PP output transformer grounded or not ?

[jogas]

HI,
some push pull schematics are showing the loudspeaker side of the output transformers grounded and some others not.
What is the difference and why a ground connection on the OT?
In principle the output side is (and should be) isolated by the rest of the circuit from the output transformer itself... or I miss something?

 

[EC8010]

When you apply global feedback from the output you need to ground one side of the output transformer secondary. It's also a safety precaution against a fault within the output transformer.

 

[jogas]

I do agree with you that a ground connection is mandatory when feedback is applied... I do not really see the need with a NFB amplifier, why you say it is a safety precaution?

 

[audiousername]

It's a safety precaution, because if there is a short between primary and secondary, B+/HT/whatever is shorted to earth, and a fuse would blow, or something.

Which is preferable to having several hundred volts at the secondary with its exposed terminals.

 

[EC8010]

Imagine an insulation failure between the primary (sitting at 300V, or so) and the secondary while one hand touches the (metal) case of the amplifier and the other touches a loudspeaker terminal...

 

[jogas]

well.... you have convinced me to ground my trannies

 

[plovati]

not only safety

To my knowledge, grounding the output transformet, PP or SE, change also the parasitic capacitance.
If the interleawing of winding is made in a certain way, the secondary grounded coils act as electrostatic screen, beaking the parasitic pri/sec capacitance and hence extend the frequency response.

 

[analog_sa]

hence extend the frequency response.

Maybe. I have tried hard to hear a difference between grounded and not and i couldn't. So i always ground for safety.