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nvme
01-06-2007, 01:05 PM
How does a x-over manage to produce an ohm load of 4 ohms to an amp if there are two drivers connected to it?

What if you mix 2 or 4 ohm drivers on a x-over?
What if you connected two mids to the same x-over?
Or what if you didn't connect a tweeter, does that change the ohm load?
Do x-over's have a power limit?
Since X-over's just offer different frequency cutoff at different slopes why is it important to match drivers to the x-over's?

Thanks.

squeak9798
01-06-2007, 01:28 PM
How does a x-over manage to produce an ohm load of 4 ohms to an amp if there are two drivers connected to it?

I didn't write this, but it's probably the easiest to understand explaination I've seen;

The woofer has a lowpass in series with it, the tweeter has a highpass in series with it, and those 2 "assemblies" are wired in parallel. Now think about how the circuit behaves at various frequencies, let's start at DC.

At DC the woofer's lowpass has an impedance of 0 and the woofer has an impedance of its Re (around 3.3ohm), since they're wired in series the woofer circuit has an impedance of 3.3ohm. At DC the tweeter's highpass has an infinite impedance and the tweeter has an impedance of its Re, since they're wired in series, the tweeter circuit has an infinite impedance. Now as far as the amp is concerned, you have a 3.3ohm impedance wired in parallel with an infinite impedance, and the result is 3.3ohm.

At very high frequencies you have the opposite, the tweeter's highpass has an impedance of 0 and the tweeter has an impedance around 4 or 8ohm depending on whether it's a 4 or 8ohm tweet. On the other hand the woofer's lowpass has an infinite impedance, the woofer has something like a 10-20ohm impedance (it rises at high frequencies due to the inherent inductance), and so now the woofer's circuit has an infinite impedance and the tweeter's circuit has a 4 or 8ohm impedance, wired in parallel you just end up with the tweeter.

At frequencies around the middle you get a mix, up until the crossover frequency the HP and LP filters will be very close to 0 and infinity respectively (or vice versa depending on which side of the xover freq you're at). When you get close to the xover frequency, the filters start to switch, the one that was 0 starts to rise towards infinity, and the one that was very very high starts to fall towards 0. At the crossover frequency, if you have say 4th order LR filters, each filter's impedance will be exactly equal to the impedance of the speaker that it's controlling. So the woofer's lowpass filter will be equal to 4ohm, and the tweeter's highpass filter will be equal to the tweeter's impedance. Since the filter is in series with the speaker, the woofer's effective impedance will be double, and the tweeter's effective impedance will be double. When you wire these two in parallel, the result will be right around the nominal impedance of either of them.

If you have a 4ohm woofer and a 4ohm tweeter, the final impedance should be right around 4ohm throughout the entire frequency spectrum.

What if you mix 2 or 4 ohm drivers on a x-over?

If you place a 2ohm speaker on a passive xover designed for a 4ohm load, then the xover frequency would be cut in half (this is not desireable).

If the xover was intentionally designed to be mated with a 2ohm midwoofer and a 4ohm tweeter, for example, then when for frequencies within the mids passband the amplifier would see a 2ohm load and for frequencies within the tweeters passband it would see a 4ohm load.

What if you connected two mids to the same x-over?

If you connected them in parallel, the xover frequency would be cut in half. If you wired them in series, the xover frequency would be doubled.

Or what if you didn't connect a tweeter, does that change the ohm load?

Depending on the design of the crossover, leaving one of the circuits open could result in the amplifier being presented with a dead short, consequently risking damage to the amplifier.

Do x-over's have a power limit?

Sure.

There will be a limit where the components within the crossover can not handle the power being passed through them. But in normal circumstances, it isn't something the consumer should really "worry" about as it is likely much higher than the amount of power being applied to them.

Since X-over's just offer different frequency cutoff at different slopes why is it important to match drivers to the x-over's?

Well, for one; see above. If you connect an improper impedance load for the xovers design, you will affect the xover frequency and as such the resulting response will not be desireable.

Likewise, all speakers will perform optimally with certain xover frequencies and slopes specific to each individual speaker. Mixing speakers with xover, even if impedances are taken into consideration, may not result in optimal performance or response from the speakers. Also, if you place a tweeter that requires a relatively high xover frequency and slope on a passive xover that has a lower-than-recommended slope or frequency; you risk damaging the tweeter.

Lastly; Xovers can do MUCH more than just offer differing slopes and xover frequencies. Many have built in impedance compensation (zobel networks), passive EQ circuits, notch filters, etc etc. All of which were designed to provide optimal performance from one specific speaker, and not necessarily any speaker you choose to place on the passive. Again, this is all about acheiving optimal performance from the speaker; and placing just "any" speaker on a specifically designed passive xover very well may not acheive this.

nvme
01-06-2007, 01:35 PM
wow..Great response. Thank you for taking the time to answer my questions.

So for people running 2 or 3 mids and one tweeter in their cars how are they acheiving this? Actively?

helotaxi
01-06-2007, 02:23 PM
I wrote a good bit on this a while back, a search would probably locate it.

As for running multiple mids, it can be done a few different ways. Active is probably the easiest. Custom designed passives are another way to go about it. Back in the late 80's/early 90's it was fairly common for SQ comp vehicles to run huge numbers of drivers on a single 2 channel amp. Huge and hugely complex passive networks were needed to get this to work right and sound good.

Squeak minor point on your post about the effects of using the wrong impedance driver on a passive. Your answer was correct for a 1st order filter but higher orders behave really strangely with the wrong load on them. One half of the filter shifts one way and the other half shifts the other. The result is a stepped filter and a probably ****** response curve.

squeak9798
01-06-2007, 05:59 PM
I was trying to keep it relatively simple ;)

helotaxi
01-07-2007, 11:46 AM
Realtively simple would be "It won't work right." :D