[JohnR]

I actually know what the last four of those mean, but I am still pretty much intimidated out of doing this.

Oh, that's too bad The thing is, with anything like this, there's going to be learning curve and you have to start somewhere. The specific filter types are not really all that important (in my opinion).

[Tyson]

Also, most speakers have the internal wires installed with tabs, not solder.  So it's easy to disconnect the passive crossover and run wire directly from the binding posts to the drivers. 

[PRELUDE]

I actually know what the last four of those mean, but I am still pretty much intimidated out of doing this.

I started when I was 16 and did not know half of what you know now and remember that if you do not start somewhere then most likely you would never do it.
In this case I would say, why even touch your speakers and loose the re-sale value.You would still need a active crossover and a power amp so that does not make any difference at cost.Go and look for a similar speaker kit and do it active.The result would be better learning,less confusion and you have a full re-sale valu of your speakers.

[wgscott]

Well, I already have the amp, so the only remaining purchase is a crossover kit, to get the most out of my current investment.

As for the speakers, I'd actually prefer to mod these than build new ones. By my ear, they sound pretty good, and match the living room decor.  It is unlikely I could build something that looks better, and I doubt I would re-sell them, so I am not too worried about screwing with them for that reason (and besides, I could just hook them up again).  My main concern is that I damage the speaker trying to open it up.  I think I can handle the actual wiring part.

Also, there is (at least) one other thing I am unclear on.  Does the low (in my case 150 Hz) crossover span just the lower pair of binding posts and the woofer, or does it also connect to the mid-range.  In other words, in my current passive bi-amp configuration, does the lower-frequency crossover work only on the "bottom" channel to remove the higher-frequency signals, or does it only work on the "top" channel to remove the lower-frequency signals, or are there two, or what?

[wgscott]

I don't think this question has been answered yet. A crossover is not a "brick wall" filter - it has a gradual transition, which occurs on the "pass" side as well as the "stop" side of the nominal crossover freqency. For example, here is the frequency response of a first-order filter:



Here is a steep filter - you can see the transition is much sharper but it's still there:



If you leave the passive crossover in, your active crossover can only "add" to this kind of response i.e. make the slopes steeper. If your crossover frequency was not the same as in the passive crossover, then you would also have to boost one output near the crossover frequency to try and counter its effect. That would be problematic. With the same crossover frequency, I think it probably is doable but you would need to be aware that the response you get from each driver includes the response caused by the passive crossover. If you are OK with using steeper slopes (and I see nothing wrong with that) and are using a digital crossover (with choice of lots of different slopes), you might be able to get it to work well. However, it would still most likely be "better" to remove the passive crossover. At the least, it might be worth a try just to get started and see how it goes, use it as a way to learn about how to set up an active crossover, before deciding to remove the wiring from the drivers. At that point you might also decide to go full 3/4 way. You may decide that rather than destroy the resale value of the speaker you would rather go active with something DIY. That is all assuming that you can measure as mentioned by others.

Thanks for this.  Mind if I ask some "Devil's advocate"-type questions?

If the crossover frequencies did match, presumably the net slope you would get would be whichever is steeper to begin with, though.  I'm not sure they are additive (and as you point out, it would just make it a bit more steep).  If the active crossover had a more shallow slope, I could see how this might be problematic.  Also, why is having a resistor in a passive crossover worse than having a (variable potentiometer) resistor further upstream in the amp to reduce gain?

[PRELUDE]

I never saw back of this speaker.If you have two pair binding posts on the back of your speakers one is for low and one for high.So in order to go active you have to change that to three pair binding post, because your passive crossover inside the box for highs are on one board to handle the mid and tweeter.

[srb]

Also, there is (at least) one other thing I am unclear on.  Does the low (in my case 150 Hz) crossover span just the lower pair of binding posts and the woofer, or does it also connect to the mid-range.  In other words, in my current passive bi-amp configuration, does the lower-frequency crossover work only on the "bottom" channel to remove the higher-frequency signals, or does it only work on the "top" channel to remove the lower-frequency signals, or are there two, or what?

The CM7 crossover frequencies are 350Hz and 4000Hz.
 
There are three crossover circuits employed.  The woofer receives frequencies up to 350Hz, the midrange 350Hz to 4000Hz and the tweeter 4000Hz and up.  To my knowledge, the lower pair of binding posts feeds the woofer crossover only, and the top pair feeds both the midrange and tweeter crossovers.
 
The information I can find indicates that the tweeter crossover is a first order type utilizing a single capacitor.  If you were to actively bi-amp, you would in essence have a hybrid system by still using using the passive tweeter crossover, although I would expect that to perform better than if it were a more complex higher order crossover.
 
The best solution would be going completely active which would mean tri-amping and installing a 3rd set of binding posts.
 
Steve
 
 

[wgscott]

Sorry about the 150/350 mixup.  I looked it up once, but clearly I mis-remembered.

Assuming I wanted to stick with bi-amping rather than tri-amping, would I want to disconnect just the woofer crossover, or the woofer crossover AND the midrange crossover?  (In other words, does the midrange crossover just cut out the lower frequencies, or does it cut out both high and low, leaving the midrange)?

[JohnR]

Thanks for this.  Mind if I ask some "Devil's advocate"-type questions?

If the crossover frequencies did match, presumably the net slope you would get would be whichever is steeper to begin with, though.  I'm not sure they are additive (and as you point out, it would just make it a bit more steep).  If the active crossover had a more shallow slope, I could see how this might be problematic.

No - they are additive (magnitude in dB terms, phase in radians/degrees). Crossover are just filters, and that's how filters work... sorry, I'm at a bit of a loss how to explain it off the cuff (and without getting into theory)...

Also, why is having a resistor in a passive crossover worse than having a (variable potentiometer) resistor further upstream in the amp to reduce gain?

Not sure who said that, but the most obvious issue is that the resistor is dissipating power when it's after the power amp.

[wgscott]

No - they are additive (magnitude in dB terms, phase in radians/degrees). Crossover are just filters, and that's how filters work... sorry, I'm at a bit of a loss how to explain it off the cuff (and without getting into theory)...

It is ok.  I know the theory.  What I wasn't sure of is whether the magnitude was amplitude or intensity.  I just don't know very much about electronics or audio equipment.   

[srb]

2-way speakers are simpler in that you have a low-pass filter on the woofer and a high-pass filter on the tweeter.  A 3-way speaker is more complicated because the midrange crossover is a bandpass filter that has to limit both the low and high frequencies to the midrange, in your case 350Hz to 4000Hz.
 
If you are only going to actively bi-amp a 3-way speaker, you would want to completely bypass the woofer crossover but bypass only the high-pass filter of the midrange crossover (which the active crossover would now perform) leaving the low-pass filter in place to limit the midrange frequencies to 4000Hz.  The woofer of course would simply get directly wired to the bottom pair of binding posts.
 
This would require some knowledge to rewire the midrange/tweeter crossover if they are combined on one board, or the midrange crossover if on a separate board.
 
If you left the midrange and tweeter connected through their internal passive crossovers to the upper pair of binding posts as they currently are, you would be both actively and passively filtering the 350Hz crossover point - probably not a good idea.
 
So although passive crossover modification could be done, you can see why those who go active on a 3-way speaker generally just connect each driver to a respective pair of binding posts and use a 3-way active crossover with 6 channels of amplification.  The extra two channels of amplification cost more, but the result is better performance and no passive crossover surgery required.
 
Steve

[wgscott]

Thanks.

If you left the midrange and tweeter connected through their internal passive crossovers to the upper pair of binding posts as they currently are, you would be both actively and passively filtering the 350Hz crossover point - probably not a good idea.

That is what I was afraid of.

[BobRex]

The CM7 crossover frequencies are 350Hz and 4000Hz.
 
There are three crossover circuits employed.  The woofer receives frequencies up to 350Hz, the midrange 350Hz to 4000Hz and the tweeter 4000Hz and up.  To my knowledge, the lower pair of binding posts feeds the woofer crossover only, and the top pair feeds both the midrange and tweeter crossovers.
 

Actually there are four crossover circuits.  There's a low-pass to the woofer, a high-pass to the mid, a low-pass on the mid, and a high-pass to the tweeter.  While the midrange circuits may be combined electrically, for the sake of calculating turnovers and rolloffs they are calculated separately.

[neobop]

If you look inside your speaker, chances are the set of posts for the woofer are only hooked up to those drivers. Normally the other posts are for the mid & tweet. So you have a low pass which you can easily bypass. The problem is the mid (bandpass) and high pass are configured for a 3-way x-over. A 3-way is NOT two 2-ways.

What I was trying to illustrate before was that a passive does much more than divide the freq. Passives are designed to make drivers that are not the same, work in harmony. Why does a Mission have the tweet below the woof? It's because the particular 3rd order network has a polar tilt that that beams the highs upward. A 2nd order Butterworth (reverse polarity) will have a + 3 dB rise at x-over freq. A 2nd order Linkwitz does not. [I think it's 3dB, I'd have to look it up]. See what I mean?

There are tons of advantages going active, but you need to triamp with the speakers you now have. If you want to biamp you'd have to try running one amp for the bass and one for the mid/highs. You might have to modify the passive up top. This would be the way I'd suggest you get started:

1 - just hook it up and see how it sounds.
2 - if the top doesn't sound quite right, try to get a handle on why.
3 - figure out what kind of passive is in there now.
4 - go here to play with values http://www.diyaudioandvideo.com/Calculator/XOver/

Look for resistors in the present passive and figure out why they're there.
Chances are it will be better right away and you can figure it out later, exactly what you're going to do.
neo

[Tyson]

I would leave the mid/tweeter alone and just worry about the bass section for now.  This will let you get the largest/worst parts of the crossover out of the way, and give you a lot of the benefits of going active.  And you can use a cheaper mic since the bass/mid crossover is less critical from a measurement standpoint than the mid/high crossover.  Something like a Cross-Spectrum calibrated Behringer or Dayton will do nicely for this purpose (about $80, if I remember correctly).

[Tyson]

Thanks.

That is what I was afraid of.

This is wrong.  Keep the mid/high binding post connected to the passive crossover, and only bypass the bass passive crossover.  This will keep the passive mid/tweeter crossover in place and will allow you to actively cross between the mid and the bass.

[neobop]

There's virtually no chance that anything other than the woofer is hooked up to those posts. One of the main advantages for the designer with dual sets of posts is the physical separation of passive components. High value inductors are of particular concern. Just trace the wires from those posts and I think you'll find this to be true.

The other set of posts should have the complete bandpass for the mid and high pass for the tweet. Good luck if you decide to proceed. Let us know if you're having problems. Maybe we can help modify the rest of your passive or help select a third stereo amp. You might want to consider a 3-way active from the get-go.
neo

[Tyson]

Or, post some pics of what you find inside the speaker, that will help.

[wgscott]

It is going to be awhile before I get the time/nerve/appropriate screwdriver to dissect my speakers, but I am incredibly grateful for the offers for help.

I am currently passively bi-amping, so I can tell you with 100% certitude that the woofer has one pair of binding posts, and the midrange and tweeter share the other pair.  I can also tell you that the high-frequency passive crossover is 1st order, and I will likely keep it in place.  Adding the second Amp board to the ClassDAudio 254 kit cost me about $230, because I could use the same power supply and transformer.  Adding a third amp would be way more expensive.  Briefly, I am unlikely to tri-amp in this lifetime.

Given that, we are talking about (1) bypassing the low-frequency (350 Hz) crossover for the woofer, and (2) possibly bypassing whatever circuitry that cuts out the low-frequency for the midrange/tweeter.

Of these, I don't see (1) saving that much power, compared to (2), since most of the power is required to drive the low-frequency response, right?  But (1) is clearly easier to do.

[neobop]

I looked up your speakers. I thought B&W used higher order designs. Given your situation, I advise not to go there. Save your money for something else. If the bottom range of the tweeter sometimes gets a little harsh, a wool type absorption ring might help that somewhat.

If the speakers are inadequate for your needs, keep them in good condition so you can recover as much as possible. If you just want to try to coax a little more out of them, I'm starting to think it might not be worth the hours and expense. It's not so easy to second guess a co like B&W. Even triamping, you don't know the uncorrected response of the drivers. I try to start out with drivers which have flat response and they're hard to find. With an aluminum dome at 6dB/octave, the weakest part of the design won't be addressed.
neo