"The brown note is an infrasound frequency that is said to cause humans to lose control of their bowels due to resonance. There is no scientific evidence to support the claim that a "brown note" (transmitted through sound waves in air) exists." - Wikipedia

[2009-07-20]

One day my friend Dave called me to ask if I wanted to build a sound system for his wedding reception. It will be outdoors, he tells me, so the system will have to be pretty big. I pulled out my "life list" of things that I want to do before I die, scratch out "build huge sound system" and tell him yes :)

I had done a fai bit of reading about big speakers, mostly because of the amazing photos on speakerplans.com, so I definitely wanted to start with some horn loaded woofers. I had used my old Straight 8 copies, adapted for PA use, at a party and they sounded awesome, so I figured a line array of some sort would be good for the mids. And I had a pair of Selenium ST324 tweeters laying around, so those would do the tweeting. I was mostly worried about having enough big amplifiers to run all these big speakers. But I remembered seeing some weird movie theater amps at The Black Hole (my local source for everything that is good in the world)...

Well whaddayaknow, it turns out that these weird movie theater amps are these amazing Kintek 200W by 2 channel amplifier modules that are supposed to be totally bombproof and are even on the list of THX recommended components. I got 5 of them for $100. That's 2kW. I think "OMGWTF!" sums it up appropriately.

It turns out that sometimes you get what you pay for. Imagine that. Of the 5 amplifiers, only one worked as-is, and most of the channels were either intermittent due to the shitty relays they use for protection, or overheated badly (bias issue?). I swapped everything around until I had 4 channels from 2 amps that work. The relays are still a bit goofy sometimes, but they tend to hold once things warm up. Happy to finally have a workable pair of amps after hours of testing, disassembling, reassembling, and and adding useful input and output connectors to the amps that were built tio slide into a modular chassis, I was pretty dang happy. I had the amps up and playing music while I screwed on the last little heat sink fin. Of course, the heat sink slipped a little bit and contacted the case of one of the output transistors, which is electrically live at around 80VDC. If this were a Batman episode from the 70s, a giant "ZAP" explosion graphic would have appeared. Dang it.

Killing that amp channel was actually a good thing because I got to learn more about transistors. I am pretty much a self-taught vacuum tubes guy, with no real electronics training, so I have major gaps in the stuff I know about. I removed the output transistors and tested them on a crude 9V battery and LED transistor tester. It turned out that one of them was shorted/blown. Fixing the amp was just a matter of swapping in a replacement from one of the working/intermittent channels I wasn't using. Funny how you can explain in 2 sentences something that took you almost a week to figure out :)

So I ended up with 4 channels at 200W each. In mono, this would let me use one 200W channel per woofer, one channel for the mids in parallel, and one channel for the tweeters in parallel. I would have preferred to have 6 distinct channels for all of this, but mono is a good thing for a big loud system (you get 3-6dB of loudness just by having everything in phase), and a 3-way mono setup would let us use a cheaper active crossover.

For the woofers I decided to build this amazingly cool, and frighteningly efficient horn cabinet from speakerplans.com. Being the cheapskate that I am, I specified the Eminence Omega Pro 18 for it.

This is pretty much what happens to your brain the first time you see an 18 inch woofer in real life. They are big. Really big. Bigger than you expect them to be. For me, this was where I started thinking "holy crap these are gonna be sweet!" approximately every 15 minutes for the rest of the project.

I found that the easiest way to build a folded horn like this is to create a full-scale paper drawing of the folds, then use a centerpunch to transfer all of the measurements onto both sides of the plywood you are using for the sides of the speaker. That way you can see where the panels go on the inside, and where the nails/screws go on the outside. I also recommend that after you transfer the pattern, you throw the plans away and work from the lines you transferred to the speakers. If you try to pre-cut all of the internal panels, you will magnify every small error all the way through. If you go piece by piece, you can compensate for errors as you go.

The happy couple enjoying the fruits of thier labor, and anticipating how awesome these are going to sound.

Dave busts a B-Boy stance to celebrate the completion of the major woodworking.