Troubleshooting EQ/bandpass circuit

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Uncle Peter
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Troubleshooting EQ/bandpass circuit

Post by Uncle Peter » Wed Feb 17, 2021 11:46 am

Hello,

I'm in the process of making a graphic equalizer module (hopefully 5 or 6 band) for my rack. After a bit of research into different kinds of filter circuits, the following two pole active filter from Dale's Homemade Robotics stood out to me:
graph eq.png
graph eq.png (24.68 KiB) Viewed 133 times
The website even has a nifty calculator for calculating the resistor values based on a fixed capacitor value.

https://www.wa4dsy.net/robot/bandpass-filter-calc

I decided to breadboard up this circuit for testing purposes. For my initial test, I decided to use a TL072 op-amp powered by a 12VDC external supply with the following parameters:

Center frequency: 1kHz
3dB frequency: 233Hz
Q-factor: 4.3
C1,2=0.1uF (ceramic)
R1= 6.8kΩ (trimpot)
R2=190Ω (trimpot)
R3=13.6kΩ (trimpot)
Voltage Gain: 1

I inputted an audio signal from my rack (a "strum" from 2HP Pluck) and outputted the filtered audio to my rack's audio out, as I would any other module.

However, there was no discernable difference in the filtered audio wrt the original sound. At first, I thought the op-amp was fried, but retesting with a TL084 gave the same results. Next, I tried switching to electrolytic capacitors, but the same thing. I even tried changing the parameters for a higher voltage gain, but the audio still sounded unfiltered, just a bit louder.

This has me thinking that maybe I need to add something to this circuit in order to hear the results. I should note that, as part of my EQ circuit as a whole, this "filtered audio" will eventually be fed into a voltage controlled amplifier, before being mixed together with the rest of the filtered audio from the other bands, via a summer circuit.

Along those lines, I decided to take the output of this circuit and pass it through a unity amplifier (simple inverting op-amp config with 100k resistors). The output oddly was static, overlaid with audio that sounded quasi-filtered.

It appears something isn't quite right with the way the audio is getting filtered, but I cannot seem to figure it out.

Does anyone have any ideas as to what the problem could be?


Edit: to fix R3 resistor value from 13.6 to 13.6k
Last edited by Uncle Peter on Wed Feb 17, 2021 3:05 pm, edited 1 time in total.

djs
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Re: Troubleshooting EQ/bandpass circuit

Post by djs » Wed Feb 17, 2021 12:30 pm

Maybe it's just me, but the cap values seem a little high... what if you switched them to 10nf or lower?

Also, the "oddly static" output sounds like some sort of clipping? Can you do a scope trace before and after?
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neil.johnson
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Re: Troubleshooting EQ/bandpass circuit

Post by neil.johnson » Wed Feb 17, 2021 2:12 pm

Not sure if it's a typo but R3 should be 13.6 kilo ohms.
Nice little Deliyannis filter :tu:

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Re: Troubleshooting EQ/bandpass circuit

Post by neil.johnson » Wed Feb 17, 2021 3:31 pm

What does the output look like on a scope?

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Re: Troubleshooting EQ/bandpass circuit

Post by SphericalSound » Wed Feb 17, 2021 7:43 pm

You can look at the monotropa circuit by reverselandfill. It has the resistors and caps values for a lot of bands and IIRC he uses that same bandpass typology.

Finaly the monotropa has a last opamp that feedback the output into the input and turns the whole thing into a evil analogic warming and destruction thing. I use it in every patch

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Uncle Peter
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Re: Troubleshooting EQ/bandpass circuit

Post by Uncle Peter » Thu Feb 18, 2021 12:54 pm

SphericalSound, thank you for that info, will definitely be checking out that circuit. Need to hammer down these values before I begin building.

Neil, thanks for spotting that typo, I have fixed it.

djs, I took your advice and swapped out the cap with a lower one (I only had 1nFs on hand and unfortunately no oscilloscope). It did reduce the static I was hearing down to a hum now. That got me thinking that it was perhaps a power supply issue, so I swapped out my 12VDC supply and took +-12V directly from my eurorack, and it fixed the problem. I could now hear the filtered audio with no background noise or static! Seems that was the problem all along. Thanks for the tip!

I did also have a question about band spacing for a graphic equalizer. Since module size is limited, I've decided that I could get away with 6 or 7 bands at the most on the module. Is there a generic rule for how to properly space the bands? I've heard of overlapping the bandwidths, but I think I would have to reduce the Q-factor of the filter to accomplish this.

Here's a quick frequency response I put together showing the gains of band that I chose: 100, 200, 400, 800, 1600 and 3200Hz bands with a Q-factor of 1.7, along with 10nF caps and resistances ranging from 1.8k - 550kΩ. A -3dB dashed line is also plotted to reference bandwidths.
bode-2poleband.png
Again my main concern is trying to balance numbers of bands with Q-factor, such that each band will actually be effective at filterting, yet when all the bands are summed together, the original audio signal is replicated as much as possible.

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