XLR Pin#1 problem - Bridged LM4780

[GAH]

After further investigation I found the 10 ohm resistor had become disconnected from the chassis earth. This explains why I had to connect Pin #1 to Chassis and Signal Grd to eliminate hum from my amp. Now with the 10 ohm resistor properly connected to chassis earth (from power star) and Pin#1 connected ONLY to signal ground as you stated I'm hum free. All is sounding well.

Peter, thanks again for your help.

[Peter Daniel]

There was a similar discussion on a subject here: XLR Pin 1 question - chassis vs circuit ground

[454Casull]

Hm. I thought the proper way to do it was to connect pin 1 to chassis on both sides and then connect signal ground to chassis at only one point (star) through a disconnecting network/loop breaker circuit.

[GAH]

The chassis is connected to power entry module Earth ground and the center of copper wire between the OGs (of both amp boards) connects to chassis through 10R resistor.[/QUOTE]

Peter, why is the 10R resistor necessary? By including such a resistor is there any compromise to safety should a power fault occur?

Regards
Glen

[Peter Daniel]

This is to separate signal and AC line grounds. 10R resistor isn't neccessary, I've built amps without it and they worked equally well.

What kind of power faults do you have in mind?

[GAH]

Thanks Peter

Andrew T made a comment about safety when using a break resistor. See below. What's your opinion on this?

The 6ohm disconnecting resistor between Safety Earth (chassis) and Audio Ground (Star Ground) does not comply with the mains safety requirement of most countries.

The exposed conductive parts must be permanently connected to the Safety Earth.

The reason for this requirement is to ensure that the mains fuse blows quickly, if the mains live lead during a fault condition contacts and/or connects to other low voltage parts of the equipment.
If fault current prior to the fuse blowing passes from Live to Audio ground then through the 6ohm resistor to Safety Ground back to distribution board earth the voltage appearing on the audio ground will be high enough to kill anyone that happened to touch the equipment at the wrong time. Further, fault current passing through the resistor will more likely blow the resistor before it blows the fuse, leaving the Audio ground at Live voltage and no effective connection to Safety Earth.

I recommend either of two solutions.
The first and safest is direct connection from Safety Earth to Audio Ground. This generally gives rise to hum loops in most connected equipment.
The alternative is to use a Disconnecting Network between Audio Ground and Safety Earth. This Disconnecting Network can consist of a parallel combination of:- Power resistor, inverse parallel coupled Power diodes, Power Thermistor, HF capacitor.
The Disconnecting Network MUST survive LONGER than it takes the mains fuse to rupture and for the arc to extinguish.

[Peter Daniel]

If a resistor does not comply with the mains safety regulations of your country, use thermistor, like it's done in First Watt amps (CL60).

You can also connect directly safety ground to audio ground or use disconnecting network as Andrew recommends.

If you check my official GC building guide, you will notice that I actually recommend thermistor or Disconnecting Network:

http://www.diyaudio.com/forums/audio-sector/123003-commercial-gainclone-kit-building-instructions-post1518378.html

The resistor is for the brave ones only