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Old 11th January 2009, 12:58 PM   #1
prezden is offline prezden  Canada
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Default need help with low pass filter

Hi all
I've just completed a new sub but can't get the low pass filter to work. The filter is made from a 2K7 5W resister and 1uF 250V cap. The drivers are 2 Eastech 7".
Have I got the wrong components or have I done something wrong in soldering it up?
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Old 11th January 2009, 01:07 PM   #2
prezden is offline prezden  Canada
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Here's the bottom. The soldering is pretty sloppy. Could that be the problem.
I forgot to add that the speakers work fine without the filters. With the filters there is almost no sound at all.
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Old 11th January 2009, 01:22 PM   #3
CLS is offline CLS  Taiwan
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Do you connect this filter between amp and woofer?

If so, the 2k7 resistor is eating up almost all of the driving voltage, thus no sound.

Such RC 1st order filter should be used in line level circuit. However the combinations of the components are not 'normal', either.

Where did you get this circuit?
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Old 11th January 2009, 01:25 PM   #4
AndrewT is online now AndrewT  Scotland
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what is the source impedance feeding the filter?
What is the load impedance that the filter is feeding?

For a normal filter, Rs is assumed to be zero and Rload is assumed to be infinity.

I'll assume this just for the sake of calculation.

F-3dB = 1/2/Pi/R/C=1/2/3.14159/2700/1/10^-6=59Hz. = a low pass filter.

But, if the load impedance is an 8ohm speaker, F-3dB becomes 20kHz.
Yes, it appears to pass everything but now you have an attenuator to account for 8ohm and 2k7. That is about -50dB. You would have to increase the power amp from 50W to ~17kW to get the same volume out of the speaker. And 16.95kW would be dissipated in the 2k7 resistor.
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Old 11th January 2009, 01:44 PM   #5
prezden is offline prezden  Canada
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Thanks for your feedback.
I'm afraid that I don't understand much about audio electronics when it comes to figuring out which resistors and caps to use. I'm guessing that they are not the right size for the job.
The amp I'm using is a 25W per channel T-Amp. The speakers are 4ohm, 15-30W. They sound quite good at a very moderate volume without the filters installed. I had the filters between the amp and the speakers.

Cheers- Preston
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Old 11th January 2009, 03:44 PM   #6
AndrewT is online now AndrewT  Scotland
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an inductor in series with the speaker will result in a low pass ~-6dB/octave single pole filter.
An inductor and capacitor combination will give a two pole (-12dB/octave) LP filter.
Download a filter calculator and see what values they suggest for 4ohm speakers and 60Hz. It's going to cost you plenty though.
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Old 11th January 2009, 06:13 PM   #7
fritz39 is offline fritz39  United States
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Default Should be 2R7

Are you sure the schematic wasn't 2R7? That would make more
sense. 2R7 is 2.7 ohms while 2K7 is 2700 ohms.
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Old 11th January 2009, 10:40 PM   #8
prezden is offline prezden  Canada
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I found an online calculator which specifies a 10.6mh inductor and a663uf cap. I could not find a calculator for a resister and cap filter. Have to admit it's all a bit confusing.
I have some 9mh resistors and either a 1uf or 4.7uf cap. I've also got some 9 and 10ohm 10W resistors on hand.
Would one of these combinations work or should I buy some new components.

thanks for your help
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Old 12th January 2009, 01:21 AM   #9
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Quote:
Originally posted by prezden
I could not find a calculator for a resister and cap filter
An RC filter wiill only worjk at line level... you use it between your pre-amp & poweramp (you need a separate power amp for the woofers), here is an article:

http://t-linespeakers.org/tech/filters/passiveHLxo.html

dave
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Old 12th January 2009, 01:34 AM   #10
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Quote:
Originally posted by prezden
I found an online calculator which specifies a 10.6mh inductor and a663uf cap. I could not find a calculator for a resister and cap filter. Have to admit it's all a bit confusing.
I have some 9mh resistors and either a 1uf or 4.7uf cap. I've also got some 9 and 10ohm 10W resistors on hand.
Would one of these combinations work or should I buy some new components.

thanks for your help

Hi,
Forget about resistors. You need a big inductor (coil) and a capacitor.
Below is a low pass filter. 100Hz at ~4 ohm load.
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