ohm load for xovers

[paikiah 5,000+ posts]

Ok, this is the situation:

I previously had a xover with a single input and a mid and tweeter output. Both mid and tweeters are 4 ohms. I have always presumed 4 ohms is what the amp sees when I connect the speakers to the amp using the standard passive xovers, since it only has one input.

Now I want to make my own xovers. THe board that I am about to get, they have a separate woofer and tweeter input/output. There is no common connection. If I made this xover and wired the inputs in parallel, would that mean that the amp would see 2 ohms?

Also, when calculating the numbers for xovers, there is an ohm load. Would that be the ohm load of the driver itself (4ohms) or the load of what the amp is seeing? As you might be aware, the ohm load is crucial to the settings of the xover values.

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[mykeys 10+ year member]

I cannot answer the first qestion, but the second should be the resistance of the driver.

If you get the numbers to work out and crossover comes out ok see if you can put up a page about it. Showing the equation and what each item is and the value of the each driver. I have an for making them in a parts catalog, but I can never seem to get it to work out.. So I wanna see if Maybe a different perspective will help. I am sure there are more than just me who would like to know as well..

Mike

 

[maylar 10+ year member]

I previously had a xover with a single input and a mid and tweeter output. Both mid and tweeters are 4 ohms. I have always presumed 4 ohms is what the amp sees when I connect the speakers to the amp using the standard passive xovers, since it only has one input.

You would be correct. Here's the way it's supposed to work:

In the simplest case, you have an inductor in series with the woofer, and a capacitor in series with the tweeter. At low frequencies, the capacitor is high impedance and the inductor is low impedance. The tweeter is basically out of the circuit. The only thing the amp sees is the woofer's 4 ohms. At high frequncies, it's just the opposite... the inductor is high impedancce and the cap is low. So the woofer is out of the circuit and the amp only sees the tweeter's 4 ohms.

There are 2 crossover points... the frequencies where the inductor and the cap are 4 ohms (the 3 dB points). If the crossover frequency is the same for both, then you have 4 ohms at the crossover point (4 ohms woofer + 4 ohms inductor in parralel with 4 ohns tweeter + 4 ohms capacitor). If the crossover points are different, the impedance at those frequencies gets more difficult to calculate.

Now I want to make my own xovers. THe board that I am about to get, they have a separate woofer and tweeter input/output. There is no common connection. If I made this xover and wired the inputs in parallel, would that mean it would be 2 ohms?

Again, if the crossover points are the same then you only have one element (woofer or tweeter) that appears as the load to the amp at any given frequency. That means you'll have 4 ohms.

Also, when calculating the numbers for xovers, there is an ohm load. Would that be the ohm load of the driver itself (4ohms) or the load of what the amp is seeing? As you have told me before, the ohm load is crucial to the settings of the xover values.

Calculating the values of components for the crossovers, you use the impedance of the driver. You're basically constructing hipass or lowpass filters for the driver.

 

[paikiah 5,000+ posts]

Originally posted by maylar You would be correct. Here's the way it's supposed to work:

 

Again, if the crossover points are the same then you only have one element (woofer or tweeter) that appears as the load to the amp at any given frequency. That means you'll have 4 ohms.

 

Ok, what you told me really helps, thanks a lot Maylar.... BUT....

Ok, I wanna cross the tweeter at 3000Hz, and the mid can do whatever is lower...

Trouble is, the xover board... two inputs, two outputs. Looking at the design of the board, it is safe to say that they share NO common connection. THe tweeter section has a 4th order design and the woofer has a 2nd order design. Safe to think that I have two different xovers per channel.

If I were to wire a 4ohm and 6ohm, what would be the total load? What if 4 ohm and 8 ohms?

I ask this because I am thinking about using the midrange with the standard xover, and the tweeter with my home made xover. THerefore, it would be two different xovers.

man, network bilding can be such a pain... but I'm learning so much at the same time...//content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[paikiah 5,000+ posts]

Ok, I got the 6+4ohm thing done on another thread, so you can ignore that....

well, I gotta base my xover values on speaker ohms and not what the amp sees... kewl...

but a 4 ohm driver means its nominal impedence is 4ohms..

This tweeter I've been looking at... it's nominal impedence (Z) is 6 ohms, but DC resistance(Re) is 4.8ohms.. should I calculate xover values based on Z or Re?

 

[maylar 10+ year member]

Use the Z value.