I'm using a true 8-bit DAC to play some low quality audio through some headphones. At the moment I'm just using a 300 ohm series resistor to limit the current to 10ma.
Do I need a series capacitor here to protect the headphones?
The DAC produces voltages from 0V to 3.3V. It does not produce negative voltages. Additionally, the DAC never actually seems to drop below 0.1V.
How do I know if I need a series capacitor to protect the headphones and how do I calculate it's value? Is a 10uF ceramic sufficient? High quality audio is not required, but I don't wish to pointlessly ruin it.
Best Answer
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.
simulate this circuit – Schematic created using CircuitLab