Crossover Resistors wattage

[vassilis1984]

Hi guys!.


I am a newbie at diy audio, so I try to read and understand as many crossover designs I can.
I noticed that almost everyone at their designs is giving only the number of the electrical resistance(aka ohm) of the resistors but no one gives us the wattage of the resistor.
Is the wattage irrelevant?
I mean, can I use a 4ohm/5watt resistor instead of a 4ohm/10watt resistor and the result will be the same?

Troels gravesen uses only 10watt mox resistors, should I follow his path?

Are there any hints like 5w for tweeter? 10w for notch? 5w in series, 10w in parallel?

 

[indianajo]

Depends on how much your wattage your amp puts out.
If your speaker is to be rated 300 W RMS 600 W program power, you have a 600 w/ch amp, you need some serious wattage resistors.
First cut buy resistors bigger than the amp wattage.
As there is usually less wattage in the tweeter than the woofer, you can cut back the wattage of resistors running current to tweeter only.
You can run sims to determine more accurately, but finding RLC model of each driver can be a problem.
As freight from the distributor is usually bigger than the cost of a 50 W resistor, start up there for a living room unit & work back. Measure current across the resistor on program material using a scope or analog VOM, then the second one you can back off wattage. Usually not worth it for diy projects. Remember using scope, rms value of a sine wave is .707 * the peak to peak.
You'll notice integrated crossovers from distributors like parts-express, have both a speaker impedance rating, a crossover frequency rating, and a wattage rating.
This goes for the ESR of the crossover caps, and the wire gauge of the inductors. Choose appropriate value for the current that is flowing.

 

[Galu]

The resistor power rating depends on the crossover frequency.

As the crossover frequency rises, a lower percentage of the amplifier power is directed to the mid or treble driver.

Here's some figures to help you choose the resistor power rating required:

Crossover Frequency (Hz)/Percentage of power above crossover point

150/80
200/72
300/55
500/40
1000/25
3000/20
5000/10

For domestic speakers, the highest resistor power rating you are likely to need is 20W. Jantzen 20W Ceramic Resistors | Hifi Collective

 

[vassilis1984]

I don't have so much power, I am just a home cinema guy, so 80 rms per channel is more likely for me.

I still don't get the general idea. The majority of the designs have 5w or 10w resistors and troels designs are all with 10w metal oxide resistor regardless of the speaker or the frequency

 

[Galu]

Also take into account that, even if you have a 100W amplifier, most of your normal listening is unlikely to exceed single figures in watts.

Additionally, music signals vary in power so the average level is much lower than the continuous power level you might calculate from my above percentages.

That is why Troels simplifies the choice by stipulating 10W resistors, as they will be perfectly adequate for use in any position in a domestic loudspeaker crossover.

 

[JMFahey]

In general resistors are used attenuating Tweeters and in some cases midrange units, with the odd Zobel network linearizing some driver impedance.

In all those cases, power involved is but a fraction of total program power.

Want to calculate some actual numbers?

Fine, try something like this, adjust to suit your particular case:

Suppose it´s a 2 way system, crossover 3 kHz.

Per Galu´s suggestion, Tweeter will receive 20% of program power. (I find that too much, but let´s stick to said table)

Suppose you need a 4 ohm resistor in series with an 8 ohm Tweeter to adjust sensitivity.
It will then drop 33% of voltage applied to it, so 10% of its power, so 2% of program power.

And besides that, program power is not applied continuously to speaker system, that would mean amp distorting most of the time; typical Hi Fi use is around 10% or so of clipping level power, we are not talking DJ or Rock PA use by any means.

Do that for all resistors in typical crossovers and you´ll see that 10W dissipation covers a lot of bases.

 

[PRR]

....600 w/ch amp, you need some serious wattage resistors.....

If "most" of the amplifier power goes up in crossover resistors, something is very wrong.

It is very unlikely to find much added resistance in the woofer side, we usually want the woofer to get all the power available.

Midranges and tweeters in domestic systems are rarely good for sustained 10 Watts before they blow. It is not likely that such resistors need be over 5 Watts (however we blew a lot of 2-Watt tweeter pads in the studio).

Resistors do NOT cost that much. 10 Watt parts are cheap.

If you need a more exact answer, it is simple Ohms Law plus establishing all the conditions of use. Or build it, beat it a while, and feel all the resistors.

 

[indianajo]

If you want to calculate, go back from the wattage rating of the tweeter or midrange. (I^2)*R=W. So that is the current. since speaker drivers are highly inductive, double the current predicted by the resistance model. Then figure out using kirchoff's 3 laws for the crossover net what the current through the resistor will be. Same equation predicts the wattage draw of the resistor.
Actually the simplest crossovers put a capacitor in series with the tweeter to resist the lows, and put an inductor in series with the woofer to hold out the highs. No resistors. I've a KLH 23 speaker that only has the capacitor in series with the tweeter, nothing in series with the woofer.