Yesterday, I told you about Chris Brunhaver's suggestion to replace the digital volume control in our new DAC, the PMG Signature 512, with an analog stepped attenuator at its output.
That's a radical new idea and one that instantly made sense to me, but it took a while to wrap my head around actually changing the entire architecture of the analog output stage to accommodate it (not to mention the daggers in the engineer's eyes who would have to rework the PCBs!).
Let’s walk through why this idea is such a good one and what it means.
Imagine a DAC with a signal-to-noise ratio of 100dB. That means the signal is 100 decibels louder than the noise floor. A strong, clean number. Respectful as all get out. Now, let’s say you’re listening to music—directly from the DAC into your power amplifier, as many folks do— at 30 on the DAC’s scale.
If that control is digital—which pretty much all are—you’re not lowering the noise floor when you turn the volume down. You’re just reducing the signal. The noise and distortion floor stays where they are, and now your signal-to-noise/distortion ratio just got really bad—might be 60dB or 70dB instead of 100. The music is smaller, but the junk underneath it is still shouting.
Doh!
And distortion? That gets worse too. At low levels, it’s like trying to paint a mural with a Sharpie. You just don’t have the resolution.
If you use a preamplifier between the DAC and power amplifier, this is never an issue. It's what I have recommended, and still recommend, for best sonics.
But for those who go straight from DAC to power amp? You're stuck. And unless you're always cranking it to 100—which no one does—you're hearing degraded performance.
Chris’ idea changes that. You let the DAC sing at full power, convert the digital bits with all their available resolution and dynamic range, and then control the volume in the analog domain—right where it belongs.
It sounds obvious in hindsight (like, why the hell didn't I think of that?). But then, radical architectural changes resulting in amazing performance differences are always 20/20 in hindsight.
Tomorrow, I’ll share why this isn't just technically smart—it also sounds better.
