Hey all. I'm slightly confused with just one aspect of speakers. I bought an 8 ohm woofer and an 8 ohm tweeter, and wired them in parallel with a crossover at 5000 Hz high pass on the tweeter, and freerange on the woofer. I wired in parallel hoping for 4 ohms, but ended up with 8 ohms overall. Why is this?
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Confused 15 year old
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Re: Confused 15 year old
With the crossover in place, the two drivers are operating in 2 different ranges below the crossover. Also if you are trying to measure using a regular resistance meter, they generally test at about 1khz and is not really acurate for speakers.
Oh and welcome to the boards. Its nice to see younger guys interested in this hobby. My son built his first pair earlier this year and we are working on a sub now.

Re: Confused 15 year old
Yes, that was supposed to happen. Resistance is better used as a DC measurement, things change when you switch to AC. Instead of resistance, it is now impedance, which is similar, but impedance varies based on the frequency of the signal.
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Re: Confused 15 year old
Originally posted by mistercmason View PostI wired in parallel hoping for 4 ohms, but ended up with 8 ohms overall. Why is this?
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Re: Confused 15 year old
When you calculate impedance (as a single reference number) of drivers in parallel, you assume they're playing in the same bandwidth. Introducing a crossover changes this  it's kind of like a frequencydependent switch (except it's not all or nothing, rather a tapered blend, but for the sake of understanding that doesn't matter.) Frequencies above the XO point go to the highfrequency driver, frequencies below the XO point go to the low frequency driver.
So you're not putting the two in parallel really, since the XO causes the woofer to become high impedance at high frequencies, and the tweeter to become high impedance at low frequencies. As the input signal moves out of each driver's intended bandwidth, it gradually disappears from the circuit, as if it weren't there at all. Thus, it becomes less involved in the impedance of the whole circuit.
Since you said your XO is only a highpass to the tweeter (leaving the woofer open), there's another concept to introduce  the inductive properties of the woofer's voice coil. In essence, the winding of the voice coil itself acts like a lowpass filter, causing the driver's impedance to rise with higher frequencies. This happens in all drivers, but the degree varies by design.
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Re: Confused 15 year old
Originally posted by SirNickity View PostFrequencies above the XO point go to the highfrequency driver, frequencies below the XO point go to the low frequency driver.
So you're not putting the two in parallel really, since the XO causes the woofer to become high impedance at high frequencies, and the tweeter to become high impedance at low frequencies.
I like the way you put that. I wish that somebody had summarized it that way for me when I was first playing with this stuff as a kid. I've been at it for a long time now, and I sometimes forget that the new guys need clear explanations like this before they grasp the concept.
Originally posted by SirNickity View PostSince you said your XO is only a highpass to the tweeter (leaving the woofer open), there's another concept to introduce  the inductive properties of the woofer's voice coil. In essence, the winding of the voice coil itself acts like a lowpass filter, causing the driver's impedance to rise with higher frequencies. This happens in all drivers, but the degree varies by design.
Welcome to DIY speakerbuilding mistercmason! Have fun and ask lots of questions.
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Re: Confused 15 year old
Originally posted by mistercmason View PostHey all. I'm slightly confused with just one aspect of speakers. I bought an 8 ohm woofer and an 8 ohm tweeter, and wired them in parallel with a crossover at 5000 Hz high pass on the tweeter, and freerange on the woofer. I wired in parallel hoping for 4 ohms, but ended up with 8 ohms overall. Why is this?
Impedance is a function of frequency. Don't confuse "Impedance" with "Resistance." Both impedance and resistance are measured in "Ohms" (not "Ohmage") but impedance can and will change as the frequency changes. Audio frequencies are typically in the range from 20 Hertz to 20,000 Hertz. (20Hz  20KHz)
Pure simple resistance doesn't concern itself with frequency, because it is measured at no frequency (zero Hertz). A typical 8 ohm woofer might have an impedance that varies from slightly below 8 ohms at 1 Hertz to maybe 60 ohms or more at its resonant frequency (peak) and then back down to somewhere near 8 ohms again. After that a woofer's impedance continues to rise gradually as the frequency extends towards and beyond 20,000 Hertz.
If you use an 8 ohm woofer and an 8 ohm tweeter WITHOUT a crossover, your impedance would be 4 ohms! But who would use a woofer and a tweeter together without a crossover? A crossover is a frequency dividing network. As long as you are using a crossover with an 8 ohm woofer and an 8 ohm tweeter then your impedance will remain at or near 8 ohms straight across, not 4 ohms.
If you use a 4 ohm woofer and an 8 ohm tweeter without a crossover, your impedance would be 2.66 ohms. But with a crossover, your impedance would be 4 ohms in the range of frequencies covered by the woofer and 8 ohms in the range of frequencies covered by the tweeter.
If you use a 4 ohm woofer with an 8 ohm midrange and a 6 ohm tweeter all with a crossover, then your impedance would be 4 ohms in the range of frequencies covered by the woofer and 8 ohms in the range of frequencies covered by the midrange and 6 ohms in the range of frequencies covered by the tweeter.
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Re: Confused 15 year old
Welcome! It would seem that way, but there are many variables at work just getting two drivers to work together well and blend properly. Impedance is one of those variable characteristics and varies according to the frequency the driver is reproducing. That's the big reason NOT to use any textbook calculations...those that only ask for the driver's nominal/average impedance, like 4 or 8 ohms and an arbitrary crossfrequency. Same with premade XO's and those online calculators, they only want the nominal impedance and a crossfreq. that might not be a good point for the drivers. With the textbook/premade/online calculator methods, it's assumed that the driver is always a constant 4 or 8 ohms. Never the case. Trying to use a fixed number for a number that constantly varies is, well, you can see how doing it that way will lead to poor sound quality. You use 8 ohms in your calculations, but what if the driver is 12 or even 20 ohms at the crossfrequency? That throws off all the component values that were just calculated and the driver will not cross where you want it to. I've found one of the best writeups about this issue and the other variable parameters that need to be dealt with; Paul did a great job with his explanations at the link. It won't take very long and is written in a nontech fashion.
https://sites.google.com/site/undefinition/diy
Read everything under "Resources" at the right. It'll explain about these issues and why the fixednumber calculation method doesn't have a chance to give you a decent result. The textbook methods will pass the signal, of course, but will be far from adequate. Even though listening to the result might give the impression of "hey, it sounds OK", if you could hear a design where the XO was worked up by taking into account the variable parameters, the differences/improvements would be nightday. I tried those methods before and I never got an acceptable result. Comparing my previous fixednumbermethod speakers to some that had a properlydesigned XO, I immediately noticed the improvements in imaging, balance, lack of harshness and the smooth transition between drivers. Don't hesitate to ask questions, as none are too "dumb", redundant or "beginnerlike". The goal is better SQ and a more enjoyable listening experience.
John A."Children play with balls and sticks, men race, and real men race motorcycles"John Surtees
Emotiva UPA2, USP1, ERC1 CD
Yamaha KX390 HXPro
Pioneer TX9500 II
Yamaha YP211 w/Grado GF3E+
Statement Monitors
Vintage system: Yamaha CR420, Technics SLPG100, Pioneer CTF8282, Akai X1800, Morel(T)/Vifa(W) DIY 2way in .5 ft3
Photos: http://custom.smugmug.com/Electronic...#4114714_cGTBx
Blogs: http://techtalk.partsexpress.com/blog.php?u=2003
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Re: Confused 15 year old
Originally posted by SirNickity View PostWhen you calculate impedance (as a single reference number) of drivers in parallel, you assume they're playing in the same bandwidth. Introducing a crossover changes this  it's kind of like a frequencydependent switch (except it's not all or nothing, rather a tapered blend, but for the sake of understanding that doesn't matter.) Frequencies above the XO point go to the highfrequency driver, frequencies below the XO point go to the low frequency driver.
So you're not putting the two in parallel really, since the XO causes the woofer to become high impedance at high frequencies, and the tweeter to become high impedance at low frequencies. As the input signal moves out of each driver's intended bandwidth, it gradually disappears from the circuit, as if it weren't there at all. Thus, it becomes less involved in the impedance of the whole circuit.
Since you said your XO is only a highpass to the tweeter (leaving the woofer open), there's another concept to introduce  the inductive properties of the woofer's voice coil. In essence, the winding of the voice coil itself acts like a lowpass filter, causing the driver's impedance to rise with higher frequencies. This happens in all drivers, but the degree varies by design.
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