Monday, 19 March 2012

Digital Volume Control for fixed output DACs

Worryingly, my lovely wife has started taking an interest in the technical side of computer audio.... It appears she has been reading random websites about digital music, and even a couple of blogs. I am not sure this is entirely a good thing – for either her or me, but it did inspire me to write my own blog. I think I shall use her (usually extremely basic) questions as a start. 

So picture the scene as we sat down to coffee this morning (Weanie Beans from London – great coffee). There was a John-Lee Hooker track playing quietly in the background as I’m doing a bit of music software comparison work today.

She asked me:

“They say that you need to turn the volume to maximum on your computer. Does that mean you shouldn’t be controlling the volume on that thing?” 

“That thing” is the iPad, which we use as a remote to control the Mac Mini computer which runs our music software. 

I didn’t pursue the question of who “they” are but I thought it best to encourage her interest in How Things Work by giving my opinion of the topic. It took a while, but here’s what I explained. 

Quite a few digital music commentators say that digital volume control should never be used.  They say this because digital volume control lowers the resolution of the signal.  They are correct - digital volume control or "attenuation" does indeed reduce the resolution.  But I still believe this is outdated advice.  We must remember that volume control in the analogue domain also adds distortion.
The best audiophile music software these days has far more sophisticated volume control than in the past, and no longer impacts the sound quality as it used to.  Modern day software uses a technique called "dithering" that reduces the audible effects dramatically.  I have spent a considerable amount of time comparing the sound quality of digital "dithered" attenuation versus analog attenuation.   If the gain between your DAC and power amplifier is such that you are at a comfortable listening level somewhere around -10dB to -6dB setting on your music player then in my opinion dithered digital volume control is at least as good as the best analogue attenuation.
Secondly, by controlling the volume in the digital domain, you can output your DAC directly to your amp, entirely bypassing the pre-amp.  Now this is where things get interesting!  The 3 main reasons we use pre-amplifiers are:

1) Allowing switching between multiple sources into one amplifier
2) Impedance matching between source and amplifier
3) Volume Control

A pre-amplifier cannot "improve" the signal quality from the source that is connected to, and in my experience most pre-amplifiers degrade sound quality. 

If your DAC has the proper output impedance to work directly with your power amplifier you can plug the DAC outputs directly into the power amplifier and use the digital volume control. If you have a DAC (or any other source) that can do this I would ask that you give that a try one evening and let me know how much better your music system sounds.  Be careful that you start off with the volume setting as low as possible! 

The electronics of most pre-amps does far more damage to the sound quality than does the digital volume control. The advantages (direct transmission) far outweigh the small disadvantages that digital attenuation causes.

Here is some good information on dither: 

Of course, pre amps are around because most of us also want to have access to all of our sources.
But if you are looking for absolutely the best sound quality possible give it a try!

Along with an iPad controlling you DAC, and you never need to leave the sofa.   : )

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