True Biamping and Active crossovers info request

timijon

Member
2012-07-01 11:37 am
Hi
I have an old Audiolab 8000A and 8000P 'biamped' to some Rogers LS55 loudspeakers. The 8000A on its own has never had a problem driving the LS55s, but I have had some amps that just couldnt cope.

6 months ago, I changed the passive xover caps of the speakers to polyprops as someone somewhere recommended it.(well it was xmas and I needed a pressy...)

This system initially started with the 8000A driving the LS55s, then I added the 8000P and then I changed the caps. I noticed a small improvement adding the 8000P but cant say for sure about the crossover caps.

When changing the xover caps, I seem to remember thinking although the LS55 has 1 tweeter and 2 woofer drivers, both woofers were being driven as a bass woofer with no mid. I may be wrong as it was a few months ago when I looked but if Im right, may explain someway why a strong amp is needed for these speakers.

In reading stuff, I came across the expression 'true biamping' and found that there is a way to put the xovers after the preamp and before the amp and this is true biamping apparently. It is supposed to give sonic benefits.

Further reading has shown me these 'Active Xovers' seem to be 3rd to 4th order op amp filter circuits like the ones I had to learn about in electronics over 20 years ago.

Im sure I could revisit all that lovely old electronics again and have a go at designing something myself, but before I do, is there anything out there that is not too expensive and very good, that is recommended.

For example, I have seen someone recommend a Behringer cx2310 and for approx £60 would be fine.

Although I can understand the theory, the practical side is new to me, so would welcome anyones opinions and info.

I have scopes, dvms, sig gen, and multiple pairs of glasses, so can do a bit of diy.
 
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You can't in general just replace a passive loudspeaker crossover with an active one. The passive crossover uses passive components operating into the complex impedance of the loudspeaker drivers. The active crossover will not reproduce the transfer functions of the passive crossover, and you might end up with something completely different, which will totally swamp the "benefits" of migrating from passive to active crossovers. Unless you REALLY know what you are doing, I advise that you do not try this experiment, and it doesn't sound to me like you really know what you are getting into.

-Charlie
 
I have been using active crossovers for many years now.

I agree that you will end up with something completely different... but I think you will end up with something better!!!
Unless the passive crossover does some equalizing, then there are no problems by just going active as you intend. Even the equalizing part can be done at line-level if it is needed.
And really... there are only benefits to go active.
1) no passive components in series with the voice-coil will give a huge improvement in the dynamics and much less coloration.
2) sharper filtering gives a much cleaner and more detailed sound.

You can easily start experimenting with the cheap Behringers, that is what everybody does. Surely an DIY circuit with good opamps and good capacitors sounds better, but for a start...just go with something readymade like Behringer.

A couple of notes to your experiment:
1) Are you sure your speakers are not 2½ way...where the bottom bassunit plays only the deeper bass? (look for different coils in series with each bass unit). If yes you should probably go 3-way with e.g. Behringer CX3400, and in your DIY filter actually do a 2½ way configuration. Or just do two way anyway - try it - you might like it. (The improvement in dynamics, sound and details can maybe make up for the change from 2½ way to two way)
2) Note if the tweeter is polarity switched. That would probably be the case if your passive crossovers are 2.order. But with 4.order filtering you need to have the tweeter and the mid/woofer in phase. Anyway that is easy to experiment with - especially with the Behringers as they have phase-invert switches on the front.
3) The Behringers and other active crossovers have XLR connections, so you need to order some XLR to phono cables as well. You will need 2 male and 4-6 female XLR plugs ( depending on the 2-way/3-way thing).
Note also that each conversions between phono and XLR eats 6dB of your signal strength, in total -12dB, so you will need to turn your knob some more ( the volume-one that is).
4) Your integrated amp must have PRE-OUT MAIN-IN connections to be able to separate the preamp and the poweramp. And if you need to go 3-way then yes - you will need one more poweramp. It doesn't need to be the same kind because you have level controls on the active crossover, but the sound will change accordingly. ( Not so much though: In my experience the sound qualities of different amps becomes much less noticeable in actively crossovered systems)

Going active crossovering is one of the biggest improvements one can do, and to me it is strange that the hi-fi world hasn't picked up on this so far.
I went active many years ago and I am never going back :)

Good luck with your experiments, I trust it will be very rewarding and if it should not be then it is easy to revert to passive filtering.
 

timijon

Member
2012-07-01 11:37 am
Thanks for the replies.
Lore42: Thanks. Rod Elliots circuits 08 and 09 are similar to the ones I designed 20 years ago. I think I could get my head around that type of thing again.

CharlieLaud: Well, I wasnt expecting that! Time will tell if your right or no!

Nrik: Thanks for the info. I forgot to say, but my other 'main' system is my Merdian Digital Theatre DSP5000s (and other bits). I believe they operate this way and they do sound great.
My Audiolab system was bought last year to dabble with and if I blow it up, I can probably repair it. I decided to experiment with Vinyl find out how good one can get it. Theres a lot of smoke and mirros and snake oil out there, but with a bit of reading and using my electronics knowledge, I am getting some pretty good results with very little outlay. (I can recommend the MDF board on a bicycle inner tune for isolation for example). I should be able to tell quite quickly if theres any mileage to be had and as you say, if I dont like it, I can revert back.
As for the LS55s, I will have another look and trace out the circuit borard.

cheers
 
You can't in general just replace a passive loudspeaker crossover with an active one. The passive crossover uses passive components operating into the complex impedance of the loudspeaker drivers. The active crossover will not reproduce the transfer functions of the passive crossover, and you might end up with something completely different, which will totally swamp the "benefits" of migrating from passive to active crossovers. Unless you REALLY know what you are doing, I advise that you do not try this experiment, and it doesn't sound to me like you really know what you are getting into.

-Charlie

I was going to disagree with you at first, but your points really all have good merrits.

One other factor to add to this is that in order to convert to an active system you must first measure (quantify) each driver out of the box, and as a system in the box to understand the individual impedance curves, frequency response, SPLs, directivity, system step responses, etc.

That is not a simple task and it is not as simple as simply choosing the same crossover points (although that may be a good start).

Being able to measure is the key and that requires not only the right equipment, but the right environment to measure in and, more importantly, the right measurement techniques.