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Old 22nd May 2012, 01:12 AM   #1
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Join Date: Oct 2009
Location: Anchorage, AK
Default DIY powered mains

I'm working on a pair of BFM SLA Pros (4x Eminence Alpha 6 + piezo tweeter array) for mobile DJ work. Cabinet design is not my strong suite, so I'm looking for some advice on my plans.

Ports
Parts Express has some triangular corner ports that would give me almost exactly the area I need. The front baffle is already quite full, and I'm to understand placement isn't really a big deal, so I'd like to put these in the rear. Any inherent problems with this idea?

Amps
I have some home-designed PCBs for LM3886 chip amps. My plan is to run one chip per mid/woofer and one for the tweeter array. The Eminence 6Cs are 4-ohm drivers, and my transformer is 36vCT.

I seem to have botched my WinISD install, so I haven't quite gotten the box modeling down yet. I think this is about the right amount of power for the Alphas, assuming a 2nd-order high-pass at 80Hz. (This is mostly a fail-safe as my DriveRack already filters the tops at 100Hz, 4th order. But I'm building an internal active XO for the 2-way power anyway, so making the lower part bandpass is practically free.)

As for the piezos... well, I know next to nothing about piezos. They don't take much current, I'm told, and seem to need a sizable pad when used with a passive crossover, so I suspect a single 3886 will be plenty of power.

Cooling
I have a rather large aluminum heatsink (about 5x7", with 1" extruded fins) that I would like to use. My plan is to put two of the ICs along each 7" side, and one (the tweeter IC) on bottom. Finally, I'll have an analog thermal sensor on the top side, monitored by an AVR uC, and enabling the mute pins if a thermal threshold is reached.

If it sounds reasonable, I'd like to have a metal plate on the back panel of the enclosure with all the inputs and controls, and have the heatsink on the INSIDE of the panel. (It's large and pointy. For a mobile system, that isn't so great.)

I have virtually no experience with this type of design, so I'm wondering if there's a snowball's chance of there being adequate cooling with the natural airflow one would get from a ported PA enclosure? If it's going to generate enough heat to warrant a thermally-controlled fan with additional vents and an air shroud around the sink, that's doable. I understand this may be difficult to predict without trying it, I'm just hoping to avoid two orders to the CNC place.

So, any thoughts? This is a learning experience for me, so I'm prepared for mistaken assumptions. I have built a computer sub with the 3886, and it's doing really well, but I stuck to the plans on the box and have a fairly traditional plate amp design there. This is a bit more ambitious.
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