On Thu, 27 Sep 2007 08:50:58 GMT, "Arfa Daily"
put finger to keyboard and composed:

I know that there is always furious debate on the audio groups about this,
but in all the years that I have been mending this stuff, I have never
actually been faced directly with it ...

Yesterday, a boat-anchor Yammy turned up from one of the high-end dealers
that I do work for, and with it was a power cable that the owner wants
fitting in place of the one that Yamaha saw fit to put on when they designed
it. This cable comprises a couple of metres of (20A?) three core rubber
power cable (the sort of stuff that you would use as the flexible 'tail' to
go from a wall plate to a storage heater, or maybe a hot water immersion
heater) plus a very ordinary UK 13A power plug on one end, and a reasonable
quality IEC straight plug on the other. The cost of this lead ? 100 UKP.
That's about $190 at the current exchange rate !! And he now wants to pay
the store to get me to fit it.

Now I'm actually not very happy about modifying anything to do with hot-side
power wiring, for obvious legal reasons, but my real question is about the
number of wires. Originally, the amp is fed with a standard 2 core power
lead. When this three core lead is fitted in its place, should I connect the
earth lead to the metal chassis ? I can't see that this should lead to any
potential safety issues, but as the amp was originally designed not to have
a power ground connected, might not connecting one lead to *more*
power-conducted noise getting in, actually making the performance *worse*
than the owner thinks that he is going to achieve, by his dubious mods ?

This month's Australian Silicon Chip magazine has an article on the
hum and RF noise produced by 2-pronged DVD players. The author(s)
tested quite a few of these players by connecting them to an old
3-pronged amp. They found that audible hum was introduced when earth
current flowed from a player's switchmode PSU to the amplifier's earth
via the analogue signal leads. Additional radiated interference was
attributed to the digital noise produced by the main decoder chip, and
to the 100kHz switching frequency of the PSU.

The only solution recommended by the author(s) was to replace the SMPS
with a linear one. Doing so eliminated the hum and PSU noise.

I have a contact within Yammy, who has direct access to the design boys back
in Japan, so I think that I am going to give him a call anyway, but I would
value the opinions of others on here as well.

Arfa

- Franc Zabkar
--
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