Post 14
Edit to include a bit of explanation.
In its simplest form, a series crossover places a capacitor across the woofer and an inductor across the tweeter. From http://en.wikipedia.org/wiki/Electrical_impedance, The magnitude of a capacitor's impedance is 1/(2*Pi*F*C) with C in Farads, and an inductor's is 2*Pi*F*L with L in Henries. Ignoring the phase for a simplicity (especially since we are assuming that the drivers are pure resistances), we can assume that the reactance and nominal impedance add like parallel resistors, R=1/(1/Rd + 1/Rxo). Make up a spreadsheet to calculate the rough impedance of the inductor/tweeter and capacitor/woofer combination over frequency, then add them together at each frequency. You'll see that the net doesn't rise much above 4 ohms.
Edit to include a bit of explanation.
In its simplest form, a series crossover places a capacitor across the woofer and an inductor across the tweeter. From http://en.wikipedia.org/wiki/Electrical_impedance, The magnitude of a capacitor's impedance is 1/(2*Pi*F*C) with C in Farads, and an inductor's is 2*Pi*F*L with L in Henries. Ignoring the phase for a simplicity (especially since we are assuming that the drivers are pure resistances), we can assume that the reactance and nominal impedance add like parallel resistors, R=1/(1/Rd + 1/Rxo). Make up a spreadsheet to calculate the rough impedance of the inductor/tweeter and capacitor/woofer combination over frequency, then add them together at each frequency. You'll see that the net doesn't rise much above 4 ohms.
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Thanks for the great feedback, but this is still really confusing to me. So what your saying is that a series crossover will be like reversing my woofer and my tweeter??
I was reading about series crossovers and it didnt say anything about the impedance.
One more thing is there anything at all that could get my fiinal impedance to AROUND 8 ohms by mean of a 3 way crossover?
-I really appreciate all this feedback, but i have never done a crossover before, and need simpler terms.
-Thanks
I was reading about series crossovers and it didnt say anything about the impedance.
One more thing is there anything at all that could get my fiinal impedance to AROUND 8 ohms by mean of a 3 way crossover?
-I really appreciate all this feedback, but i have never done a crossover before, and need simpler terms.
-Thanks
Not reversing. Let's take a really simplistic way of looking at what a crossover does. It sends the high frequencies to the tweeter and lows to the woofer.
So, to the maximum extent possible, it wants to get out of the way. No matter what you do the the passband the crossover has near zero impedance. So at 100 Hz. the crossover feeding the woofer has nearly 0 impedance in series with the 4 ohms of the woofer. It has a much higher impedance feeding the mid and tweeter, attenuating their output to near zero.
Look at the diagram near the bottom of Audio crossover - Wikipedia, the free encyclopedia. Notice that even in series crossovers you get a low impedance feeding each driver in its pass band. One way to conceptualize it is thing of a crossover as a frequency controlled switch. First look at the woofer and sweep the frequency up. At low frequencies, the LPF is a closed switch and the HPF is an open switch. The closed switch effectively shorts out the tweeter. As the frequency increases past the crossover point the switches reverse so that the HPF is closed and the LPF opens. Now nothing can go to the woofer. Effectively you hjave one driver or the other connected across the speaker terminals depending on frequency.
In reality the filters have impedances that change gradually change over frequency, but the principle is the same. With some overlap, you pretty much have only one driver connected for a particular frequency.
Using the same analogy, the three way parallel filter that you have is like a three position switch controlled by frequency. At low frequencies it sends everything to the woofer, with the other drivers off. At middle frequencies the switch moves to send everything to the midrange, turning off the woofer and the tweeter.
Other than placing a high power resistor between your amp and speakers, the amp is going to see a ~4 ohm load. If you put a resistor in series with the speaker you will A. lose half of the amp's power as heat, and B. change the low end response of the woofer.
Sticking to funky analogies, you have a hammer and and a saw. Both useful tools for what they are supposed to do. However, you really want a wrench when you want to remove a nut. Get a rid of the saw and buy a nail or get rid of the hammer and find a log. Trade in one or both to get a combination that works. Either buy some drivers that can give you the impedance you need or an amp that can handle a 4 ohm load.
You can use surplus parts and build yourself a chip amp for not much more than your current budget. A pair of LM3886s in parallel will handle the load well. See forum sponsor Apex Jr.Home Page for power transformers, chips, PSU capacitors, rectifiers and heat sinks. DIY Chip Amplifier Kits, PCB's, Components and Information. has boards available for a few dollars. Add a couple of resistors and small capacitors and you're in business. You can probably build an amp for under $150 if you are creative.
So, to the maximum extent possible, it wants to get out of the way. No matter what you do the the passband the crossover has near zero impedance. So at 100 Hz. the crossover feeding the woofer has nearly 0 impedance in series with the 4 ohms of the woofer. It has a much higher impedance feeding the mid and tweeter, attenuating their output to near zero.
Look at the diagram near the bottom of Audio crossover - Wikipedia, the free encyclopedia. Notice that even in series crossovers you get a low impedance feeding each driver in its pass band. One way to conceptualize it is thing of a crossover as a frequency controlled switch. First look at the woofer and sweep the frequency up. At low frequencies, the LPF is a closed switch and the HPF is an open switch. The closed switch effectively shorts out the tweeter. As the frequency increases past the crossover point the switches reverse so that the HPF is closed and the LPF opens. Now nothing can go to the woofer. Effectively you hjave one driver or the other connected across the speaker terminals depending on frequency.
In reality the filters have impedances that change gradually change over frequency, but the principle is the same. With some overlap, you pretty much have only one driver connected for a particular frequency.
Using the same analogy, the three way parallel filter that you have is like a three position switch controlled by frequency. At low frequencies it sends everything to the woofer, with the other drivers off. At middle frequencies the switch moves to send everything to the midrange, turning off the woofer and the tweeter.
Other than placing a high power resistor between your amp and speakers, the amp is going to see a ~4 ohm load. If you put a resistor in series with the speaker you will A. lose half of the amp's power as heat, and B. change the low end response of the woofer.
Sticking to funky analogies, you have a hammer and and a saw. Both useful tools for what they are supposed to do. However, you really want a wrench when you want to remove a nut. Get a rid of the saw and buy a nail or get rid of the hammer and find a log. Trade in one or both to get a combination that works. Either buy some drivers that can give you the impedance you need or an amp that can handle a 4 ohm load.
You can use surplus parts and build yourself a chip amp for not much more than your current budget. A pair of LM3886s in parallel will handle the load well. See forum sponsor Apex Jr.Home Page for power transformers, chips, PSU capacitors, rectifiers and heat sinks. DIY Chip Amplifier Kits, PCB's, Components and Information. has boards available for a few dollars. Add a couple of resistors and small capacitors and you're in business. You can probably build an amp for under $150 if you are creative.
1. it doesnt work without crossover
2. with the exception of series xo all driver are always in paralel
3. the nature of series componts in a paralel xo prevents that your amp sees the drivers as being in paralel
a cap or inductor in series with a driver will raise inpedance in their "stopband"
the "stopband" is the frequency you try to attenuate
4. additonal paralel components "regulates" this by having the opposite effect on impedance
sorry, but I havent had time to real it all, and it may have been covered in other posts
and its a simplification
2. with the exception of series xo all driver are always in paralel
3. the nature of series componts in a paralel xo prevents that your amp sees the drivers as being in paralel
a cap or inductor in series with a driver will raise inpedance in their "stopband"
the "stopband" is the frequency you try to attenuate
4. additonal paralel components "regulates" this by having the opposite effect on impedance
sorry, but I havent had time to real it all, and it may have been covered in other posts
and its a simplification
Sorry but i have to ask one more thing. I have later discovered that my tweeter is actually 8 ohms but putting the multimeter on the right setting. So i did some research on this.
Dayton Audio XO2W-4.5K 2-Way Crossover 4,500 Hz | Parts-Express.com
I know its a premade crossover, but im not going for award winning sound. It says you can use a 4 ohm or and 8 ohm woofer (which is perfect) and an 8 ohm tweeter(which i have after testing more).
-Thanks
Dayton Audio XO2W-4.5K 2-Way Crossover 4,500 Hz | Parts-Express.com
I know its a premade crossover, but im not going for award winning sound. It says you can use a 4 ohm or and 8 ohm woofer (which is perfect) and an 8 ohm tweeter(which i have after testing more).
-Thanks
That's interesting, since the data sheet says otherwise.
2 issues:
You're still going to end up with a ~4 ohm speaker at the low end, where most of the power is, and this will continue to give your amp fits.
Either your MTX woofer won't reach high enough for a 4K5 crossover (you quoted published range up to 150 Hz) or your mid won't go low enough. You're still stuck with a hammer and saw in need of a wrench and bolt.
You may find something acceptable for now but lacking deep bass using the mid and tweeter and that crossover.
2 issues:
You're still going to end up with a ~4 ohm speaker at the low end, where most of the power is, and this will continue to give your amp fits.
Either your MTX woofer won't reach high enough for a 4K5 crossover (you quoted published range up to 150 Hz) or your mid won't go low enough. You're still stuck with a hammer and saw in need of a wrench and bolt.
You may find something acceptable for now but lacking deep bass using the mid and tweeter and that crossover.
bs,
At 15 I probably knew less than you do. My first project was a few years later and I had no idea what I was doing but it was fun.
Series crossovers won't do what you want. Take two 4 ohm woofers and wire them in series and you get 8 ohms. However, a series crossover won't do the same thing unless your tweeter is able to cover the same range as the woofer!
You could really use a simple and cheap project. At the kind of budget level you are aiming for, you are probably best to buy something really cheap and have some fun modifying it. Try a second hand place or garage sales and pick up some old boom boxes. You could brace and reline the box with stuffing. You could try a better tweeter, or improving the crossover.
Or if building from scratch perhaps try something with a full range driver.
You can also buy mini monitors sometimes that are dirt cheap, then do some mods.
About off the shelf crossovers, and textbook ones based on online calculators, I wrote this article:
Red Spade Audio: Off the shelf passive crossovers
I've explained a few things about crossovers in there and why you don't want one of those things.
At 15 I probably knew less than you do. My first project was a few years later and I had no idea what I was doing but it was fun.
Series crossovers won't do what you want. Take two 4 ohm woofers and wire them in series and you get 8 ohms. However, a series crossover won't do the same thing unless your tweeter is able to cover the same range as the woofer!
You could really use a simple and cheap project. At the kind of budget level you are aiming for, you are probably best to buy something really cheap and have some fun modifying it. Try a second hand place or garage sales and pick up some old boom boxes. You could brace and reline the box with stuffing. You could try a better tweeter, or improving the crossover.
Or if building from scratch perhaps try something with a full range driver.
You can also buy mini monitors sometimes that are dirt cheap, then do some mods.
About off the shelf crossovers, and textbook ones based on online calculators, I wrote this article:
Red Spade Audio: Off the shelf passive crossovers
I've explained a few things about crossovers in there and why you don't want one of those things.
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