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Old 31st January 2006, 09:19 PM   #1
Puggie is offline Puggie  United Kingdom
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Default 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
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Old 31st January 2006, 09:22 PM   #2
Stocker is offline Stocker  United States
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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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Old 31st January 2006, 09:26 PM   #3
Puggie is offline Puggie  United Kingdom
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Well its for a PD 2150 sub (www.precision-devices.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 90-100Hz 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
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Old 1st February 2006, 12:12 AM   #4
owdi is offline owdi  United States
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Default Re: passive notch filter, how much power do they take?

Quote:
Originally posted by Puggie
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
A lot. You can figure it out pretty quickly.

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
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Old 3rd February 2006, 01:19 AM   #5
hermanv is offline hermanv  United States
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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 sub-woofer, 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 10-25 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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