Help with Crossover

[Sterben]

I am building a speaker, preferably using dayton audio's reference series 1 1/8" tweeter and 8" woofer wired in parrellel. However, both speakers are 4 ohms and I need help figuring out how to make a 2nd order crossover that will bring the ohm load up to at least 6 ohms, without significantly decreasing the wattage available to the speakers themselves. Any takers?

 

[stillspin'n]

Search second order crossover on the web and several calculators will come up, just put in speaker impedance and at what freq you want to cross them over at.

 

[PR Audio]

Search second order crossover on the web and several calculators will come up, just put in speaker impedance and at what freq you want to cross them over at.

This was not the OP's question...What he wants is to make his 4 Ohm speakers (drivers) at least 6 Ohm. My best guess is that 6 Ohms is the lowest his amp can handle without stressing too much. Answer is simple, it can be done, but not recommended. Just Google how to convert a 4 Ohm speaker to 8 Ohm.

To make your 2nd Order follow the instructions of a good XO calculator. But, remember that impedance in changing.

You could technically place a 2 Ohms or 4 Ohms resistor in series, but it is not recommended, plus you would need a resistor that is the same Wattage your amp puts out at full volume. So, if your amp puts out 50WPC at 8 Ohms, you need a 50W resistor in series.

Here is some interesting info taken from another post:

What are you asking of the amp? If you take the amp's 8 ohm rating, you can derate it to see what safe 4ohm load it can drive. An amp that can drive an 8 ohm Speaker at 150W can drive a 4 ohm Speaker at 75W.

The maximum current the amplifier is designed for can be figured from its output Power (P1) and designed load impedance (Z1):

I=sqrt(P1 / Z1)

Now, for a given load impedance (Z2), we can find the maximum voltage (V2) that the amplifier can output into this load before exceeding it's designed maximum current output:

V2=I*Z2

And from this, we can find the Power (P2):

P2=V2*I=I^2*Z2=sqrt(P1/Z1)^2*Z2=P1/Z1*Z2

Simply put, the maximum power an amp can safely put into a lower-impedance load is the original power times the new impedance over the old impedance:

P2 = P1 * Z2/Z1

So if your amp can put out 150W into an 8 ohm load, it can safely put 75W into a 4 ohm load, but can be overdriven by a too-high input signal.

A mid or high amp won't be driven this hard, but a sub ampmay very well be.

 

[JURB]

The main thing you're going to need is a coil with like one ohm or two of Xl at 20 Hz. My Bostons are like that, they call themselves eight ohms nominal but the woofers are four ohms or less. They are like 3.1 ohms DC resistance. The nice thing about such an arrangement is it will give you a steep bass curve and help the bottom octave(s). So you have a MINIMUM coil inductance to shoot for. That will unfortunately set a maximum crossover frequency. Hope the tweeter can handle it.

Typically you have a cap across the woofer, in this case the value should be quite low.

In this case, the woofer will be quieted down to the point where a simple resistor can be used on the tweeter. Of course you still need a cap in series.

 

[67dylan]

What speakers are you using? I've built quite a few DIY speakers from the Parts Express Speaker Building page and think it is a great place to build from: http://projectgallery.parts-express.com/category/speaker-projects/ You could purchase a Parts Express crossover as well, but the sound won't be as good as a custom crossover. If you want to build your own custom x-over, good luck. It requires some crossover/speakers design software, a laptop, a mic (I use a Berringer ECM800), and a pre out (I use the M-Audio Mobile Pre USB), some cables, a way to mount the speaker, but it can't be in a box it has to be free air, and a mic stand. It is time consuming and requires experience to be any good. Then you will need patience as modeling is just that, so our ears will not always like what is modeled. Like I stated earlier, Parts Express Speaker Building page is a great place to start and may have a design using speakers you already have. BTW, don't try and 'change' a design to use your speakers as it was built for certain speakers and a change in speaker means a change in design.

As for changing the load from 4 ohm to 8 ohm, you will create quite a heat source and lose power. The resistor won't be a 5W resistor either, it will have to handle the power from the amp, so I think it will be quite large. That is the downside. The best thing to do is replace the speakers with 8 ohm versions or buy 2 more speakers, both 4 ohm and then you should end up with ~6 ohm (3 speakers per cab). You will run into problem if you make both woofers use the same frequency so you'll want to the upper woofer to handle up to the tweeter/woofer x-over point and the lower woofer to handle an octave lower than that point. That will make it a 2.5 design.

I'm not trying to rain on your parade, I am trying to make you aware designing crossovers is no easy task. Building a proven design is easier and gives you valuable experience if you have the equipment to test what has already been tested and build what has already been built. You learn what the designer did and maybe try one yourself. The only pair I built/designed myself is a small pair of 2 way speakers using Dayton Audio 4" and there 3/4" tweeter I use in my back patio. I found out the work was too much time and not worth it to me.

 

[Sterben]

Sorry about the delay fellas, I went a different route and got a buddy to build it. 2nd order, L-R, polypropylene caps and 18 gauge solid core inductors, the good stuff!