How much amplifier power is actually lost via a passive xover though? Enough to even add more than 1dB of sensitivity?
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The crossover in general wastes about half your amp power. It is generally reckoned to be about 1 db per order of the electrical filter. So a fourth order filter would take 4 db. However the lower the crossover point the more power is lost due to the massive series inductors. So a fourth order crossover at say 200 Hz might well loose you North of 6db.
Then the other issues is HF and midrange L-pads. Say you have a woofer 87 db sens and a tweeter 93 db. In the operating range of the tweeter you are loosing 6 db right there. However there is not that much power in the tweeter pass band.
However in a three way things are very different. Say you cross at 400 Hz and the mid is 3db more sensitive than the woofer. That would be usual. Say you have to pad for another 3db of band pass gain for a total of 6 db then you are loosing a lot of power.
In the last scenario an active design saves well over 50 % of the amp power.
That is another strike against passive three ways.
Domestically you have power to spare, but in large room applications these losses are serious and cause crossover failures by causing too much heating in the crossover components, to say nothing of the dynamic thermal compression from component heating.
That is why you no longer encounter passive crossovers in pro applications.
From my experience under domestic conditions when you do see crossover failure it is nearly always from a passive three way.
Hi-Fi sound tell me they see crossover failures in high powered three ways like B & W 800 series with significant frequency.
I think these issues are enough that these high end three way designs should have active crossovers on the woofer mid crossover point as a minimum.
For most domestic two ways this is not going to be a big factor, mainly because there is power to spare, and under most circumstances you do not get significant heating in a tweeter L-pad under domestic conditions. However I never specify anything less then 15 watt resistors even in tweeter L-pads.