Roles of notch filter components?

[fernandov]

I've been experimenting with LCR values in parallel notch filters used to tame broad peaks in drivers. It seems as if the inductor and capacitor values directly affect the filter's range; i.e., how broad or narrow the filter is applied. But I'm a bit stumped as to the value of the resistor in the circuit.

I understand the circuit requires additional resistance to work, but swapping values doesn't necessarily correlate to the amount of attenuation the notch provides. Does anyone know how increasing or decreasing resistance affects the filter?

[Chris Roemer]

I've been experimenting with LCR values in parallel notch filters used to tame broad peaks in drivers. It seems as if the inductor and capacitor values directly affect the filter's range; i.e., how broad or narrow the filter is applied. But I'm a bit stumped as to the value of the resistor in the circuit.

I understand the circuit requires additional resistance to work, but swapping values doesn't necessarily correlate to the amount of attenuation the notch provides. Does anyone know how increasing or decreasing resistance affects the filter?

The resistor simply regulates how much voltage can flow through the notch. Very low resistance (with the LCR in series) gives maximum notching, which is usually not what you want. Make the resistor large enough and it effectively removes the notch from the circuit. When the LCR are used in parallel, the opposite is true. The LC components set the frequency.

Chris

[benchtester]

My simplistic way of thinking about it is:

The cap and inductor determine the roll off point of each side of the notch. So they determine the width of the notch. Without a resistor the depth will be a function of the width (wider is deeper). If you add the resistor, you can reduce the depth (lower Q). So the resistor is particularly useful for a broad but shallow notch.

If the cap and inductor are parallel (tank circuit), the resistor also dampens oscillations.

[fernandov]

My simplistic way of thinking about it is:

The cap and inductor determine the roll off point of each side of the notch. So they determine the width of the notch. Without a resistor the depth will be a function of the width (wider is deeper). If you add the resistor, you can reduce the depth (lower Q). So the resistor is particularly useful for a broad but shallow notch.

If the cap and inductor are parallel (tank circuit), the resistor also dampens oscillations.

Then how would you adjust the amount of attenuation the filter provides? If you think of it in terms of a parametric EQ, I'm looking for a way adjust the height of the notch. Substituting resistor values for a given LC combination doesn't seem to affect this.

[Chris Roemer]

Then how would you adjust the amount of attenuation the filter provides? If you think of it in terms of a parametric EQ, I'm looking for a way adjust the height of the notch. Substituting resistor values for a given LC combination doesn't seem to affect this.

Are you modeling this, or trial and error building?
How about more info, a schematic, some specifics?

chris

[fernandov]

Are you modeling this, or trial and error building?
How about more info, a schematic, some specifics?

chris

Sure Chris. Coincidentally I dug up an old thread where you had suggested a notch for someone else for an Usher 8137a woofer. That's what got me experimenting since I have the same drivers. You had previously suggested a .25mh coil, an 8uf cap and a 40 ohm resistor. The notch is located between a stock PE 2k crossover board and the Usher.

[donradick]

Then how would you adjust the amount of attenuation the filter provides? If you think of it in terms of a parametric EQ, I'm looking for a way adjust the height of the notch. Substituting resistor values for a given LC combination doesn't seem to affect this.

Jeff Bagby's PCD makes this very easy to play around with.
I think Speaker Workshop can do it also.

[fernandov]

It seems that the LCR filter has the same effect when the components are connected in parallel and wired in series with the woofer vs. being connected in series and used as a shunt across the woofer terminals. My measurements are almost identical, both eliminating the 3dB peak centered around 1500Hz.

The values I am using are a 12uf cap, a 15 ohm resistor and a 1mh inductor. The driver is an Usher 8", 8137A. In both cases the notch is applied between the x/o and driver, and there is no Zobel circuit being used.

That being said, is there a preference for using a series or parallel notch filter in this particular way?

[PWR RYD]

IIRC I read somewhere that you want to use notch filters the following way:

Series notch filter - all three components in series with each other and that leg is in parallel with the driver - when you are trying to notch out something that is in the pass band of the filter (i.e. below the xo frequency in a LP and above it in a HP)

Parallel notch filter - all three components in parallel with each other and that group is in series with the driver - when what you are trying to notch out is outside of the pass band of the filter (ex. a tank circuit on a woofer to notch out a cone breakup well above where you are crossing it)

I hope I remembered that correctly. If not someone please jump in a straighten me out

[Sydney]

This may help:
"Series LC circuits give minimum impedance at resonance, while parallel LC (“tank”) circuits give maximum impedance at their resonant frequency."
from
http://www.allaboutcircuits.com/vol_2/chpt_8/6.html

[donradick]

IIRC I read somewhere that you want to use notch filters the following way:

Series notch filter - all three components in series with each other and that leg is in parallel with the driver - when you are trying to notch out something that is in the pass band of the filter (i.e. below the xo frequency in a LP and above it in a HP)

Parallel notch filter - all three components in parallel with each other and that group is in series with the driver - when what you are trying to notch out is outside of the pass band of the filter (ex. a tank circuit on a woofer to notch out a cone breakup well above where you are crossing it)

I hope I remembered that correctly. If not someone please jump in a straighten me out

That's what I remember Wolf saying - who is pretty much a master at this stuff.

[fernandov]

IIRC I read somewhere that you want to use notch filters the following way:

Series notch filter - all three components in series with each other and that leg is in parallel with the driver - when you are trying to notch out something that is in the pass band of the filter (i.e. below the xo frequency in a LP and above it in a HP)

Parallel notch filter - all three components in parallel with each other and that group is in series with the driver - when what you are trying to notch out is outside of the pass band of the filter (ex. a tank circuit on a woofer to notch out a cone breakup well above where you are crossing it)

I hope I remembered that correctly. If not someone please jump in a straighten me out

Perfect! That seems to make sense and I detect a slight preference when A/B ing the two. Very well said!