Active notch filter


2013-02-05 10:26 pm
I have recently fired up an active Linkwitz Riley 24dB filter using the PCB provided by ESP (Linkwitz-Riley Electronic Crossover). This works excellent, really... but here is the thing... I know next to nothing about op amps :) and now I want to add a notch filter to cancel out a 4dB peak at 850Hz a baffle step compensation from 460Hz and below.

I have a couple of questions...

Does anyone have a PCB or layout that explains how to do this? I mean, I can read all about the basic filter circuit, calculate the R/C values and all but I'd feel more safe using a printed PCB or at least have a look at a wiring diagram that tells me how to wire the whole thing. I really have no idea whether to put the notch and baffle step circuits before or after my existing crossover etc.

Does anyone have like a active-notch-filters-for-the-mentally-challenged booklet or something? ;)


2003-02-11 9:02 am
Problem with notch filter is that it affects phase (Otherwise crossover design is a piece of cake).

Issue is how to calculate the proper Q (sharp or shallow notch). Flexibility can be achieved from using parametric equalizer with variable Q, so you can tune by ears (instead of calculating).

ESP has parametric equalizer project, but with no control on Q. May be it will suit your requirement, may be not. It does if you have access to simulator.

You need 1x opamp for input buffering (or not, depends on your pre/xover)
You need 1x opamp for output gain.
You need 1x opamp for gyrator (selecting the frequency), either shelving or peaking/notch.

In Linkwitz site there is a collection of opamp circuits required to build active crossover (and its calculation). You can read from there.
I have recently fired up an active Linkwitz Riley 24dB filter using the PCB provided by ESP ... and now I want to add a notch filter to cancel out a 4dB peak at 850Hz a baffle step compensation from 460Hz and below.

To me, a notch filter is designed to ...REMOVE a frequency, ie remove 120HZ hum.

I do not think you want to remove all 850HZ sound.

I am a bit confused when you added, "a baffle step compensation from 460Hz and below"

Are you really talking about a more fundamental issue - maybe better to address that rather than trying to compensate........... Just my two cents.
Well, you are right but..............

What is a Notch Filter?

A Notch filter is a filter that passes all frequencies except those in a stop band centered on a center frequency. A closely related Knowledgebase item discusses the concept of the Q of a filter. This Knowledgebase item focuses on high Q notch filters - the type that eliminate a single frequency or narrow band of frequencies. A closely related type of filter - a band reject filter, is discussed in a separate knowledgebase item. The amplitude response of a notch filter is flat at all frequencies except for the stop band on either side of the the center frequency. The standard reference points for the roll-offs on each side of the stop band are the points where the amplitude has decreased by 3 dB, to 70.7% of its original amplitude.

The -3 dB points and -20 dB points are determined by the size of the stop band in relation to the center frequency, in other words the Q of the filter. The Q knowledgebase item will have additional information, but it is hard to talk about the roll-off points of a notch filter without defining the Q, which is the center frequency divided by the bandwidth.

What is a Notch Filter?
Depends on the notch dept. He wants to cancel a 4dB peak with what would need to be a notch filter with a 4dB dept. Don't see the problem there.

I probably ought to written more completely................

The problem, as I perceive it, is that most folks see a notch filter as I described. I was concerned he was going the wrong way.

What if the 'bump' is asymmetrical? Design a asymmetrical notch filter?

It seems a bit of overkill but I concur with a previous post about using a parametric equalizer.

Though I still submit that going back to see the root cause is a step in the right direction.


2013-02-05 10:26 pm
Hi guys, thanks for your inout.

Let me clarify...

I have a 2-way speaker based on scan-speak 8545k and 2906/9300. I have now successfully implemented an active crossover which also lets me attenuate the tweeter a few decibels to get a good treble and upöer midrange.

The 8545k has a prominent breakup around 850hz that makes the speakers unbearongly sharp, especially with complex soundstages. Therefor I need to add at notch filter that attenuates the frequency band around 600-1000hz with a few decibels.

I could easily add a passive notch filter, but I would rather do this with op amps now that I have gone through the trouble and created an active crossover in the first place.

I think I can get my head around the calculations eventually, but I would really want to do this on a ready-made PCB. Does anyone know where I can get one? Does someone have a pcb layout that I could copy?