Help with math regarding a 3-Way Speaker Crossover circuit

Thread Starter

mjsummers

Joined Jan 2, 2013
12
I was asked to design a circuit for a 3-Way Speaker for some home made speakers as a way to keep my skills from developing cobwebs since I graduated college. The attached pdf file is my circuit, it is a single channel circuit. The circuit has 4 bandpass filters, three L-Pad Drivers, and a notch filter in addition to a tweeter, mid-range, and woofer speakers. My Problem is one of calculating the impedance that will be seen at the amplifier. I am looking for an impedance between 4 and 8 ohms. I have included the dc resistance for the inductors. I have text notations of speaker power by the resistors that make the l-pad for each speaker. The signal input is 100W (RMS) from the amplifier.


Any help would be greatly appreciated, if you need any other information please let me know.
 

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Brownout

Joined Jan 10, 2012
2,390
Start with the impeadance of the speaker(s) and work back towards the source, using parallel and series formulae for each node/loop. Use the formulae:

XL=ωL for inductors, and
Xc=1/jωc for capacitors.
 

Thread Starter

mjsummers

Joined Jan 2, 2013
12
thanks for the advice, how do i account for a signal that covers from 20 Hz to 20kHz in the formula?
 
Last edited:

Brownout

Joined Jan 10, 2012
2,390
BTW, there are design methods for standard filters, for example, the Butteroworth filter, which take into account injection point and load impeadances. Use those formulas and you won't need to be concerned with calculating those impeadances, they will be automatic. I suggest goolging "butterworth filter" etc.
 

bountyhunter

Joined Sep 7, 2009
2,512
I recall there are phase issues depending on what order the filter is. It's been a while since I did crossovers. You definitely want a butterworth so it will have a flat bandpass.

If I recall, a first order LPF is OK for the woofer, second order is also OK. The midrange needs a second order bandpass, and the tweeter gets a second order HPF.
 

Thread Starter

mjsummers

Joined Jan 2, 2013
12
I have included butterworth filters in to the design already. Im using a 3rd order butterworth filter that is designed to handle the tweeter/mid and mid/woofer crossovers so there isnt a noticable difference in the switching of the speakers.
 

Thread Starter

mjsummers

Joined Jan 2, 2013
12
Everything for the design is included in the PDF. I would like to make sure that I will not blow the amp or the speakers when I connect them, thus the reason for the calculation. The amp in question is discontinued and the speakers are going to be made out of a wine barrel
 

bountyhunter

Joined Sep 7, 2009
2,512
I was asked to design a circuit for a 3-Way Speaker for some home made speakers as a way to keep my skills from developing cobwebs since I graduated college. The attached pdf file is my circuit, it is a single channel circuit. The circuit has 4 bandpass filters, three L-Pad Drivers, and a notch filter in addition to a tweeter, mid-range, and woofer speakers. My Problem is one of calculating the impedance that will be seen at the amplifier. I am looking for an impedance between 4 and 8 ohms. I have included the dc resistance for the inductors. I have text notations of speaker power by the resistors that make the l-pad for each speaker. The signal input is 100W (RMS) from the amplifier.


Any help would be greatly appreciated, if you need any other information please let me know.
There's a design tool here. You need to know the crossover frequencies, filter slope, etc.

http://www.diyaudioandvideo.com/Calculator/APCXOver/
 

Audioguru

Joined Dec 20, 2007
11,251
Why are you throwing away more than half the power of the amplifier?

Get rid of the resistor in series with the woofer since it destroys the excellent damping of the woofer resonances provided by the extremely low output impedance of a modern amplifier. Also get rid of the resistor parallel with the woofer.
Oh, the amplifier is discontinued? Is it an old vacuum tube amplifier with a high output impedance that needs an old woofer that damps its own resonances??

With the resistors at the woofer removed then the resistors attenuating the midrange and tweeter speakers might be able to also be removed.
 

Thread Starter

mjsummers

Joined Jan 2, 2013
12
Why are you throwing away more than half the power of the amplifier?

Get rid of the resistor in series with the woofer since it destroys the excellent damping of the woofer resonances provided by the extremely low output impedance of a modern amplifier. Also get rid of the resistor parallel with the woofer.
Oh, the amplifier is discontinued? Is it an old vacuum tube amplifier with a high output impedance that needs an old woofer that damps its own resonances??

With the resistors at the woffer removed then the resistors attenuating the midrange and tweeter speakers might be able to also be removed.
I was trying to determine how much power the speakers will actually see. This is a first draft of the circuit before I buy the parts to make the card for the speaker box. The amp is a 7.1ch yamaha rx-v659 that is no longer made
 

Audioguru

Joined Dec 20, 2007
11,251
I was trying to determine how much power the speakers will actually see.
Will you plug your ears then run the amplifier at full blast all the time?
Most people don't. They simply turn down the volume control.
The volume control is at THE INPUT of an amplifier, not at its output.
 

Audioguru

Joined Dec 20, 2007
11,251
The amplifier produces 100 real Watts per channel at a low distortion.
Its Damping Factor when it drives an 8 ohm speaker is at least 120 so its output impedance is only 0.067 ohms or less for excellent damping of the resonances of a speaker.
 

Thread Starter

mjsummers

Joined Jan 2, 2013
12
The amplifier produces 100 real Watts per channel at a low distortion.
Its Damping Factor when it drives an 8 ohm speaker is at least 120 so its output impedance is only 0.067 ohms or less for excellent damping of the resonances of a speaker.
Dont know if you saw the resistance for the woofer (4OHMS not 8) besides the tweeter is rated for 45W RMS and the mid is 60W RMS and the woofer is 140W RMS, so aside from the input the speakers fall within +/- 60% of the rated output of the amp, hence my concern
 

Audioguru

Joined Dec 20, 2007
11,251
The amplifier is spec'd to produce 140W into 4 ohms at fairly high distortion.
Simply keep the volume control turned down a little (half of full power is only a little less louder) so you do not break your midranges, tweeters and your hearing.

I used a pair of little speakers for many years on my Yamaha 70W per channel amplifier. The speakers were rated at 40W each. The woofer on one speaker stopped playing and when I opened the speaker I saw that the woofer was stamped "5W Korea".
 
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