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Problem with parametric equalizer
Problem with parametric equalizer
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Old 1st March 2014, 02:36 PM   #1
karha096 is offline karha096  Sweden
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Unhappy Problem with parametric equalizer


I have built an Active crossover based on Rod Elliots designs which works well. Recently, I have attempted to add a parametric equalizer that is based on this design Parametric and Sub-Woofer Equaliser, with the purpose of attenuating a few dBs of an unwanted peak in the response of the woofer.

I have attached a few photos that illustrate what the oscilloscope says, along with the curcuit.

The circuit works (more or less) but it sounds like sh-t... The filter, which has both variable frequency and variable Q, can be adjusted as intended and if I lower the Q-value, the distortion is reduced. As far as I can tell, the attenuation of the sine wave is in the form of a cutoff of the top of the wave rather than a smooth attentuation of the whole wave, and by looking at the square wave, there seems to be heavy distortion.

The op-amps are Burr Brown OPA2134PA, the signal path caps are polypropylene.

I should tell you that I am a happy amateur, with very limited knowledge about what I am doing here, but what the heck, that shouldn't stop me right?

Is there an electronics wizard out there that could help me figure this one out?
Attached Images
File Type: png parametric eq.png (3.9 KB, 84 views)
File Type: jpg circuit_photo.JPG (159.9 KB, 85 views)
File Type: jpg sine.JPG (133.8 KB, 82 views)
File Type: jpg square.JPG (142.6 KB, 78 views)

Last edited by karha096; 1st March 2014 at 02:48 PM. Reason: Added details about components
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Old 1st March 2014, 02:56 PM   #2
mickeymoose is offline mickeymoose  Canada
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What are the powersupply rail voltages? What are the voltages on the display? Are the top traces the outputs?
The squarewave out sort of indicates low LF and high HF gain. E
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Old 1st March 2014, 03:01 PM   #3
Michael Bean is offline Michael Bean  United States
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Is the resistor in the feedback path of the filter circuit really supposed to be that low, 470 ohms? And another thing is the layout of your circuit. Generally you should strive for a more compact layout that avoids long, looping connections that create stray circuit inductance. It appears that the power supply decoupling may be insufficient also. There should be some small value electrolytic caps in parallel with the disc caps, anything from at least 10uF on up to what ever physically fits is appropriate here.


Last edited by Michael Bean; 1st March 2014 at 03:15 PM.
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Old 1st March 2014, 03:08 PM   #4
karha096 is offline karha096  Sweden
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Power supply rails are -15V and +15V.
The top traces are outputs.
Not quite sure about the voltage of the inputs, I will check and get back to you.
What are the causes and effects of "low LF and high HF gain"? (Sorry if this is a stupid question)
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Old 1st March 2014, 03:10 PM   #5
karha096 is offline karha096  Sweden
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Mike, check the link Parametric and Sub-Woofer Equaliser under "Simulated inductors". According to Rod Elliott, the feedback resistor should be 470 Ohms.
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Old 1st March 2014, 03:13 PM   #6
karha096 is offline karha096  Sweden
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Default Ringing

I should also tell you that if I lower the input frequency to a sine wave in the order of a few hundred Hz, there seems to be a ringing och some sort on the output sine wave.
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Old 1st March 2014, 04:24 PM   #7
sgrossklass is offline sgrossklass  Germany
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Looks like bad power supply sag. What do you have for supply decoupling? I wouldn't shy away from 100 F or more for each voltage here. Generally +/-15 V and ground should be low impedance.
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Old 1st March 2014, 05:08 PM   #8
karha096 is offline karha096  Sweden
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In terms of decoupling, i used this recommendtation from Elliot:
"Remember that all opamps need bypass capacitors on the supply pins, and I suggest that a 10uF electrolytic is used from both supply rails to earth for the whole board, and 100nF ceramic capacitors should be used as close as possible from each supply pin to earth."

My interpretation of this was to add 10uF electrolytics from the supply rails to earth and 100nF ceramics from the supply pins of each op-amp to earth. I am a bit uncertain though, should I add additional capacitors? Should the capacitors 10uF caps be 100uF?

The power supply is this: Power Supply for Preamps
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Old 1st March 2014, 05:17 PM   #9
Mooly is offline Mooly  United Kingdom
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Problem with parametric equalizer
A quick check you can do. With NO signal applied the DC voltage on all the IC pins (apart from supply) should be 0.00 volts.

Looking at your scope shots and I suspect a construction error.
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Old 1st March 2014, 11:08 PM   #10
geraldfryjr is offline geraldfryjr  United States
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In Rod's circuit he shows 4 filter sections with Four 10k pots in parallel.

The combination resistance of the 4 pots is 2.5k and yow show that you are using a single 10k pot.
This will change the gain of your boost and cut stage to extreme boost and cut!!

Try replacing the 2.7k resistors on both side of the pot to 10k and see if that helps.

Also if you are feeding a square wave into a filter circuit you will get such waveforms, but, it does look like the filter is ringing and trying to oscillate and this is caused by having too much gain as I mentioned above.



Last edited by geraldfryjr; 1st March 2014 at 11:28 PM.
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