Please help: building a notch filter

[BillDitwell]

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.

[Conrad Hoffman]

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.

[CharlieLaub]

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:

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

[BillDitwell]

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.