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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.

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  • \$\begingroup\$ You have almost 5 mA standing current into your headphones - I doubt that this will burn them up. But can your ears deal with the pop when you plug or un-plug the headphone jack? Your threshold of "high quality is not required" is subjective. \$\endgroup\$ – glen_geek Nov 14 '17 at 15:56
  • \$\begingroup\$ Ah. Would a series capacitor remove this pop? How large would this capacitor need to be and how might I calculate it? \$\endgroup\$ – Shanee Vanstone Nov 14 '17 at 16:08
  • \$\begingroup\$ See @Trevor answer for "pop" - he's got it right. The 300 ohm series resistor provides protection, but yields low volume - don't lower this value too much. A 10 uF capacitor value would be OK for low-quality voice. \$\endgroup\$ – glen_geek Nov 14 '17 at 17:12
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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.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ The 10uF ceramic should come with the warning "for low quality audio only" :) And you might not even need headphones anymore :) \$\endgroup\$ – rackandboneman Nov 14 '17 at 20:22
  • \$\begingroup\$ Is there a reason for the value to be 10 uF except the OP mentioning it? I've seen way smaller values used in old examples. I never checked the bass on them, but if I read your answer correctly, that should've been lacking in the result of those older examples, right? \$\endgroup\$ – Mast Nov 14 '17 at 22:17
  • \$\begingroup\$ @Mast if the signal is already biased at ground it is not needed. \$\endgroup\$ – Trevor_G Nov 14 '17 at 23:31
  • \$\begingroup\$ @Trevor Of-course, but the entire purpose of this cap is to remove any remaining DC offset. \$\endgroup\$ – Mast Nov 15 '17 at 9:20
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    \$\begingroup\$ We're in agreement here. What I was trying to convey, and failed horribly, was in saying there may be a risk of unintentional DC on the line. You can either implement filters to make sure this doesn't happen, or place one simple capacitor to decouple the DC noise from the equipment that doesn't like DC. \$\endgroup\$ – Mast Nov 16 '17 at 8:09
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It is advisable to put a series capacitor to bring down the DC offset in the input signal. Otherwise it may damage your headphones. When you add a series capacitance, along with the input impedance of your headphone, it will form a high pass filter. You can design the value of capacitor needed depending on the attenuation you want on incoming audio frequencies. You know 3-DB attenuation happens at the cut-off frequency f = 1/2*pi*RC , for the C you choose.

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