|Project by ferstler||posted 06-08-2009 02:54 AM||1834 views||2 times favorited||4 comments|
This is a mixed project that involved both woodworking and electrical modifications.
I have four Allison Model Four systems in my main audio-video system that are used for surround-channel duties. I picked up all four quite a few years ago. All were in sad cosmetic shape when I got them, and I did change out the tweeters and woofers to upgrade them to some extent. I also removed a rear-panel switch inside that allowed one to adjust tweeter balance, and sealed up the cutout on the back where the switch lever extended through to the hookup junction area. I left in place the tiny and hard to use connectors on the back and also made no major changes to the crossover network itself. These are two-way systems, with dual tweeters and an upward-facing 8-inch woofer. Facing it upward both allows it to propagate midrange frequencies better and also allows it to deliver smoother mid bass to the room, and using dual tweeters on 45-degree, side-angled panels helps to disperse treble energy better, which is good for both main and surround speaker duty. Remember, these speakers deal with the latter job, with the primary front-channel work handled by some much bigger models.
Being retired, I finally decided to upgrade and refinish the surrounds, and followed some of the guidelines Roy Allison himself came up with when he decided to upgrade the speaker for a new company that was going to start making the devices again. (That rebirth fell through, but I at least obtained the schematic for the upgraded model, and used much of that info for my own upgrading work.)
The first photo shows the backside of one of the four speakers. The strip of glue along the top edge was an attempt by the previous owner to seal what he thought was a gap between the back panel and the top. I sanded away the excess, and checked to make sure his sealing job worked. You can also see the small, difficult to use connectors and a tape piece that covers the filled in lever cutout above them. Note also the hangers on the top corners. These speakers have been hung about six feet up on the side walls of the main listening room, and the hangers, which I installed years ago, are plenty solid for the job. The grey pads on the lower corners are there to keep the cabinet from marring the wall when the unit is hung. To remove the old crossover network and connectors I used a mallet to punch in the area inside of the circular cutout. I then discarded most of the materials, but kept the original inductors (chokes), because they never wear out. I had to scrape off quite a bit of old glue on the interior of the back panel after removing those plates.
The second photo shows the four crossover networks in the background and a panel that I made that was attached internally on the back panel where the old hookups were located. The panel, plus three others were then attached inside by glue and screws. Also shown is a new five-way binding post cup that will replace the old connectors. There were one of them for each speaker, too.
The third photo is of one of the crossover networks up close. It uses first-order filtration for the woofer low pass and second-order filtration for the dual-tweeter high pass. The capacitor is a polypropylene job and the choke is a 0.27 mH unit. (Normally, it should be 0.8 mH, but the speakers are using Allison Model Five woofers, which require a different value; the original Model Four woofers were shot.) The two little tan wafers are self-resetting polyfuses, with the smaller one for the tweeters and the bigger one for the woofers. There is also a one-ohm resistor in series with the tweeters and a 0.72 mH choke in parallel with them. (The choke steepens the tweeter slope and protects those now rare drivers better than the original first-order network.) The barrier strip at the upper right allows the tweeter, woofer, and input wires to be quickly installed or removed.
The fourth photo is of two of the speakers after the initial restaining work. The speakers were first only lightly sanded, because I did not want to chance rubbing through the thin veneer. The stain was a mix of walnut and mahogany that I came up with and had used before on other projects. Note the installed panel, visible through the woofer cutout on top, on the rear interior of one of the speakers. The crossover networks (one is shown on the speaker to the rear) were screwed to the bottoms of the cabinet interiors. The restained speakers were then given two coats of Minwax satin urethane.
The fifth photo shows the backside of one of the other speakers after the new rear panel was installed on the inside and the gold-plated, five-way binding post cup was also installed. This speaker had a darker back panel than the one shown in photo number one. However, that first unit, older than this one and in rougher shape, was given a coat of flat black paint on its backside later on to make it look more finished. We want the unseen areas to look good too, right?
The sixth photo shows one of the finished speakers mounted on the room wall. Surround-channel speakers do not usually have a huge amount of work to do, but I did at first hook up each pair in place of my main systems to make sure the electrical modifications did the job. The speakers sounded as good as this one looks.