Bypassing electrolytics. How do you know when you can?

[honkyjonk]

I know it has something to do with the amount of DC (if it's DC they're supposed to be filtering) present at that point in the circuit. If there is a small enough amount, then having an electrolytic at that point in the path is unnecessary, right?

So, one would measure this in reference to ground? And at the positive leg of the cap? Unless it's part of the negative rail network? Hmmmm.

What I'm unsure about is exactly where to measure the DC on the cap. And also, what amount of DC is generally okay to be present.

[Knights Who Say Neve]

Why do you want to pass DC in that part of the circuit?

[honkyjonk]

Well, the story is, a number of my mixing board input channels now have I think 18 of the 28 electrolytics bypassed. This was possible because of less filtering needed after a chip upgrade. Admittedly I copied Jim Williams' mods that were in the mixing board I bought and the upgraded channels sound good, but I have enough chips to do at least 4 of the buss strips too. They're National LME47910NA's

I guess I was just thinking if there are a bunch of lytics that don't need to be there in the input strips after an upchip, the same would be true for a number of the buss strips.

[vorian]

Well, the story is, a number of my mixing board input channels now have I think 18 of the 28 electrolytics bypassed. This was possible because of less filtering needed after a chip upgrade. Admittedly I copied Jim Williams' mods that were in the mixing board I bought and the upgraded channels sound good, but I have enough chips to do at least 4 of the buss strips too. They're National LME47910NA's

I guess I was just thinking if there are a bunch of lytics that don't need to be there in the input strips after an upchip, the same would be true for a number of the buss strips.

So your channels are working/sounding great? Don't touch anything! There's no reason to go removing them unless they're causing some sort of problem. If you've already completed whatever mod Jim suggested/used, chances are good that all the parts still remaining need to be there. Again, I can't think of a compelling reason to remove caps unless they're causing a problem. It may be true that the chip upgrade requires less filtering, but that's not the only thing electrolytic caps are used for anyway.

[honkyjonk]

I'm not changing the input strips. I just thought I might be able to bypass some caps in the buss strips, as I'll be upgrading the chips there too.

[Max Gain]

Eletrolytics can do several different functions.
A. They act as a battery in parts of the circuit needing bias but not electrically near B+ - rails. This can provide head room and gain linearity.
(also can minimize "thump" powering on and off.)
B. They can be used as noise filters, quite often the High pass culprit for
the 20hz in the "20 to 20K" spec.
C. They block DC while passing signal; something like a Xfmr's function.

All that being said, they can really not be the optimum circuit component for audio. but they are cheap and can make test specs look great!!