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-   -   Need an Active 2nd Order Low Shelf Filter aka Low Shelving Filter (http://www.diyaudio.com/forums/analog-line-level/234448-need-active-2nd-order-low-shelf-filter-aka-low-shelving-filter.html)

elmura 19th April 2013 11:41 AM

Need an Active 2nd Order Low Shelf Filter aka Low Shelving Filter
 
1 Attachment(s)
Hey Guys,
I've been trying to design a simple, linear, active, second order low shelf filter with around 6dB attenuation without much success. Research on the web has proved a massive time consumer without luck.

The design of the low frequency shelf cut specs are around 5-6dB between shelves and a 1kHz corner. Needs to have low phase shift, and low group delay variation and have low noise & THD.

I've got a high pass shelf working based on Linkwitz Shelving Filter, but I need a low shelf cut.
Thanks

AndrewT 19th April 2013 12:38 PM

that is a first order filter.

DF96 19th April 2013 01:21 PM

Quote:

Originally Posted by elmura
Needs to have low phase shift, and low group delay variation and have low noise & THD.

You can't independently choose filter slope and phase shift, as causality links them. For low phase shift you want a first order shelf.

The circuit shown in post 1 does not seem to go with the plot. Circuit is low pass, plot is high pass.

AndrewT 19th April 2013 01:28 PM

plot and circuit are both high pass. They match each other, F-3dB = 8kHz

DF96 19th April 2013 01:45 PM

Sorry, yes you are right.

CharlieLaub 19th April 2013 03:34 PM

2 Attachment(s)
See if the attached circuit works for you...

You can design this yourself, using the LT designer I wrote:
CFL Linkwitz Transform Designer with Monte Carlo Sensitivity Analysis

There are other ways to make the second order shelving filter. You have to implement a biquadratic filter, and you can do that using various "biquad" circuit topologies.

As others have said, phase shift and group delay are a direct consequence of the frequency response and are independent of which circuit you choose. The distortion is dependent on what amplifier (op amp) you use and the resistances in the circuit, etc. and is something that you can tweak.

-Charlie

elmura 26th April 2013 05:12 AM

I've tried the Linkwitz Transform circuit and yes, it works quite well for the response curve. Nice spreadsheet CharlieLaub. Very interesting and looked like some work to put together.

! However ! Regarding the Linkwitz Transform:
- There are too many components in the signal path
- It inverts the signal
- Lots of noise & distortion added by the network

Is there a simpler, cleaner way? Similar to the high pass circuit I posted? Even if it is 1st order.

DF96 26th April 2013 10:21 AM

Yes. Non-inverting opamp, with CR in parallel in the feedback arm and R to ground from the inverting input. Make sure the opamp is unity-gain stable. Unity gain at high frequencies; gain set by resistors at low frequencies, rolloff set by C (in conjunction with R's). Should be in any decent electronics textbook, or Google.

elmura 26th April 2013 12:30 PM

I believe you just described a low shelf boost, not cut.

DF96 26th April 2013 12:55 PM

You want HF cut, don't you? That is what I described. Two equal resistors, with a cap across the feedback one, will give you a first-order HF cut shelf of -6dB.


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