewhere between the drivers, not off in a corner.
A 4th order active crossover at 200 Hz will allow these to get very loud while avoiding over-Xmax. If you don't need that kind of power handling, you could cross over at 150 or lower to reduce subwoofer localization.
Enclosure details
The baffle as seen in the photos is removable. It is held in place with socket head cap screws, 1/4-20 x 1.5" long. Part 91251A546 from Mcmaster.com. Inside the enclosure there are 4 gussets in the corners, with each holding a Parts Express hurricane nut. This is one of a few designs using this box. It's nice to just slap on a new baffle if I want to try something else.
The socket head cap screws are countersunk. A nifty bonus of this is that neo magnets can be pressed into a grill frame and the grill will stick right to the bolts. No ugly ball and socket type grill fasteners, and it looks great with the grill off while still allowing the baffle to be removed. I don't have the grill done yet but it's a concept that I know works well.
The baffle finish is Rustoleum texture finish. I like this stuff better than the Truck bed liner that is popular for a black texture. The Rustoleum stuff has a finer grain and dries harder. A couple coats of primer are required first, mostly to seal the end grain. The enclosure is veneered in walnut with a hand rubbed tung oil finish. As I mentioned above, you're on your own with finishing. I don't consider myself very good at building boxes, which is part of the reason I use removable baffles. I only have to build one box and I can put many projects in it.
I leave the enclosure type and tuning up to you. This enclosure as shown has a port, but it was plugged for this project and the response curves you see above. Average broken in t/s parameters are: Qts-.56, Fs-81 hz, Vas-2.4 l. Pop these values into a modeling program like WinISD and decide what your preference is. Whatever you come up with, keep the baffle dimensions the same and change the volume by increasing or decreasing depth. You could use these full range, cross them over actively to a sub, use them in a small bedroom system or a home theater. The alignment that works best will depend on your needs. Whatever you pick will be a tradeoff between bass depth and power handling.
All that said, I think sealed is a better choice for these due to the driver's high Qts. Bass reflex should be an option only considered if power handling is a serious issue. Sure, you can get some decent bass extension in a largish enclosure, but high Qts drivers have terrible group delay when vented. Without getting too vulgar, if the right kick drum or bass strum comes along, it's going to sound like a fart at the tuning frequency. Sealed has a much smoother group delay. Sure there's less bass but it's tighter and cleaner. That choice is up to you. It doesn't hurt to try both, and the enclosure requirements for either are both in the ballpark and you can always cork the port like I did. Korbel Brute Dry works great. Open a bottle and then stick the cork in before it fully expands. :)
Don't neglect countersinking the drivers, even the tweeter. It makes enough difference to make it worth while, not to mention I think it looks kind of stupid if it's not done. In addition, the inside of the woofer cutout must be scalloped. In other words, it needs a 1/2" roundover or 1/2" chamfer around the inside everywhere except where the screws are. This promotes good airflow and slightly smoother woofer response. You can use wood screws or you can use #10 socket head cap screws with hurricane nuts like I did.
Summary
The strong points of this speaker are great imaging, soundstage, clean midrange and small size. The negatives are very low power handling and bass depth. Slightly high driver cost might be a negative also, but these Audax drivers seem to be on sale a lot lately.
Keep an eye on those sale pages folks. Parts Express had the drivers half price for a while, and Maplins in the UK had the woofers severely discounted. Madisound has an unshielded version on sale, which won't work for this design. Audax is getting away from the DIY market, and as such many vendors may be discounting some items to clear inventory. The fact that these drivers can be found for cheap is the reason I am posting this project.
Page done by John "Zaph" Krutke © 2005
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