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Old 8th October 2012, 01:43 PM   #1
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Default need help with passive notch filter design

I have an OB speaker with a dipole peak around 1150hz. I don't know how to design a RLC notch around this frequency. The peak is about 5-6 db.Thanks.
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Old 8th October 2012, 02:05 PM   #2
tvrgeek is offline tvrgeek  United States
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Two quick answers you won't like: First, there are several WEB sites to do the calculations for you. Second, they will be wrong, so you will need to use trial and error to get it right. You might also go read the comments on ZAPH buried in one of his speaker designs on why you use a series or parallel filter and when.

BTW, you need to determine the Q of the peak. Careful measurements to find the -3dB points.
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Old 8th October 2012, 02:23 PM   #3
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TVR, First of all i do like your answer because of it's thruthful. How many times we use a so called calculator only to get it "wrong" So I appreciate the heads up so if I use a calculator it is only to be used as a starting point and i will not be surprised when I have to redo it. Thank you so very much. I can tell you I have already been to 2 websites and got 2 different results, just as you said.

So I will do some careful measurements to determine the Q and go to Zaphs site. Your assistance and honesty is greatly appreciated. I will let you know what I find.
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Old 8th October 2012, 03:34 PM   #4
Pano is offline Pano  United States
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Also be sure you know the impedance at the peak (1150Hz). It's probably near nominal at that point, but you want to know for sure.

Notch filters aren't easy, you may have to do several trials to get it right.
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Old 8th October 2012, 04:03 PM   #5
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Quote:
Originally Posted by Pano View Post
Also be sure you know the impedance at the peak (1150Hz). It's probably near nominal at that point, but you want to know for sure.

Notch filters aren't easy, you may have to do several trials to get it right.

Thanks pano. I have patience and XO parts are relatively cheap so I know with persistence I should be able to attack itbut may take awhile. I'll do a before and after measurement and then modify as needed.
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Old 8th October 2012, 06:33 PM   #6
Jay is offline Jay  Indonesia
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Originally Posted by jimbones View Post
Thanks pano. I have patience and XO parts are relatively cheap so I know with persistence I should be able to attack itbut may take awhile. I'll do a before and after measurement and then modify as needed.
What design tools do you have on hand? Where do you get those 1150 numbers? Where do you want to put the notch filter, on woofer or on the input of crossover?

For 5-6 dB, it should be relatively a deep notch where for effective notch you want a LRC in series (in parallel with load).

At 1150Hz (middle of audio frequency), such notch filter will for sure kill some dynamics/sonic of the speaker. You need to pay attention to sonic versus distortion (due to peak) trade-off. Flattest response is usually not a smart goal here.

For 5 to 6 dB notch you will need 4.7 to 4 Ohm resistor.

For LC, simply use the basic resonance formula: f=1/(2*pi*V(L*C))

You will need L from 1mH to 1.8mH, depends on the Q or shallowness of the notch. 1.8mH will give the sharpest notch (high Q), 1mH will give shallower notch. Sharp notch is dangerous especially when not precise, because it may create an overshoot, augmenting frequency you actually want to dampen.

For 1150Hz, here is the LC combo (using common inductor size):
L=1.8mH, C=10.64uF
L=1.5mH, C=12.77uF
L=1mH, C=19.15uF

This is a theoretical notch on pure flat response curve on both ends. If you want to put the notch on woofer, you must know that the woofer response is decreasing (rolling-off) towards the high frequency end, and the lower frequency side is usually the "troublemaker". Because of this, it is usually better for sonic to use a shallower notch, or have bigger capacitance.

In this case, 1mH-20uF-4R would probably subjectively sound better than 1.5mH-12.77uF-4R.
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Old 8th October 2012, 06:59 PM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by Jay View Post
For 5-6 dB, it should be relatively a deep notch where for
effective notch you want a LRC in series (in parallel with load).
Hi,

No you don't, you want LCR in parallel, in series with the load.

In the absence of any other x/o components the above will
simply load the amplifier, though it would work if it and the
driver is preceded by a large baffle rolloff inductor.

rgds, sreten.
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Old 8th October 2012, 07:24 PM   #8
Jay is offline Jay  Indonesia
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Originally Posted by sreten View Post
Hi,

No you don't, you want LCR in parallel, in series with the load.
Yeah, it is better to see real situation. A 6dB notch at 1150 with filter in parallel with the load is a real devil. But the other type of notch will have almost no effect if distortion is the object to be reduced.
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Old 8th October 2012, 08:22 PM   #9
xjr100 is offline xjr100  Europe
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Quote:
Originally Posted by jimbones View Post
I have an OB speaker with a dipole peak around 1150hz. I don't know how to design a RLC notch around this frequency. The peak is about 5-6 db.Thanks.
Can you post measurements of freq.response?
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Old 8th October 2012, 10:24 PM   #10
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I will do a sweep and get back with some data before I do anything. Thanks,
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