Balanced to Unbalanced Line Conversion

[SGregory]

Safety.

[Loren42]

Originally Posted by Steve Eddy

Silly question.

What's the point of connecting the signal and power supply grounds to the chassis in the first place?

se

My educated guess is that it would be biologically hazardous for the potential difference (voltage) between signal ground and the chassis to rise too high.

Imagine plugging in a RCA plug and getting a nice shock because the audio ground is 150 volts higher/lower than the chassis ground potential.

[Steve Eddy]

Originally Posted by SGregory

Safety.

What, in case the power transformer's primary shorts out to the secondary?

se

[Steve Eddy]

Originally Posted by Loren42

My educated guess is that it would be biologically hazardous for the potential difference (voltage) between signal ground and the chassis to rise too high.

Imagine plugging in a RCA plug and getting a nice shock because the audio ground is 150 volts higher/lower than the chassis ground potential.

How does the audio ground come to be 150 volts higher or lower than the chassis ground if neither the power supply or signal grounds are tied to the chassis?

se

[DF96]

If you want your family to stay safe you have to design equipment on the basis that a mains live connection will break loose (or mains transformer insulation breaks down) and will then be long enough to touch something inside the amp which is connected to something outside the amp which you or one of your children is touching at the same time. This may seem unlikely, but it happens. Grounding all external metalwork means that fuses blow instead of people getting killed.

Grounding all external metalwork can introduce hum loops, so ground breakers will break the hum loop while maintaining safety.

[Steve Eddy]

Originally Posted by DF96

If you want your family to stay safe you have to design equipment on the basis that a mains live connection will break loose (or mains transformer insulation breaks down) and will then be long enough to touch something inside the amp which is connected to something outside the amp which you or one of your children is touching at the same time. This may seem unlikely, but it happens.

Yes, but there is plenty of consumer audio gear out there which meets Class II specs and doesn't even use the mains safety ground.

And if there's no way for the AC mains hot to come into contact with the power supply/signal ground or signal output, i.e. if no AC mains hot lead was long enough to reach any of those points, then tying the power supply/signal grounds to the chassis would provide no additional safety benefit.

There is however at least one reason to tie the power supply/signal ground to the chassis other than safety, and that's so that the chassis can be more effective at RFI shielding. And if you tie it to the chassis through a pair of diodes, you're not going to get any advantage there as you'll never likely have 0.7 volts of RFI between the chassis and power supply/signal ground.

So if there's no chance of the AC mains hot contacting power supply/signal ground or signal out, but you still have the chassis tied to the AC safety ground, then the better way of tying the power supply/signal ground to the chassis is via a series RC network (say 50 ohms and 0.01uF).

That provides a high impedance at audio frequencies and helps reduce ground loop noise, but a low impedance at radio frequencies so the chassis can better serve as a shield.

se

[BrianDonegan]

This is getting pretty far OT and covered by a million other threads already, but...

Yes, but there is plenty of consumer audio gear out there which meets Class II specs and doesn't even use the mains safety ground.

Such devices typically tie post transformer GND (0V) to the chassis, then fuse the transformer secondaries. Device failure and somehow shorting to chassis would cause the fuse to blow. Tying the audio GND (common) to the chassis then also protects the device inputs/outputs from conducting higher voltages.

[Speedskater]

Originally Posted by Steve Eddy

Yes, but there is plenty of consumer audio gear out there which meets Class II specs and doesn't even use the mains safety ground.

se

But meeting all the Class II specs is not an easy task.
Then you have to have the unit tested by an accredited agency.

[kevinkr]

Originally Posted by Loren42

Just to be sure:

I place a 4.7 - 10 ohm 1/2W - 1W resistor in parallel with the diodes which has always proven sufficient as a ground loop breaker and doesn't leave the audio gnd floating. In some cases I add a small ceramic cap to assure the chassis and system ground are close to the same RF potential. Further in such instances I will also use small ceramic caps (typically 0.01uF) from the RCA ground buses to the chassis.

[DF96]

DIY Class II safety is difficult to achieve. Therefore the alternatives for DIY are grounding or danger. The mods won't allow us to encourage danger, and most of us would not wish to. See other threads - this topic seems to come up regularly.