Number 4 on our list of 9 areas to pay attention to is making sure there's no DC on the output of the DAC. This might seem meaningless and something everyone just sort of does as a matter of course but how you do this can have a sonic impact and it's important to do it right.
Music is AC and not DC. AC meaning the signal alternates back and fourth producing music. DC is like the voltage that comes out of a battery and while necessary to power our DAC output stage, we don't want any of it appearing on the output of our amplifier. DC being fed into your power amp or preamp isn't a good thing and if you get too much can causes popping noises in the speaker and sometimes even damage to the speakers if it's a power amp you're feeding.
All amplifier stages produce some level of DC on their outputs. You don't want anything more than a few millivolts. In the old days we simply placed what's known as a blocking capacitor on the output of the preamp or phono stage to keep DC from passing. Over the years we've learned that capacitors in the signal path have a poor sonic signature and are to be avoided. This entered an era of what was known as the "direct coupled" days - where we eliminated any capacitors in the signal path.
If the designer chooses an IC op amp, then direct coupling the DAC's easy as most IC op amps have very little residual DC on their outputs (yet another reason why so many use IC op amps). But if the designer chooses the better sounding discrete amplifier circuit then keeping DC out of the output becomes a bigger challenge.
There's not space to go into all the ways to do this, but the simplest way is to use a capacitor as part of the feedback loop so the stage doesn't amplify DC - and then hand trim each unit for best results. This works but isn't our favorite because it still has a capacitor and hand trimming has only limited success and can vary with temperature.
The best way to trim DC from a discrete amplifier is with a servo - which is yet another electronic circuit outside the signal path that constantly monitors the output for DC and takes action to correct it. The trick with designing such a circuit is to make sure the corrective action is very slow - like a second or two to make the change. Handling it slowly keeps anything from affecting the sonic qualities of the amplifier stage.
