DAC audio - Is a series cap required?
You need the cap to block the DC component. The DC can damage your headphones, put a severe strain on your output driver, and waste power.

The size of the cap depends on the resistance of the headphones. The cap and headphone resistance forms a high pass filter. The RC value dictates how much bass you will get out of the headphones, so the bigger the cap the better.

However, if your headphones are ear-buds, you will not be getting much bass anyway so a 10uF ceramic would definitely be enough. 

The 300R resistor is just a big attenuator, especially if your headphones are like 8R.

Re "the POP" You will get that whether you use a cap or not. In order to improve that you need to pre-charge the cap using a bypass resistor. There will still be a pop when you first apply power to your circuit though.







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