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Active crossover for 804S and Rythmik subs?

Hello everybody.

I've been thinking about this quite a bit and would like to validate with experienced users. This step entails relieving the speakers and amp from playing below 100/150 Hz, approx. Sorry for the long ramblings.

My system is made of Lamm LL2 pre, McIntosh MC275 amp, B&W 804S speakers and two Rythmik 12"-subs.

I tried the subs as reinforcement, meaning from my pre going straight into the amp and in parallel also straight into the subs. I also use REW (in-room response measuring software), and a measuring mic, and by tweaking the Rythmiks controls achieved a rather nice in-room frequency response.
Lately I tried going from pre to each subs internal x/o and from there to the amp. The subs plate amp include a crossover with a fixed point at 80 Hz, so the amp now sees 80 Hz and up. While the measured frequency response is not as even, I think I like it better than in reinforcement configuration.

So I'm thinking a better x/o and shorter/better interconnects should help, and that the key constraint is the sub's frequency response at the higher frequencies.

A guy over at hometheatershack.com made a ton of sub measurements and tested the 12" Rythmiks. They are flat until 70 Hz and drop 5 dB by 100 Hz, and an additional 7 dB by 200 Hz. http://www.hometheatershack.com/forums/subwoofer-tests-archived/5756-diy-rythmik-audio-direct-servo-12-sealed-56l.html

I can't find measurements of the 804S, but the 804D, the newer model, was recently measured by John Atkinson. http://www.stereophile.com/content/bowers-amp-wilkins-804-diamond-loudspeaker-measurements
Kal Rubinson mentioned on his review the 804D and the 804S both sounded very similar and were spec'd the same. Maybe safe to assume they have similar bottom end response...

So going with the 804D measurements, they drop about 4 dB at 50Hz compared to their level at 100 Hz.

I guess all this means is the best x/o point is likely to be between 150 and 100 Hz. Right?

What else should I take into consideration?

I'm thinking of using a Marchand XM44. Can you think of better options? I would like to keep it under $1500.

And if all the above ramblings were spot on (probably not!), then what slope should I use for the XM44? They offer 6, 12, 18, 24, 30, 36, 42, 48 dB/octave.

Thank you!
lewinskih01
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lewinskih01 OP
909 posts
 
Bob,

Thanks for posting.

If I understood what you wrote, are you trying to find the lowest frequency such that both the subs and main speakers are flat?

Not really. My goal was to cross over at the highest frequency the subs would work well at. But then I wasn't necessarily positive I was right. What do you think, and why?

Remember that the crossover is not of infinite slope, so the response of each will continue to add to the aggregate level both below and above the crossover point.

I'm not tracking you here. Would you be so kind to rephrase/elaborate?

Interesting suggestion about the miniDSP. I wasn't familiar with it. Have you used it? Seems similar to the DSpeaker unit, but with more flexibility and cheaper. Is it low price, but not sound-degrading cheap? I'm intrigued. Thanks for the suggestion!
rodman99999
6,145 posts
 
Hello Lacee- There is no adjustment available for the Hi-Pass. The goal of the design was transparency and phase coherency, where it's most critical. I really can't imagine why any adjustment, other than the crossover point(determined by the cap and amp's impedance), would be necessary for the mains amp. All level matching is done in the Lo-Pass section. I'm guessing no one sees enough demand for actively bi-amping systems, to go into production. When Carl Marchisotto designed the system(Nola Grand Reference IV), pictured on this site: (http://www.nolaspeakers.com/), he used the DQ-LP1, to actively bi-amp it. Of course; he helped design the unit, back in the day.
rodman99999
6,145 posts
 
I should have said Grand Reference III. Harry Pearson gave it five stars. Read the paragraph under the heading, 'Variable Low Pass Filter', here: (http://marcs.members.sonic.net/nola/grandref.htm)
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