Op Amp Theory

[camljenkins94]

I doubt that this is where I'm supposed to put something like this, but I couldn't really find any other place to put it that made sense.

I'm sure that this basic topic has been done to death, and I can already hear Dirk Hendrik's eyes rolling all the way over in Amsterdam , but the comparison of op amps is something that I find fascinating. However, I have a theory when it comes to op amp substitution in distortion/overdrive pedals: The choice of op amp doesn't really matter if the clipping diodes are in the feedback loop as in a Tube Screamer. In this circumstance, the op amp is never leaving its linear region, so it makes sense to me that there shouldn't be much of a difference between a TL072 and a 4558. However, if the clipping diodes are going to ground after the op amp such as in a Rat, the choice of op amp suddenly becomes more important because the op amp will sometimes be going non-linear, and the individual characteristics of different op amps will suddenly become present.

However, I still feel like these differences are small or negligible in most cases. The only time that a notable difference can be heard, I feel, is when a high performance op amp, such as an NE5534, is used in place of a very slow op amp, such as an LM308.

What do you guys think? Am I simply being a cork-sniffing moron, or is there some truth to my theory?

[POTL]

Hey.
I conducted an experiment a couple of years ago, I called my two friends, one musician who is a little keen on the effects, the second radio amateur, who sometimes plays music: LOL
We collected several mockups of popular pedals, like OCD and Screamer and JHS Angry Charlie (it was a long time ago, I don’t remember exactly what it was, but it’s not very important) and decided to check how much the sound changes when replacing operational amplifiers, we had NE5532 / JRC4558 / TL072 / TL082 / LM833N popular and inexpensive amplifiers, I can say that no one could notice the difference in the blind test, the differences were very small.
Similarly, we tried several options for limiting, several different types of diodes, LEDs and mosfets.

The difference between ordinary diodes was not great, the mosphets on the background of their tone did not seem to be something new (we did not have germanium diodes), but the LEDs were very pleased. We made the following conclusion that in the constraint its appearance is more important - soft or hard, as well as symmetry and asymmetry, a good result gives the same softer limitation with a resistor, as in the Bluesbreaker circuit.

[Manfred]

Watch the video:

There are differences audible to me.

[phatt]

Don't forget noise problems are a big issue,, A chip that might sound better might also have a poor noise figure.
Phil.

[Frank_NH]

And don't forget the "glitch" associated with response of some op amps when they hit the rails...(via mictester)

" I also avoid the use of some op-amps in feedback clipper circuits - many types tend to "glitch" when switching gains as the diodes go into conduction, which adds a really unpleasant extra distortion artifact to the abuse that I'm already imparting to the signal! The ones that don't glitch include the venerable 4558, the 5532 (though some versions of more dubious Chinese original glitch horribly), the LM358 and some of the more exotic Burr-Brown types."

https://www.freestompboxes.org/viewtopic ... ch#p183728

[camljenkins94]

Absolutely, Frank. The TL072 is well-known for its glitchy behavior under clipping, so it's one that I don't exactly like using for non-linear applications, even though it's fairly quiet. One curiosity of mine is that the Orange CR120 uses TL072s in a non-linear fashion (actually having them clip by hitting the supply rails), yet sounds pretty good.