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Grid hum despite line transformers
Grid hum despite line transformers
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Old 8th January 2019, 10:28 AM   #1
JanErik is offline JanErik  Finland
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Default Grid hum despite line transformers

I have an active crossover for bi-amping and have now built an input selector/volume control using ESP32, relays, opamp buffer and PGA2310.
Originally I was going to feed it from the filters grid transformer. It employs a dual half-wave rectifier to double supply voltages and SMPS modules down to +12V and +5V (in series). -12V linear. This, though, resulted in grid hum.

With a separate AC wallwart, it is silent.

Now once again I tried to feed it from the crossovers transformer and connected a line isolator/transformer on the lineout, but it still hums! How is this possible? The signal comes out clear otherwise. I can not see how there would be a ground loop anymore.


On another forum I got the suggestion to put 10nF commutation capacitors over the rectifier diodes, could this help?

Last edited by JanErik; 8th January 2019 at 10:56 AM.
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Old 8th January 2019, 11:07 AM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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With half wave rectification you need smoothing capacitors twice as big.
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Old 8th January 2019, 11:27 AM   #3
DF96 is offline DF96  England
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A ground loop is not the only possible source of hum. Why not show us the PSU you built?
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Old 9th January 2019, 07:29 AM   #4
JanErik is offline JanErik  Finland
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Quote:
Originally Posted by nigelwright7557 View Post
With half wave rectification you need smoothing capacitors twice as big.

Yes of course, and it is silent with a separate 18VAC wall wart.
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Old 9th January 2019, 07:55 AM   #5
JanErik is offline JanErik  Finland
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Quote:
Originally Posted by DF96 View Post
A ground loop is not the only possible source of hum. Why not show us the PSU you built?

I cant draw at the moment but it is quite basic - has two half wave rectifiers; 1N4001 and respectable size electrolytic plus 100nF ceramic.

Negative side uses 7912 regulator and that feeds opamp and PGA2310.

Positive side uses one chinese SMPS/buck converter to 12V (opamp and PGA2310) and from 12V another buck converter down to 5V for digital logic.

Electrolytics and ceramics on all rails after regulators. 100nF on all chips' supply pins.
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Old 9th January 2019, 10:48 AM   #6
DF96 is offline DF96  England
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If you cannot show us what you built then we cannot do remote diagnosis. In many cases hum from a PSU is caused by poor grounding - most people seem to naturally get this wrong even though there is plenty of information around on how to get it right.
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