BFD for OB woofer EQ and crossover? - Home Theater Forum and Systems -

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post #1 of 3 Old 01-10-10, 10:07 PM Thread Starter
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Paul Janda
Join Date: Jan 2010
Location: Scottsbluff, NE
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BFD for OB woofer EQ and crossover?

I'm planning a diy open baffle speaker system with fullrange drivers on top and "helper woofers" (some of the most efficient 15"ers I can find) on the bottom. I'll be bi-amping the system, most likely with low powered tube amps, but maybe a chip amp on the woofers if necessary (50W). So, I'll be crossing over at line level, or thereabouts (maybe between gain stages in a tube amp). I'll take care of the crossover to the fullrangers with passive components, but I think a BFD might be just the ticket for the woofers.

I'd like to cross the woofers over somewhere between 100-200hz (exactly where will depend on sims and measurements of the actual setup). I'll be looking for an acoustical second or third order slope [the fullrangers will have 6db/octave from the baffle, then either one (line level) or two (line and speaker level) first order crossovers], though the electrical slope will probably need to be more complex (based on preliminary simulations). Hopefully phase won't be a big problem. I'll either end up 90 or 180 away on the fullrangers. I'll also need to apply considerable shelving to counteract the first order rolloff of the woofer below the limits of the baffle (dipole cancellation) and to some extent, to counter a boundary-related bump between 100-150hz (ultimately something like a first order low pass that is -6db at 100hz). I'll only get it flat down to 45-50hz, otherwise I'll run out of amplifier power. In addition, I'd love to tackle some room modes if they become a problem (though with dipole bass, they should theoretically be less of an issue).

It seems to me that I could accomplish all of that with a BFD. I know it isn't really a crossover, but it seems it can be used as such. In addition, I could add at least a first order passive line level filter in front of it if I have to (I can tolerate some insertion loss, I could go second order too if necessary). Here is the question: Can I do it all with the 1124? It has a higher input impedance than the 2496, which will come in handy for my application (tubes like high input impedances). I don't need the extra resolution, but do I need the extra bands of the 2496 to do everything I want? Is there anything I'm missing?

This is my first post here, so thanks a bunch for your help. In the spirit of full disclosure I actually make/sell the fullrange drivers I'll be using. I do all sorts of audio DIY, but I haven't been to the HT boards much. I have been thinking about this project for awhile, but it just occurred to me that a BFD might be what I need! I was very happy to find a forum with folks who know the machine well.

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post #2 of 3 Old 01-11-10, 10:20 AM
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Re: BFD for OB woofer EQ and crossover?

Welcome to the Forum, Paul!

It was a bit difficult to follow everything in your second paragraph, but nevertheless I don’t think the BFD is what you’re looking for.

To start, it is a very noisy unit and as such is best suited for subwoofers – i.e. below 100 Hz or so.

Second, it has no shelving filters.

Third, it’s unsuitable for use as a crossover. True, it can be set up to “mimic” a low pass crossover filter, but that requires a plethora of filters, and in the end it is impossible for an equalizer to deliver the same kind of “infinite slope” a crossover does, that continues to drop each octave out from the crossover frequency.

And let’s take a closer look at the “plethora of filters” required to make a BFD mimic a crossover filter. As you may know, equalizer filters alter frequency response by shifting the phase of the signal passed through them (see this article for a more technically detailed description). Using an equalizer as it’s intended, that’s not a big deal because it’s essentially counteracting phase issues caused by the room that are altering a speaker’s perceived response (i.e., sound bouncing around the room and arriving at the listening position at different times).

But that’s not the case when you try to force an equalizer to behave like a crossover by using a bunch of filters. Each filter is introducing phase, but none of them are counteracting anything from the room. You’re just needlessly adding artifacts the signal that don’t need to be there.

If you want crossover filtering and equalization in a single package, you might consider the Behringer DCX2496.

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post #3 of 3 Old 01-11-10, 01:57 PM Thread Starter
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Paul Janda
Join Date: Jan 2010
Location: Scottsbluff, NE
Posts: 4
Re: BFD for OB woofer EQ and crossover?


Thanks for your reply. I know the DCX2496 would solve all of my problems, but I'm a cheapskate. I also feel like it is a good deal more machine than I need. If they sold a half DCX, that would be perfect for subs.

Thanks for the article. I suppose I should have sought that explanation out earlier, but it makes perfect sense. I was thinking of digital filters as a magic black box. Your comment about noise also got to me to thinking. I doubt it would be an issue between 100-200 hz.

Wouldn't the addition of a passive first order low pass filter go a considerable way towards solving these problems? It would provide my needed shelving. Given the noise concern, I could put it between the BFD and the amp. Again, I'd imagine something -6db around 100hz. So, I'd end up using a band or two to get the rolloff just right from 100-300hz, and then I'd only need another couple of bands to be sure nothing nasty was in the octave and a half towards 800 hz (which would be -24db), right? Wouldn't that leave me plenty of bands to address issues between 50-100hz?

I need to read a bit more about this machine.

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