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!