Re: Digital volume control question....

In article <7_adnbmqDIagjO7ZRVnygA@xxxxxxxxx>, Nick Gorham
<nick@xxxxxxxxxxx> wrote:
Serge Auckland wrote:

I'll accept that pots, capacitors, resistors etc. sound different when
I see objective measurements that indicate that there should be a
difference and the results of properly conducted double-blind tests
that show that there was a difference.


http://members.aol.com/sbench102/caps.html

I've only had a quick look at the above, and the pages linked to it.
However they don't seem to me to be particularly relevant to the question
of capacitors normally having a 'sound' in normal use.

The first thing that struck me about the page was the following which I
quote:

"The signal level was held constant at about 70 volts RMS at 600 Hz across
the capacitors. (for about 26mA signal current). This is probably more
than you would normally expect,..."

My reaction to the last phrase was,,, "indeed!" :-)

I can't recall ever building or using an audio amplifier or tuner that had
anything like this large an *audio voltage* across any of the *capacitors*.
Note the above is 70v RMS *between the capacitor terminals*. i.e. not just
an input where most of the voltage appears elsewhere. I have certainly
built (and use!) amps which have audio voltages this large inside them, but
not between the terminals of any of the signal capacitors.

Given the above, the plots are hardly surprising. Indeed, they seem to me
to agree with work by Doug Self, who simply concluded that we should
avoid significant ac across any of the capacitors in the signal
paths if we used caps like electrolytics. In general, this is quite easy in
my experience.

FWIW The results also seem to agree with measurements I've seen published
elsewhere, and with those which one my honours project students obtained
some years ago in similar tests.

Also, the nonlinearities shown would produce easily measureable levels of
THD. However if you measure many amps that have capacitors in the signal
paths they deliver far lower levels of nonlinearity even when the caps are
outwith any feedback loops. The reason simply being that the ac voltages
on the caps are orders of magnitude smaller than 70v RMS.

I can have a more careful read of the page(s) if anyone thinks it
worthwhile, but I can also comment on another quote:

"The "current monitor" resistor used for all curves was 100 ohms. The
"integrator" was passive, using a 100k and 0.047, 0.1, or 0.47 uF
capacitors depending on the frequency and level. The "X" input to the
scope was fed with an additional series capacitor and adjusted for exactly
"90 degrees" phase shift (typically ran 0.01u into the 10 meg scope probe
resistance) to reduce systemic errors. The generator is absolutely
isolated, being the secondary of a transformer capable of 300 volts into
1k from 15Hz to 1kHz. The bias voltage was added "in series" with
this from an HP regulated and isolated supply. The junction of the current
monitor resistor and the capacitor under test serves as the scope and
system "reference" point."

For a measurement engineer/scientist the above would prompt many questions.
It is not at all clear if any of the results arise due to imperfections in
the system described as we are not given the details of any other tests
used to calibrate or assure the performance of the measurement system. If
caps have such effects, it seems a shame to have so many in the measurement
system without having any info on what effects *they* may be producing...

However can anyone say what domestic amps actually employ ac audio signals
across the signal caps that are this large in nomal use at frequencies like
600Hz? If not, I am not sure why the results show anything about what may
be 'audible' in real amplifiers, etc.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scots_Guide/intro/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html
.

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