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post #1 of 15 Old 05-14-06, 08:11 PM Thread Starter
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Crossovers

Well I hope some of you guys can help with this. I have an HT system with bookshelf speakers for my front L/R mains sourced from the pre-outs of my receiver and driven by my main amp. Also I have a Paradigm X-30 Xover between the receiver and the main amp. The subwoofer output of the X-30 sources my subwoofer amp which drives an old dbx 15Ē sealed subwoofer that I have modified.

I havenít received my DSP1124P yet (still waiting after 2 weeks!) but I have used my Rat Shack SPM to evaluate several subwoofer locations. The best position I have found is right up front between my main speakers. This arrangement sounds really good, but I have been reading lately an article by Dr. David Griesinger on the spatial effects of low frequency room modes http://world.std.com/~griesngr/asa05.pdf. His thesis states that our ability to localize sound is dependent on a combination of sound pressure modes and sound velocity modes. Now I know the conventional wisdom is that you canít effectively localize low frequency sound, but he is a big fan of stereo low frequency drivers.

So I am thinking seriously about adding a second subwoofer in stereo with the first one, but if I do so I will need a full stereo 2-way crossover instead of my X-30. I have researched a lot of home and pro gear and at the same time I have read a lot of disappointing personal reviews about Xover noise and coloration. The only Xoverís that seem to consistently come out on top are those manufactured by Marchand. But at $450+ this is over my budget.

My questions (finally!) are:
Am I nuts thinking this will be worth the cost and effort? Keep in mind I already have sufficient power to drive a second subwoofer and another dbx subwoofer would not be very expensive.

Is there a reasonably priced Xover (<$250) that wonít destroy the sound quality of my system? As a point of reference I am on a limited budget and have a fairly modest HT system. However my foremost interest is with sound quality, not extreme SPL.

Thanks

Reed


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post #2 of 15 Old 05-14-06, 10:32 PM
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Re: Crossovers

I think brucek might want to address this for you... I'll notify him of your thread.
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post #3 of 15 Old 05-14-06, 11:27 PM
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Re: Crossovers

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Is there a reasonably priced Xover (<$250) that wonít destroy the sound quality of my system?
I can't imagine there is, although others might disagree...

In fact, in your present configuration, since you're using bookshelves (that I assume don't extend very low), I probably wouldn't have put the X-30 in the path between the pre-out and power amp. I would feed full range direct to the main amp and let the bookshelves roll off naturally.

Then (sourced from a Y-splitter at the pre-amp) feed the X-30 for your sub. Then adjust its crossover to match with the mains. (easy to do with REW program which you'll need to learn when you get a BFD anyway)..

But anyway, you want to add another sub. To keep the cost down, would you consider using the above arrangement and just add another X-30? One for each sub fed from splitters at the preamp. When you add a BFD, the two X-30's would be driven from the two BFD channels independently. Yes, you'd have to match the crossover and volume independently, but that may end up being a bonus......

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post #4 of 15 Old 05-15-06, 12:16 AM
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Re: Crossovers

That's what I did, and it worked fine. Most bookshelf speakers roll off at 60-80 Hz, which is ideal for matching up to a sub-woofer. You don't need to add a crossover, and you don't want to add a crossover because it only introduces another source of phase-shift. REW will show you if there is a cancellation notch where the bookshelf woofers and the subwoofer meet. I had one, which I was able to correct, best, by adjusting the time-of-arrival delay through the AV-amp. There was also a phase-control on the subwoofer which also worked, but I didn't like that control because it was not lockable, so I set it to full CCW.
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post #5 of 15 Old 05-15-06, 12:29 AM Thread Starter
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Re: Crossovers

Thanks for your responses. Before I had the subwoofer in use, I had the mains do just that; roll-off naturally. The Quads are great speakers from the highs on down through the midrange, and then they roll-off gradually from 45Hz (-3db) on down. These speakers are near field monitors with great detail and imaging which is my personal preference in sound. But they definently need lower end support.

I had read all the praises being bestowed on active bi-amping and I figured the xover hook-up I have would be similar to that, especially if I go with the stereo subwoofer setup. It just so happens I don't have all the speakers in one enclosure. However to make up for the non-planar speaker arrangement (subwoofer against the wall behind the mains) I needed phase compensation. That is one of the reasons I chose the X-30.

I had thought about your idea of just getting another X-30, but I wanted to keep things simple and have all the crossover functions in one box. But the more I look at what's available, the better that option is starting to look.

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post #6 of 15 Old 05-15-06, 12:41 AM Thread Starter
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Re: Crossovers

gasmollin,

Don't think I was ignoring you but you responded whille I was writting.

I have a 7.1 receiver with room correction which I would think take care of any gross phase miss-alignment at least on a per speaker basis, except for miss-alignment between the mains and the subwoofer(s) since they are sourced from the same pre-outs.

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post #7 of 15 Old 05-15-06, 01:18 AM Thread Starter
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Re: Crossovers

Brucek,

When I get my BFD you are saying to insert the X-30 after the equalizer. Why is this prefered over feeding the BFD from the X-30?

Thanks

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post #8 of 15 Old 05-15-06, 08:55 AM
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Re: Crossovers

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except for miss-alignment between the mains and the subwoofer(s)
Exactly, and this is where you use REW and the X-30's own phase (and crossover) adjustment to individually match each sub. That's the bonus I spoke of.

Quote:
are saying to insert the X-30 after the equalizer. Why is this prefered over feeding the BFD from the X-30?
The X-30 line amp should be situated between the BFD and the subwoofer power amplifier to provide the most gain control (which you will especially require since you don't have a subwoofer processor trim as would be provided if using a standard 'subwoofer out' jack). The BFD's maximum input level (while standard) cannot be exceeded. The BFD is not an amplifier, it's a unity gain device. Since it's a digital device and you're feeding an ADC/DAC chain you want to optimize its input level to take advantage of all the bits available.
By this I mean to take maximum advantage of the dynamic range provided by the analog to digital converter in the BFD or any ADC for that matter, you need to provide it with a maximum input signal level that takes advantage of the greatest number of bits to represent the resultant voltage samples.

If the range from your weakest signal to the strongest signal was only half the required maximum input level, you would be robbing yourself of dynamic range and subsequently increasing your signal to noise level. To get the most out of this system you would like the maximum signal sent to the BFD to "just" not turn on the red LED. If the yellow LED comes on in explosions etc, that would be fine. The BFD has no compressor or limiter on its input, so you can easily overdrive it and clip the output.

This means once you've reached this maximum input level, you can't increase the level any more even if you wanted to. So it's the BFD's output analog signal that you want to amplify.

The BFD's unity output (or less if filters are applied) can be amplified and phased to your hearts content with the line amp. If the line amp can create 18dB gain, then it will 'all' be available to you, instead of being limited if you placed the line amp before the BFD.

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post #9 of 15 Old 05-15-06, 01:09 PM
 
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Re: Crossovers

I’d advise being careful running bookshelf-sized mains full-range, at least for movies. The soundtracks of action movies typically have the low frequencies boosted tremendously, which can overdrive small drivers during demanding passages, especially if you like to listen at, or your room demands, high volume levels. I found that the 8-inch woofers in my speakers were bottoming out, even with a crossover set at 60 Hz. You might find that the risk of damage to your speakers isn’t worth the improvement in phase characteristics.

Quote:
His thesis states that our ability to localize sound is dependent on a combination of sound pressure modes and sound velocity modes. Now I know the conventional wisdom is that you can’t effectively localize low frequency sound, but he is a big fan of stereo low frequency drivers.
I used to have my subs set up in stereo, and found that I could localize them via air pressure. As such I did notice a difference later I when put them both in the same corner. At first it was distracting, but like most changes in life, after awhile you get used to it and don’t notice it anymore.

Quote:
So I am thinking seriously about adding a second subwoofer in stereo with the first one...
I’d think long and hard about that. Unless you have a perfectly symmetrical room, getting them balanced and equalized so that they have linear response - i.e., within 1-2 dB of each other across their range - is usually pretty tough. REW will make it a lot easier, however.

Here’s the situation I was faced with:





As you can see, the right one was in a corner, the left one on a flat length of wall, and near an opening. To the left of it was another opening, going to our entryway. Needless to say, their individual response was drastically different. For instance, the right one had a big peak at 43 Hz, the left one fell like a brick below 40 Hz. Trying to equalize the left one so that it had the same extension that the right made it regularly bottom out during demanding action scenes.

When I finally put the both in the corner, I got almost a 6-dB gain in output – which is what you typically get when you co-locate a second sub where you only had one before. In other words, due to its poor location, the second one had been contributing absolutely nothing to maximum SPL. About all I was getting from the stereo arrangement was a perceived balance in air pressure from both sides – hardly worth the performance tradeoff, I found out.

Of course, different subs will get different results, as will different rooms, so your mileage may vary. You might not have a problem equalizing your second sub in a poor location so that it performs as good as the one in the optimal location, if it’s robust enough.

However, some things here are universal. Like, assuming your first sub is adequate (i.e., fully pressurizes the room and doesn’t out during demanding passages), you’re putting out a pretty big hunk of change for a new sub and related electronics, for a rather small perceptible improvement. You’ll have to decide if it’s worth the expense or not. If you spend a lot of time sitting in your chair listening to music, and you’re really picky about your sound, it might be worthwhile to you. However, if your primary use is movies, I think you’ll find there’s not much value in it, because I assure, you’ll be focused on that guy hanging on for dear life, not the finely balanced stereo subs.

If you do go ahead with it, you’ll find the second one will be much easier to integrate if you have a symmetrical room. It’ll be easier to equalize them, since their response will be pretty much the same going in, and you should see at least a few dB in improved maximum SPL – but not the full 6 dB you would with dual co-located subs.

Quote:
...but if I do so I will need a full stereo 2-way crossover instead of my X-30. I have researched a lot of home and pro gear and at the same time I have read a lot of disappointing personal reviews about Xover noise and coloration. The only Xover’s that seem to consistently come out on top are those manufactured by Marchand. But at $450+ this is over my budget. Is there a reasonably priced Xover (<$250) that won’t destroy the sound quality of my system?
I recommend the AudioControl Phase Coupled Activator Series III. You can ignore the bass processor and use its built-in crossover, which has 24 dB/octave filters with Linkwitz-Riley alignment. The distortion, S/N ratio etc. specs are stellar – I can dig them up if you like.

You can’t buy the PCA III new anymore, but you can see them fairly regularly on eBay. They listed for $300, so you should have no problem picking one up for under $200. The only small downside is that they use plug-in chips for the crossover frequency, which you’ll have to order from AudioControl (or make your own with a resistor network, as I did) if you want a frequency other than the stock 90 Hz. The upside is there is no frequency selector knob that people can mess with, and it’s actually more accurate and reliable than a knob

Regards,
Wayne
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post #10 of 15 Old 05-15-06, 01:25 PM
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Re: Crossovers

Quote:
brucek wrote:
In fact, in your present configuration, since you're using bookshelves (that I assume don't extend very low), I probably wouldn't have put the X-30 in the path between the pre-out and power amp. I would feed full range direct to the main amp and let the bookshelves roll off naturally.
I had always worked under the assumption that, if you didn't have to, you shouldn't let a speaker run full range, i.e., don't let it roll off naturally. If you cut off the lower (higher power stuff) your mids and highs are cleaner and you don't don't run the speakers at the lower ranges which they won't reproduce well any way. You can also have the speakers roll off in a predictable way so you don't have to EQ as much. I really have no data/hard facts/etc to back up my assumptions, it's just what I had assumed based on the general info I've picked up. I guess I'll have to re-arrange/re-examine my internal paradigm on this subject. Always good to learn something new.

As for a reasonably priced crossover, I found a used Rane AC22 for $150 on ebay. If you look a little longer, I'm sure you can find one for <$100. Granted, they are used, but for <$100, I think I'd take that chance. If you were going to go with a crossover, would this be a way to go?

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