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31st January 2006, 09:19 PM  #1 
diyAudio Member
Join Date: Oct 2004

passive notch filter, how much power do they take?
IF I wanted to build a passive notch filter to flattern out an impedance hump in a speaker (and the resulting hump in the passive Xover response) what percentage of the power sent to the speaker would the filter recieve. the speaker is an 8Ohm VC, and the (currently planned) notch consists of a 14Ohm resistor 6.5mH inductor (is laminate core ok) and a 6.8uF cap.
So if my speaker is recieving 1000W (its a big sub) how much power will the filter be required to take? what wattage values will I want for the components? thanks 
31st January 2006, 09:22 PM  #2 
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Join Date: Dec 2003
Location: Austin

power at what frequency? I haven't done anything like the maths involved, but it seems to me that the only frequencies where the filter will take much power would be the ones where the filter has its main AC reactance. The rest of the time I am guessing most filters will take less than a watt (not even warm).
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31st January 2006, 09:26 PM  #3 
diyAudio Member
Join Date: Oct 2004

Well its for a PD 2150 sub (www.precisiondevices.com), and with a 16uH inductor in series as a low pass filter (starts to roll off about 100Hz) and the peak is centred around 90100Hz so I presume that is pretty close to worse case scenario.
and before someone says I know active would be better but thats not what I want to know 
1st February 2006, 12:12 AM  #4  
diyAudio Member
Join Date: Sep 2003
Location: Bellevue, WA

Re: passive notch filter, how much power do they take?
Quote:
Assuming you have a 4 ohm nominal sub, it takes roughly 60 volts to get 1000 watts rms. When you wire a 6.8mH coil in series with the subwoofer you end up with (assuming the sub is a 3.2 ohm resistive load, which I know it's not) 60v @ 20hz, 888 watts total, 700 watts to sub, 187 watts to inductor 60v @ 40hz, 733 watts total, 478 watts to sub, 255 watts to inductor 60v @ 80hz, 544 watts total, 263 watts to sub, 281 watts to inductor 60v @ 160hz, 359 watts total, 114 watts to sub, 244 watts to inductor 60v @ 320hz, 213 watts total, 40 watts to sub, 173 watts to inductor So you need at least a 300 watt inductor. I'm pretty sure my math is right, but don't take my word for it. I've been figuring this stuff out from DIY websites and forums. Dan 

3rd February 2006, 01:19 AM  #5 
diyAudio Member
Join Date: Apr 2004
Location: Northern California

If I understand your post, you are talking about a network to smooth out the low frequency resonance bump (Fs) in a woofer.
The values you supplied 6.5mH, 14 Ohms and 6.8uF resonate at about 720Hz when connected in series accross the woofer. This is an unusual resonance frequency for a subwoofer, there is probably a mistake. Calculating the resistor dissipation is very tricky because it depends on how much of the time power at the resonance frequency is present. At other frequencies the resistor dissipates almost nothing. If a film capacitor is used and a large wire gauge coil the reactive components will dissipate almost zero power. Additionally even a 1000 Watt amplifier only delivers about 100 watts of average power because most music has a 10:1 peak to average ratio. So given that all notes are present and only 720Hz notes dissapate much network power, a 1025 watt resistor will be fine (except if this speaker is being used as a guitar amplifier speaker then you will need at least a 100 watt resistor). If you change the values, my results will not be valid. 
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