Crossover & Ohms

[Stockinv]

As a newbie I've seen the terms "crossover" and "ohms" when discussing stereo and/or home theatre. With regard to amps, I know it's ideal to have watts doubling as ohms half, i.e. 200 watts at 8 ohms, 400 watts at 4 ohms, etc. Could someone explain these two concepts? Specifically, what is crossover, and why is the relationship between watts and ohms important for achieving the best sound? Thanks

[Glen B]

As a newbie I've seen the terms "crossover" and "ohms" when discussing stereo and/or home theatre. With regard to amps, I know it's ideal to have watts doubling as ohms half, i.e. 200 watts at 8 ohms, 400 watts at 4 ohms, etc. Could someone explain these two concepts? Specifically, what is crossover, and why is the relationship between watts and ohms important for achieving the best sound? Thanks

A crossover is a passive electrical network that forms part of a loudspeaker system. Its job is to split the output signal from an amplifier into three frequency ranges (high, middle and low) and send each to the appropriate speaker (also called driver) in that loudspeaker system. The drivers in a loudspeaker system (i.e., woofer, midrange and tweeter) are limited in the range of frequencies they can handle, hence the need for a crossover.

An amplifier that can increase or double its output power into lower impedances is considered ideal because a speaker's impedance can sometimes drop significantly low enough at certain frequencies to create a load that is difficult to drive. An amplifier that cannot deliver enough current when a speaker's impedance drops, will sound congested and strained when driven hard, maybe even overheat and shut down in extreme cases. The ability to double power into lower impedances by itself though does not guarantee the best sound. Things such as proper matching of speakers and other components in the system will determine a good end result.

[Stockinv]

Thanks for your response. One favor: could you discuss "impedance"?

[ThomasW]

All these terms are easily available via a google search.

The easy answer is that impedance = the resistance of a loudspeaker measured at a given frequency.

Here's the more technical answer

http://stereophile.com/features/99/index5.html

[Gordon Moore]

could you discuss "impedance"?

You kind of already have in a way...

I know it's ideal to have watts doubling as ohms half

impedance (R) aka: resistance is measured in ohms : R = V (voltage) / I (current)
current (I) = the flow of a charge and is measured in A (amps)
Power (P) is measured in watts (W) = 1 Joule/s or P= I (current) * V (voltage).

That's all I remember from grade 9 physics :B

In a way a speaker is nothing fancier than a big resistor. Speakers have ohm ratings (2ohm, 4ohm, 8ohm etc...) and will "resist" the current as it draws from the amp. The lower the resistance rating the greater the draw on the amp. A speaker isn't really a static resistor so that sometimes it will draw more or less current than it's rating states. I believe those ratings are so you properly match your speakers with the correct power amp.

ie: low impendance drivers (4 ohm) would mate well with "high current" amplifiers so you don't burn out the amp and it's sufficiently suited to high current draws.

Thomas is the expert though (so I'd pay more attention to his answers than mine), so if I've flubbed, I'm sure someone will correct me.

[Rolex]

If I'm not mistaken, resistance has to do with DC current, where as impedance has to do with AC current. I could be wrong, however. I believe they are esentially identical, just called two different things. I think it's important to point out that crossovers do not always split frequencies into three ranges. Two way speakers, for example, will only be split into two frequencies ranges. (Midbass, and high).

To make things even more confusing, when you use a tube amp, wattage does not double as you halve the impedance, it stays the same. I have no idea why, I'm not nearly the tube expert that others are.

[soundhound]

Impedance = total circuit resistance. Inductors as Thomas implies change resistance at a given frequency, resistors do not.

[Jam_Master_J]

If I'm not mistaken, resistance has to do with DC current, where as impedance has to do with AC current. I could be wrong, however. I believe they are esentially identical, just called two different things. I think it's important to point out that crossovers do not always split frequencies into three ranges. Two way speakers, for example, will only be split into two frequencies ranges. (Midbass, and high).

You're right about the resistance/impedance. Resistance usually represented with "R" refers mainly to DC circuits. Impedance usually represented with a Z mainly refers to AC circuits.

Heres an example, when finding the impedance of the circuit at a certain frequency a capacitor has a impedance of z=1/(i(complex number not current) * omega(angular frequency) * C(capacitance value). Basically this says that the impedance changes by frequency. For a capacitor, as the frequency approaches zero(ie it becomes basically DC) the impedance becomes infinite or it behaves as an open circuit. As frequency rises, it becomes more like a short circuit. The opposite is true with inductors. As you can see, this have important implications in regards to crossovers, because they effectively can be used to filter out frequencies we don't want certain drivers to play.

Crossover design is a large subject, and I only have a rudimentary knowledge but that's a quick rundown.