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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
Post removed 
lewinskih01 OP
909 posts
 
Thanks Rodman.

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)

So what is the crossover point built into this unit? And how difficult is it to change it?
rodman99999
6,125 posts
 
Check out the manual(copy/paste the site, referred to
previously)). It contains a
table allowing you, based on the input impedance of your
amp, to select a value for the filter cap(and perhaps
resistor). If you can solder: VERY simple. If not: refer
back to Regnar, and they can do a comprehensive check up,and
update, including the cap value), still remaining far below
your target costs. I
always purchased my filter caps from Michael Percy as he
balanced them within 2% for me, and the DynaCaps were
execellent: (http://www.percyaudio.com/Catalog.pdf) of
course; I was pretty anal about balance and transparency.
He has a minimum purchase, BTW.
lewinskih01 OP
909 posts
 
Bob,

Thanks for clarifying. I did understand now - I think!

In my case, the speakers are rated to go well below 80 Hz and they do, but I suspect the combination of them distorting at lower frequencies plus the tube amp being driven hard to get down low is why I'm preferring the option of crossing over at 80 Hz. So I'm looking to cross over above 80 Hz because I suspect sound might improve further.

I hear you about the subs potentially becoming "localizable" above 80 Hz, though. I might be able to mitigate that running the subs in stereo and near the main speakers. Need to try again with alternative layouts and see if I can keep the frequency response in check and achieve this, hopefully allowing for crossing over above 80 Hz.

MiniDSP looks intriguing. I will look into it further this weekend and will contact those guys. I'm not too enticed about the A>D and later D>A conversion post my DAC, but want to check with them if this can be done with their boards at the computer itself, before ever hitting the DAC.

Interestingly nobody's arguing for the traditional x/o, such as the Marchand. I hope I'm not getting too creative here and that's a good and safe way to go.
rodman99999
6,125 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)
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