Matt's DIY speaker
This is from my archived web pages. The exact Audax woofer is no longer made. The boxes from Parts Express have also changed
Speaker selection and design
I wanted a 2 way speaker to be used with my sub-woofers. A 2 way system requires a woofer no
larger than 8" (some say 7") so you get good off axis response at the upper range of the woofer.
A 6.5" speaker seemed best.
I looked at many woofers from Audax, Vifa, Focal, Dynaudio, and Peerless. I finally decided on
Audax HM170Z0 Aerogel woofer (top left). Aerogel is a "new" material that is suppose to have the benefits of kevlar, carbon
fiber, and paper but none of the disadvantages. It didn't hurt that it had the cool looking
"phase plug" like the fancy speakers do in the high end magazines. The Audax Aerogel speaker was the best value. It is a little more than the standard
kevlar or carbon fiber speakers but not nearly as much as something from Dynaudio.
I used the suggestion in the '97 issue 4 of Speaker Builder magazine to determine if the woofer
was suitable for a sealed enclosure. The article by Marc Bacon (pg. 18) uses the Fs/Qts to determine
if a sealed or ported box is best. If the Fs/Qts is less than 82 then sealed is best. If the number
is above 100 then ported. Any where in between indicates either method will work.
The tweeter needed to have a fairly low resonant frequency (Fs). I wanted to crossover the
tweeter 2 octaves above the Fs. This is one of the many suggestions in the Loudspeaker Design Cookbook by Vance Dickason.
The Vifa D25AG-35
has a Fs of 850HZ due to its extended rear chamber. I have heard
different types of dome tweeters and have developed a taste for metal dome tweeters. I think the
harsh sounding stereo type (pun intended) metal domes have got is from badly designed tweeters. A good
metal dome should be crisp and accurate but not harsh. I think the Vifa does a very good job.
You have probably seen the same Vifa tweeter or its shielded version in some high end
commercially made speakers. Snell uses the Vifa tweeter in it's E.5 $1400 per pair speakers reviewed in
the September '98 issue of Audio magazine. When I saw the pictures of the Snell speakers I said to myself "those
have to be my Vifa's, they look identical". Sure enough, in the review the author mentions the tweeters are made
PSB is suppose to use the same Vifa tweeter. I could not confirm this. You can pick the tweeter up
for about $30. Such a deal.
The Box woodworking
I am happy with the pre-made box that came from Parts Express. It is put together very well. Parts Express
has since changed the boxes they carry slightly. The new boxes have the back veneered, mine
did not. They have also made the sides and back out of 1" MDF instead of .75" MDF and the front
has gone from 1.75" MDF to 1.5" MDF. The coin (quarter) sitting against the bottom of the front baffle (picture to the right) gives you
an idea of size. The most challenging part of the project was the flush mounting
of the speakers. Flush mounting is done to keep peaks and dips from occurring in the frequency
response. The tweeter was no problem. A circle attachment was attached to a router. Once you
set it for the radius and depth you are ready to go in. The woofer is a totally different story.
The metal frame on the woofer is not round. Read that as pain in my behind. You need to trace
the speaker on a piece of wood and cut it out just shy of your pencil trace. Then you take and sand
the little that is left until you get right up to your pencil mark where you traced the speaker.
Now hopefully the speaker just barely fits through this hole. This requires a whole lot of sanding
or ultra-precision with a jig saw and still some sanding. Remember that the bearing on the
router bit is going to trace every little imperfection on to your front baffle so everything needs
to be nice. Your freshly sanded cutout becomes a template for your speaker baffle board. The
template is attached to your baffle (where you are going to mount the woofer) and then a router
bit that has the bearing over the bit is needed. These are not real common but can be found in
a "full on" tool shop. Carefully set the depth remembering to include the depth of your template.
Always practice on a spare sheet of wood before you tear up your speaker baffle. I like the Aerogel
Woofer but if there is a next time I think I will try to stick to a speaker that has a round frame. The flush
mounting of the woofer took more time and sanding that I like to recall. It does however look very
nice once you are done.
I mounted the woofer very close to the tweeter. The closer the better (more Vance Dickason's words
I found some nice screws intended for metal at the Home Depot. Their heads fit flush into the
tweeter mounting holes.
I used a bi-amp five way binding post plate (see picture at right) for speaker connections.
I was concerned about the plastic plate buzzing or rattling so I gave it a thorough coat of silicone.
I actually smeared silicone all over the inside of the plate and surrounding area. The mass of the silicone as well as it adhesion
to the surrounding wood keeps it from buzzing.
I used a small bead of silicone to seal the tweeter. I used a black putty like substance to
seal the woofer. No one seems to know exactly what it is called but my friend got some from
Parts Express when he ordered some woofers. I used what he had left over but was about an inch
short. The guys at a car stereo installation shop recognized it immediately and gave me a foot
of it free. I didn't not want to use silicone for the woofer because it would be hard to get
the woofer out again.
The woofer whole is the biggest and best access point to the box and I wanted to use
something that sealed well but did not adhere strongly.
The crossover is passive and is 12db/octave for both the tweeter and woofer. I used Solen
capacitors and 16 gauge Sidewinder coils sold at Madisound. The resistors are non-inductive and
rated at 10W 4 ohms. Everything is mounted to a perf.
board using plastic ties. I reversed the polarity of the tweeter connection as this x-over is approximately
180 degrees out of phase at the x-over frequency. The reverse
polarity connection keeps a response dip from appearing at the x-over frequency (3000HZ). This was confirmed with a SPL meter.
You can see the crossover schematic and 2
graphs generated by the program.
You can also find more pictures there.
Matt Tucker Escondido CA.