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How long do the allison two midrange and tweeter last? I have a pair dating back to around 1976 and am having the woofers refoamed. How do I check to find out if the midrange and tweeters are working?

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The foam around the woofer perimeter is urethane, and after some time it has to be replaced. So, you are on the right track, there.

The flat foam around the midrange dome's perimeter and used as a thin damping ring between the tweeter dome's edge and mounting plate is polyethylene, and should last for many decades. (Allison determined that polyethelene just did not "terminate" properly for woofer-surround use, but it worked great for tweeters and midrange drivers.) There is also a bit of cotton batting under the tweeter dome, but it should hold up much longer than the foam packing found under a number of conventional tweeter domes of the 1960s and 1970s, such as the classic AR-3a tweeter. There is no packing under the Allison midrange dome.

I have tweeters going back to the mid 1970s and they seem mechanically OK, as do the midrange drivers I have on hand. (My main speakers are triangular-cabinet IC-20 units that each use four tweeters and four mids; with the two woofers at the bottom arranged just like the Model Two.) The only qualification might be the silicone cooling grease in the tweeters and the ferrofluid in the midranges. I am not sure if that stuff can dry out, or not, and there have been questions raised about their longevity. The grease in the tweeters should not do this, but I am not sure about ferrofluid.

In any case, when I do full-range measurements (using an averaging technique with an RTA) my two IC-20s measure a tad better through the midrange than they did ten years ago (they are now 19 years old), but with some rolloff above 10 kHz that they did not have when they were newer. It may be that my measurement microphone is at fault here, so I am not sure of the ramifications of the tests. My listening room also has a bit more in the way of overstuffed furnitre than it did ten years ago, too. After all, the midrange seems even flatter, and I am talking about flat. Note that most good systems actually dial in a bit of rolloff in the treble, so any response-related aging of the tweeters might work to advantage. One way to compensate for any rolloff you might be concerned about will be to set the three-position blance control on the back of the system to the flat or second position instead of to the third, concert-hall curve slope. Most modern digital recordings work better with one of the flatter settings, anyway.

I assume your tweeters and midrange drivers are at least making sound, and you are curious if they are still performing up to factory-new quality. To respond, I see no reason why your Model Twos should not be musically competitive with many current systems, or better. If you are curious enough, get hold of a good test disc and see if you can hear test-tone signals above 10 kHz. If you cannot her up that high, or higher, then any tweeter deterioration issue will be academic.

Howard Ferstler

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