🚀 go-pugleaf

John Chapman writes:
>>2) A musician's audio power amplifier hand made somewhere in
>>   France. There is a HUGE transformer to step down the AC and
>>   it only has the one 220 v primary winding. Probably needs
>>   250 Watts of power.

Philip Peake writes
> I would be interested to see *any* power supply which needed
> one side grounded ...
> 
> Since all of the equipment described (PC excepted) has transformers,
> there is no problem.

  A word of warning here:  Mr. Chapman didn't mention the specific type of  
musician's amplifier.  It is a common practice for manufacturers of electric  
guitars and amplifiers to electrically connect the guitar strings and bridge to  
the return (shield) of the cable, and to connect this return to ground inside  
the amplifier.  The result is a nice ground plane behind the guitar (formed by  
the musician touching the strings!) that eliminates induced hum.

  This is fine as long as the amplifier is plugged into a properly-grounded AC  
circuit.  But if some roadie wires up the stage power system wrong, it's only a  
matter of time before the guitarist touches something that is really grounded  
while touching his strings.  The results, needless to say, can be tragic.

  To make matters worse, some amplifiers simply connect the signal return to AC  
_neutral_ rather than ground;  That's bad enough for power systems with a few  
annoying ground loops, but it would definitely spell danger if it were the case  
for the handmade amp described above used with U.S. 220VAC.

  The bottom line:  check out the amplifier.  Make sure that the hot AC wires  
don't get past the transformer (internally or externally).  And make sure that  
ground wires are actually connected to ground, and are isolated from the amp's  
neutral.

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