Transformer Hum

akunec · 27th February 2005, 11:21 PM

I am building an 8 channel amp with 4 3875s on one side and 4 parelleled 4780s on the other. Each side has its own 500VA 21V dual secondary tranny. I got the first side to work with the 4 3875s and it sounds great. However, I noticed that on the second side, when i turn it on with only the bridge rectifier wired, i get a hum coming from the tranny. I get all the proper voltages at the end of the rectifier. The other side will only make a hum when there is a short circuit or something. Does anyone know what is wrong with it?

Russ White · 28th February 2005, 01:31 AM

Only time I have had a hum is when the lm4780s were not isolated on the heatsink. Make sure you use a isolator as those ICs are not isolated and will carry negative voltage and pickup all kinds of noise.

DonoMan · 28th February 2005, 01:34 AM

They sell those chips in isolated packages... I really would recommend you guys start buying those ones.

Russ White · 28th February 2005, 01:35 AM

No they don't, the lm4780 is only available non-isolated. Unfortunately.

akunec · 28th February 2005, 01:38 AM

When I don't even have the 4780s attached, I still get a hum. They only thing that is wired is the rectifier board.

jeff mai · 28th February 2005, 01:39 AM

Funny thing is, the poor guy doesn't even have any chips attached to his PSU yet. You guys are solving a problem he doesn't have!

Some transformers have mechanical hum. If it's really severe you might claim to the maker that it's defective. Or you may have some DC on the mains. Search on "DC trap" in this forum to see what can be done about it.

DonoMan · 28th February 2005, 01:42 AM

oh. 3886 does but not 4780. sorry :P

27th February 2005, 11:21 PM	#1
akunec diyAudio Member Join Date: Aug 2004 Location: Alberta	Transformer Hum I am building an 8 channel amp with 4 3875s on one side and 4 parelleled 4780s on the other. Each side has its own 500VA 21V dual secondary tranny. I got the first side to work with the 4 3875s and it sounds great. However, I noticed that on the second side, when i turn it on with only the bridge rectifier wired, i get a hum coming from the tranny. I get all the proper voltages at the end of the rectifier. The other side will only make a hum when there is a short circuit or something. Does anyone know what is wrong with it?

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28th February 2005, 01:31 AM	#2
Russ White diyAudio Member Join Date: Jan 2005 Location: Nashville, TN, USA	Only time I have had a hum is when the lm4780s were not isolated on the heatsink. Make sure you use a isolator as those ICs are not isolated and will carry negative voltage and pickup all kinds of noise.

28th February 2005, 01:34 AM	#3
DonoMan diyAudio Member Join Date: Feb 2005 Location: Massachusetts	They sell those chips in isolated packages... I really would recommend you guys start buying those ones.

28th February 2005, 01:35 AM	#4
Russ White diyAudio Member Join Date: Jan 2005 Location: Nashville, TN, USA	No they don't, the lm4780 is only available non-isolated. Unfortunately.

28th February 2005, 01:38 AM	#5
akunec diyAudio Member Join Date: Aug 2004 Location: Alberta	When I don't even have the 4780s attached, I still get a hum. They only thing that is wired is the rectifier board.

28th February 2005, 01:39 AM	#6
jeff mai diyAudio Member Join Date: Aug 2002 Location: Melbourne	Funny thing is, the poor guy doesn't even have any chips attached to his PSU yet. You guys are solving a problem he doesn't have! Some transformers have mechanical hum. If it's really severe you might claim to the maker that it's defective. Or you may have some DC on the mains. Search on "DC trap" in this forum to see what can be done about it.

28th February 2005, 01:42 AM	#7
DonoMan diyAudio Member Join Date: Feb 2005 Location: Massachusetts	oh. 3886 does but not 4780. sorry :P

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