8"woof with 3" xo---is this a tree i should not be barking up?

[Bill poster]

This sort of approach can work very well. As has been pointed out, it is hardly a new idea.

If using a first order crossover, I'd suggest doing it at 500 to 800Hz. This will prevent the 3" from running out of steam when the woofer is barely moving. It's also the likely location of the baffle step, which will save you a few db.

Others prefer to cross over lower to preserve that full range effect. This is a legitimate approach also. Personal preference. But it will require an active or line level passive crossover.

If using a passive crossover, the main thing to be concerned with is the impedance curve. A textbook value cap and coil simply won't work. I prefer to put the mid/hi driver in a short TL, vented out the back. This will smooth the Z enough that a cap highpass will work, but it should still be crossed over a couple octave or more above Fs. A small cabinet tuned to Fs with the port stuffed with polyfill will also work, as will a compensating network.

The woofer will need a zobel.

Or go active.

You've covered everything here, really well put.
I thought to make a closed TL to help with the Tangband W2-800's impedance spike (using in a 2 way) but decided on a notch filter and an x/o 3 octaves above fs

 

[BHTX]

Aside from overcoming the issues posted by Greg B, a cap on the 3" and a 2nd order linkwitz on the woofer with one of the drivers wired out of phase to make a pseudo 2nd order LR response should work ok, shouldn't it? -6 dB point should be somewhere between 300 and 500 Hz, depending.

 

[BHTX]

Actually, with a small TL extending the response of the 3", having two caps on it would probably work better, and increase power handling too.
So, I suppose the thing to do is figure out the best place for the crossover, in regards to baffle step, and choose a wide range woofer with a sensitivity to compensate for it. Different applications will need different amounts of baffle step compensation. 500Hz for the FF85K looks good, in regards to power handling.

 

[Bill poster]

interested to hear opinions on the crossover points in relation to baffle step for this type of design, I guess a little spacing out with x/o points around 600hz or so can create a dip in the right place to help BSC?

 

[Vix]

I've been thinking about a 3+8" speaker that would be a combination of some things you've already seen, but not all at once. Here is the mix: FAST, Metronome, Half-chang. How can that be? 3" fullrange, for example Visaton b80 or your favorite one. 8" woofer: Visaton W200S or similar. Box? On the outside it would look like a Metronome, but it won't be a Metronome. The upper part of the met would be divided and sealed chamber, and would accommodate the 3" fullrange. The lower part of the Met would in fact be a BR box, but not with the typical port, but with the slot, exactly like it was done on Half-chang, albeit with slanted side walls. Mouth could face the backside, so on the front it would look like a typical Metronome speaker, while on the back the lower part would look like the mouth of a backloaded horn. Maybe sometime I'll manage to draw it, but I hope you get the idea. Result should be high WAF plus a good sounding speaker. Does it make any sense or I drunk too much?

 

[Bob Richards]

I think it's a great idea. I've been thinking about doing that myself. I'm in love with the Peerless 3 inch TG9FD10-08 driver (Madisound), but it's efficiency is only about 85dB (the 4 ohm version doesn't have as good a high end response), so you'd probably be padding the woofer to match it, which is not popular (about a 2 ohm resistor in series with the woofer).

The 2 ohm R would let the woofer resonate a bit more (less damping), but it may actually sound more relaxed and warm (my experience). Yes, throwing away 3dB is like cutting the poweramp in half, but read on.

The Peerless has a vented pole piece, a well vented flat spider, shorting ring on the pole piece, glass fiber cone, rubber suspension, and the surround suspension isn't so big as to cause substantial "cavity effect" (like the Daytons for ex.). Roger Russel, formerly of McIntosh, uses these drivers in his $19,000 Line array system (Google his name).

The only drawback with this and perhaps any 3 inch driver is that it's hard to mount them with good rear air flow clearance. Try to use 1/2 inch MDF or wood for this driver. Use 3/4 inch MDF for the woofer cabinet part. The drivers with the smaller dyminium (sp?) magnets all seem to have lousy frequency response graphs.

I routered the back side of the mounting holes with a 1/4 inch radius router bit, when screwed and glued the drivers in (front mount) with silicone rubber glue. With your woofer, I'd put the crossover at 600-800HZ, 1 pole passive. 800HZ is actually better from the point of view of imaging, but is a tradeoff with directivity. You might not need the 2 ohm R on the woofer at all, since you might want the 3 inch to be slightly attenuated (my experience).

Check the impedance graphs of each driver to see what the impedance is at the frequency you want to do the crossover at. It varies significantly over frequency in most cases.