Is there a way to get rid of a buzz in a state variable filter?

I have built an audio equlizer for my independent study at school. There are a total of four state variable filters. They are summed together using a summing op amp circuit. Everything is working fine, except for a buzz I have. I have not calculated what frequency it is at, but it sounds as though it is between 200 and 400 Hz.

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ejp10878 years ago
Do you have a schematic of the circuit that I could look at? One quick thought might be lowing the gain on some of the op amps or perhaps increasing the supply voltage. Since these are filters, you might be getting a higher voltage from the 200-400 band than the rest and so when you go to amplify that you might be hitting the rails of the op amp. That would cause a distortion in the sound. I've dealt with a lot of filters but not any state variable filters. What you are describing just seems to sound like your signal might be getting over amplified at those frequencies. Decreasing your gain or raising your supply voltage should fix this. I hope this helps.
ejp1087 ejp10878 years ago
What kind of a buzz is this exactly? Does it sound like the sound itself is being distorted, or is it more of a completely separate sound that is being added to the signal?
sound91 (author)  ejp10878 years ago
The buzz isn't actually coming from the filter. Sorry, I misspoke. It seems to be produced when I combine the filters using a summing amp. Here are the schematics for the summing amp and filter. The buzz is definitely not the music being distorted. It is there even if I don't have a source connected.

http://webpages.ursinus.edu/lriley/ref/circuits/node5.html#fig:opinv

http://www.daycounter.com/Filters/StateVariableFilters/State-Variable-Filter-Design-Equations.phtml
ejp1087 sound918 years ago
Hmm...I'm not too sure where the problem could be. If the buzz comes after you hook them up to the summing amplifier then you could try switching the out the op amp incase something is wrong with that. You could also try using a different brand and/or model of op amp. You could ask your teacher or do some research on the internet to find out what op amps people usually use for audio frequencies.

Another possibility could be that your supply voltage is not completely constant. Are you using a bench-top power supply or a small power supply that you plug into the wall? A bench-top power supply should be fine, but if you are using a smaller plug-in one the voltage might not be as constant as it needs to be. You can fix that by putting a capacitor in parallel with the Vcc and -Vcc pins (one on each). This might help. I skimmed it real quick and I saw some stuff that seemed applicable.
sound91 (author)  ejp10878 years ago
For the capacitors, I'm using .1 uF, and for the resistors R is 360 ohm. The frequency reisitors vary by filter. For my feedback resistor in my amp, I'm using a 10k ohm pot that is audio tapered.