Do you always attach ground to chasis?

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If it's a "hot chassis" design, i.e. the circuit is not isolated from the mains, you don't want to connect potentially lethal voltages to exposed metal where the meat puppets can touch it and get hurt. IIRC such circuits need to be double isolated per UL.

In general you want to bond metal chassis to a safety ground to provide a safe return path for any dangerous voltages that may leak or be present due to some failure, and provide a low impedance shunt to quickly open any protective circuit breakers.

JR

 
I always attach an earth to chassis, but not the audio earth. Remember that the audio earth is both a reference point and a signal return, and the chassis is for safety earth and screening.

I take the incoming mains earth to the chassis directly from the mains connector. To this ground point I also take shields and screens.
From that chassis ground pont I lead out a low value resistor to the main audio reference point which is normally the star point between the smoothing capacitors. All audio circuitry is then referenced to this star point.

These comments apply to line level equipment (preamplifiers, etc.). I have never designed a guitar amp, but would apply the same principles.
 
http://www.gbaudio.co.uk/data/ground.htm
Ground loops

http://www.tcaas.btinternet.co.uk/jlhearthing.htm
Grounding

http://sound.westhost.com/earthing.htm
Grounding

http://www.dself.dsl.pipex.com/ampi.../balanced.htm#7
Balancing and grounding

http://rane.com/note151.html
Grounding and shielding from Rane

http://rane.com/note110.html
Interconnection from Rane

http://www.elecdesign.com/Articles/...cleID=5944&pg=1
Grounding in ADCs

http://peufeu.free.fr/audio/extremi...mplement_2.html
Supply and ground in digital systems

http://www.geofex.com/Article_Folders/stargnd/stargnd.htm
Star grounding in tube amps

http://openbookproject.net//electricCircuits/DC/DC_3.html
Mains shock hazard / electrical safety

There is an interesting free presentation download (zip file) here:
http://home.comcast.net/~zenkai-audio/
'Grounding and Transportable Sound Systems'
 

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