Power supply musings

[Harry Bissell]

Martin Czech wrote:

> > IMHO  (not too "H" I might add) the DC power supply ground should not be
> > grounded. I open my equipment and remove such grounds from the audio path,
> > though I would leave the metal chassis connected if ALL in/outs were
> > isolated.
> >
> > There are often 3 grounds in an audio chain... One is the audio shields...
> > One is the mains ground... and one is the sneaky path from chassis to
> > chassis.
> >
> > If you break the audio shields, you are asking for noise problems...so I
> > break the mains ground... and isolate the chassis from each other (humfrees
> > are a commercial variety of the technique I use, nylon shoulder washers)
> >
> > Noise is gone. Some say that I am at risk this way (safety)... I think the
> > audio cables are more than adequate to handle the fault currents long enough
> > for the breaker to trip. Most Audio gear can have the "3rd pin ground"
> > safely plugged into one central common point. I pulg into the wall at
> > exactly one point, and run a dozen outlets right from that point.
>
> If it is ensured that your transformer has certain isolation properties
> and your internal wiring IS safe, you don't need to have mains ground
> connections at all.  Most music instruments are that way, they have only
> two pin connectors, even if they are 19" metal "bricks".
>
> Modern fault current detectors will sense any differential current
> in the two mains lines, so in case of problems they will switch off.
> Older models just sensed to return current in the mains ground , this
> will of course not work if the faulty aparatus has no such ground.
>
> I think such an interrupter is a very good investment, at least for your
> workbench if not for the whole house.
>
> The bad thing is that you can't kill your wife by throwing the hairdryer
> or shaver into the bath tub any more. (THIS IS JUST A JOKE. DON'T TRY THIS
> AT HOME).
>
> OTOH, in case of secondary high voltage like in tube amps a mains ground
> connection to signal ground is a MUST.

) This is a very delicate subject... I would have the Chassis connected to earth
ground through a capacitor here (like Fender amps)...  It is common for most tube
giutar amps to have a switch to allow "floating" chassis, or ground to one side of
the line or the other... This is a setup for a nasty shock. With the capacitor in
series, you are current limited (you will get knocked on your @ss but not killed)
without it you will be very unhappy. Those amps that have the chassis hardwired to
"neutral" have no place in my studio or stage use. I insisted my bass player replace
a Traynor he had with a hot chassis... "before the fvcking thing kills me..."



>
>
> >
> > The ground of the non-center tapped transformer can't be mains grounded if
> > it uses a full-wave bridge rectifier... half-wave can be. It is really hard
> > to generalize like this because all designs are different. In my 360sysems
> > Midi Bass (rack) ther was a 5V regulator whose tab (ground) touched the
> > chassis. This was audio suicide. When isolated (mica washer) all problems
> > were gone
>
> Yes, half wave. This is the big disadvantage of this simple transformer circuit.
> Not recommended for severall amperes, but good enough for a stomp box.
>
>
> > Watch out for MIDI cable grounds, they should have the shield grounded ONLY
> > at the transmitting end of the cable, Never at the receiver. This can be a
> > fourth ground problem.   Flames welcomed !!!! :^) Harry Bissell
>
> It can be, but I think only with a faulty internal grounding circuit.
>
> m.c.