If for some reason your power amplifier puts DC into your loudspeaker you'll wind up letting the magic blue smoke out and killing your loudspeaker. "Magic blue" smoke is engineering humor: it's what we jokingly say when a component blows up and smoke spews from its innards - "oops, we let the magic blue smoke escape". As my wife Terri would say, "nerd!".
Amps should only produce AC and, after all, that's all that music is: electricity that moves back and fourth between positive and negative at a prescribed rate according to the musical source. Once that movement stops and we put out DC, our speakers fry and that's a rare but bad thing.
Yesterday we learned that designers have an easy way to make sure DC never gets into our equipment - they merely place a blocking capacitor on the input - DC cannot get through and everything's safe. Problem is, these blocking capacitors add an unwelcome sonic signature so we eliminate them and call their elimination a feature: Direct Coupling.
The easiest way to direct couple an amplifier and keep the DC from being amplified is to move the input capacitor to another place in the circuit that isn't directly in the signal path. When you do this, the amp no longer makes DC entering the amplifier bigger, it just passes it on (hopefully) at a level that won't damage anything. While this is certainly the most popular method and has a number of advantages: the expense of the circuit doesn't increase, the DC levels are acceptable, it sounds better than where you started and the marketing department can say it's direct coupled - it isn't necessarily the best.
Technically, if you just move the capacitor from one part of the circuit to another, the capacitor is still in the signal path and it doesn't eliminate DC, it only stops the amp from making the DC bigger.
A far better method is to add a DC servo - but that's a lot more difficult and can be something of a black art to implement it properly.
A servo is a simple electronic system that constantly measures the output of the amp for the presence of DC and if it finds any, adjusts the amplifier to eliminate it. Technically it too is in the signal path but the beauty of this system is that if you're careful, you can't hear any difference between the servo in or out of the circuit.
Explaining how this is done is well beyond the scope of our series, but suffice it to say you need to make sure it's slow enough and affecting only the operating parameters of the amp in a way that doesn't affect the sound.
PS Audio, as well as many other fine high-end solid state brands use servos in all of our products to make sure we have no capacitors in the signal path to muck up the sound.
It may seem esoteric, but I assure you, direct coupling has major benefits not to be ignored.
