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high-pass for 3-way fEARfuls
greenboy: This was a collaboration of sorts. Knowing the Alpha6 and the 6ND410 were very similar in their high-end rolloff points and slopes, I contacted Duke and described my idea of how to address either midrange driver and all the recommended waveguides with the same crossover - and why the compromises in off-axis performance would be negligible for bass guitar - if not for other sources. Duke agreed to do measurements and design to my idea if I would ship him the 6" x 6" JBL waveguide.
I'm very pleased how this turned out. Duke rules.
Duke LeJeune: For those of you interested in adding a reasonably priced high performance tweeter to your fEARful cabs, I've designed a high-pass filter for use with the Eminence ASD-1001 mounted to the 90° by 90° 6" x 6" JBL PT-B99HF-1 waveguide (will work fine with other recommended compression drivers and waveguides as well - greenboy).
A high-pass filter is essentially half a crossover. The idea is to let the midrange driver (either the Alpha 6A or the 6ND410) roll off naturally, which both of them do smoothly in the 6 kHz ballpark. The tweeter then just fills in the top of the spectrum. Rarely do the audio gods smile on us little folk, but in this case they have: The response curves of the Alpha 6A and the 6ND410 are similar enough that a tweeter section which works well with one is going to work well with the other, assuming the difference in efficiency can be accounted for. That is done by using a variable L-pad.
The correct polarity is the same polarity is the midrange driver; in other words where the polarity of the midrange is reversed relative to the woofer, the polarity of the ASD-1001 should also be reversed relative to the woofer. This takes into account not only the phase shift of the crossover, but also the inherent phase rotations of the drivers as well as the driver offest (the voice coil of the ASD-1001 is physically about 1/2 wavelength farther away than that of the midrange unit at the crossover frequency).
The ASD-1001 is a PA-style compression driver so it's quite a bit more capable than what's commonly found in bass cabs. And with a 6.5 kHz third-order (18 dB per octave) high pass filter, it's not being pushed hard at all in this application.
The JBL waveguide is a constant-directivity device, which means it doesn't beam the high frequencies like some horns do. The tonal balance stays fairly uniform across at least a 90 degree frontal arc, so that nice, wide sweet spot characteristic of the fEARfuls can be extended up to the top of the spectrum. Also the design of the waveguide itself contributes to smoother sound as compared to many horns.
Total cost for the waveguide, driver, crossover components and L-pad (not counting shipping) is well under $60 using very good but not exotic parts. You can shave off about $8 or $9 more by making your own fixed L-pad out of resistors; here is a description of how to do so: fEARful thread #3, post #818.
¤ highpass parts:
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