# DC blocking caps

Where is the logic when using polarized DC blocking caps in a circuit?

I am not talking about -3dB point, but about the polarity orientation.

For example:

this is taken from USB Headphone Amplifier Design Walkthrough - Phil's Lab #101 on Youtube

How to determine the correct orientation?

AND: shouldn't we use a drain resistor after C29 to discharge it?

• Generally you avoid polarized caps whenever possible. But at 100uF you will probably only get ceramic caps rated at some 16V which might be too low. So you might be forced to use polarized caps. Either that or the design is just old. It wasn't that long ago (relatively) when 100uF wasn't even available for ceramic caps. Mar 1 at 12:04

## 3 Answers

Assuming the op amp Vss is connected to ground, not to a negative voltage, the output of the op amp will have a DC bias, it will always be positive in relation to ground. The positive pin of the capacitor is connected to the op amp output to filter out the DC.

As everyone said, place the (+) to the node with the highest DC potential.

And about C29, it has a direct DC path via the pot to the output of U6B, which can source or sink current, I'm sure.

The logic is you put the cap positive side towards the more positive voltage.

If there is no DC bias over the cap then the polarity does not matter.

• "If there is no DC bias over the cap then...": a polarized capacitor should not be used. The ac signal will reverse bias. Mar 1 at 10:39
• @RussellH a few hundred mV of reverde bias should be handled just fine by a general purpose electrolytic. The average DC bias will still be zero. Mar 1 at 11:15