Any benefit for passive bi- or tri-amp with LM3875?

[wchick]

I built an inverted gainclone few years ago (LM3875) where the left and right channel share one power supply (external, 250VA toroid). Recently, I add another PS and go dual mono. The sound improved quite a lot and I feel a little more power. Now I'm tempted to go bi-amp: for each channel, one 3875 feed tweeter, the other feed woofer, but passively because I have no idea to work with active XO (and don't have time). I assume I don't need to adjust gain for each chip, correct? And what benefit would I gain from this set up?

Thanks.
Wallace

[preiter]

Good article about that here:
http://sound.westhost.com/articles.htm

but the answer is that yes, there are potentially huge gains, but to get the best out of a bi-amped setup you are going to need to actively cross it over.

If you have a separate amp for your tweeter, for example, and you send it to a passive highpass filter, your amp will still be driving all of those low frequencies and the crossover will be dissipating the extra power as heat. Not at all what you want.

Cross it over actively, before the amp, and the amp will only be driving the frequencies that the speaker needs.

Bottom line is, if you are going to all the trouble of bi-amping a speaker, do it right and buy or build an active crossover. It's not all that complicated. That same website has plans for one.

And as a bonus, you no longer need a passive crossover.

[wchick]

Thanks for your advice and the link. I got an impression somewhere that if you replace the stock passive crossover of the loudspeaker with an active one, you might alter the sound signature of it. I don't know if it is true. Can someone comment on it?

Wallace

[Mad_K]

That is true. The loudspeaker driver interacts with the filter in front of it and the filter designer needs to take this into account. It is not trivial to try to calculate the response needed for the active filter to be exact replica of the passive one. This is best done by measurements, and adjusting the various filter parameters to get what you want.
That being said, you still might get good results with easy drivers and steep active filter slopes. It all depends on the speakers. If it is exotic, I would probably leave the original filter. If the speakers are relatively cheapo, I would probably try the active route; you'll for shure learn a lot and have plenty of fun

[SpittinLLama]

Although you may not like the sound you can do some filtering on the LM3875 by changing the input capacitor (if there is one) and the feedback capacitor (also if there is one). By changing these to smaller values on the tweeter amp there will be less gain and less signal coupled at lower frequencies. For the bass amp there is nothing to be done to filter the highs but then this is not really the problem. Just an idea for cheap filtering so the tweeter amp uses much less power and less heat.

-SL