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Old 4th June 2011, 07:56 PM   #1
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Question Please help: building a notch filter

I am currently building an active version of Zaph's Hi-Vi B3S single driver speakers (Zaph|Audio), with a built-in Dayton DTA-2 amplifier circuit.

Using parametric EQ software on my PC, I recreated the response curve of Zaph's passive filter, and the results sound amazing (seriously; $13 drivers, a $22 amplifier and $5 MDF boxes have no business sounding that good!). Now if I could just figure out how to build the same filter with resistors, capacitors and op-amps, I'd be one happy, cheapass audiophile.

I already found a very well-explained and workable BSC filter circuit (Baffle Step II), so that's one less worry.

I am still at a loss when it comes to the 8.1KHz notch filter. I found many examples of circuits for such filters, but they were designed to completely remove a band of frequencies, whereas I only need to slightly lower the gain. Basically I need 8.1KHz, -8dB, Q=3 (about 1/3 octave bandwidth if I'm not mistaken). The simpler the circuit, the better. Any suggestions?

If I manage to pull this off, then I'll be sure to post a detailed writeup with pics and all.

Thanks in advance.
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Old 4th June 2011, 08:03 PM   #2
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It sounds like you just need one section of a multi-band equalizer. Do a search on "equalizer circuits" and you'll find many. They usually have a pot for boost or cut- either set what you need with a pot, or replace it with fixed resistors.
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Old 6th June 2011, 03:42 AM   #3
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If you are looking to make an active notch filter, I can supply you with a printed circuit board and a bill of materials for creating a notch filter at that frequency and Q. The circuit will only use 1/2 of the PC board, a dual op amp, and a few parts. The response should be like this:
Click the image to open in full size.
The above plot is from a simulation I put together of the as-built circuit using real world components having Fc=8.1k Hz and Q=3. There is a slight gain loss of about -0.5 dB.


I have developed the board design so that a range of active circuits can be created using the same board, depending on how it is populated with components. You can design it for whatever frequency and Q you want, if you decide to modify the notch frequency or depth for instance, or want to use the other half for a HP or LP filter, delay, etc. You will need to solder all the parts to the board and provide a regulated power supply to use it.

Send me a PM if you are interested.

-Charlie

Last edited by CharlieLaub; 6th June 2011 at 03:45 AM.
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Old 7th June 2011, 11:24 PM   #4
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Conrad: thanks for the great suggestion. I knew there was an obvious solution, and that I just wasn't searching for the right one! Looks like I'll just have to build a gyrator filter circuit (Guitar Pedals: Gyrator Filter Calculator), then pad it to get the desired depth.

Charlie: thanks for the offer, but for now I'd rather try "rolling my own" with a generic PCB. I'll let you know if I change my mind.
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Old 3rd May 2013, 04:27 AM   #5
sol2013 is offline sol2013  Australia
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Default PCB

Hi Charlie,
Thanks for your post. I'm interested in your PCB. Please respond to the follwing queries, thanks :
1) Do you also provide the components or at least recommend the appropriate ones ?
2) What notch depth can I get ? I'm looking for at least -60 dB in order to use it in distortion measuring set up at 1kHz
3) Please quote price for the PCB (inlcuding any components, assuming you do supply them) and postage. I live in Australia.

Many thanks Charlie.
Sol

Quote:
Originally Posted by CharlieLaub View Post
If you are looking to make an active notch filter, I can supply you with a printed circuit board and a bill of materials for creating a notch filter at that frequency and Q. The circuit will only use 1/2 of the PC board, a dual op amp, and a few parts. The response should be like this:
Click the image to open in full size.
The above plot is from a simulation I put together of the as-built circuit using real world components having Fc=8.1k Hz and Q=3. There is a slight gain loss of about -0.5 dB.


I have developed the board design so that a range of active circuits can be created using the same board, depending on how it is populated with components. You can design it for whatever frequency and Q you want, if you decide to modify the notch frequency or depth for instance, or want to use the other half for a HP or LP filter, delay, etc. You will need to solder all the parts to the board and provide a regulated power supply to use it.

Send me a PM if you are interested.

-Charlie
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