My Radio Shack 40-1354 fullrangers steered me onto the single driver path, and I wanted to take the next step in quality, efficiency, and resolution.

Martin King has done some really great work with TQWT speakers, and has put many great designs up on his website, quarter-wave.com. He has a set of Mathcad worksheets that allow one to play with various drivers and enclosure parameters.

I chose to use the Fostex FE-166E drivers because I thought that the 6.5" Fostex drivers were big enough to give output down to 40Hz or so, and small enough not to have the peaks that the 8" Fostexes tend to have in the upper midrange. Also, I heard a pair of Cain Audio Abby's at VSAC, which I believe use this driver, and they sounded really great.

I discovered some really cool Baltic Birch void-free veneer-core plywood (from Russia even) at a local lumberyard that seems to cater to cabinetmakers (Alpine Lumber in Santa Fe). Unfortunately this stuff comes in 5' by 5' sheets, so I am limited to a horn length of just under 60".

Martin King's original design uses the Fostex FE-164 driver, which I was not able to find a source for. I tried modeling his design, with the line length shortened from 60" to 58", with various available drivers. I was suprized how similar each frequency curve was. The ML-TQWT design seems to be fairly tolerant of various driver parameters. This is one thing that gives the Voight Pipe/TQWT design such appeal to DIY-ers. You can easily build a TQWT and drop any driver in it with good results. The last model is of a Cain-style mass loaded adaptation of the "original" voight pipe that appears on the Lowther club of Denmark site.

The cabinets ended up being 12.5" wide, just under 60" tall, 4" deep at the top, and 16" deep at the bottom. The ports are 3" diameter, and between 1" and 2" long.

Martin King suggests a baffle-step correction circuit in his similar design. I implemented it with Madisound iron-laminate core inductors and Solen caps. Solen's aren't really my favorite, but they are easy to find so I end up using them quite a bit. The circuit really helps smooth out the response of the driver and bring the low end up relative to the mids and highs, but it is a shame to throuw away those couple dB of efficiency.

One thing that really helped these speakers come into thier own is a new pair of speaker cables. I have never believed that cable made such a difference, and have been using a single cat-5 cable for years. I brought these speakers to an Atomic City Audionerdz listening party and it was suggested that we put a little more copper between the amp and speakers to beef up the upper bass and give them a little more weight. Well, I braided 4 cables of cat-5 together, connecting all strands of each cable to a single terminal to keep the capacitance down, and it really helped these things achieve the tonal balance I believe they should be capable of.

They are still pretty green and need much more break-in time. They were pretty damn hot at first and practically screamed you out of the room. Real dentist's drills! Now they have calmed down a bit and they should continue to smooth out over the next weeks/months.

Most TQWT designs have the upper portion of the pipe stuffed to reduce the amount of non-bass that comes out of the port, and to reduce reflected sound from affecting the speaker cone. I got a couple bags of polyfill at Wal Mart for this. Like most people I hate to go to Wal Mart. I'm not sure why, but it just seems wrong. Even the parking lot is annoying. I parked next to a car with 2 flat tires and had to push shopping carts out of the way to even get into a parking spot. So after all that, I open the bags of stuffing and find out that it isn't the long fiber stuffing that forms into mats, but this short fiber stuff that makes millions of little pills. Probably perfect for stuffed animals, but not good for speakers. No way this stuff would stay in the top of the speaker. And so on the night before the Audionerdz get-topgether where I had hoped to play these speakers, I was totally screwed. There aren't any fabric or crafts stores (or Wal Marts) in Los Alamos that are open past 4:30 and it was 4:45. Fortunately we have a sort of department store that sells pillows! Well, aas I was browsing the pillow selection, I discover that they actually advertize pillows as being stuffed with Dacron Polyfill II! It was as if these pillows were meant for speaker builders :) Unfortunately 1 Queen sized pillow cost more than all 4 bags of Wal Mart polyfill.

A really great idea from Martin King's project is to install screw eyes in the inside of the pipe at the bottom of where you want the stuffing and then lace some string between them to form a net to hold the stuffing in place. Works like a champ and shouldn't affect the sound at all.

Another future tweak I want to try is to beef up the front baffle somehow because I think there is quite a bit of upper midrange coming off the baffle. When you put your ear up to it, you can hear some very intense resonances in the very high midrange. I would rather not stuff it any more, as I want them to be as efficient as possible, but a little bit of wool felt might be a good thing too. I think a piece of MDF laminated onto the front baffle should do quite a bit to deaden it. I think I will just cut it into a fancy shape and just glue-and-screw it on there.

[update 2003-12-06]:

Here the speakers are with thier new layer of 1/2" MDF to add a little bit of damping to the front panel.

The cheap stamped-frame drivers I tend to use always look like they would really flex and ring with that big old magnet dangling from a that flimsy looking basket. One easy way to stiffen things up is to use some PC-7 epoxy to just smear on a little reinforcement. Note the plastic knife - it is a pain in the ass to use a nail to smear this stuff right next to a big magnet!

Nick Cave's "The Boatman's Call" rotates in the background. Maybe the Tech-12 is not the ideal audiophile approved turntable, but I don't have my Rek-O-Kut with Ladegaard air bearing tonearm running yet :)

Also notice the coverless Nakamichi CD player. This 6-disk piece of crap will periodically freak out and commit shuffle-suicide by running all the moving parts into each-other and hopelessly jamming up. That's why the cover has been off since I got it for $30.

The idea is to fill the gap between the magnet and the frame to couple the 2 together more rigidly, and also to add some strength and maybe a little damping to the legs of the frame as well.

It might not look too pretty, but between the additional baffle reinforcement, and the addition of PC-7 to the driver, the bass has improved quite a bit. It feels much quicker and more defined, with more weight to things like bass guitar.

[update 2004-4-26]:

Since building these speakers, I have heard really great things about the Fostex FE 168 Sigma - the old one before the sigma series got really weird with that serrated surround and whatnot. If anyone has a pair to sell, please contact me at "jsn at boozhoundlabs dot com"