the Dahlia Debra Dick Olsher updates his DIY loudspeaker design

 Since the publication of the Dahlia do-it-yourself speaker article in February 1986 (Vol. 9 No. 1), I've been inundated with requests for clarification of construction  details and additional information. I'm very pleased that the Dahlia has been received so favorably and wish I could personally acknowledge each and every  one of your letters. To have  done so, however, would have kept Me very busy for a long time, and would have precluded additional research work on my part.  Instead, let me take this opportunity to respond to the questions raised most frequently and to describe an exciting upgrade to the Dahlia. First, there are no  construction Plans available from Stereophile: what you see is  what you get. Yes, JGH implied there might be such plans, but he was actually referring to the construction details provided in the original article.

The Enclosure

Two minor clarifications are in order for fig. I-"Enclosure Layout,,. The crossmember of the "H" brace does indeed run from front to back-as shown in the  top view; side view has  been corrected to reflect this. The distance from the top of the cabinet to the center line of the tweeter was inadvertently omitted, and is 4.75 ".  My original plans for the Dahlia  called for a 20 degree tilt-back of the front baffle, not only for time-alignment purposes, but also to optimize the tonal balance in the  upper mids. Without the sloping front, the radiation pattern exhibits a slight suckout around 3kHz. The tilt-back provides better symmetry in the radiation pattern and  results in a neutral midrange balance. Rather than complicate the woodworking, I opted for a simple box with a tilt-back stand, only to realize after the fact that  appropriate stands are hard to come by. As some of you discovered, the  Chicago Speaker Stand (model RJ100) that I recommended is no longer available. I have  resorted to using a particle board base under the Dahlia with a 15 degree tilt-back. This  base is then positioned on top of a 15" tall Chicago Speaker "Hercules" stand,  as shown in the accompanying photograph of the Dahlias. A 20º base would be even better,  but then the entire assembly becomes unstable and the Dahlia ungracefully flips over backward.

In retrospect, the best approach would have been to bite the bullet and embrace the sloped cabinet, and this is my current recommendation for those of you with the  woodworking savvy. just be careful to maintain the present internal volume (1. 3 6 cubic feet) and front- baffle dimensions by increasing the depth.

 There were several questions concerning alternative cabinet materials. Peter Crosby of Claremont, New Hampshire, has the best approach. He is apparently  planning to cast the  cabinet out of concrete, which is eminently rigid and damped, and is expecting nothing short of "concrete" results. The initial investment in  researching and perfecting the  proper molds is, however, quite considerable - at least compared with the cost of the materials involved. Peter is expecting the final  cabinet to weigh from 100 to 200  pounds-not exactly UPS shippable! Sticking with wood, an interesting composite is a sandwich of 0.5" plywood and 0.5"  particle board, glued and screwed together. Although  more difficult to work with, it is far less colored than a comparable thickness of either material alone. The  mass-market approach would be to use 1" particle board or plywood, and this is what I recommend at the minimum.

Diffraction Control

Several readers have complained of not being able to locate any 1/4 1 felt for diffraction control around the tweeter. I have routinely used weather-strip foam  tape for this  purpose. The stuff is 1/4 ' thick by 112 1 wide with an adhesive backing, and is readily available in local hardware stores. At least three rows of  tape should be glued around the  tweeter faceplate. It's not a bad idea to leave about a 1/4 ' between rows, which increases the HF- absorption efficiency of the overall pattern.

Wool Stuffing

 Be careful not to overstuff the cabinet, this is not a Thanksgiving turkey! Contrary to what the folks at the Rio Grande Wool Mill may tell you, the proper amount of  long fiber wool  is as I originally indicated: one-half to one pound maximum per cabinet. I have experimented with as much as one pound, but feedback from the  field seems to favor the lower limit. The danger in over damping the enclosure is of choking off the vent contribution. The bass-reflex action diminishes as the cabinet becomes more lossy.

The Crossover

 1 may have been a bit understated in extolling the virtues of the SiderealKaps. These are still, by a large margin, the best caps for the Dahlia crossover. The  improvement in treble clarity and detail is very noticeable compared with the French FRC (Solen) metallized polypropylene caps. Unfortunately, the Sidereals  are very expensive; many budget-minded audiophiles may find the cost prohibitive. Also irksome is the Sidereal's tendency to be on the low side of the nominal  capacitance value. For example, a 3uF cap  may measure 2.8, still within the 10% spec, but for this expensive a cap the tolerance should be no worse than 5 %. Be  sure to measure the caps you buy, and beef up the values to within 5 % for the best results. The best coils I've found so far are Madisound sidewinder coils. These are  16-gauge air-core type with very low DC resistance values and very good tolerances.

The Woofer

 Audax claims that they've experienced serious production difficulties with the TPX drivers and have been unable to meet initially advertised parameters. For example,  the original TX-2025-RSN was touted as having an overall Q of 0.46 and a free-air resonance of 38Hz, while current production is typically of the order of 0.6  for Q and 49Hz for the free-air resonance. The effect of all this on the bass alignment of the Dahlia is to increase the response peak at 6OHz to 4dB and  decrease the half-power frequency to 55Hz. This prompted me to take a closer look at the Audax catalog. The TSN version of this driver, which sports a slightly  larger magnet and thus a lower Q of 0.5, appears to be a better  choice-it gets much closer to the original bass alignment. The bass peak with the TSN version  should be no greater than the 2.5dB originally intended. I asked Audax to send  me samples of the TSN cones for evaluation. What I got, however, were more RSNS,  so as yet I have no final report on the matter. Since the TSN and RSN versions are identical except for the Q parameter, I recommend trying the TSN for new  construction. I would not personally go to the trouble of modifying existing Dahlias, as the sonic differences  should be small. In either case, be sure to use the 8-ohm version of these drivers.

The Dahlia Debra

I've left the best for last. You should recall from my original article that I concentrated my design efforts on midrange excellence. Subsequently, I was forced  to take a closer look at the upper octaves. It's not so much hat the Audax dome is bad-it is, in fact, a Creditable performer, but it is a tad too bright, sluggish, and  fuzzy compared with the best lomes money can buy I began a systematic search for an alternative tweeter for the Dahlia.

 Candidates would have to meet three important criteria: sonic excellence, physical compatibility with the existing front baffle cutout, and no crossover changes. The  latter two criteria, of course, were aimed at facilitating the upgrade of existing Dahlias. I looked at tweeters from Focal and Dynaudio, as well as other expensive domes.

 The winner is: the MB Electronics model MCD-25M titanium dome. This is a West German I ' dome of very good construction quality and excellent specifications. I sat on  samples of this dome for almost a year because of negative  feedback I had received in the past about metal domes. Such domes are supposed to be out of control in the time  domain, with severe ringing in the ultrasonic range  that translates to harshness and sizzle in the treble. When I finally got around to testing it, the MB titanium dome proved to be. very well behaved, with no nasty  peaks out to 4okHz. Treble response is flat to 25kHz, and rolls off gently to 4OkHz. The MB dome is a perfect fit in the present Audax front baffle cutout, and  works very well with the old crossover, providing a simple impedance-equalizing network is added to the high-pass filter section. This network  consists of a 0.56uF  cap in series with a 7-ohm (5 watt) resistor, and is soldered directly across the tweeter input lugs (see fig. 2).

 I have dubbed the end result the Dahlia Debra, borrowing my daughter Dahlia's middle name to denote the upgrade. This seems more elegant than such mundane  designations as "Mark II". The new tweeter dramatically improves the treble quality, with faster and cleaner transients. There's more focus and detail above  3kHz than before. By  comparison, the old Dahlia is smeared and veiled-sounding through the upper octaves. Sibilants, for example, are reproduced oh-so-naturally,  without gratuitous spit or splash. Despite the clear superiority of the titanium dome, however, it may be an unwelcome guest in some situations. It is much less forgiving  than a soft dome, and therefore more  revealing of solid-state treble nasties or problems in the front end. Be forewarned.

The MB tweeters are not cheap-around $50/pair-and are available from just Speakers in  San Francisco. As I see it, the Dahlia is now complete. Some readers  have inquired about a subwoofer, and the answer is no. I would much rather conceive a three-way design with  deep bass response, and I can tell you that such a design is in the works.

One final note. You may have seen advertisements for Dahlia kits from several sources. just Speakers even offers fully assembledDahlias. You should know that  neither I nor Stereophile has in any way licensed or endorsed commercial production of the Dahlia, so  that I cannot vouch for any of the commercial products. There is nothing wrong, however, with using the design for commercial  gain. I think it's a good design and it is in the public  domain. As always, your comments are welcome.
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