Keith G wrote:
"Serge Auckland" wrote


(If it is, I wonder why more manufacturers don't use them?)

As far as I know there are two sorts of "digital" volume control. One is a
digitally controlled analogue attenuator, that changes its attenuation
according to the numeric code it is given. This can be linked to a
physical rotating control, driven from up-down counters etc. The second
sort is a DSP function that multiplies the digital audio signal by varying
co-efficients and outputs a digital signal who's values are an attenuated
(or can be amplified) version of the input. I would imagine that the
volume control of your amplifier is the former as it takes in and gives
out analogue. The latter would require A-D and D-A conversions.


OK. Makes sense.


Your Marantz CD player with the variable output could be of
either sort but I suspect more the latter, as changing the digits before
the D-A conversion would mean that the variable output would work on both
the digital and analogue outputs simultaneously.


And again.


As Don Pearce mentioned, the provision of a "digital" volume control is
often done for reasons of cost. Digital volume controls track left and
right channels virtually perfectly (comfortably within 0.1dB) from full
output to extremely quiet, and don't generate any significant noise when
changing levels. Normal carbon pots can be relatively quite noisy, and
even conductive plastic pots will find it difficult to track both channels
to better than 2dB at high attenuations.


Interesting.


Carbon pots can be as
much as 6dB out at low levels.


Even more interesting (but not surprising).....


This will move the stereo image around as
you change volume settings. An electronic attenuator chip is a lot cheaper
than a conductive plastic pot, and the actual mechanical control can be a
cheap device, as all one is sensing is position.

As to audio quality, a conductive plastic pot will have zero effect on
audio quality. It is virtually pure resistance, and that doesn't have
non-linearities or generate noise (other than Johnson noise, which is not
terribly relevant.)


Oh I don't know - I would have thought a noisy Johnson could be a bit
embarrassing at times!! ;-)


A digital attenuator will have a finite level
before overloading, and will generate some noise and distortion. However,
that can be so low that it is essentially swamped by the inherent noise
and distortion of the rest of the amplifier circuit. The only disadvantage
of a "digital" volume control I can think of is that volume changes can
only be made in discrete steps, typically 1dB at low volumes, perhaps 2 or
even 3dB at high volumes.


-1.25 dB steps across the range on this amp it appears....

Excellent answer Serge - thanks. Makes me think there's even less of a good
reason for manufacturers to avoid them if, as Don says, they are 'properly
implemented...???

I would have thought a standalone 'digital attenuator' (with remote?) would
be a good thing for some valve amp owners - I wonder if such a thing is
available??




I would have thought that as soon as the word "digital" is mentioned
many valve amp owners will run a mile.....

There are a number of passive volume controllers available albeit at
ludicrous prices for what is basically a good quality pot in a tin box;
and not forgetting the multi-tapped transformer controller which will
have all the benefits and cons of a digital controller, but again at
vastly increased cost and price.

S.