Seas Usher TM
This is the final saga in the story 4 different driver/XO combinations or projects that used the same enclosure. These cabinets with 38 cu. ft. internal volume were built by diyspeakercabinets.com for Ed Frias' AR.COM kit. This was its lifecycle:
1) AR.COM: Peerless 850122, Peerless 812687 / Madisound prebuilt XO's
2) U-Pee: Peerless 850122, Usher 9950-20 / Wayne J's PeeCreek XO
3) MyPee: same drivers / My first self-designed XO
4) Seas Usher TM: Seas ER18RNX, Usher 9950-20 / My second XO.
I wasn't completely happy with the MyPee XO. I never got the baffle step and tonal balance right. Furthermore, I felt the Usher tweeters deserved a better partner. I chose the Seas ER18RNX because of its use in several other well-regarded and documented designs, published measurements and because it fit exactly in the driver cutouts!
Going for LR4 targets, I ended up with third-order electrical highpass and second-order lowpass. The XO employs a notch for the 4-6kHz breakup of the mid-woofer.
I received initial comments from PE board members whether LR contour on the highpass was necessary. I found that it plays a key role in this design. Without it the sound was too bright and slightly tinny / metallic. With the LR at values as modeled, the details were killed off.
So I went to work unwinding the coil in steps, going from .25mH and landing at .20mH. I also played around with the resistor that is in parallel with it and ended up at changing it from 1.5 to 1.25 ohms. I was surprised how sensitive the sound was to changes in the resistor: bring the value down too low by a little bit and the sound gets too forward in a hurry! I believe this is because changes here affect the region from 1.5 to 4kHz, where our hearing is most sensitive.
This project has been a personal success as this crossover is the first I have done on my own where I achieved satisfying results. The 9950 is confirmed as my favorite tweeter and the ER18RNX has the clarity and lack of distortion to keep up with it. I think I achieved the right tonal balance in the end and have a much richer midrange response than previous iterations.
If I weren't constrained by re-using the cabinets, this TM should ideally go in .5 internal volume enclosures for better bass extension. Still with modeled extension down to around 50Hz it sounds quite good.