Lightweight Cab Project

exmaxima1 · #1 12-05-2011, 06:42 AM

Recent activity on this forum about lightweight cabinets inspired me to build a lightweight cabinet as well. I didn't want to spend much so I opted against the overpriced neo drivers. The key features are:
--1/2" plywood construction throughout. A few small sheets were in the trash after a machine was delivered from Germany. I'm not sure what it is, but it is essentially void free and fairly light.
--I used quite a few braces internally to stiffen the panels. Whenever possible, I tied as many panels as I could together.
--That's a lowly Delta 12LFA, but It's fine for my application. Vented with a 4" dia x 7" long Aeroport. Internal dimensions are 12x15x20, which equates to about 2 Ft3. Net volume is about 1.75Ft3.
--I also added a 6" mid driver to extend the response and widen the dispersion. It is in a separate tubular sub-enclosure and simply high-passed via a 20uF cap.
--The finish is water-based truck bed liner. I can't recall the brand but it is that great stuff everyone discusses on this forum. I love that stuff and will certainly use it again in the future.
--Inputs are a pair of Speakons. One is the combo version with a 1/4" plug option.
--Grill is a pvc coated screen used on machinery guards (also found in the trash). Basic plastic corners and rubber handles.

Total cost was under $150. And the finished weight came in at 35 lbs. It was lots of fun to build and I am amazed at how nice it sounds. I may never buy a factory cabinet again!

exmaxima1 · #2 12-05-2011, 06:43 AM

Her's some pics of the internals

gregoire1 · #3 12-05-2011, 07:07 AM

Wait...wait...wait, what 6 is that? What's the crossover freq? Whats the volume of the 6's box? I have so many questions!
Great looking cab on any budget!

exmaxima1 · #4 12-05-2011, 07:21 AM

Quote:

Originally Posted by gregoire1

Wait...wait...wait, what 6 is that? What's the crossover freq? Whats the volume of the 6's box? I have so many questions!
Great looking cab on any budget!

I'm embarrassed to say it's a Dayton PA driver---had I known this project would have been so much fun, I would have spent a little more on an Eminence. But the wholesale cost was so low, I figured I'd try it. Crossover point is about 1000 @ 6dB/oct. Mid enclosure is .1 Ft3 internal volume.

will33 · #5 12-05-2011, 08:38 AM

That's some helluva bracing.

All in all, nice work. The capped tube makes a fine subchamber. My only concern would be crossing the 2 drivers. Aside from interference, etc., the mid is only 12db down @ 250hz. Aside from affecting power handling, with the woofer running full range there will be an impedance dip where the two overlap, so you'd essentially have an 8ohm cab in the lows and highs and a 4ohm cab in the mids. Something to consider if you ever add another cab to the stack. All in all, it's a good cab, but I'd look into adding a couple parts to the crossover so you roll the top end of the woofer down and pass the mid a little steeper. This can be difficult to get right without some test equipment and a lot of experience but, eminences cab design pdf's will have an in box impedance curve for the woofer, if you can find that for the mid, you can use those to plug into a parts calculator and get something decent as far as driver protection, etc. Not as highend as a pro can do it but useable, safe and decent.

exmaxima1 · #6 12-05-2011, 09:42 AM

Quote:

Originally Posted by will33

That's some helluva bracing.

All in all, nice work. The capped tube makes a fine subchamber. My only concern would be crossing the 2 drivers. Aside from interference, etc., the mid is only 12db down @ 250hz. Aside from affecting power handling, with the woofer running full range there will be an impedance dip where the two overlap, so you'd essentially have an 8ohm cab in the lows and highs and a 4ohm cab in the mids.

It is indeed a pretty simple design. The 12" speaker has an inductance of 1.75 mH, so it starts to roll off naturally well below 1Khz, while the cap on the midrange complements it to maintain a minimum impedance of about 6 ohms. I modeled several multi-pole crossover designs and they all exhibited much deeper impedance dips. I cannot see how you can add a shunt inductor to the midrange driver and NOT get a lower system impedance----I've wired up a few and did impedance sweeps, and the dips do exist. In any case, the midrange driver was touted as a full-range design, and it seems to be able to handle the slow slope of my simple crossover. Again, my goal was to build a low-cost lightweight cabinet. My "next" design will be more elaborate

will33 · #7 12-05-2011, 10:03 AM

Cool, you're on the right path then. Probably a bit ahead of me in the crossover department. The trick to keeping the impedance up is to use a true 2-way and get them set so the response is even through the crossover region but doesn't overlap too much. Then in real world application, the coils heat up, the interaction between the coils and crossover changes and everything's out of whack again.