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Old 4th April 2007, 10:09 AM   #1
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Default Crossover - anyone help, please?

I'm a bit dumb about all this, but I want to convert a previously bi-amped PA cabinet to single amp use. The cab has a 10" 8ohm LF driver and the HF is two 5" 8ohm mids with a piezo tweeter. Is it possible to construct a passive 2-way crossover to feed >250Hz to an 8ohm LF and 250Hz< to a 4ohm (2x8ohm) HF? Or am I trying to defy the laws of physics again? All help gratefully accepted, cheers, labougie.
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Old 4th April 2007, 12:00 PM   #2
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To design a xover you must know all the references from the speakers you are using as brandname, model, T/S parameters, frequency curve output, to say the least...
That been said, your xover parts might cost you as much as your speakers (and time), but if you want to try anyway...
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Old 4th April 2007, 03:17 PM   #3
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Default you don't necessarily need an XO

depending on the bass driver-- it probably has a finite top end, and piezo tweeters don't need crossovers due to their design-- they usually kick in around 2500-3500 hz because they're capacitive, and function largely as their own crossover. by the sounds of the box-- it's not a high end design (piezos are usually second in line in quality to compression drivers), so the upper end of your bass driver may end up being a little raggedy transitioning to the tweeter.. but despite that, it may work out anyhow-- just don't put a lotta juice through it at first. wire both to the terminals, and listen at moderate volumes.. see what you end up with.

the only thing you MAY need is an attenuating capacitor in line with your tweets so that it's not a ton louder than your bass driver-- go to the CTS website for a good article on piezo tweeter usage!


good luck!

d
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Old 5th April 2007, 07:02 AM   #4
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The low crossover frequency (250Hz) you have in mind will need large value components to function.

The simplist x over would be a 160uF capacitor (or 100u,47u and 10u for neerest preferred values) in series with your parallel mids (4 ohm), and a 5mH inductor in series with your woofer.
Use non-polarised electrolytic capacitors.
I would also be inclined to pop a resistor in series with the piezo to bring down it's output. Play with the value to match the level by ear (They might even have one in as they are).

There are lots of more complex x overs, but these values will give you a simple 250 Hz first order type.
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Old 5th April 2007, 06:11 PM   #5
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Thanks bobhayes - I'll try that. The piezo DOES already have a resistor in series so no problem there. What total load will this present to the amp? Cheers.
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Old 6th April 2007, 10:52 AM   #6
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The load to the amplifier will be frequency dependant.
At frequencies below about 200 Hz it will be around 8 ohms. This is where most of the energy is used, so you can consider it the nominal impedance for AV or music/speech.
There may be an uneven load from around 200 to 400 hz, where the crossover and drivers are interacting the most, above which the load will be around 4 ohms.
There are lots of resources around on the web for helping with crossover design, as well as 'by ear' tuning, so once you can see if the speakers have potential, there are lots of possible improvements that can be made.

Have fun.
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Old 9th April 2007, 09:15 AM   #7
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Hi bobhayes - I've got another issue which I'm sure you'll have the answer to. I've built a PA system and I needed the LF to give me all the power it can down to 70hz (the E string on an acoustic guitar detuned to D). The result of this is that below 70hz and specifically below 50hz I'm going over xmax, so I need a high pass filter to block anything below (say) 65hz. I've been looking at The12volt.com which has all the calculators, but I notice that a 65hz high pass filter seems to start attenuating above 65 and is actually 3db down AT 65 - what centre frequency should I go for if I want the attenuation to START at 65? Cheers.
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Old 9th April 2007, 01:54 PM   #8
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The reason the calculators give you the -3dB frequency, is for crossover networks. Assuming zero phase difference, the theoretical combined response for a low pass and high pass filter together with the same -3dB point, should be flat (or near to flat).
It's never that simple in practice.

I would definately be using an active filter ahead of your amps (or built into your amps) for a 70Hz high pass. You could build a 4th order (very steep roll off) filter for a few bucks, whereas the components for a 4th order passive filter will be expensive, waste power, and be heavy too.

Try on another forum for possible circuits whether tube or silicone based.

Good luck.
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