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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
rodman99999
6,160 posts
 
Ideally, to keep the system time/phase coherent; the voice
coils(acoustic centers) of all the drivers, should be on the
same plane(vertically). That would actually require the
subwoofers to be aligned with the mains. Many claim that the
lowest freqs are non directional, BUT- they contain a great
deal of the recording venue's ambient information, and(if
delayed) can scramble, what would otherwise be, a more precise
sound stage and image.
(http://en.wikipedia.org/wiki/Loudspeaker_time_alignment)
Post removed 
dentdog's avatar
dentdog
500 posts
 
Call Jim Smith. He's a firm believer in double subs and probably the expert on placement. At least after his Room Play session you will know everything is optimal.
lewinskih01 OP
909 posts
 
Dentdog, I'm in southamerica, so it would be rather expensive to have Jim visit. I will get his book, though. I have gone through a number of books, but haven't gotten to that one. Thanks for pointing it out.

Been doing a fair amount of reading and asking around and here's what I'm getting at. Feedback most welcomed!

MiniDSP suggested using Dirac on the server plus a digital crossover right out of my server (their nanoDIGI), followed by two stereo DACs (yes, I would need another DAC on top of mine) and getting rid of my pre. This option went out the window. I have not, to date, used my DAC to drive my amp because I prefer it with the pre in place, plus adding another DAC sounds weird, plus adding so many digital devices connecting thru S/PDIF feels like significant jitter might be introduced. And lastly I would be left to only using a software digital volume control.

One option that is growing with me is replacing my DAC and preamp with a digital processor, along the lines of what Bob suggested. I'm looking into DEQX HDP-4. It does x/o on the digital domain, correction for speakers and for room, including time and phase corrections, converts to analog, has a subwoofer output, and volume control.
The biggest issue is I cannot audition, and I love my pre and taking such a leap of faith to ditch it together with the DAC feels rather unsettling, at least for now. I'm concerned the DEQX might not sound as good as a pre as my Lamm, and/or might not be as good a DAC as my Metrum. But I found a guy who claims to have replaced a $30k DAC with this DEQX and is very happy. BTW, he also uses Dirac on the server.

What are your thoughts on the DEQX HDP-4?
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