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
One of the most transparent HI/LO pass filters available, since the 80's, is the Dahlquist DQ-LP1. The High-Pass is passive, the Lo-Pass active. One of the advantages; there is just one capacitor(and perhaps a resistor in parallel, if needed to match your amp's input impedance), which is actually the filter before the mains amp. You can pick the quality of that capacitor, and thus the transparency. The are rare, and thus bring more than when they were introduced, if in excellent condition. I did a variety of mods to the one I used for almost 30 years. Probably the most important, was to change the RCAs to gold WBTs(very simple, if you can solder) This site for info: (http://www.allegrosound.com/Dahlquist_DQ-LP1_AllegroSound.html) These guys used to work for Dahlquist and still offer support: (http://dahlquistspeakers.com/Dahlquist_Speaker_Service%20DQ-LP1.htm) Here's a site that offers the manual: (https://sites.google.com/site/mpbarney/home/dahlquist-dq-lp1)
The Marchand is quite good and since you don't need additional sub controls, should work well.

If you just want a high pass between the preamp and amp for the main speakers, you can look at Harrison Labs.
There's one on eBay: ( http://www.ebay.com/itm/Dahlquist-DQ-LP1-Active-Passive-Subwoofer-Crossover-/221445734709?pt=Vintage_Electronics_R2&hash=item338f31c135 )
Hello Rodman.

Thanks for the input. I have come across this unit before. I remember you liking it.

It is not clear to me how this unit is operated. The frequency knobs are for adjusting the low-pass x/o frequency, right? Is there a way to adjust the high pass if there is only a cap on that side of the signal?

Sounds like a purist approach to active x/o. I wonder why there are no more, newer, newer units employing this approach?

Cheers!
I guess all this means is the best x/o point is likely to be between 150 and 100 Hz. Right?
If I understood what you wrote, are you trying to find the lowest frequency such that both the subs and main speakers are flat? If so, it's not necessary. 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. 80 Hz should be fine with your main speakers, unless that is a particular problem point in your room. Besides, the higher you set the crossover the more likely you'll be able to localize the subs, which wouldn't be good.

I would think that you should be able to get as smooth a response high passing the main speakers as you did before.

I'd consider the mini-DSP to accomplish crossover and EQ duties.
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!
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.
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)
The crossover point is usually determined by the response of the main speakers. So really small stand mount speakers will force the sub to come up to meet them and will increase the chance that the sub can be localized. 80Hz is generally agreed to be low enough so that the sub cannot be localized. If the main speakers (and amp) are comfortable below 80Hz, then the crossover can be set lower. In any particular room that might provide a smoother bass response.

What I was trying to say regarding the filter slope, is that the slope won't be infinite (vertical line) so the response of the main speakers below the crossover point will add to the response of the sub and the response of the sub above the crossover point will add to the response of the main speakers. In theory, the aggregate sum being a flat horizontal line. Not sure that I did any better explaining myself, but it's the same as the crossover between any two drivers.

I've not used the miniDSP, but I plan on getting one to play with in my office system. Kal reviewed one in Stereophile http://www.stereophile.com/content/music-round-64.
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?
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.
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.
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)
I might be able to mitigate that running the subs in stereo and near the main speakers
I've never really bought into the stereo sub thing, but keeping the subs close to the main speakers will help, but, of course, negates one of the advantages of a sub -- independent placement. Though time delay becomes an issue as Rodman notes and no way to really correct for it in the analog 2-channel world. One of the reasons I went with an AV prepro several years ago.

I recently contacted SVS about the possibility of a new product for them that would've been an implementation of the NHT X2 filter. They weren't interested. Ed actually argued against high-passing the main speakers in a 2-channel system.

It'd be nice if you could use the miniDSP as a digital in / digital out processor and then feed the DAC of your choice.

Good luck with your research.

-- Bob
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.
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?