Happy new year! I truly hope we all enjoy a peaceful, happy, healthy and prosperous new year. I am sure that would be something we could all benefit from.
2012 was a great year for us, filled with life, adventure, family and a wealth of good things. Hope yours was as well.
I promised that today we'd get started on the output analog stage of our DAC we're learning about. If you've been following along in the series you'll know that we've smoothed out the final steps from our analog signal through our filter and we're ready to send the music through the output stage.
One of the questions we might want to ask at this point is why have an analog stage at all? After all, we have the perfected analog signal ready to go, so why must we put it through yet another stage of electronics and risk mucking up the sound? There are a couple of good reasons why we would want to do this, so let's examine those first.
After we've run the signal through a filter, especially if we've used a passive filter to smooth out the steps, we need to terminate that filter into a high impedance load and then deliver it to the outside world through a low impedance driver. It's the same sort of issue we might find with a power amp: we have a weak analog signal from a preamplifier that we need to gain adjust and then provide enough power to drive a pair of loudspeakers. In this case, of course, we're not driving loudspeakers, but rather connecting cables which place demands on the stage driving them.
The second reason we need an output stage is gain. In most cases a well designed current to voltage converter (the part that converts the current output of the internal DAC to a voltage), doesn't produce enough gain to get us our minimum output voltage of 2 volts and, in many cases, 3 to 4 volts on the output. Here the last analog output stage can give us that added lift in gain.
The third reason we'd like to have an output stage is common mode rejection (CMR). In many DAC designs, including our own, we use a balanced output DAC. The advantage of using a balanced output DAC (essentially two DACS), is a subject we've discussed in past posts at length. In brief, CMR is the ability of a device to take two inputs and reject whatever is common to both inputs. In this case, distortion can be a commonoccurrenceand using this balanced approach allows us to significantly reduce 2nd order harmonics that were generated as part of the conversion process of the DAC.
So there are a number of advantages to having a DAC output stage and pretty much all DACS feature one. As I've said in past posts, the majority of high-end DACS as well as all commercial DACS use IC op-amps for this task, a practice we do not follow for sonic quality reasons. Certainly a few DACS have taken the upper road with discrete output stages and this is to be applauded.
Now that we understand why we need this stage we'll dig into what it's all about tomorrow.