Passive & Active Crossover Questions

[GirlyHandedDog]

I'm very much a novice, so please forgive my lack of knowledge and naivete. Now that that's out of the way, I can explain that I took Bill's advice and started reading from the library on Rane's website in order to get a better handle on live sound reproduction. Here are my questions:

1 - If using DR250's along with Titan 48's, is it necessary for me to use any kind of a passive crossover, or would it be possible and more beneficial to allow something like a Driverack handle all of the crossing over.

2 - If the answer to the aforementioned question is "no," can someone help me understand why use of passive crossovers is recommended in the plans?

I guess what I'm trying to wrap my head around is the info on the Rane website (forgive the multi-paragraph quote):

"Each time a passive crossover shows up, it comes with problems.

One of these is power loss. Passive networks waste valuable power. The extra power needed to make the drivers louder, instead boils off the components and comes out of the box as heat -- not sound. Therefore, passive units make you buy a bigger amp.

A couple of additional passive network problems has to do with their impedance. Impedance restricts power transfer; it's like resistance, only frequency sensitive. In order for the passive network to work exactly right, the source impedance (the amplifier's output plus the wiring impedance) must be as close to zero as possible and not frequency-dependent, and the load impedance (the loudspeaker's characteristics) must be fixed and not frequency-dependent (sorry, not in this universe; only on Star Trek). Since these things are not possible, the passive network must be (at best), a simplified and compromised solution to a very complex problem. Consequently, the crossover's behavior changes with frequency -- not something you want for a good sounding system.

One last thing to make matters worse. There is something called back-emf (back-electromotive force: literally, back-voltage) which further contributes to poor sounding speaker systems. This is the phenomena where, after the signal stops, the speaker cone continues moving, causing the voice coil to move through the magnetic field (now acting like a microphone), creating a new voltage that tries to drive the cable back to the amplifier's output! If the speaker is allowed to do this, the cone flops around like a dying fish. It does not sound good! The only way to stop back-emf is to make the loudspeaker "see" a dead short, i.e., zero ohms looking backward, or as close to it as possible -- something that's not gonna happen with a passive network slung between it and the power amp.

All this, and not to mention that inductors saturate at high signal levels causing distortion -- another reason you can't get enough loudness. Or the additional weight and bulk caused by the large inductors required for good low frequency response. Or that it is almost impossible to get high-quality steep slopes passively, so the response suffers. Or that inductors are way too good at picking up local radio, TV, emergency, and cellular broadcasts, and joyfully mixing them into your audio."

Thanks everyone in advance for any help.

[Greg Plouvier]

The simple answer is some flavor of speaker processor like the driverack is better. You will need more amplifier channels though. The horn loading does offset some of the losses passive crossovers cause. Passive crossovers are recommended in the plans because A: Bill knows how to design them, B: they are the most cost effective.

[Ken Lustgarten]

Active crossovers are better period. That said there is a point of diminishing returns. The passive crossover in the DR's is at a higher frequency and does not waste so much power. It does not make sense to use another amp just for high frequencies that draw very little power where the extra gear and set up that is needed would provide only a small gain. For this application the passive is good enough. On the other hand the crossover between a sub and a top should always be active. While it is possible to do it with a passive the higher power requirements make it a very poor choice. Using an active crossover between subs and tops also gives a lot of flexibility in dialing in the amount of bass that is required for a given application and lets you properly limit both subs and tops. It also lets you dedicate a larger amp for power hungry subs and a smaller one for tops. I use a Driverack PA+ and have been very happy with it.

[GirlyHandedDog]

Thanks for confirming what I was thinking. It's occasionally nice to know that you're actually starting to learn a few things.