PP output transformer grounded or not ?

jogas · 2nd May 2005, 12:07 PM

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 · 2nd May 2005, 01:30 PM

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 · 2nd May 2005, 01:43 PM

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 · 2nd May 2005, 01:49 PM

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 · 2nd May 2005, 01:49 PM

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 · 2nd May 2005, 02:34 PM

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

plovati · 2nd May 2005, 02:39 PM

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 · 2nd May 2005, 02:58 PM

Quote:

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.

2nd May 2005, 12:07 PM	#1
jogas diyAudio Member Join Date: Jun 2004 Location: Milan	PP output transformer grounded or not ? 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?

2nd May 2005, 01:30 PM	#2
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	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. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

2nd May 2005, 01:49 PM	#4
audiousername diyAudio Member Join Date: Jul 2004 Location: Melbourne, Victoria	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. __________________ Jason

2nd May 2005, 01:49 PM	#5
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	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... __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

2nd May 2005, 02:39 PM	#7
plovati diyAudio Member Join Date: Mar 2005 Location: near Milan	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. __________________ Piergiorgio

2nd May 2005, 01:43 PM	#3
jogas diyAudio Member Join Date: Jun 2004 Location: Milan	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?

2nd May 2005, 02:34 PM	#6
jogas diyAudio Member Join Date: Jun 2004 Location: Milan	well.... you have convinced me to ground my trannies

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