Restoring old Allison Model Fours

[Howard Ferstler]

I have had four Allison Model Four systems in use with my main system as surround speakers for some time. (The main channel speakers are IC-20s, with a home-built center that uses Allison drivers, and a Velodyne F1800RII to handle the low bass.) They were probably built in the late 1970s, and were in rather rough shape when they arrived at my place. (The previous owners stored them in rather sub-par locations for years, I think.) I replaced the beat-up tweeters with newer, screen-covered versions (built maybe a decade after the speakers themselves were made), and replaced the absolutely wrecked Model Four woofers with Model Five woofers that I had in storage. I also removed the corroded level controls and sealed the rear-panel lever openings. The speakers were hard wired in the "flat" mode. For a while that is all the upgrading they got.

However, I recently went all out and brought the speakers fully into restored condition. I lightly sanded the boxes (not enough to remove much of the stain), restained them to make the tint more uniform, and hit them with three coats of Minwax polyurethane. This did not restore them to a fully sanded down and pristine finish, because I was afraid that sanding too much would wear through the thin veneer. However, they are better than before. I also gave a flat-black spray job to the backs of two of them, both of which were previously unfinished in the back at all. The other two were already lightly stained in back.

The attached photos highlight what I did.

1. The backside of one of the units prior to the upgrade. Note the early-version binding posts. Those would only take very small wires and even with small wires they were tricky to use. Later versions used five-way binding posts, and my latest upgrade also switched to five-way binders. The black tape covers the sealed level control cutout. Note also the wall hangers and the gray pads at the rear corners that are foam spacers to keep the speakers from dinging the wall.

2. The upgrade in process, showing the two gutted boxes outdoors on my workshop's workdeck. The staining had already occurred, but the polyurethane work was not yet done. Note the new crossover network sitting on the unit behind the one in the forground. You can also see a new rear template through the woofer cutout in the foreground speaker.

3. The four crossovers, plus one of the five-way cups, plus one of the templates that would replace the existing rear template. The new template would be installed in place of the original that contained both the old binding posts and the interior crossover network. The new templates were both glued and screwed into position on the rear interior.

4. One of the new crossover networks. These would be screwed to the interior bottom. Note the screw-type barrier strip that allows one to detach the driver and infeed wires for any future mods or fixes.

5. Schematic for the new crossover network. Note the two "polyfuses." The tweeters get one and the woofer gets the other, bigger one. The diagram shows three barrier strips, but there was actually only one, with the drawing showing three for illustrative purposes. The network also has the tweeter high pass done second-order style, to better protect the tweeters than the first-order version in the original. (Note that Roy Allison himself went to a second-order network when he redid the Model Four for the Kentucky outfit some time back.) The capacitor is a polypropylene type. Parts were purchased from Parts Express, except for the .0.72 mH choke, which is from another Allison network that I cannibalized.

6. Rear panel of one of the restored units, showing the new five-way binders. As with the pre-restored version, the hooks at the top corners allow the speaker to be hung on the wall. The grey pads at the rear corners are still on hand to keep the speakers from dinging the wall.

7. One of the finished speakers hanging on the room wall.

The units sound just fine (I hooked up each pair in my main system as left and right speakers to make sure they sounded and measured OK), complement the IC-20 L/R mains and home-built (with Allison drivers) center speaker in the system, and hopefully they will last for a long time.

Hioward Ferstler

[speaker dave]

However, I recently went all out and brought the speakers fully into restored condition.

Hioward Ferstler

Hi Howard,

Looks like very tidy work and a handsome end result. I especially like the subtle color that Walnut develops over time. Minimal sanding preserves this better than a heavy refinishing.

Can you comment on the conversion of the HF network to second order (how you decided on the values, best tweeter polarity, etc.)

Thanks,

David

[speaker dave]

Attached are two RTA derived room curves that I ran using my moving-microphone averaging technique to get a good idea of the actual input to the room.

Howard Ferstler

Hi Howard,

Thanks for the detailed explanation. I agree that the two versions (first order and second order) look extremely close through the crossover region, on the order of typical unit-to-unit variation.

I can't guess why the top octave difference. It must be differences between the tweeters rather than the network. You could try the trick of bypassing the treble series resistor with a small value cap, but you would pick up very little since it is only 1 ohm that would be bypassed.

Regards,

David