AW: To earth, or not to earth...
Eric at Svetlana Electron Devices
svetengr at earthlink.net
Fri Dec 5 19:01:32 CET 1997
At 07:57 AM 12/5/97 +0100, you wrote:
>But it is the case with all guitar tube stuff, because signal ground goes
>to the strings and guts of the guitar.
As far as I know, all modern guitar amps have 3-wire power cords with
hot, neutral, earth. This change occurred during the 1980s. Older
amps often have no earth pin, plus the notorious switch which
puts a small capacitor between the chassis and one side of the
AC line! That stupid scheme was responsible for plenty of
electrocution deaths. And it was done because it was a cheap way
of reducing AC hum due to groundloops and induction to the
>In my understanding of VDE/IEC stuff, our types of DIY belong to
>Protection Class I Devices. That is: Possible Hazard of dangerous
>voltage at chassis, because you don't always have the right material
>(trafo, mounting stuff) to get class II. And class I means: Chassis
>ground to supply earth, i.e. all conductive parts that can be touched
>by the user, even potis, if there are metal. Most industrial stuff
>(19" ware) belongs to class II, because they have double insulated
>trafos, special power chords etc., so there is no possibility that the
>metall chasis will get hazardous voltage. In this case the chassis is
>I think: keep all 230 Volt stuff into a separate metal enclosure. Put
>it far away. Ground it. This way you keep also magnetic fields etc
>away. Good mixing consoles do it that way. Use good, shielded trafos,
>that is ground shield between prim. and second. If the low voltage
>side is properly mounted, so there is no possibility ever to connect to
>chassis or power (carefull at chassis openings etc.) the audio device
>may not be grounded. But in this case, you'll loose some hf shielding.
>I'd ground the audio devic, too. But no connection to signal ground,
>thus no hum.
>Best connection between 2 "boxes": Both metal enclosures to the *same*
>supply earth. Banlanced cable between, with shield to chassis at
>*both* sides. No connection between chassis ground and signal ground.
Excellent advice. The most expensive audiophile tube amps use
such a scheme. Some, even using balanced lines. (Remember, this is
home audio equipment.)
>> If there is such a regulation, what about laboratory power supplies?
>> They always have earthfree outputs.
>They are for "expert use" only.
Correct. If the device is not for use by the average consumer, you
can circumvent many CE regulations. (The equipment MUST be
heavily hazard-labeled, and they are now apparently requiring special
safety-type binding posts for any power connection carrying more than
24vdc, so that the voltage can't be touched accidentally without accessing
it with a cable.)
Svetlana Electron Devices
Portola Valley CA USA
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