James, good points brought up early in the design, which should rightly be addressed early.
Whilst there are the obvious advantages of control with full active crossovers, the additional equipment required is a major concern, especially if only two boxes were being used. In the case where you may run amp racks behind each stack, there would be the temptation to run both hi-mid and highs off each side of one power amp. This would result in the HF being connected to an amplifier probably at least double the power it needs to be. As long as everything is run clean and within limits, there is no problem, but an accident (feedback squeel, dropped mic, etc) could easily kill the horns unless carefully set limiters are employed.
I tend to favour using the passives on the top for the reasons you suggest, however the box needs to be designed to cater for this. If the hi-mid and HF drivers are mounted the same distance back from the front of the box (therefore time-aligned), a passive crossover is easy. If there is a small difference in distance, then various techniques like differing crossover slopes and frequency overlap/underlap can be used to compensate the time difference. Once the difference in distance exceeds 1/2 wavelength at the crossover frequency, then coherently using a passive crossover is probably impossible.
Time-aligning the drivers would suggest a very long horn for the HF, which could be very detrimental to its performance, but there are various ways around this, such as including the HF portion as part of the hi-mid flare (as I did in one of my dual 8"+1" boxes), or extending a short horn with a higher flare rate out to the full width of the mi-mid flare.
If the general concensus is to either use passive or have it as a serious option, then we need to allow for this BEFORE we get into the HF geometry. Although this would more than likely preclude the use of a commercially available HF flare, don't let this be a deterrant to using a passive crossover if most would prefer to do so.
As this is a community project that we would like any of us to be able to build, then the availability of FULL and accurate plans is a must. NOT just a couple of section views with dimensions like most plans available on the net, but full individual dimensioned parts diagrams and sheet cutting diagrams etc. With the 3D CAD setup I use, producing accurate parts drawings is easy. So even if the design is quite complex, and uses a lot of small, funny shaped bits with wierd bevels on them, IF you have accurate drawings of each bit, then cutting and assembling is a breeze.
The point of that last paragraph is:- if you would prefer a passive crossover, don't decide against it on the grounds that it may make the box too complex.
Cheers
Graeme
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