understanding star grounding

[pacificblue]

Originally posted by AndrewT
Pacific,
would you care to comment?

Hi, Andrew,

My post answered post #105 and its attached schematic, not #108.

Post #108 is one example, where the poster must find the best compromise whether the advantages of shielding the signal wires are bigger than the disadvantages of increasing the capacity between signal wires and ground. Or maybe the increased capacity is also an advantage, because it could accidentally create a convenient RF-filter. There is too little and too imprecise data to decide that from a distance.

Although there is a small possibility that this schematic could lead to hum. If there is a star-point for signal return on the PCB (signal ground, IN- or whatever it is called there), the potentiometer's gound wire and the RCA's ground wire should go there individually and both be connected to that point.

From the feedback circuit's point of view it is also better to connect the speaker return to the PCB's ground. That entails however sufficiently big traces one the PCB for the current involved.

Regards
David

[endia]

hi,
what type of capacitor should i use for 100 nf in Rod Elliot's article?
http://sound.westhost.com/earthing.htm
ordinary film or polyester caps are sufficient or should i use class X2 type?
and also what is the voltage rating for this cap?
thanks..

[AndrewT]

Hi,
if the diodes are doing their job to pass peak and fault currents then the cap should never see more than a few volts.
I have used both ceramic 50V and MKT 100V in this location. It seems to make no audible difference and I cannot see a safety issue with using 25V here.

[pacificblue]

A ceramic or normal film capacitor will be sufficient as long as the bridge rectifier is either working normally or internally shorted. If the unlikely event of a phase to amplifier ground short ocurrs at the same time as the even more unlikely event of an internally open-circuited bridge rectifier, the 10 Ohm resistor will go up in flames anyhow and it doesn't matter, if the capacitor does the same or not.

[AndrewT]

Originally posted by pacificblue
....as long as the bridge rectifier is either working normally or internally shorted. If the unlikely event of a phase to amplifier ground short occurs at the same time as the even more unlikely event of an internally open-circuited bridge rectifier,........

I bit the bullet and tested a similar arrangement to the ESP Disconnecting Network.
Wired the mains Live from the fused IEC socket to the network with the other side of the Network to Safety Earth.
I blew the fuse holder apart when I switched on. The majority of the glass and copper fuse disappeared with a big bang.
The Network components measured as unaffected by the Fault current and looked unaffected.

[pacificblue]

Yes, I didn't want to give another impression. It is so unlikely that four high current diodes go open circuit at the same time, even Murphy's Law may not apply anymore.

If the rectifier is internally shorted, there is virtually no voltage across the resistor and capacitor, so any resistor and capacitor would probably live forever on that condition.

And if the rectifier is in working condition you will get 1 or 2 V across them until the fuse reacts. You could probably even get away with a 1 W resistor and a 6 V capacitor there.

A 35 A bridge rectifier usually survives several 100 A of short circuit current per diode, before it fails. This circuit is as safe as you can reasonably design one.

[endia]

hi,
i'm getting a little obssesive about safety earthing nowadays.
and a bit confused between these two schemas..

in digi's schema safety earth is bonded to chassis..

http://www.diyaudio.com/forums/attac...amp=1213600606

but in original network schema, safety earth is going to rectifier's shorted ac legs..

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

are these two wiring same? or am i missing something?
thanks a lot..

[AndrewT]

the first is wrong it shows the Safety Earth going to a terminal block and from the Terminal block to chassis.

The second is correct but misleading.
It shows the Safety Earth going straight to chassis but the diagram implies it could come off the terminal of the rectifier.

All exposed conductive materials must be permanently connected to Safety Earth.

Take the Safety Earth wire direct to chassis with a short robust mechanical fixture, or even welded but not soldered.

From above the permanent fixture take a connection from the Safety Earth to the Audio Ground. This can be a direct wire connection or as shown in fig2 of ESP through a Disconnecting Network.
BUT,
that connecting wire and/or Disconnecting Network must survive longer than it takes the mains fuse to rupture and for the arc to extinguish. This fault current can run to kA.
The bigger the mains fuse the longer it takes to disconnect the fault.

[Juergen Knoop]

be careful with spade connectors!
Last year I opened a >10 year old amplifier and found that the 6,3mm spade connectors had lost their contact force.
Lots of mains jacks, filters and rectifiers come with spade connectors.
regards

[AndrewT]

I don't count a spade connector as permanent!