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Active Bi-amplification with Fixed Passive Crossover Present???

#1
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6/9/09 at 1:10pm
  • Joined: June 2009
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Hi everyone. I found an older related thread from about 5 years ago and read through it in its entirety, but no one seems to have brought up the issue I have in mind. For reference, this was the old thread:
http://www.hometheaterforum.com/htf/...provement.html

I'd like to actively bi-amp my Paradigm Monitor 7's (older version) but the manual clearly states that there is no way to directly access the drivers; and thus removing the internal passive crossover is not an option. Whether or not that is even a good idea is not the issue up for debate though, as the prior thread pretty much covered everything in this regard.

My question is this:
Is there anything wrong with actively bi-amping these speakers using an active crossover and sending those low-level outputs to the amplifier/s which will power the low and high frequency terminals on the speaker???

preamp/receiver -> active crossover -> amplifier -> speaker (passive xover elements present, but terminals decoupled)

It seems this would have the benefit of reducing the load seen by each channel of the amplifier/s, as well as getting rid of the intermodulation distortion that occurs when the amplifier has to amplify the full spectrum. The internal passive crossover of the speaker would then just have to pass each signal straight through to the drivers without any filtering (I realize things aren't quite so simple though as there will still be loads/inductances present in the passive crossover).

I've yet to see this mentioned, so is there something inherently wrong with this idea? Is it worth doing? For what it matters, the spec sheet advertises that the passive crossover utilizes a 3rd-order Butterworth filter that maintains phase coherence.

Thanks in advance for any and all opinions or advice.
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#2
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6/9/09 at 3:35pm
  • Joined: August 2000
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Re: Active Bi-amplification with Fixed Passive Crossover Present???

The passive crossover will definitely have an impact on the sound. You can always remove the drivers and re-wire the internals to bypass the crossover.

As for the 3rd order crossover, every order will make the mid and the high frequency driver 90 degrees out of phase. There are other tricks to work with this but bypassing the passive crossover and utilizing your electronic crossover, you will not have any phase issues at all.

-Robert
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#3
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6/10/09 at 1:36am
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Re: Active Bi-amplification with Fixed Passive Crossover Present???

Thanks for the reply.

I realize that the remaining passive crossover elements will have to effect the sound in some way, but nonetheless, do any of you with more expertise (than myself, I mean) think that the plan I mentioned will still offer a substantial improvement in sound quality due to the reduction in amplifier intermodulation and reduced load? Basically, is it worth doing?

I agree that the drivers must have leads buried somewhere in the box, but from what I've read on other forums, and the manual itself, there really isn't a way to utilize any such connection without really tearing things up (and perhaps messing up some box dynamics). It's a 2 1/2, btw. After examining things I've given up on the notion of directly driving the drivers, though that was my original plan.

As for the phase coherency, Paradigm are no slouches so when they claim phase coherence is maintained I'm inclined to believe them. I know you're saying that a 3rd order filter would be 270 degress out of phase (while the 4th order L-R would be 360=0, as we all know) but there must be an additional shift or something regarding the fact that two mids are driven which maintains coherence. I just don't think Paradigm would make that claim otherwise.

Hmmm... it's also a 1.8kHz passive crossover, so more of a mid/high separation rather than mid/low in the box.

Thanks again.
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#4
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6/10/09 at 4:26am
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Re: Active Bi-amplification with Fixed Passive Crossover Present???

Do you have the appropriate equipment to determine the crossover point, driver levels and reproduce the filters present in the passive crossover?

I'm not familiar with your speaker but if you aren't brave enough to open it up and re-wire the internals, then I don't think you should attempt this. Without removing the passive crossover you are only going half-way with your project.

What are you using for an active crossover?

-Robert
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#5
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6/11/09 at 4:48pm
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Re: Active Bi-amplification with Fixed Passive Crossover Present???

Thanks again for the feedback.

I have been trying to avoid unnecessary details, but the bottom line is that I will not be removing or bypassing the internal crossover from this set of Monitor 7's. This is why I tried to get by earlier by just saying the issue wasn't up for debate. So I'm just trying to get a sense of whether the active crossover is still a worthwhile addition. A different set of speakers and I'd have no issue modifying them.

I know the crossover freq (1.8kHz), and have already modeled the Linkwitz-Riley style crossover design I intend to use in SPICE. I have everything I'm going to need (mostly from my days in EE labs but a few things I've bought since) and have some experience assembling boards. Building the crossover is not the issue for me, though if anyone has advice on where they obtain quality components for their audio builds that is welcome.
In any case, please just assume I'll be using a flat, phase coherent active crossover. I know I could always just buy one, but what's the fun in that?

I don't believe I need to determine the driver levels/sensitivities if I'm not removing the internal xover.

I'm simply seeking opinions and knowledge on whether this would be a worthwhile addition. Assume someone already had a passively bi-amplified system, where both amplifiers send the full spectrum to both the low and high frequency speaker terminals. Would the addition of an active crossover before the amplifiers be a good idea? (Could it possibly be a bad idea and degrade the sound somehow?)
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