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10th April 2010, 08:21 PM  #1 
diyAudio Member
Join Date: Aug 2006

Constant Q parametric equalizers (SVF topology)
Hello DIYaudio community!
I'm currently designing a 3 band parametric equalizer based on the 2nd order state variable filter as proposed by W. Kerman, L. Huelsman and R. Newcomb (StateVariable Synthesis for Insensitive Integrated Circuit Transfer Functions). I like its bandpass response because it is really easy to tune. I've read articles about the use of this filter topology as a parametric equalizer (to adjust Q, center frequency and boost/cut). I think the most inspiring I read is the one by Ethan Winer (Spectrum Analyzer and Equalizer Designs) and I used his circuit as a basis. Since I didn't want to copy his circuit I studied to see how I could improve it. One day I read Rane's Constant Q Graphic Equalizers article (http://www.rane.com/pdf/constanq.pdf) and now I'm obsessed with that idea, since I haven't been able to accomplish it. According to the author "The amplitude function must be entirely separate from the bandpass filter function" so I'm using a modified noninverting SVF and bandpass gain is totally independent from Q. Take a look at the graphs I obtained with LTSpice at various levels of boost/cut, you can see that Q is not constant at all levels. I would like to hear if someone has worked or is working on a similar project to share experiences. M. Fricke (Sorry about my bad english) 
11th April 2010, 04:01 AM  #2 
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Join Date: Jan 2010
Location: Midwest U.S.A.

If I can find it and digitize will post it here. If I do not remind me via private message. Good luck Sum
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12th April 2010, 03:44 AM  #3 
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Join Date: Aug 2006

Thanks Sum, it would be nice to see what real constant q curves look like...

12th April 2010, 03:21 PM  #4 
diyAudio Member
Join Date: Jan 2010
Location: Midwest U.S.A.

Magic Filter
Wish I could claim this one but I cannot. Please thank Mr. Ike Wakibiashi for this one. He put a lot of work into it. Using half way decent parts I have gotten Q=2000 with this set up, WOW!
He gave this to everyone who ask so I pass it freely to this forums users. Standing on the shoulders of giants, makes me a little taller. Ike was one of those giants.
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16th April 2010, 03:12 AM  #5 
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Join Date: Jan 2010
Location: Midwest U.S.A.

Was curious did you get this to work and did it do what you wanted?
Sum
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16th April 2010, 04:18 AM  #6 
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Join Date: Aug 2006

Hi Sum:
Thanks for the circuit, I will start "spiceboarding" the circuit, in previous days I was immersed in the noninverting SVF equations, I studied a Rane schematic (http://www.rane.com/pdf/old/pe17sch1.pdf) of an equalizer that used this topology. With the common NI implementation one is unable to achieve Q values lower than 1 (you can using the inverting SVF but then Q is dependant of gain and viceversa). Q can be less than 1 (using the NI SVF only if): a) R107 and R108 resistors are not equal but that will modify gain. b) RC values are not equal (Rane's trick) however I haven't figured out what effect will this imbalance have on frequency and how to tune for the values needed so I will further study it. 
24th September 2011, 08:30 PM  #7 
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Join Date: Sep 2011
Location: Winnipeg, Canada

Are there any simpler ones than Winer's circuit that allow boosting/cutting, with the frequency and Q all independent(the response frickecello posted is exactly what I need)? I'd like to build 3 single band parametric EQs and cascade them for conceptual simplicity. The amount of opamps used won't be a problem, but I need a design that doesn't use any dual gang pots.
Last edited by dewasiuk; 24th September 2011 at 08:32 PM. 
24th September 2011, 09:14 PM  #8 
diyAudio Member
Join Date: Sep 2011
Location: Winnipeg, Canada

Sorry I forgot to mention that I actually need constant Q as well. I gave the response a better look lol.

25th September 2011, 02:19 PM  #9 
diyAudio Member
Join Date: Jan 2010
Location: Midwest U.S.A.

The trouble is this is a second order function which means two integrators which leads to a dual pot. Do not use audio taper for frequency. Use a quality linear taper instead and my posted circuit will work fine. You could use a digital resistor ladder with some encoder and control circuit or possibly some device set up as a variable resistor. Linear taper dual gang are not so bad as audio taper pots for tracking and matching the linear ideal values.
Good luck and post if you find a suitable circuit.
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