Back when Rick asked for some suggestions for the tweeter, I offered the DXT as a possibility, but my #1 recommendation was SB26ADC-000-4 for its better distortion performance (posted earlier, my bad).
Rick had initially suggested the SB26STWGC-4. but the top end is rough and would need extra components to EQ (mandatory)
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The SB26STWGC-4 is an SB26STC (well regarded) with a waveguide. The small waveguide it uses just doesn't have enough gain (5 dB) to keep up with the distortion performance of other, better choices:
I then looked into ring radiators, which would need a waveguide to keep up with the Purifi's power handling capabilities
- Revel uses a variant of the SB26ADC-000-4, which shows great distortion performance here and here. Great price. Troels makes it work OK (but not great) off axis here. The SB26ADC can be improved with a waveguide such as Augerpro`s free design here. The CAD file shows that the 4in waveguide would fit, and maybe even the 5ìn (see here and here).
- An oldie but still goodie is the SB26STAC, MarkK tests and here
- DA25TX00-08 is an option I want to explore some day based on this
- Seas DXT is a good option for its built in waveguide but it has much more distortion 1-2k than the SB26ADC. I was also surprised to read this diva complaint
- DX25TG09-04 and Monocor WG-300 waveguide, tests results here show very nice directivity and great distortion results. But the waveguide is big and the SB26 has better performance
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- XT25TG-30 and Monocor WG-300 waveguide, tests results here also show very nice directivity but the distortion is much higher, the DX25 combo wins in the battle of the ring radiators
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The SB26ADC-000-4 with a waveguide is the front runner for me.
CTRLs diffraction models then simulated options. For a much more old school approach to this simulation but one that includes the tweeter's native frequency response as well, see my old tutorial here hosted at Charlie's page. There are better free tools around now, but the design concepts will be the same.
I was thinking about Augerpro's waveguide with the SB tweeter as well when I first saw the topic.