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Background

While doing research looking for new speakers to use in my music/video system, I ran across a link to Siegfried Linkwitz's web page.

I was already familiar with some of Mr. Linkwitz's work as the co-inventor of the crossover topology which came to be known as Linkwitz-Riley, as well as his work with Audio Artistry. You can refer to the Audio Artistry web page for some reprinted reviews, as well as the review at Stereophile Magazine.

Mr. Linkwitz has been kind enough to publish much of what he has learned about loudspeaker development, specifically relating to dipole loudspeaker performance and active crossovers, on his web page.

System Components

The Phoenix system consists of three parts:

Main dipole panel

The main panel of the Phoenix system is a mid-tweeter-mid arrangement with an open baffle. Since the tweeter has an enclosed back, this results in a dipole radiation in the lower midrange, a directional radiation in the treble, with the change from dipole to directional smoothed a bit by a bit of poly fill in the back of the baffle (and in my case, grill cloth which covers the back side of the main panel).

The only thing that I did differently in my implementation of the Phoenix was due to my lack of a table saw. In order to avoid having to cut long straight edges with a guide board and a hand circular saw, I used pre-cut MDF shelving. I have to admit that this was only a semi-successful experiment. It did avoid the problem of cutting straight edges, but the MDF is not quite as strong and non-resonant as I had hoped. If I ever rebuild the main baffles, I will probably use the baltic plywood that Mr. Linkwitz recommends.

Following are some pictures of the progress of my Phoenix system. Each small picture links to a page with a larger version of the same picture. Just click anywhere in the small picture to go to the larger version.

Main panel raw pieces This is a picture of the MDF shelving after cutting to length, and routing the first insets for the midrange drivers.

The sequence for cutting the mounting holes is to first cut the "rebates," then cut the smaller diameter hole all the way through. A circle jig from Jasper Audio is invaluable for cutting the holes. I used a model 200 which I bought from my local Woodcraft store. They mostly sell mail order, so check out the link if you need some high quality wood working equipment. I have to admit that they are usually a little too upscale for my budget, so I get most of the equipment at Home Depot, and just go to Woodcraft for the specialty supplies I can't get elsewhere.

front of main panel This is a front view of the main panel after cutting the driver mounting holes, and gluing the side pieces and braces.

Another slightly experimental aspect of the assembly is relying on glue only to attach the sides and internal braces to the front panel (no screws). On solid wood or even plywood I would not be very worried, because the glue joint is as strong as the wood, but I'm not sure yet how well the MDF will hold up. It is strong when pressure is applied in some directions, but does not have much strength in other directions. I'm afraid one of the weak directions is torsional force on the edge, so I'll have to see if the joints hold up long term, or if the MDF cracks from the weight.

rear view of main panels This is a view of the rear of a panel, showing the braces which are installed just above and below the tweeter cutout.

main panels from all three sides This is a view of the main panels and the rear supporting spine from all three sides. This is just to give an idea of the proportions (depth vs. width).

rear view of main panels after painting This is a rear view of one panel next to the front view of a second. The rear cavity has been painted black at this point.

main panels with plastic laminate This is a view of the front of all three panels after being laminated with Wilsonart high density plastic laminate (but before the driver cutouts have been cut throught the laminate). I found a laminate which matched fairly closely the surface of my Mitsubishi television, so I decided to use that as a front surface finish (since these will eventually end up in a front/center/right configuration around the television).

complete main panel next to surround speaker This is a completed main panel awaiting the drivers. The white plaster column behind the panel conceals a surround speaker. Not obvious in this picture is the hook-and-loop fastener along each edge to hold the grill cloth in place.

main panel with grill cloth The grill cloth will cover the rear of the speaker, not the front. That is partially for aesthetic reasons, and partially because the Phoenix is designed to smoothly transition from dipole radiation in the bass to directional in the treble, and the grill cloth (along with a bit of poly-fill) will help smoothly roll off the high frequencies coming off the back of the midrange driver.

detail of Speakon connector What isn't visible in the previous picture is that the cable connection to the main panel is through a Neutrik Speakon® connector. The picture to the right is a close-up photo of the cable and connector for the tweeter and mid-range drivers. The picture isn't very impressive, but if you've ever dealt with having to make sure four separate wires get connected to the proper terminal without miswiring, you can appreciate the convenience of the Speakon connectors. The advantages of the Speakon® connector are that the connector locks in place, so there is no danger of the cables being accidentally pulled loose, and the connector is 4 pole, so that the tweeter and midrange channels are always connected correctly, there is never a danger of accidentally connecting the low frequency amp to the tweeter (which could damage the tweeter pretty quickly).
The connectors used are NLMP4R on the speakers, and NLMP4 on the amps.

Sanus speaker stand I decided on some Sanus stands from Best Buy for the main panels. I had originally considered stands which hung the main panels from cables, as suggested by Mr. Linkwitz as a way to reduce structrural vibration transmission, but found the main panels too heavy to make that practical.

main panel on stand The panels sit nicely on the stands, although perhaps a little precariously given how much of the weight is at the top of the panel (because of the top mid-woofer).

Dipole woofer

The dipole woofer is described very adequately on the Phoenix woofer page at Linkwitzlab.com, so I will direct you there for all the technical details.

cut pieces for woofer This is just to give a better idea of how you go from pieces of wood to the woofer shown on the Linkwitzlabs page. Here are all the cut pieces lined up on the floor. For reference, the large pieces are 14"x19" and 16"x19".

woofer pieces clamped for gluing Here are the top and inner pieces clamped and glued.

second view of woofer pieces clamped This is just a view from a different angle of the woofer pieces clamped for gluing.

woofer top section with drivers This is the woofer with top and baffles glued together and drivers installed. The only step left is to screw the base plate onto the assembled woofer.

Assembled woofer Grill cloth can wrap completly around the assembled woofer to hide the baffle openings if desired.

Active crossover

Dipole speakers require compensation for the bass roll-off caused by the open baffle. The Phoenix system incorporates that compensation into an active crossover which also handles dividing the low, middle, and high frequencies to feed to the woofer, midrange, and tweeter drivers respectively. Note that this is done at line level, not speaker level, so the system requires separate amplification for each driver.

Amp and crossover components The crossovers and amplifiers for each channel are mounted together, with amplification provided by LM3886 power op-amps. The IC's are mounted on model PM21 PCB's from Marchand Electronics. This picture is the crossover PCB, amplifier PCB, and power supply tacked together temporarily on a piece of plywood.

The crossover circuits were modified by the addition of a balanced input circuit using a simple to construct instrumentation amplifier buffer. The RFI filter RC networks were not installed on the Phoenix board and were instead built as input connector filters placed directly behind the XLR input connectors. The ceramic chip capacitors specified in the Phoenix PCB bill of materials were replaced with surface mount capacitors soldered directly to a solid ground plane, with a brass sheet soldered to the ground plane and bent to form part of the mounting bracket for the filter PCB. That mounting arrangement gives a low inductance path from the filter capacitors back to the chassis shield. I found that for optimum RF rejection I had to use ferrite beads rather than just bare wires to connect the XLR connector to the input of the RC filter. I used the RFI tester I built to check the susceptibility of the input to RFI demodulation.

For a more thorough discussion on the benefits of balanced impedance connections, see the writings of Bill Whitlock and the late Deane Jensen at Jensen Transformers, the Clean Audio Interface Guide from Benchmark Media, and the June 1995 issue of the Journal of the Audio Engineering Society.

Amp and crossover in chassis The crossovers and amplifiers are mounted in rack mount enclosures from Middle Atlantic Products. Markertek carries their enclosures, search for "CH-3" in the search engine on the home page. Markertek is also a good source for the Neutrik XLR and Speakon® connectors used as input and output connectors.
Having finished the enclosure assembly, I would probably opt for enclosures from Hammond or Gray Enclosures over the Middle Atlantic enclosures. The MA boxes were about $25 cheaper per enclosure, but were more diffucult to assemble than I would have liked.

Diffuse surround sound speakers

These surround speakers are not technically part of the Phoenix project, but I'm including them here since they are also another Linkwitz design. Mr. Linkwitz has a nice description of his surround sound configuration on his web site, including a link to the construction details if you would like to build your own (as I did).

They are diffuse surround speakers with limited high frequency response. They would work well for the original Dolby recommendations for matrixed surround, and also work well in most uses for discrete surround as well (e.g. Dolby Digital). They will probably not be a good match for surround music which places direct instruments in the surrounds, but they work well for ambience, and are a fair match for reproducing the surround array effect from a movie theater.

surround enclosure This is the surround speaker enclosure, made from 4" PVC pipe, mounted on a wooden base. An end cap forms the bottom of the enclosure, and an adapter flares to a 6" opening to accept the surround driver.

decorative cover for surround speaker My wife apparently has different tastes or a different level of tolerance than Mrs. Linkwitz, so I was requested to find an alternate look for the surrounds. This is a decorative plaster pillar, with holes bored in the top and bottom so that the PVC pipe speaker enclosure can fit inside.

close-up of surround driver This is a close-up of the driver mounted on top of the PVC pipe, which is installed in the decorative plaster pillar.

surround speakers in living room This is a view of the surround speakers installed in my living room (looking from the TV to the back of the room).
You may notice that the left surround is not placed symmetrically with respect to the couch, due to the fireplace hearth in the way (out of frame in this picture). I haven't decided what to do about this yet, or how noticeable it is. My only options are to move the couch slightly, which causes other problems in the room, or move the surround up onto the hearth.

System Installation

If you have the Adobe Acrobat reader (or equivalent, such as Ghostview on Unix or Linux), you can take a look at how I plan to configure my Phoenix system here:
plan view of system
elevation view of system

system installed as left/center/right in living room The linked picture is a photo of my Phoenix system installed as the left/center/right in a 5.1-style video and music system. The center channel does not have the dipole woofer because of space constraints. The surround channels are handled by the single driver speakers described above.


Page last updated: 16 July 2002


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