Homebrew CW audio filter for uBITX - thumping/distortion on leading edges

Rob French (KC4UPR)


I've implemented a simple audio band pass filter for CW on my uBITX v5.  It uses 3 sections of a TL974 op amp running off of 5V regulated (the 4th section of the op amp is used a buffer for a line out).

The three sections are identical, and implement a bandpass filter with a center frequency of about 700 Hz (I've attached a screenshot of the audio response as I tuned across a CW signal on 40 meters).

The problem I have is that when a signal is tuned up in the passband, plus or minus a couple hundred Hz, I get a strong thumping or distortion effect on the leading edges.  I've attached an audio clip of listening to a CW station.  The audio distortion is most prominent on dots, and less so on dashes.  I've also attached some spectrum plots of (a) the clip as a whole, (b) dashes, and (c) dots.  In the dot spectrum, I can clearly see that there are some additional artifacts that I assume are contributing to the sound I get.

Does anyone have any recommendations on how I can modify one or more of my filter sections to remove these artifacts?  I designed the filter using components I had on hand.  I don't have a schematic drawn up at the moment, but I'm looking for awareness of general principles I may have overlooked. 

I designed it using this page:  https://www.electronicshub.org/active-band-pass-filter/

I used the Multi Feedback Active Band Pass Filter for reference.  My values are:
C1 & C2 = 11.4 nF (specific values .01 uF metal film caps I had on hand)
R1 = 82k
R2 = 180k
R3 = 3.3k

Any thoughts greatly appreciated!


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