[sdiy] Signals leaking into the PSU?

[Mike Bryant]

> As I suggested: Read Bill Whitlock's papers. He says it better than I can in an email.

I have done in the past.  But his main job is to sell transformers to perform isolation.


> Those papers point out the difference between chassis ground and signal ground. The optimum wiring diagrams show three wires for cables: signal, signal ground, and shield. Shield is yet another consideration that is distinct from safety ground.

Agreed.  Which is one reason why IEC 62368-1 introduced the Functional Earth symbol to identify such equipment.


>Whitlock points out that airplanes have no connection to Earth, and still have properly grounded audio (and other) signals. This is his way of diverting attention from the wall socket.

The whole plane is the earth.  It is more than a large enough electron sink as all free electrons go to the outside of the fuselage.  Otherwise the plane would die on the first lightning strike.


> I'll also point out that some folks with deep wallets run balanced power, with 60 VAC on each of the two wall socket blades (separate from safety ground on the third prong), opposite in polarity to produce 120 VAC in the power supply transformer. In these studios, nothing has signal ground coming from the wall socket.

Not in the UK they can't !   Here the earth pin is intended to be at neutral voltage.  It can wander a few volts sometimes but never to half the line voltage.  The one exception to this is on building sites where portable 235 to 110V site transformers are used for power tools to reduce risk from a cut cable, and the output is centre tapped around ground on a BSEN60309 socket.  But these are not approved for any other use, and specifically not for enabling the use of imported USA mains equipment.

> Basically, if you review all of the papers, none of them suggest using the wall socket *directly* as a signal ground reference.

Agreed.  But that's not the same as no connection.  As I said, in the UK, exposed metal chassis must be connected to the mains earth.  You can choose to isolate all the electronics from it if you wish, but on a Eurorack that's almost impossible, and in any case it will usually connect to mains earth through the mixer unless you use a transformer based DI box.

Also if you perform live music, hold conferences with your own AV, or so on, the location's house electrician is supposed to check the chassis grounds on all your equipment are in place before he turns on the three-phase.

> Behringer probably gets around safety regulations by moving the 120 VAC conversion to a wall wart, where they can just use a third-party product that's already approved. Nothing but low voltage enters the product, and thus the product doesn't need a metal chassis or safety ground.

That may be true under the US regulations, but doesn't meet the current UK IET interpretation of a Class 2 FE device where the whole system is PAT tested, not the individual item.  You can use a wall-wart provided the item doesn't expose the output voltage across a major surface, for example a laptop can have USB sockets with up to 5mA of leakage (which I personally think is too high - the old 0.75mA limit was met by most equipment), but the case should be plastic or double insulated metal.  Trading standards confiscate huge numbers of imported items every year breaking this rule.  For audio equipment as I said the whole situation is being looked into but currently IEC 62368-1 lumps AV with IT.  The problem is if you change the testing for one type of equipment, before you know it things like desktop PCs could come with no earth pin.



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