The s8 |
Purpose |
High Fidelity Output. |
| Design |
Lots of cheap drivers. Piracy. |
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First of all, let me say that these speakers are based on the Straight-8 speaker kit by Bottlehead Corporation/Electronic Tonalities. I say "based on" because I did not build them from the kit and do not have all the specs for them. All I have is whatever information is published on the bottlehead site and discussed on the bottlehead bbs. This is kind of a weird situation because I do not want to suggest that using my methods will produce a speaker of equivalent quality to the Bottlehead Straight-8. I am just using the same number of the same drivers in a similar enclosure. That is where the similarity ends. My construction methods are different, I have done my own enclosure and port design calculations, My crossover is probably different from the unpublished Straight-8 crossover, and most importantly - I have not coated and modified the drivers to the level of perfection that the Bottlehead crossovers are modified. I don't even know what all they do to them. My so-called s8 speakers are bound to be sonically different from the Straight-8s, so please do not use this project as a basis for comparison to the Straight-8s. Basically, I feel kinda bad that I am publishing a design for speakers so similar to the Bottlehead kit. I am a huge fan of everything they do at Bottlehead, and I think you should buy their Straight-8 kit instead of screwing around like I did. The drivers used are the MCM 55-1870 5" full range aluminum cone driver and the MCM 53-325 1" titanium dome tweeter. These 5" fullrange drivers are amazing. They are under $10 each, which is nice since we need 16 of them for this project. I have also used them open-baffle in my car and they sound quite nice. Visit the Whamodyne Files and the DIY section of the Single Driver Website for more projects using these drivers. The tweeters were chosen primarily for their efficiency. They are also quite inexpensive at $10.46 each. I coated the 55-1870s with Dammar varnish, which does seem to smooth the response a bit. I coated the 55-1870s I used in my car with Zinzer bullseye Lacquer (or something like that), which seems to do a fine job also.
Construction is pretty straightforward, the cabinet volume is 5000 cu.in. which (minus the volume of the drivers themselves and the ports) is somewhere near the recommended .35 cu.ft. per diver. All cabinet panels are 3/4" MDF and the joints are all what I think they call "rebate" joints. I didn't use any internal bracing - I feared my cabinetry skills were not up to the task. Although the cabinets seem dead enough not to color the sound, deader is better and I should have gone for it. First I finished the cabinets with Minwax Polycrylic but it streaked like just about anything does on MDF, so I let it dry and pumped 2 cans of Almond spraypaint onto each speaker.The drivers are set in counterbored holes with Silicone adhesive. There are no screws anywhere except for a few T-nuts and screws to mount the tweeters.
I only have 6 barclamps, so I used lots of straps and wedged doorstop shaped pieces of wood between the straps and the cabinet and the strap to either tighten things up (or maybe just to stretch the straps out for no reason).
My favorite part was siliconing the drivers in. I got lucky and everything fit just right, with a little bit of silicone coming out of the screw holes to reveal the lack of screws. Plus you gotta love any project that uses big jugs of water (jugs left over from Y2K preparations) and gravity for clamping. The surrounds of the drivers do not protrude past that little foam ring that all speakers have for no good reason, which made clamping that much easier.
The crossover is textbook first order at 4kHz. I used a copper foil inductor and a Solen capacitor. After break-in I padded the tweeter down with a 7 ohm resistor ahead of the tweeter section of the crossover which seems to let the mids and tweeters blend better. I would have liked to make the crossovers internal, but I wanted to be able to get to them to make adjustments and with the drivers siliconed in, that could be trouble. If I had thought about it beforehand, I would have built in a removable hatch in the back of the speakers with the crossover and binding posts built on that.
All internal wiring is Belden plenum rated CAT-5 cable. The inside of the cabinets are lined with 3/8" synthetic carpet padding. In the future I would like to experiment with different port lengths, using various alignments. These babies sound surprisingly good, especially in the lower ocataves. While not as poerful as my previous homebuilt speakers which used the NHT 1259 12" drivers for subs, the bass is much quicker and goes nearly as low. Mids are great. Resoultion and sense of space are much better than anything else I have built. Speaker position seems to affect this quite a bit, and my listening room (ok, my living room) is quite asymmetrical, which does not help at all. Highs are a bit edgy, but I haven't quite got the tweeter section of the crossover sorted out yet. They should really sing once I have my Paramours built. As Nelson Pass says, modesty prevents further auto-review :) Thanks to Bottlehead Corporation, Doc Bottlehead who I hope will forgive me for this article, Paul Joppa who gave me a nice bit of encouragement on the crossover, and everybody participating in the Bottlehead forum. -------------------------------------------------------------------------------- Update [2000-11-29]:
The whole thread:
-------------------------------------------------------------------------------- Update [2000-12-6]: I played around with the crossover a bit with the intention of crossing the tweeter over a bit higher so that the 5.6kHz resonance will be centered in the trough between the highpass of the tweeter and the lowpass of the midrange. The midrange is already crossed over at 4kHz, which is about 1.5kHz below the center of the resonance, so as a ballpark guess I replaced the capacitor on the tweeter with a 3uF which puts the crossover point about 1.5KHz above the resonance. The results are a much more relaxed tweeter and noticeably less overlap. I was able to run the Vifa tweeter unpadded without it being too bright, which confirms that the brightness was indeed due to the overlap of the mids and the tweeter. I think I will stick with the Vifa tweeter instead of switching back to the original MCM tweeter, as it makes the overall tonal balance very relaxed and old-school, but has enough high frequency detail for my taste. One other thing I have noticed about these speakers is that the midrange is strongly dependent on listening position. If you stand so that your ears are above a horizontal line defined by the tweeters the midrange is distant and refuses to image at all, but if you then sit down so that your ear is somewhere in front of the woofer array everything snaps into focus and the midrange is sweet and deep. I wonder if a 16 driver MTM configuration (I suppose MMMMMMMMTMMMMMMMM would be more accurate) like is used in the Ultrawhammodynes would extend this sweet spot floor to ceiling. I might have to compromise and use maybe a 12 driver version so they will fit in my house, which has a pitched ceiling that is only 7' at the low end. 4 or 8 more drivers would also give a welcome bit more efficiency. hmmm... -------------------------------------------------------------------------------- Update [2002-1-22]: After happily listening to these with the Vifa tweeter and very different crossover than the bottlehead specifications, I decided to go back to the original tweeters and update them with Doc Bottlehead's new crossover recipe. Basically he raises the crossover point of the tweeter, adds a zobel network notch filter type circuit across the tweeter terminals, and makes the woofer low pass section second order. I implemented everything but the tweeter zobel. I tried dammar on the tweeters to tame a bit of peakiness, which tamed them a bit. The sound now is more what I would call new school - less forward midrange and more emphasis on high frequency and detail. Frequency response wise, they sound very flat. The vertical sweet spot is emphasized, as you now need to be sitting down for the midrange to do its thang. Overall, I would consider the changes subtle, but positive. I hope to add the zobel once I get the parts for it. I really hadn't given the MCM titanium dome tweeters a fair shot. I was crossing them over way too low, which made them really glare. I should have played with the crossover more instead of chucking them and popping in those Vifas. My new rule of thumb is that if a tweeter sounds too bright, first try crossing it over higher :)
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