Refinishing and upgrading speakers

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Project by ferstler posted 06-08-2009 02:54 AM 2217 views 2 times favorited 4 comments Add to Favorites Watch

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.

Howard Ferstler

4 comments so far

View a1Jim's profile


117115 posts in 3600 days

#1 posted 06-08-2009 02:59 AM

that came out great

-- wood crafting & woodworking classes

View kiwi1969's profile


608 posts in 3465 days

#2 posted 06-08-2009 03:00 AM

Looks great, but I had no idea doing this was so complex! I think I will just make new boxes for mine. Thanks for the ideas.

-- if the hand is not working it is not a pure hand

View Dan'um Style's profile

Dan'um Style

14173 posts in 4006 days

#3 posted 06-08-2009 03:22 AM

excellent ! looks really good on the wall

-- keeping myself entertained ... Humor and fun lubricate the brain

View ferstler's profile


342 posts in 3543 days

#4 posted 07-08-2009 10:58 PM

This has nothing to do with woodworking, but I need to make a point about my restoration original posting, just in case somebody manages to come up with an old pair of those Allison Model Four speakers and wants to do the same kind of fixup work that I did. The four speakers I modified are used as surround speakers in the 8.1 channel system I have in my main room that has Allison IC-20s for the main speakers and a custom, 5-driver speaker for the center channel feed. (Some time back I also posted an article about the construction of that center speaker.) However, they would perform superbly as main-channel speakers, too.

Anyway, I had been confused about just what kind of choke (inductor, or copper coil) value to use with the woofer series part of the crossover network. The original Model Four schematic was cryptic when it came to noting inductance values for the woofer filter (it mentions how many turns on the spool, but nothing else), and later models like the CD-7 indicated roughly 0.72 to 0.86 mH, depending on the model. Woofer and cabinet volume differences obviously were involved, and it was because of that range of values that I decided to split the difference and use 0.8 mH in the updated Model Fours.

Interestingly, the earlier Model Five schematic shows the value to be 0.27 mH, which is a far cry from that of the others. The dual choke notation on the Model Four schematic says 220 turns, with 190 for the tapped junction, and that would be in the 0.72 to 0.8 range and not close to 0.27, even with the enclosure sizes being roughly the same for the Model Four and Model Five and the woofer-driver impedances also being the same. The 0.27 mH chokes used in other Allison systems involved just 121 turns.

I had tried 0.8 mH, as noted on the schematic copy I posted in the first essay, but ultimately that still resulted in most of my room/power readouts showing a sag in the midrange. (In one location the sag was slight, but I was fudging there, and basically fooling myself.) The woofer was simply not reaching up high enough to dovetail decently with the tweeters. I was basically confused as to what kind of woofer the Model Seven and LC-120 had, since the woofers I have show the number 5 scribed on the cone backside, indicating they were designed for the Model Five. However, they came out of LC-120 models that have a larger enclosure. Note that the LC-120 and Model Seven have the same crossover network.

Anyway, I opened two of the cabinets up and replaced the 0.8 mH chokes with 0.27 mH versions, that I was lucky enough to have in storage.

Previously, I did some rough driver curves (knowing the limitations of this kind of measurement, but just for a reference) and the woofer response with the 0.8 mH choke rolled off continuously from 100 Hz on down to 500 Hz, with a really fast rolloff above that frequency. However, with the new choke the woofer response was flat on up to 1 kHz, with a modest rolloff above that. I also measured one of the pristine Model Fours I use as surround speakers in my second system (those have only had the woofer surround changed, with everything else stock, including the Model Four woofer), and the upgraded woofer curve was almost identical to that of the upgraded Model Four with the 0.27 mH choke and Model Five woofer.

Consequently, the woofer “response rolloff towards the midrange” problem has been solved.

I still have a bit of a room/power dip at 2 kHz, because the upgraded speaker has that second-order filtration with the tweeters (rather than the first order of the original), and that has them rolling off faster down low than with the stock model. However, the dip is not all that bad and the faster rolloff better protects those hard to replace tweeters. (Allison classic fans will be aware of issues with hunting up tweeter replacements.) The additional upgrading work was not all that tedious, thanks to the ability to remove the crossover network and the barrier strips I used to hook into the driver wires.

A scan of the additionally upgraded crossover network is attached. The only edit over my earlier version is the choke change in series with the woofer. I also replaced the original photo of the completed crossover network with one that shows the new choke in place of the old one.

Sorry if this post if off topic, but I did want to make sure that down the line somebody did not head off in the wrong direction with a restoration project of their own.

Howard Ferstler

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