Volume control: analog pre-amp vs. digital feed

default -- Wed, 07/29/2009 - 13:36

I am feeding my Cambridge Azur 840C's DAC with a feed from a Sonos Z80. The Cambridge is in turn going to my Classé CP-700 pre-amp.
The Sonos has a volume control (digital) as, of course, does the Classé preamp (analog). I am currently setting my Classé to a fixed level (64 on its display screen) and I use the Sonos volume control to raise or lower volume as need be.
My question is this: what is the best mix of volume I should be using? Digital vs. analog? I have seen it stated that the digital volume should be set to maximum (0 dB?) and the analog become the only volume control. I've seen it stated that digital volume controls add noise, if poorly designed, as they increase the volume, therefore it sounds like I should minimize digital volume.
Any ideas / suggestions are appreciated.
Thanks in advance, Dave Boyd

Robert Harley -- Thu, 07/30/2009 - 14:18

I recommend setting the Sonos output level to maximum (full scale) and then controlling the volume with the Classe preamp. Digital-domain volume control operates by multiplying each sample by a number less than one. For example, for 6dB of attentuation (a reduction by half), each sample is multiplied by .5.

There are two theoretical problems with this approach. First, every 6dB of attenuation is equivalent to reducing resolution by one bit. A 16-bit system now has the performance of a 15-bit system. If you listen at very low levels, you're throwing away a considerable amount of resolution.

Second, errors can be introduced when performing this math, specifically rounding errors. If the accumulator (the section of the DSP that stores the intermediate values) isn't wide enough, it simply truncates the least significant bits and introduces noise and distortion.

Having said that, however, it is possible, in theory, to design a digital volume control that might outperform an analog volume control (which itself isn't perfect), but such a design would be expensive and requires a serious design effort.

Although I recommend controlling the volume with the Classe, you could try an experiment: set the Sonos to full-scale and turn the Classe's volume control way down. Then set the Sonos volume way down and turn up the Classe's volume control so that you have the same playback level as before. Listen for a reduction in resolution, added noise and distortion, and a truncation of reverberation decay.

Dave Boyd (not verified) -- Tue, 08/04/2009 - 09:14

Thank you sir for advice. May I pass a copy of your response on to the Sonos forum as I don't ever recall this being discussed there?
Thanks again, db