This speaker system was developed and built during the Winter of 1998. There was so much hype on the news about the impending storms yet nothing was happening that I decided to name this project "El Nino". I figured that I would be done with this project, the news people would still be talking about imaginary storms, and I would have the last laugh. Boy was I wrong. When the storms finnally started in January I was on my third prototype. The rain continued into June and when it was all over and done the rainfall total at my house in the mountains above Santa Cruz topped 72 inches. Some of the surrounding areas got over 100 inches. When it finally stopped I was ready to build the final version. All of the MDF that was in my garage all winter needed to dry out but I got it all together and painted by the middle of July.
I had been toying with the idea of building a dipole speaker for some time. The idea became a requirement after hearing the Audio Artistry Beethovens at the Stereophile Hi-Fi'97 show in San Francisco. I was really impressed by these speakers and their $30000 price tag! I thought I could do something similar for a lot less money.
I began with driver selection. Zalytron had some Focal 5K413S's on sale because their pretty yellow cones were painted black. They also had the Focal TC120TDXT tweeter on sale. This tweeter has a vented pole piece so it would be perfect for my dipole project. For woofers I chose two Vifa M22's for each side. My first prototype looked a lot like the Beethoven(flat 12 inch wide by 48 inch high baffle) and after working out some basic crossover details with Liberty Audiosuite(LAUD) I sat down and listened for a long time. The speaker configuration was MTM+WW with the two woofers mounted one above the other as close to the floor as possible. The idea here was to use the floor bounce notch as one pole in the lower crossover. Crossover points where at 400 Hz and 3000 Hz. A modified Parapix Amplifier with built-in crossover was used to power the two bass drivers. The modifications consisted of changing the crossover points to 400 Hz). My homemade, Nelson Pass designed, Zen Amplifiers(20 watts per channel into a 4 ohm load) were used to power the mids and tweeters. A passive crossover was used between the mids and the tweeters. Just for fun I decided to play around with baffle shape to try and get some of the crossover "work" done by the baffle. I also wanted to try to get some time coherency by setting the tweeter back behind the plane of the mids. The great imaging of my previous speakers(the pyramids) I attributed to the narrow baffle width. After playing with all those variables through most of the winter I ended up with something entirely different than what I started with.
The time alignment issue suggested a change to a TMM layout. Even with that arrangement I needed to tilt the baffle back about 20 degrees to make everything line up. The off axis response of the mids got a little weird when the listening axis got way off center so I decided to change to the Morel MDT-37 horn loaded tweeter. This solved several other problems as well. It has a much smaller faceplate diameter than the Focal, it doesn't have the 24KHz ringing of the Focal, and it integrates into a very narrow baffle quite nicely. The voice coil is about an inch behind the baffle providing the needed offset. I like the smooth sound of the silk domes also.
The Focal midrange drivers are just that, midranges, not mid/woofers. As such they want to crossover to a woofer at no less than 400 Hz. There is a large resonance at 400 Hz with the existing configuration so I have tried various remedies. Eventually I will try moving the crossover frequency up to 500 Hz. This will be easy when I get a 4th order electronic crossover built. My initial effort at integrating the woofers and midranges have been a little ragged but I must admit that I have learned a great deal about LAUD, measurement techniques, floor reflections, resonances, and patience through all this.
The horn tweeter provides an unexpected benefit. It protects the soft dome from accidental damage since I did not plan to use any grill cloth except on the bass unit. The phase plug in the Focal mids provides some of the same benifits since it extends out past the baffle. This allows the entire unit to be placed face down on the table when working on it.
I'll be the first to admit that some of the decisions where based on esthetics as much as science but lots of testing using LAUD went on throughout the entire process. I want to thank Bill Waslo for putting together such a nice, easy to use program.
Encouraged by the results of my Pyramid speakers I chose to use two Vifa M22 8 inch bass drivers per side. I generally prefer sealed box bass speakers but I also prefer smaller drivers and the list of available 8 inch drivers suitable for sealed boxes is very short. These Vifa drivers can be mounted in about 1.5 cubic feet with a 3 inch port 21 inches long. This produces what is known as an EBS(extended bass shelf) alignment. By using a box volume that is a little larger than the traditional QB3 alignment and a port length a little longer than usually required the response falls off gradually below 50 Hz. The in room response is boosted somewhat producing a flat response to below 30 Hz. This eliminates a lot of the boominess sometimes associated with ported speakers. That's what the theory says anyway. One problem I have with these particular boxes is high frequency noise radiating back out of the port. I get quite a boost in port output between 200 Hz and 375 Hz. I have reduced it somewhat by lining the boxes with 1 inch thick fiberglass but this is a problem still to be delt with. One change that I expect to make soon is to build a better electronic crossover with a 4th order slope rather than the 2nd order that I am using now.
The Paramount Pictures Subwoofer Amplifier is a small amplifier built by HTP International for Paramount for use in a self powered subwoofer. Paramount Pictures is not in the subwoofer business anymore and they sold off all their inventory. Apex, Jr., an electronics surplus store in the Los Angeles area bought up a few hundred of them and has been selling them, with transformer for about $35 each. More information and schematics can be found at the Parapix Page. In their stock configuration they produce about 60 watts into each of two channels. They are not, however, stereo amps. They are dual mono. They can take line level or speaker level inputs, stereo or mono, and combine the input into two identical output signals. These are ideal for powering a dual voice coil woofer(see my subwoofers page) or for driving two separate drivers as I am doing here. The built in low-pass crossover is variable from about 60 Hz to 120 Hz and is second order(12 Db per octave). There is also a fixed first order(6 Db per octave)low-pass crossover at 120 Hz. the combined effect of these circuits is a third order low-pass crossover. Also included is a first order passive high-pass crossover which is fixed at about 90 Hz. This can be used on line level signals or at speaker level signals.
I made some simple modifications to my Parapix Amps to raise the crossover frequency to 400 Hz. I changed R9/1 and R11/1 from 18.2K ohms to 5K ohms. Then remove C2/1. These simple changes produced a second order lowpass crossover variable from 90 Hz to 450 Hz. The high pass crossover for feeding your satellites is not variable but I put a 10K ohm resistor in parallel with R1 and R2 to lower their effective value from 18.2K ohm to 6.4K ohm. This produces a first order(6 Db per octave)high-pass crossover at 450 Hz. There is an extra unused op-amp in the circuit and the bass-boost circuit(which I don't use) has two op-amp stages that would be available. With this I could build a third order low-pass and high-pass. I'm still thinking about that one. The parts quality is mediocre at best but it sure is convenient. Other options are a complete stand alone variable 4th order crossover or I could build a crossover right into my preamp. Maybe next Winter.
The amp is mounted in its own compartment at the back of the bass modules. Although not shown here the Zen Amp sits on the shelf at the back of the speaker. So this arrangement needs an interconnect from the preamp to the Parapix Amp(about 6 feet), and another from the Parapix to the Zen(about 18 inches), internal speaker wires from the Parapix to the bass drivers(about 2 feet long) and speaker cables from the Zen Amp to the high-end module(about 8 inches). That's it!
The speaker cabinets are constructed entirely from medium density fiberboard(MDF). This is a great material for sculpted work because it has very little grain. It can be cut and shaped any way you want and then finished up as smooth as you have the stamina to do. After the basic shapes were set, I started filling screw holes and nicks with lightweight auto body filler. Sand, sand, sand, sand. All inside square corners were given a small fillet of filler. Sand, sand, sand, sand. When the final shape was acceptable I gave it six coats of automotive filled primer. Sand, sand, sand, sand. Then came six coats of Torrid Teal Urethane. Sand with 1000 grit. Then came eight coats of two-part catalyzed Urethane Clear. This was sanded with 400 grit wet, 1000 grit wet, brown rubbing compound, white rubbing compound, and then two kinds of showcar polishing compounds.
The paint cost over $200. The EPA and the AQMD and various other environmental groups have outlawed the more common paints so now we have to use exotic paints without any thinner in them. Its really hard on us amateurs trying to keep up with the changes. Even regular spray guns are illegal in my county. Only high volume low pressure sprayers are allowed.