Yesterday we traced the digital audio path from the input of the DAC all the way to the output of the DAC itself, which outputs a series of current steps, and are in the process of converting those current steps to voltage steps.
You can use a simple resistor to make the current to voltage conversion and several manufacturers do, but it's not an ideal load for the DAC chip. What the DAC chip wants to see on its output is zero impedance: no resistor or resistances at all. But in order to convert current to voltage we're going to need something that either is a resistor or simulates one and that simulation is typically handled with an op amp.
As we learned, an op amp has two inputs: a plus and a minus. The plus input is high impedance and the minus input is essentially zero impedance. So we use the minus input to connect the output of our DAC up and voila, we get voltage on the op amp's output. But a few of us nutso high end designers would rather not place an op amp right at this junction because they simple don't sound great.
The first clue that an op amp right at this position is a sonic no,no happens when you start to swap op amps out to see if the sound changes. Oh my, it changes mightily depending on the op amp's characteristics. The faster op amps sound better than the slower ones, generally speaking, but all sound a bit bright and transistory in my opinion. Speculation as to why this should have such an impact on sonics usually revolves around the rapid speed at which the DAC is changing it's current output and how those rapid step responses affect op amp performance.
The whole issue is, however, easy to skirt if the designer wishes to - but unfortunately most don't. Of the few that do, Nelson Pass and I prefer to use a pair of transistors instead of an op amp - others may have some different scheme I am unaware of. Nelson prefers MOSFETS and I prefer bipolars here, but that's a matter of choice that is really dependent on everything else in the chain.
The cool thing about using a simple pair of transistors is that if you connect them properly they are still extremely low impedance but don't have any feedback related hardness issues to contend with as do op amps. Further, we can ensure that they produce low noise, high speed, low distortion outputs that are perfect for feeding into the next stage of our chain: the analog filter.
Tomorrow we get rid of what we don't want.