AW: To earth, or not to earth...

[Eric at Svetlana Electron Devices]

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
guitar pickups.

>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
>not grounded.
>
>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.)

Eric Barbour
Svetlana Electron Devices
Portola Valley CA USA