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Old 14th November 2003, 11:39 PM   #1
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Default Does anyone here use active (digital or otherwise) XOs or EQs or...

Tried searching the archives, didn't find much in the tube forum, and this is where I want to ask this question. I'm assuming most people here have some level of concern for the 'purity of the signal'. I also think most people here might be somewhat 'old school', which might skew the responses

Anyway, my question is, do any of you use room correction systems, or active crossovers, or equalizers, or anything else which falls into that general category of products? I'm a little leery of putting extra boxes in the signal path, but I often wonder if the benefits of using such devices might outweigh any sonic degredation, and my hesitation is mostly unfounded. One approach that I would probably be comfortable with would be to use something only for bass, so put it on the 2nd set of linestage outputs and use it to EQ/tame my subwoofer's response in my room.

Do any of you have fully multi-amped systems with active line-level crossovers? If they're analog, are they tube or SS? And does anyone use any product that digitizes the signal, processes it in some way, then D-A's it to go out to the amps/speakers? Specially something that's full-range, and not just for subwoofers.

I guess the other school of thought would be to use non-electronic solutions, i.e. place the speakers/sub as best you can, and treat the room. At least, I think that's the traditional way of doing it.

Thanks in advance,
Saurav
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Old 15th November 2003, 01:46 AM   #2
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My sub towers weigh well over 200 pounds each, perhaps over 300. It's not as though I can just drop them on the scales and check. As such, I tend not to move them very often. I built them, then sweated, grunted, and strained to get them into a semi-reasonable position in my listening room. The bass had a strong hump at about 80Hz or so. Looked at the placement, decided I'd given up too soon, and grunted and sweated some more. The result was nearly flat response--just from moving the subs. No electronic changes.
A good rule of thumb for speaker placement is 1/3 of the way down the room. Then listen 1/3 of the way from the rear wall. If you look at the way resonances form, this gives you a fighting chance of getting decent response, as it avoids even multiples (or sub-multiples) of the wavelengths.
Try it. It's free and it works well.
That said, I use simple slopes (e.g 6dB/oct) to flatten the response of my woofer panels and my subs. To get good results, you need to know what your speakers are doing. Use some form of test gear to determine what's actually happening. Many people build subs according to programs that work from the Thiele-Small parameters and assume that they're getting results like they see in the cute little graphs. Nope. As an example, drivers such as the Shiva, Titanic, et. al. are anything but flat. Waaay off. Cone resonances and such do not show up in the simulations--but they do in the listening room. And people have this funny, persistant feeling that something's missing. Well, yes, the bass is missing. Put one of those drivers in an optimal T-S box and you'll find that 100Hz is as much as 10-15dB higher than 20Hz. (Notice that they rarely, if ever, show a measured response curve for the drivers; it's always a simulation. Why? Because if they showed a true response curve, they would sell very few drivers.) In the case of the Dayton Titanic drivers, a 6dB/oct low pass at about 20Hz will flatten the response up into the mid-hundreds. Then you can put in another slope to be the 'real' crossover.
Approach eq with caution. Like fire, it is a good servant, but a bad master.

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Old 15th November 2003, 03:34 PM   #3
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A good rule of thumb for speaker placement is 1/3 of the way down the room. Then listen 1/3 of the way from the rear wall.
Hmm... I could give that a try again. My listening room is my living room, and while my wife seems to be more accomodating than most other audio enthusiasts I know, I still have limits I'd rather not try to push So I've tried both front corners, as well as both sides of the TV stand (which would put it maybe 1/4 into the room). I settled on the corner that's at an end of the 'L' shape. That gives me OK response, there's a large room hump at 35-40Hz, and a suckout somewhere in the 65-75Hz region.

I've read of people corner loading a sub to excite maximum room modes (and my room isn't a cube, all 3 dimensions are fairly different), and then using a parametric EQ to tame the response. That seems like something I could do, which is why I'm curious.

Quote:
To get good results, you need to know what your speakers are doing. Use some form of test gear to determine what's actually happening.
I have an SPL meter and a couple of CDs of test tones. Certainly not as good as a full-blown acoustic measurement system, but it's better than just trying this by ear. Though sometimes I trust my ears and ignore the SPL meter, especially with in-room measurements of treble frequencies. The meter shows large peaks and dips that I don't hear, and the response swings around a lot when I move the meter a few inches. So I figured my ears/head average out whatever reinforcement/cancellation my meter is seeing.

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Approach eq with caution. Like fire, it is a good servant, but a bad master.
I'll remember that
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Old 15th November 2003, 08:06 PM   #4
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I use a (horrors) IC based equalizer of my own design to give my system a low frequency boost below 100 Hz. The EQ contains a copper-to-copper bypass switch and can provide a variable level of boost. This 7th order equalizer is of the "shelving" type.

I added this equalizer to enhance the bass and sub-bass when I listen at very low levels (as when I have guests and the music is in the background).
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Old 16th November 2003, 04:22 AM   #5
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You might also want to give the biophonic EQ a shot, http://headwize2.powerpill.org/proje...=equal_prj.htm
Scroll down to fig 12. I'm running that circuit in my integrated amp with a bypass switch. It's just subtle enough to have an affect without any obvious coloration (not that there's anything wrong with that).

Cory
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Old 16th November 2003, 11:16 AM   #6
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As you know you can't EQ a system without pink noise and a good analyzer. Without spending hours test tones won't do a bit of good.
Any shop worth their salt has on hand something simple to operate such as an old "Crown" or "White" analyzer or something along that line. They probably will let you borrow it. Sometimes it is very unlikely that you can setup a book based listening environment in a specific area of the room therefore filters can be built to take care of the peaks in response or Eq's can be utilized.

I use digital EQ's in commercial systems and they sound quite nice and will blow away the op amp based units of 10yrs ago. Some op amp units are worth getting a hold of and rebuilding with higher performance IC's.
I'm not saying that you need to buy the expensive op amps as the FET variety will do quite well.

I have on hand some "White" inductors that will make nice filters to tame your frequencies that are too hot.

I too have used 6th order tuning on low frequency systems(subs) in order to better tune to a specific room in addition to being able to use Theil small parameters to jam a speaker requiring a large box into a much smaller one thus increasing performance and saving floor space.

Just my .02

Joe
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Old 16th November 2003, 03:48 PM   #7
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Hi,

At this point, I'm just looking to tame the bass in my room, I'm not trying to achieve full-frequency EQ. And I don't really want to build my own, I think if I try this, I'll just buy a parametric EQ and play with that.

Thanks for all the advice.

Saurav
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Old 18th November 2003, 02:32 AM   #8
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my computer speakers are bi-amped with a 24dB/octave crossover, linkwitz-riley alignment at 2khz. there are a number of factors which prevent me from commenting on the quality though, among them being the source, enviroment, speakers, unexpected resonance in the cabnits, lack of power with inefficient speakers...
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Old 18th November 2003, 02:53 AM   #9
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Default The secret of comedy is.. TIMING!

I've made loudspeakers with active crossovers for some time, and think they're a great idea because they allow you to achieve the crossover slopes and equalisation you want without upsetting driver damping. Crossover implementation is an electronic problem.

Loudspeaker bass alignment and room interaction is an acoustic problem. Trying to fix it acoustically with an electronic solution will always be wasteful of power. I believe you call this a kludge.

Digital crossovers allow timing problems to be solved really easily. Practical analogue crossovers can't do this. Digital crossovers can emulate analogue filtering controllably, predictably, and repeatably.

However (this always happens), by definition, an active crossover receives changing voltages depending on volume setting. If A/D and D/A were perfect, and had true 24 bit resolution, this wouldn't be a problem. Unfortunately, conversion isn't perfect, and the intervening DSP and necessary rounding also causes errors. Not applying the full signal exacerbates those errors.

Having said all that, I personally think that the ability to correct timing errors may be more significant than anything else...
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Old 18th November 2003, 04:12 AM   #10
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Quote:
Loudspeaker bass alignment and room interaction is an acoustic problem. Trying to fix it acoustically with an electronic solution will always be wasteful of power. I believe you call this a kludge.
Agreed, but domestic constraints intrude upon my desire to find elegant engineering solutions to these problems And unlike my problems with my main amp, I think I have plenty of power to spare on my subwoofer amp.

I understand what you're saying, I can't boost a room null by throwing power at it. But I should be able to get better overall results than what I have now, right?

Quote:
Having said all that, I personally think that the ability to correct timing errors may be more significant than anything else...
That's what I think about too - the blanket "digitization is bad" phobia that so many mainstream audiophiles seem to have is probably unwarranted when weighed against the improvements that can be realized with this technology.

I'll probably work on a full-blown multi-amped setup once I'm living in some place relatively permanent. For now, I'm financially and aesthetically restricted to pretty much what I have. It's funny, one of my wife's friends told her she was 'very accomodating' for letting me keep these speakers in the living room
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