Need an Active 2nd Order Low Shelf Filter aka Low Shelving Filter - Page 2 - diyAudio
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Old 26th April 2013, 11:51 PM   #11
elmura is offline elmura  Australia
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I was after a low frequency cut. I have a HF shelf boost, and LF shelf boost. What I'm having trouble finding is an active LF shelf cut. Have you seen one?
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Old 27th April 2013, 12:32 AM   #12
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Location: Minneapolis, Minnesota, USA
elmura,

I think you could adapt my tone control circuit (here: A usable tone control) to your needs, just scale the filter components to the desired frequency and pick a attenuation level, and you're there.

Mike
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Old 28th April 2013, 03:13 AM   #13
elmura is offline elmura  Australia
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Interesting work Michael. I've been playing around with it in Tina simplifying it to my requirements. It works great.
Questions:
1. Is this a biquad?
2. What do you adjust to alter corner frequency?
3. Is it possible to do the same thing with one opamp for bass shelf cut? ie. a simpler design?
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Old 28th April 2013, 03:15 PM   #14
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elmura,

If you take a look at the tech-pub by Dennis Bohn at Rane Corporation (here: http://rane.com/pdf/acceler.pdf), it should answer many of your questions, this circuit is based on that work. The original circuit utilized a dedicated opamp for each filter; by scaling the first filter's resistance/impedance level I was able to eliminate one opamp from the filter section.

1) No, it's not a biquad, it's a non-traditional type cascade of two first order filters that, because of the way thet're implimented here, will provide a steeper than normal slope while limiting phase shift.

2) If you read through the whole thread I started (here: A usable tone control), you probably noticed the original version had filters where the corner frequencies were easy to adjust. After evaluating the first circuit I still wasn't happy with the way it affected the mid-band frequencies. That's when I decided to try the accelerated slope type of filter, but in using those I gave up the easy frequency adjustment the original had.
To change the filter corner frequency, a simple mathematical scaling of the cap and/or resistor values works well, just be sure to scale both of the cascaded filters equally.

3) The circuit is already about as simple as it can be. It requires a summing inverter at the input, a summing inverter at the output, and what ever filter stage(s) in between to provide the transfer function desired, a typical topology for many graphic and parametric equalizer circuits.

As a final comment, the circuit is simple, easy to build and tweek, tolerant of less than ideal physical layout, and best of all when done right won't get in the way of the music.

Mike

Last edited by Michael Bean; 28th April 2013 at 03:25 PM.
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