Electronics > Beginners

Why can you use electrolytic caps as coupling caps?

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Yuu:
I've been reading about headphone amps and I see circuits like this by people who know much more than me

and I wonder why this is okay? Everyone knows electrolytics only like to be biased one way at DC.
Like, looking at the input cap for example, you will have input bias current on the op amp and that will give some DC offset. Maybe it's negative or positive. Depends on direction of I_B. Maybe it's normal for it to go into the op amp and so the right side will be negative. That doesn't tell you anything about the voltage on the input side though. Being a newbie, I have no clue what DC offset might be there coming from DAC. So this is something I'm unsure about and would love to hear what the people here think and learn why this is okay.

TimFox:
An electrolytic capacitor can be considered as a capacitor in parallel with a "crappy" diode, so one should pay attention to the polarity of the voltage across it.
A better solution than shown in your circuit is to used a "non-polarized" electrolytic, which is basically two capacitors in series (with opposite polarity).
See:  https://techatronic.com/non-polarized-capacitor-detailed-information-what-is-a-non-polarized-capacitor/
However, if the voltage across the polarized capacitor itself is very small, the unit will still work as a capacitor, so long as the diode current is negligible.
Put a large reverse voltage across the polarized capacitor and it may well explode (literally).
In my audio work, however, I avoided using a normal electrolytic capacitor without a substantial DC voltage across it.

Yuu:
I've seen this same thing in audio power amplifiers for speakers and stuff. One day I'd like to make one and probe the voltages. Anyway, I think you're right. I think they use these electrolytics because they can count on very small DC voltages across them. I need to get around to reading more.

magic:
IIRC common wisdom is that elcos can take maybe up to 0.5V reverse bias without major trouble.
I presume it takes some non-zero voltage to trigger the electrochemical reactions involved in DC current flow.

The input side would be OK unless there is horrible DC offset from the upstream gear.
Input current of NE5532 flowing through 10kΩ to ground is just a few mV - not a problem.

But for output coupling? That looks a little dodgy :wtf:
It certainly robs you of one of the benefits of AC coupling, which is protecting the load from output stage failing short.
Not even sure what's the point of this capacitor at all - amp output is zero-centered without it, with just a few mV offset.

If you are concerned about A1 offset amplified by closed loop gain, put a cap between the stages like in Objective2.

TimFox:
That voltage seems reasonable for the parasitic diode in the capacitor.
Before electrolytic construction was used for capacitors, it was used in "electrolytic detectors" (Fessenden's invention for an RF detector diode) and "electrolytic rectifiers" (for power applications).
On the latter, see  https://www.nature.com/articles/114462d0  for a 1924 technical article.