I'm currently breadboarding the reconstruction/anti aliasing filter for the bitcrusher. Last night I noticed heavy distortion when the frequency approached 15kHz. Nothing helped, I even tried buffering the signal.
Today I switched the TL072 op amps for 4558 opamps. And it worked straight away! I then swapped for some other TL072 with other numbers and it still worked.
I then tried using the original TL072s as simple buffers. When approaching 10kHz the sine input turned more and more to a saw wave.
I popped in some other op amps I had laying around. LM1458 worked fine. Two versions of TL074 worked fine. But another batch of TL072 and one TL082, all with numbers fairly similar to the original TL072, failed in the same way.
Failing:
81SHC8M
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| At 20v p-p distortion is seen as early as 10kHz |
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| Heavy distortion at 15kHz |
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| Even worse at 20kHz |
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| Reducing the amplitude to 14v p-p doesn't help |
Working:
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| Working fine at 20v p-p, 35kHz |
I can only conclude that my stash of op amps has been infested by fake or faulty op amps. I tried googling the issue and found this:
https://sound-au.com/fake/counterfeit-p4.htm
https://forum.allaboutcircuits.com/threads/fake-tl082-are-actually-a-cheaper-model.141005/
https://www.muffwiggler.com/forum/viewtopic.php?t=179427
That site compares two TL072s - one with the number 18MDSHY which is assumed fake, and one 85AK87M, which is assumed original.
Funny thing, my *TL082* is numbered 18MDSHY... and the TL072s are 81SHC8M and 61SHC8M (My print is not bad in any way btw). My working TL072 is 58CVY8M and TL074s are
LM1458 is WHY28M. It works fine at 20-30kHz but exhibits the same behaviour at 35kHz. That may be ok for an LM1458 though, I don't know.
Guess I'll have to dump my stash then :-(
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