• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Grounding outputs?

Why are the negative output terninals normally returned to ground in a NFB amplifier with output transformer?

I can understand why if feedback is used as the reference point is needed, but why ground a galvanically isolated output to ground point?

Is there some performance issues?
 
Hi!

Each node in a good design has a defined potential. If the secondary is left floating it can build up an electrostatic charge from capacitive coupling between primary and secondary.

Some transformers, especially high impedance transformers will have a wider bandwidth if the secondary is not grounded. In such a case connecting the negative end of the secondary through a resistor of a few KOhms will still avoid build up of electrostatic charge and minimze impact on bandwidth

Best regards

Thomas
 
I can understand why if feedback is used as the reference point is needed, but why ground a galvanically isolated output to ground point?

Is there some performance issues?

It may be galvanically isolated but leakage is a big deal. While testing some Hammond 25 watt SE transformers I left the secondary floating and touched one end of the load resistor. It sure did wake me up :hot: There was a good 80 volts or so between the secondary and ground. Now I always ground them.
 
It may be galvanically isolated but leakage is a big deal. While testing some Hammond 25 watt SE transformers I left the secondary floating and touched one end of the load resistor. It sure did wake me up :hot: There was a good 80 volts or so between the secondary and ground. Now I always ground them.

This got me thinking of another transformator issue. Depended on where the live part of the mains is connected in the mains input socket, the potential between ground (mains installation ground) and chassis may vary 15-150VAC.

This potential will, when system is interconnected be equilized between different parts via ground currents. There is quite some sonic difference between good and bad.

Is the any way to determine the correct way? Normally I test with a voltmeter...

I guess the difference is due to capacitive coupling between windings and chassis?
 
While testing some Hammond 25 watt SE transformers I left the secondary floating and touched one end of the load resistor. It sure did wake me up :hot: There was a good 80 volts or so between the secondary and ground. Now I always ground them.
Been there, felt that!

Always ground the appropriate secondary terminal directly or indirectly (bandwidth consideration).
 
Valve outputs aren't too worried by a shorted output, so if a single fault occurs between the 'hot' speaker wire and earth then it's not such a big problem (compared to say an SS amp).

Floating outputs require two wiring faults to occur to short the output, which has its advantages for other applications such as power supplies.

A floating output can also be advantageous for DC power supplies wrt safety of staff from contacting one of the outputs - which is worth understanding. I believe the contrasting experience of astouffer and THD+N relates to the level of parasitic capacitive coupling from OT primary to secondary and the frequency being tested and the equivalent circuit being formed by the human body.

Tim