Dave and Martin's Speaker Project

Martin and Dave's Speaker Page
A Chronicle of Speaker Design

Martin and I are EEs that worked together at Zoll Medical. I have since moved on to a startup. In the year or so that we worked together we discovered a connection in the desire to create. The subject of home audio came up and I have dabbled in speaker design and building in the past. My current speakers aren't very current: home-brewed Bose 901 MKI clones, built in late seventies. They continue to perform well to this day in my living room and in my basement. Martin's speakers are small monitors whose foam surrounds have long since turned to dust. It was time for us both to upgrade.

We looked at commercial tower speakers and liked the fact that they were close to proper height for listening without requiring stands, that they had a small footprint, and had nice clean sound. The clean sound is what appeals to us. No screeching highs, no thumping bass, just clean, flat, low distortion sound. We both like all kinds of music. Me mostly singer-songwriter stuff, seventies rock and roll, some modern. I also listen to TV with these speakers so maybe a sub-woofer is in the future... We considered building sub's into the towers like Cambridge Soundworks Newton's, but the enclosures would need to get real big. So a separate sub tucked into a corner someday...

So one rainy Saturday in the '05 summer I looked around the basement, and saw some old bookshelf speakers I had built ages ago, and some particle board scraps, and went to town. With a 'plan' drawn on the back of a receipt. I built the proto box. I scavenged two 6.5" Pioneer woofers and a teeny Pioneer tweeter out of the bookshelfs. In there I also found some crossover parts, terminals, and polyester batting. Within a few hours the first proto box was built and I was listening to it. It was a definite case of 'ready, fire, aim' (build first, design later). So I built it and set it up against the Bose 901s, some old EPI 8" bookshelfs, and started listening. It didn't sound too bad! The crossover was a first order (inductor for the woofer and cap for the tweeter). I invited Martin over, we experimented with positioning a baffle inside in order to vary the volume, and added a port tube of 1 1/2" PVC. We experimented with different ports, and decided that with decent drivers, decent materials, crossover, design, and construction, we could build decent sounding speakers.

The first speakers would be two way towers. Dual 6.5" woofers, soft dome tweeters, 100W+ capability, even though we'd be running them at conservative apartment and suburban listening levels. We perused many speaker builder's web sites, and found some good design ideas.

I found drivers on Ebay at good prices. Peerless 6.5" woofers for $10 apiece. I also found some 100W titanium dome tweeters (so much for soft domes) for $20 a pair. My approach on most purchases is to consider carefully all the options and possibilities, weigh them carefully, then go for the cheapest! In the case of these speakers, I mounted them in the proto box, re-twaked the crossover, and was very pleased with the results. Martin wasn't convinced that my 'ebay deal' drivers were the best, but when I pointed out that I could get another 4 Peerless woofers for under $50, he couldn't resist. He bought a pair of proper Vifa silk dome tweeters from PE for $50 a pair. It was time to build some real speakers.
-- Dave
11/15/05

Here is the first pair of speakers we built. The tweeters are Martin's Vifa's and the woofers Peerless. Construction of the rear enclosure is 3/4" MDF glued together. The corners are caulked and the insides lined with two layers of roofing felt plus about 1" of polyester batting. The front baffles are screwed into hardwood cleats. For now we wanted to be able to play with the front baffles changing drivers, positions and porting. We'll probably stick with this approach forever and use gasketing to seal the front properly.

We have a plan for finishing them, but that's coming.....

Here is one of the passive 6db crossovers, front and rear view. Components from parts express. The European style terminal block made the wiring and the connections to the speakers easy. Crossover frequency is 2.4KHz. The l-pads are to allow to adjust the tweeter level to match the woofers.

With two 8 ohm woofers in parallel, the speaker's impedance at low frequencies is 4 ohms, but a reasonable amplifier can handle this load. Just don't use two speakers at the same time.

Dave, 11/18/05

We began the contraction of the second pair on 12/1/05. With most of the dimensions established, cutting the parts went quickly. The internal baffle is being used as a spacer to keep the center from sagging while the glue dries. The joints are only glued, no screws used so far except the removable front baffles. We could have gone either way on screws: the veneer will cover screw heads, but Titebond glue is very strong and we think quite adequate.

On the front baffles, we recessed all the speakers and the port tube flanges. The baffles don't yet have their 3/8" hardwood strips on the sides and tops to match the veneer.

The enclosures are lined with one layer of roofing felt followed by about 3/4" of polyester batting for dampening. Since the boxes are a bit of an experiment, we wanted to be able to remove the baffles in the future. So we used wooden cleats on the insides to screw the baffles into. We didn't want to screw directly into the MDF since it would eventually weaken. The cleats are recessed 1/16" and a 1/18" rubber grommet is installed on the cleats to seal the enclosures. Then the front baffles are installed with 1 5/8" screws. This is the first pair, done in white oak veneer.

Drivers and ports mounted on the baffles and wired up. The drivers were gasketed with speaker gasket tape. Note the 3/8" strips of solid white oak trim around the baffle. This pair has Titanium dome tweeters, not the VIFAs.

The second pair veneered in Walnut and stained with Minwax Natural. We think these look terrific with their flat black baffles and bookmatched veneer.

Martin celebrating the completion of the second pair. We were very pleased with both the sound and the look.

Overall dimensions: 42"H x 8.5" W x 12 3/4" D
Main chamber: 7.5" W x 27"H x 11"D = 1.3 cu ft less the battens, batting and speakers is about 1.1 cuFt.

Parts list for two speakers:

Qty	Description	Part number	$Total	Source
	Cabinets
5	2' x 4' sheets 3/4" MDF		$25.00	Home Depot
1	2' x 8' sheet veneer, wood selection to taste		40.00	Rockler
1	3/4 x 4" x 5' hardwood		10.00	Rockler
1	about 12' of 3/4" x 3/4" hardwood cleats (scrap)
24	#8-32 T-nuts (Speakers)		6.00	Home Depot
8	#8-32 Hex nuts (Ports)		1.00	Home Depot
64	#8x32, 1" RH screws (Black preferred)		5.00	Home Depot
30	1 5/8" drywall screws, black		3.00	Home Depot
	8' 30# roofing felt			Home Depot
1	Roll 1" polyester batting		4.00	Fabric store
	Titebond glue, flat black spray paint, Minwax Natural stain, polyurethane, sandpaper			Home Depot
	Subtotal		$94.00
	Speakers
4	Peerless 6.5" woofers. (Note 1)		60.00	Ebay
2	Vifa D27TG-05 silk dome tweeters	264-524	42.90	Parts Express.
2	2.5" adjustable port tubes	260-386	4.10	Parts Express.
1	Speaker Gasketing Tape	260-540	4.23	Parts Express.
25'	16 Ga speaker wire		2.00	Radio Shack
8	3/16" female disconnects (for woofers)	095-287	3.25	Parts Express.
	Subtotal		116.48
	Crossovers
2	Inductor, .27mH 18GA	255-216	4.88	Parts Express.
2	Cap, 8.2uF polypro., Dayton	027-426	8.50	Parts Express.
2	L-Pad, 8 ohm 50W mono,	260-252	10.50	Parts Express.
2	Dual banana plug, red and black	090-471	3.50	Parts Express.
2	European style screw terminal, 8 position		3.00	Radio Shack
2	5" square plastic or sheet metal scraps
	Subtotal		26.38
	Total		236.86

Note 1:
These were surplus OEM drivers manufactured by Peerless for NHT. Other 6.5" drivers can be used and may need a different cabinet volume. This can be accommodated in these cabinets by adjusting the internal baffle up or down and/or adjusting the port length and diameter.

Drivers
Our criteria for driver selection was fairly simple. 2 woofers and one tweeter that could handle 100W RMS each, flat response and low distortion at normal listening levels. We wanted overlap on the woofer and tweeter response curves so a simple 1st or 2nd order crossover design wouldn't be fighting the speaker response curves. This is hard to do with 8" and larger woofers but very achievable on 6.5" woofers. We wanted high frequency response of the woofers to be 5-6KHz. And the low frequency response of the tweeter to be about 2KHz, for a 2.5-3 KHz crossover cutoff. You generally don't want the crossover to be in the midrange area between 500Hz and 2KHz since that's where the ear is most sensitive. The Vifas are spec'ed from 2.5 to 30KHz but their response plots look good down to about 1KHz (Vifa D27TG-05 Specs). The Peerless are spec'ed about 50Hz to 6KHz. We chose the crossover to be 2.5 KHz for both the 1 and 2 pole designs.

We weren't too concerned about driver sensitivity. Using two woofers in parallel is like having another 3db of low and midrange sensitivity. Despite this, the tweeter sensitivity would still be a bit higher and so a pad (attenuator) would be needed to compensate. The Vifas are 6 ohm speakers but an L-pad set to anything but MAX will tend to make them look like 8 ohms to the crossover. L-pads seem to be taboo among audiophiles, but these sound fine and are rated 50W. Since these speakers are one big experiment, having a tweak on the tweeter is a good thing.

We could not find data on the Peerless 831773 surplus woofers, so we just measured their Thiele-Small parameters. They look similar to Peerless 833599's except the magnet is a bit smaller diameter than the 833599. (Peerless 833599 Specs). These are polypropylene cone, rubber surround, steel basket speakers. A goal for the woofer was no foam surrounds since they tend to deteriorate over time. Martin has had them crumble on him. My Bose 901's have the original CTS drivers with rubber treated paper surrounds and are in great shape after 30 years. So rubber it is.

All in all we're real happy with the sound of the drivers and 1st order crossovers. But I have a nagging feeling that 2nd order may sound cleaner, possibly flatter. Next project....

Now that I have these great new speakers...

The latest here in nerd-land is that with these efficient and clean speakers I can now hear every little flaw in the homebrewed ca. 1978 amplifier and 1990 preamp I use. With Bose 901's mounted at ceiling level the amp never ran at less than a few watts so low level distortion was not an issue. I built up a better bias circuit to clean up crossover distortion. It allows the amplifier quiescient current to be adjusted and eliminates the crossover distortion.

Then I found that the cheesy 6' RCA cables I was using picked up 60Hz from the rest of the system. The amp runs at full gain always so it's input is super sensitive to hum which the speakers faithfully reproduce. The Bose's were >10dB down at 60Hz (with the equalizer making up the difference) so no problem. I suspected crummy shielding. I went thru all the cables I had until I found quieter ones. I can't bring myself to buy Monster RCA cables so I bought proper plugs and will build up cables using RG58. Don't get me wrong, crappy cables work perfectly well for line level stuff (CD output, etc.), but for low level (phono and amp input) good shielding is important.

The preamp chip I use is the TDA7318 which doesn't have loudness control. With the Boso's you would just set the equalizer to adjust the bass and treble boost so it sounds good. The new speakers really needs loudness compensation. So I ordered a new TDA7313 preamp chip with loudness and will add it in to the hardware and software when it arrives.

The CD player always puts out a bunch more volume than the rest of the components. Now that I'm obsessed about loudness it bothers me. So I wired up a set of cables with fixed 1/4 attenuators for the CD output. All CD players I've owned seem to do this. I wonder why.

While I was in the preamp I re-did the IR remote software. It worked, but only with an ancient universal remote. I got a new 8-in-1 do-all remote at Circuit City ($20) and updated the codes in the preamp to look like a more modern Sony reciever. Now we can be proper couch potatoes with one remote doing it all.

The processor controlling the preamp is an old 68hc11 wire-wrapped with lots of support chips (RAMs, ROM, FPGA) to drive the graphics LCD display. I'm prototyping up a new single chip Atmel processor with a nicer looking and smaller, brighter, smart LCD (no controller needed) just for fun. If the old C code ports well I may put it in the preamp. Hey it's going on 15 years old and needs to be played with.

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Last Updated: Jan 28, 2006