CV, mux'es and op amps.

 I am working hard to select the appropriate mux and op amp to use for my CV multiplexer.

As stated in previous posts, the OB6 uses a combination of TL062 and 74HCT4051, very common components. When using these (or actually TL072) on my breadboard I see tiny dips of around 30mV-50mV every time the cap is recharged.

I've tried various op amps for buffering the DAC, different buffer op amps and different mux'es. None perform significantly better than what I have now (on my Saleae scope that is). 

Here is a list of what I've tried:

Muxes:

DG4051

CD4051 (worse)

DAC buffer op amps:

TLV9352 

AD711     

OPA2137

TLC071

CV Buffer op amps:

LT1014

TL062

The only thing I have yet to try is the 8ch DAC used by the OB6. It supposedly does not require a buffer.

This is the config I'm going for:

DAC feeding into a 2x gain TL072 op amp using two 10k resistors that feeds a CD74HCT4051 through a 470R resistor. 

The CV buffer is a TL072 with a 1nF C0G cap.

Fake op amps?

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

At 20v p-p distortion is seen as early as 10kHz

Heavy distortion at 15kHz

Even worse at 20kHz

Reducing the amplitude to 14v p-p doesn't help

Working:

TL072 - 58CVY8M

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 :-(

TL0x2 opamps - difference

- TL062 is the low power version of the TL082. 
- TL072 is the low noise version of the TL082. 

So: the TL062 uses less power, but is more noisy than the TL072.

From: http://www.freestompboxes.org/viewtopic.php?f=17&t=328