Keith G wrote:
A week or two ago I bought a little cheapo ss amplifier from Argos for
fun/summer/curiosity/all day long radio, MP3s &c. and wuz so taken with it I
bought a couple more (similarly cheap) components from the same range to go
with it:

http://www.apah69.dsl.pipex.com/show/outbreak.JPG


(OK, I admit it - I was driven by the 3-way, multi-purpose remote control!!
:-)

Anyway, the clarity I'm getting from this little bugger from any number of
different sources is quite exceptional and I'm wondering if the 'digital
volume control' has anything to do with it?

I know the speakers (firewood horns - Pinkies) are 'on song' now and will be
contributing mostly to the sound quality and I am convinced that normal
(carbon wiper) volume pots do the sound no favours whatsoever, but is there
any reason the 'digital volume' (much like a computer soundcard, I guess) is
likely to be helping in a significant way?

(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. 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.

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

S.