Thanks for bumping this back up. Bob, thanks for all the good work.
There is a Dope from Hope about the change over from the spring clip (one piece phase plug) to the two-port phase plug (solder lug). It shows extended response up to 6 kHz on the two port.
But, it shows that the glitch (peak) above 6 kHz is there on both. It just moves a bit. I've read some posts here and there claiming that the peak is characteristic of only one model. That does not seems to be the case at all. There seems to be anomalies above 6 kHz on all drivers of this type including the Electro Voice units.
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I don't have any personal experience with the P-trap. It has a lot of supporters though. As intended, and as Bob shows, it filters out the peak.
It was first suggested to me by a good fellow (Mr. B.L.) who is a big Klipsch fan. Mr. P. set up his system in general and installed the P-trap. Mr. B.L. told me that his daughter remarked very favorably on the change without being told about it. "What did you do to them?"
Therefore, even if some ears don't hear a difference, others probably do. I don't know if it is young ears versus old ears, or female ears versus male ears, etc.
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The Vacuum Tube Valley article on the K-Horn investigated the P-trap and said it cured a "shouty" horn sound.
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The Heyer Audio review reported some issues attributed to the crossover zone to the tweeter. Just maybe, the real problem was the glitch. Pure speculation on my part.
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In my view, part of the problem is that the K-400 has on axis gain up above 3 kHz and this may still be climbing above 6 kHz. Hence the glitch-peak from the driver come through. It could be that other horns which people substitute do not have the same gain characteristics. Therefore the glitch-peak is less pronounced. So it could be that the alternate horns "solve" the glitch-peak by not amplifying it. But the P-trap would do the same at far lower cost.
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I'll let Al K comment. It is my understanding that Al.K's extreme slope crossovers also filter out the glitch-peak. People have good things to say about those crossovers.