Picking woofer to midrange XO point

[Saurav]

Hi,

Relevant information:

Woofer: Eminence Beta 12CX (12" fullranger), should be good up to about 1.5kHz
Midrange: Audax PR170M0 (6.5") should be good down to about 250Hz with a steep filter
Amplification: Gainclone for the woofer, 2A3 SET for the midrange (and tweeter above it), active XO between the two.
Baffle width: 14"

I've fallen in love with the midbass dynamics that you can get from a high efficiency 12" driver, and don't want to lose it. At the same time, I don't want to take it up too high, because that defeats the point of using a midrange driver. So, I was thinking of something between 300 - 400Hz. I'm not sure what instruments/vocals fall into that range, but I'm hoping that it'll give me some of the punch of the 12CX while also retaining the better midrange of the PR170M0.

Using the f = 4560/Wb formula from the t-linespeakers website, I get a -3dB point of 325Hz for the baffle step compensation curve. This is right in the middle of my proposed XO frequency.

So my thoughts so far - I have enough opamps, so I'll just build a 4th order L-R crossover at say 350Hz, and add the BSC circuit for 325Hz to both the low pass and high pass sections. I guess I could come up with some kind of optimized filter which would use less stages, but do I need to? Also, at 400Hz the wavelength of sound is around 34", so I can ignore the driver offset between the woofer and the midrange. Or I could add the delay stages too, if I have opamps left over. I'll be referring to the Linkwitz page for all of these circuits.

Anyway... like I said, those are my thoughts so far. I would appreciate it if you could point out any big holes that I'm missing, or any suggestions about a diferent XO frequency to choose, or anything else in general that I have missed.

Thanks,
Saurav

[Bill Fitzpatrick]

I've been bi-amping since forever and have always preferred a lower crossover frequency - in the 100Hz area.

[Saurav]

Well, I've already bought the midrange drivers And I do have a subwoofer under the 12CX that is currently biamped/crossed at 100Hz. I think I'll move that down to 80Hz though, I can't justify having a big 12" woofer doing only 100Hz to 300Hz

I should have mentioned that I have a subwoofer, so the 12CX won't be handling all the bass. It's not in a cabinet that's big enough to do that anyway.

So, with that additional piece of information... does that change anything?

[roddyama]

Originally posted by Saurav
Well, I've already bought the midrange drivers And I do have a subwoofer under the 12CX that is currently biamped/crossed at 100Hz. I think I'll move that down to 80Hz though, I can't justify having a big 12" woofer doing only 100Hz to 300Hz

I should have mentioned that I have a subwoofer, so the 12CX won't be handling all the bass. It's not in a cabinet that's big enough to do that anyway.

So, with that additional piece of information... does that change anything?

80Hz would be good, or maybe even a little lower. Checkout what kind of closed box alignments you can get using that driver. You might find that closed box alignment will provide 12db of your 24db/oct cut-off. Plus you get better transient response from the driver.

I wouldn't take the 12" much above 300 - 400Hz.

[Saurav]

Thanks. My boxes are already made, and I think it's set for around 60Hz. I've been using the 12CX's with a coax compression tweeter, now I want to use them as woofers and add open baffle midrange + tweeter on top.

Lower than 80Hz... I could take it down to the sealed box rolloff, and add just a second order electrical filter on that. I have an EQ on the subwoofer, and I want the room problem frequencies to come only from the subwoofer, so I can stay in control of them. I'll try this out though, maybe the 12CX's position won't excite the 63Hz mode, and then I could use it with a ~ 60Hz XO.

Plus you get better transient response from the driver.

How does that work?

[planet10]

With an active XO at the BS frequency, you don't need a baffle-step filter...

dave

[roddyama]

Originally posted by planet10
With an active XO at the BS frequency, you don't need a baffle-step filter...

dave

Good one Dave. You will only need to set the relative levels.

[Saurav]

Why not? I've seen active BSC circuit schematics. Or are you saying that I can just adjust the gains/levels of the two channels (woofer & midrange) and achieve the same effect? If that is true, then that's my answer, I pick 325Hz as the crossover point. Can I still do a 4th order L-R crossover, or would I need to change that? I'm not sure how differet output levels will work with the L-R XO, and if the combined result will exactly (or more or less exactly) cancel out the BSC curve.

[chris ma]

Originally posted by planet10
With an active XO at the BS frequency, you don't need a baffle-step filter...

dave

Dave,
How do one find the baffle step frequency? you know I don't really know much please explain in junior level for me.

Regards,
Chris

[Saurav]

The easy answer:

f = 4560/Wb

Where Wb is the baffle width in inches.

Which makes me think - the woofer is in a closed box with a 14" baffle. The midrange will be on a 14" open baffle (or 18", since I'll have to add some wings to make sure it doesn't fall over). The BSC frequency for an open baffle should be different right?

This'll probably become a lot more fun once I build the baffles and start taking measurements