[sdiy] Hum Problem Kaput

[synthos at xs4all.nl]

> History. The USA old standard was two wire 120V (hot and neutral... no
> ground).  Then they went to a combination of three wire (hot, neutral,
> ground) and polarized two wire... Neutral is larger and cannot be reversed... or
> plugged into an original two wire system.
That happened in Europe too.
If you are in an old building, you should check which of the 2 kinds of juice you have.

> The original system would allow you to reverse plugs... so you might
> have a short
> to hot in one chassis, and a short to neutral in another... and when
> you ToUcH ThEm
> BoTh YoU gEt iT !!!!   Musicians have died of this....
[...]

>> You cannot buy
>> anything similar here in Germany (but OTOH mains voltage is twice as
>> high here).
Doesn't really matter that much for safety issues.


>> A safe way would be to disconnect circuit ground from mains ground
>> (protective earth) but leave the connections between mains ground and
>> anything that has to do with mains voltage (power transformer, front
>> panel with mains switch etc.) intact. Not easy with a Mackie mixer...
> Right... so you let the Mackie be the ground point... unground the
> other gear... then let the audio shields carry the fault current to the Mackie... and
> blow the fuse / breaker.
The problem is that you shouldn't confuse 2 concepts which are widely confused...
There is safety grounding, and there is signal grounding. 
In theory, all your audio gear should be grounded for safety.
But then you will run into problems when you're are building a complex network of
signals, as you will make groundloops in the process of connecting signal grounds.
(these loops can be considered as coils, in which the magnetism of the current in the
wires around you generates electricity flow, audible...)

If you decide to unground some gear that is still connected to the mains, there are
two issues to take into account:
- Your mixer, or central ground, might not be suitable for handling safety ground
well enough.
- If you unplug something from the mixer or central ground (say, if you decide to
patch into another channel, you are leaving unpatched gear ungrounded, making
it unsafe! Or you might get a few sparks while repatching, as you only had a few
volts difference....

It's often quite disappointing to fry your computer, sdiy gear
or your fellow artists with a not too well thought over grounding plan.


Dave Krooshof







> Its 'safe'... but allows a wider range of equipment destruction in a
> fault. OTOH if the gear is noisy it cannot be used for the purpose 
> anyway... so in the words of
> Ian Pace (Deep Purple) "I bought it, I'll bloddy well boot it!"
> 
> Most ground problems come from shitty, ignorant design...
You should always ground the design faults first.
Then the mistakes, then the ignorance, then the equipment.