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Duties of the output stage

— By Paul McGowan —
The analog output stage of a DAC is a critical element in the DAC's construction. It has a number of duties to perform and there are seven ways from Sunday to perform them: all important and all critical to the sonic success of the DAC. From the output of the current to voltage converter we run the stepped signal through a low pass filter that cleans up all the jaggies we had from the DAC output. Now we have a rather limited bandwidth analog signal we need to pass through the output analog stage and make sure we do the following:
  1. Amplify the signal to where we want it in terms out loudness. This ranges anywhere from 2 to 10 times what comes in, with an average closer to 3.
  2. Remove any common mode distortions and noise - that is if we are using a balanced DAC configuration
  3. Add any additional low pass filtering the designer may wish to add
  4. Make sure there is no residual DC on the output to feed into a preamp or amplifier
  5. Buffer any analog style volume control if the DAC has one
  6. Provide sufficient linear performance to never negatively effect the performance
  7. Provide a single ended and balanced output (if the DAC has this)
  8. Have output current capabilities sufficient to drive long lengths of cables without degradation
  9. Mute the output for any pops or startup noises occur
This is an admittedly long list of tasks that the output analog stage has to contend with. I would guess most of you hadn't really put all this together in your understanding of what this entails - which frankly isn't musch different than that of any high-end preamplifier. In fact, most of the work designers do to make sure their output stages are good sounding and well serving of their products were developed in preamplifier designs they've produced. In our case, we've designed many models of preamplifiers and our DAC output stage, with but few exceptions, was refined over decades of designing preamplifiers which have the same basic needs and requirements as does this all so critical output stage. We'll cover each of the 9 items, one per day, in some detail.

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