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In Reply to: Bi-amp crossover question/Rane AC-22 with 3.6's posted by bfrank53 on March 9, 2007 at 13:46:11:
Several points:* The 3.6 is such a wonderful speaker that it's a shame to drive it with second-class electronics. I have no idea what the Rane sounds like but - as a piece of pro gear - it may not deliver as good a sound as, say, a Marchand active XO. You can buy Marchands in either kit form or fully made up ... and either 2-way or 3-way.
* Secondly, having to make use of cable adapters *will* degrade the sound ... but possibly only a little (depending on their construction and materials).
* Re. what slopes to use - the 3.6 passive external XO (bass lowpass/mid highpass) is:
- 24dB bass LP
- 6dB mid HP.However, IIIas and 3.3s/3.5s have an 18dB bass LP slope, so with an active XO replacing the passive, I suggest an 18dB slope would work just as well. If Al Sekela is to be believed, the reason the 3.6 has a 24dB LP slope is because this puts a cap across the driver, which damps out-of-band resonances (which makes a lotta sense! :-)) ). This is a good enhancement (on the 3.5) for a passive setup but in an active setup, the amp itself damps out-of-band resonances, so the "advantage" of going to 24dB is moot.
The frequency which the LP & HP slopes cross at (ie. the "crossover frequency") is almost exactly 200Hz. However, modelling this in lspCAD shows a pronounced hump at this frequency - which I suggest could be improved upon, with an active XO.
Does the Rane allow you to have asymmetric slopes ... eg. 18dB on the bass and 6 or 12dB on the treble?
If you are limited to 24dB slopes then choose 200Hz. In this case, you need to reverse the mid/treble connections, as the bass and mid drivers need to be in phase.
If you can have asymmetric slopes then I would suggest you try:
* 18dB LP/6dB HP or 18dB LP/12dB HP (and keep the drivers connected the way they are with the passive XO).
* I would experiment with setting the -3dB points of the bass LP and mid HP filters apart slightly, to tame the hump at 200Hz. First try 18dB @ 200Hz for the bass & 6 dB @ 250Hz for the treble - and go on from there! :-))Regards,
Follow Ups:
You may want to try reversing connections on the bottom end with some combinations to hear how it sounds. With my 3.5s and the 18dB LP/12dB HP @200Hz, I found there to be a big mid-range suck out that was fixed when speaker leads on the bottom were reversed.
this is why i suggested 300hz. i haven't measured the 3.6r, but the 3.5r has a bit of a dip around 250hz and drops off quickly below 200hz (green line). what would the combined frequency response look like 24db per octave at 250hz?
Hi helmholtz,A 24dB L-R crossover is designed to be flat (at whatever frequency you choose).
This is bcoz the LP and HP filters are both down -6dB (half signal) at the specified frequency ... and two half-signals sum to one-signal.
You could certainly choose 24dB L-R @ 250Hz but this will move the bass LP curve up a bit from stock and, given the bass panel is much bigger (and slower) than the mid-range panel, you might be pushing it up a bit too high. Hence I suggested to stick to the same "crossover frequency" (ie. where LP and HP curves interesect) as the stock situation ... 200Hz.
However, the sum will be flat in either case.
Regards,
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