If you are only interested in looking at audio on an oscilloscope, one decent method is to use a "high quality" USB audio interface.
The Behringer UMC202HD is a decent unit that can be found at several music stores for around $100. Behringer enjoys a mixed reputation, apparently if you get a good unit they will work fine "forever."
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My three have been in nearly daily services for 4 years now. They are used as the analog to digital converter as a test instrument and to feed AF into a PC for audio analysis/decoding on a shortwave radio and one is used as an actual musical instrument input device.
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This circuit is used for the oscilloscope input conditioning.
PC Oscilloscope Interface
https://sound-au.com/project154.htm
And these circuits are used for radio to PC, PC to radio
PC To PA System Interface
https://sound-au.com/project133.htm
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I used parts I had on hand so my circuits are slightly different.
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We use similar circuits to feed PC to/from our church's PA system and I carry a pair with my laptop just in case I find myself pressed into service as PA source or recorder.
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The PC to PA system interface will also function as a basic PC oscilloscope interface.
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For those who wish to study audio distortion, Mr. Elliot has some interesting, and downright weird, techniques.
Intermodulation - Something 'New' To Ponder
https://sound-au.com/articles/intermodulation.htm#s4
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And
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Distortion Measurement System
https://sound-au.com/project232.htm
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One can get by with a less then stellar USB AF device. Some well respected AF amplifiers show some surprising weaknesses when tested this way.
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