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Old 30th December 2010, 01:23 PM   #1
Skorpio is offline Skorpio  Denmark
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Default 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?
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Old 30th December 2010, 01:35 PM   #2
DF96 is offline DF96  England
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If NFB is used from the secondary then a ground reference is necessary, as voltage always has to have a reference.

If no feedback is taken from the secondary then a ground is still helpful for safety reasons; it may reduce interference pickup too.
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Old 30th December 2010, 03:59 PM   #3
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Quote:
Originally Posted by DF96 View Post
If NFB is used from the secondary then a ground reference is necessary, as voltage always has to have a reference.

If no feedback is taken from the secondary then a ground is still helpful for safety reasons; it may reduce interference pickup too.
Seconded on all above..
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Old 30th December 2010, 04:48 PM   #4
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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
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Old 30th December 2010, 06:15 PM   #5
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Quote:
Originally Posted by Skorpio View Post
<snip>
but why ground a galvanically isolated output to ground point?

Is there some performance issues?
Hello,
Safety!
If you want that floating feeling center tap and ground that.
DT
All just for fun!
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Old 30th December 2010, 06:46 PM   #6
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Quote:
Originally Posted by Skorpio View Post

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 There was a good 80 volts or so between the secondary and ground. Now I always ground them.
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Old 30th December 2010, 06:56 PM   #7
Skorpio is offline Skorpio  Denmark
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Ok, thank you for all the good and relevant answers...

My secondaries will now be grounded!
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Old 30th December 2010, 07:04 PM   #8
Skorpio is offline Skorpio  Denmark
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Quote:
Originally Posted by astouffer View Post
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 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?
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Old 30th December 2010, 09:06 PM   #9
DF96 is offline DF96  England
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In DIY the chassis should always be grounded, either directly or via some safe arrangement to avoid a hum loop. There is plenty of discussion about this in other threads.
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Old 31st December 2010, 04:32 AM   #10
THD+N is offline THD+N  United States
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Quote:
Originally Posted by astouffer View Post
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 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).
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