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redpackman

Allison Four cross-over pictures

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There's considerable discussion about the Allison Four. I just picked up two in order to restore them. Put in new surrounds, spiffed up the switch in the cross-over with some De-oxit and am in the process of refinishing the exterior. I am fortunate to have all six of the drivers working, and they are all original Allison.

I agree with the many who comment that it is a stunningly impressive speaker. I'm just breaking-in the surrounds and already the sound is so very, very pleasing.

As for the cross-over. I've tried to take some pictures from a number of different angles that may be of interest to Allison buffs or those who want to consider restoration or fixing them.

The cross-over consists of what you see in the pictures: one yellow polyester 4uf 100v capacitor. I checked it out and it was within specs. Alliston certainly used better quality caps, apparently, than some of their fine competitors in that era (AR, KLH, Advent etc.) who used components that by now have usually drifted a long ways from their original specs. The rest of the crossover consists of one "air core tapped" choke (pictured below the main board) and a few resistors in addition to the switch that adjusts the tone for room conditions, etc. There is also one .5 ohm resistor connected in series between the two tweeters and is glued to the bottom of the cabinet and out of sight.

I don't understand the circuitry of cross-overs, so I'm not going to try to draw the schematic. You're out of luck there. But I have a camera. I hope these are helpful.

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post-123677-0-30405700-1411180040_thumb.

post-123677-0-03355400-1411180043_thumb.

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Here's the schematic

post-101828-0-27270800-1411185824_thumb.

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The Model Four crossover was the most complex one Roy designed for a two-way speaker, excepting perhaps the still later Model Four that he configured for the Kentucky outfit that brought the speaker back to life. Most of those he used with other two-way models simply installed ballast resistors in series with the tweeter. With the Model Four, not only did he use those attenuating ballast resistors, but he also installed a tapped connection to the choke, which allowed it to better dovetail when the tweeters were adjusted in level. Because of this, the level control is more complex than usual.

Howard Ferstler

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