1876 hum - grounding issue?

derMichi · 2nd May 2006, 03:05 PM

Dear all,

recently, i've built a gainclone based on an lm1876, as suggested in the datasheet. it seems to work fine, apart from one strange problem: when I plug in my laptop, i notice an audible, though not very loud 50Hz-hum. as soon as I unplug the laptop from the amplifier, the amp is dead quiet, with no audible hum at all.
The laptop plugged in my faithful old denon-amp doesn't hum at all.

Of course i'm using shielded cable.

Does anybody have an idea? Should I disconnect the grounded shield from the cable on the amp side? But in this case, i would lose the reference, wouldn't I?

I'm thankful for any advice!

Cortez · 2nd May 2006, 07:41 PM

Ground must go from source to the amp.
But if there is a plus shielding wich is also connected to the ground,
then you could try to disconnect it.
Or the earth from the amp. Whats earthed in your system ?

derMichi · 3rd May 2006, 08:07 AM

i guess that the laptop is earthed somehow - i'm not sure about that.
my amplifier - ehm, good question. it's not in an enclosure, and the ground reference is the middle of the two secondaries on the transformer. could be that the amp is floating

but then - it should be grounded via the connection to the laptop.

anyway - i'll ground the amp and see if the problem is gone then.

2nd May 2006, 03:05 PM	#1
derMichi diyAudio Member Join Date: Sep 2004 Location: Munich, Germany	1876 hum - grounding issue? Dear all, recently, i've built a gainclone based on an lm1876, as suggested in the datasheet. it seems to work fine, apart from one strange problem: when I plug in my laptop, i notice an audible, though not very loud 50Hz-hum. as soon as I unplug the laptop from the amplifier, the amp is dead quiet, with no audible hum at all. The laptop plugged in my faithful old denon-amp doesn't hum at all. Of course i'm using shielded cable. Does anybody have an idea? Should I disconnect the grounded shield from the cable on the amp side? But in this case, i would lose the reference, wouldn't I? I'm thankful for any advice!

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2nd May 2006, 07:41 PM	#2
Cortez diyAudio Member Join Date: Dec 2002 Location: Hungary	Ground must go from source to the amp. But if there is a plus shielding wich is also connected to the ground, then you could try to disconnect it. Or the earth from the amp. Whats earthed in your system ?

3rd May 2006, 08:07 AM	#3
derMichi diyAudio Member Join Date: Sep 2004 Location: Munich, Germany	i guess that the laptop is earthed somehow - i'm not sure about that. my amplifier - ehm, good question. it's not in an enclosure, and the ground reference is the middle of the two secondaries on the transformer. could be that the amp is floating but then - it should be grounded via the connection to the laptop. anyway - i'll ground the amp and see if the problem is gone then.

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