[ericj]

Analog audio is always alternating current - AC. Most audio devices have some DC offset -- this means that there's a portion of the signal that ls always above or below 'ground' even during complete silence.

When the DC offset is low, it's not a big deal. But a large DC offset reduces the incursion or excursion of the driver by moving the 'neutral' position of the driver in or out from where it would normally be at 0 volts.

If the DC offset is large enough, it can overheat the coil in the headphone driver or cause the driver to exceed it's maximum excursion, which ruins the driver.

Capacitors block DC voltage. Some amplifiers have input coupling capacitors because if you have a source with a DC offset of 10mv and an amp with a voltage gain of 11 (and no DC servo on the output), you end up with 110mv of DC offset at the output of the amplifier - which starts to be a problem.

It's unavoidable that an input (or output) coupling capacitor can color the sound, and some people prefer to avoid their use wherever possible.

So, some amps are built without coupling capacitors. And many of those have no DC servo either. There is always some chance of a bad DC offset at the output of the amp with these, depending on the DC offset of the source.

But most good sources don't have any significant dc offset in their output.