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In Reply to: RE: Utah 2a with foster horns posted by sanman on February 22, 2015 at 14:59:48
I THINK the 2a's have a 3 1/2 inch cone tweet and 8 inch woofer. The big cone tweeters play quite low, but so do the horns. The cone tweeters are attached with screws from inside the cabinet. Try just removing the cone tweets entirely and i think the slots in the horns will slip right over the existing screws from the back. Just use the OEM cap that is currently on the 3 1/2 inch cones on the horns, but that cap you are using might be about the same value.
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I'm afraid of having a hole in the middle...
"When the demon is at your door, in the mornin' it won't be there no more"
Steely Dan
I have not looked at the specific response curve for the Foster horns, but I know the following.
1. Utah woofers are paper cone and designed to play high up and smoothly roll off without an inductor. One of the reasons these speakers were inexpensive is there were not a lot of crossover parts.
2. I have not looked at the FR of the Foster tweeter, but I am very familiar with the similar Pioneer version which is rated to play down to 2000 Hz. http://www.parts-express.com/pioneer-ahe60-51f-3-1-2-horn-tweeter--270-050
I would be shocked if the Foster was not very similar. THey MIGHT even be the SAME tweeter.
THe crossover frequency for the 12-inch woofer on the bigger 3-way Utahs is 2500 Hz. The three-way Utahs have a sealed 8-inch mid which plays for only 1000 Hz and crosses over to the 3 1/2 inch cone at 3500 Hz
I suspect the 8-inch woofer in the 2a crosses over to the 3 1/2 inch at about the same 3500 Hz frequency, with the single OEM cap handling that a leisurely 6 db-per-octave slope. In which case a horn that plays down to 2000 Hz on a 6-db-per octave slope works perfectly, since that is almost exactly the slope of the 3 1/2 inch cone.
In a number of upgrade Utah models, they replaced the 3 1/2 inch cone tweter with a very similar round horn so what you are doing is not inconsistent with what Utah engineers themselves did.
My hunch is that if my reasoning is correct the OEM cap value is a fine choice for the horn too.
Bottom line is that I would be shocked if the horn didn't work well as a drop in replacement for the cone, using the OEM cap, while adding sparkle to the treble. I would be shocked if there were a significant hole in the FR.
I'm not sure if the 2a has a tweeter level control or not, but if it does not and the result of this drop-in experiment seens treble heavy (the horn IS higher output, but the cone seems treble shy by current standards), you need to add a variable level control into the cuircuit to fine tune the treble-bass balance, but that is it. Conveniently, the Utah 3-ways already all have tweeter level controls which can be used to fine tune the balance between the treble and bass when the horn replaces the cone.
There are not many speakers out there, vintage or otherwise, where you could pull off a simple tweeter-swap stunt like this with good results, but the Utahs are one of them!
Doing this to a pair of Utah 3-ways was my first (and arguably my most successful) DIY speaker project. I have the round horns sold by Radio Shack in the mid 1970s (and probably made by Utah!)
David
David
I have had mixed results with dropping in these tweeters. I did drop some in to a pair of, if I recall correctly, allied speaker company 2 way AS speakers, with similar paper drivers. I added a 2 uf cap to the crossover. They would be good candidates for the L pad you suggest as they are still a tad bright.
I am intrigued by the concept of dropping these in and will try it out soon as I get the above project further along!
Thanks, David.
"When the demon is at your door, in the mornin' it won't be there no more"
Steely Dan
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