Electronics > Beginners

Is this a ground loop ?

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lordvader88:
I have a cheap/simple guitar amp here that has a quite noticeable 60Hz hum, and it's not dependent on the volume setting, or the house it's in.

So I took it apart. The mains transformer is bolted to a metal plate that's also the front panel (with the knobs/inputs/etc). Mains earth GND is connected to it, but I unhooked that for the sake of using my Tek2430 on it. And I can quite clearly see the hum, it's +/-25mA on the speaker, not a simple 60Hz sine wave.

There's 2 DC COM connections to this metal plate as-well. 1 is from the TDA2003 amp IC, it's metal package tab is bolted to the plate.

The other is just another a wire on the other end of the PCB ground trace, it's bolted to the plate.

So when I unhooked that wire, the hum goes away and it's no longer on the scope (fades into the microV noise anyways).

The metal of the transformer is ohmically OL from pri/sec. windings.


So why would that wire even be there ? Just as over-the-top safety in case the TDA2003 came loose ?

I'm not hooking it back.

Brumby:
We need to know exactly what earthing connections you have in that Amp.  What goes where and what you are proposing to never reconnect.

There has been some very ignorant advice given in some quarters - even published in magazines - on how to stop that annoying hum - and they invariably turn the amplifier into a potentially lethal weapon.

Please be very clear in drawing up a diagram for us - so that we can see exactly what you are doing.


Just disconnecting an earth wire makes me (and I am sure many others here) very nervous - and until we can be certain of the risk - please treat your Amp as if the chassis and circuitry were mains live.




If you think I'm being over the top about this - I'm not.  People have DIED because an earth lead had been cut.

borghese:
Many people think that the negative rail and GND are the same thing; in any case you must connect the case to GND and eventually also the negative rail connecting it only in one point (the power amp return)

tggzzz:

--- Quote from: Brumby on July 28, 2018, 07:38:44 am ---Just disconnecting an earth wire makes me (and I am sure many others here) very nervous - and until we can be certain of the risk - please treat your Amp as if the chassis and circuitry were mains live.

If you think I'm being over the top about this - I'm not.  People have DIED because an earth lead had been cut.

--- End quote ---

Yes. You didn't use enough emphasis.

lordvader88:
The PCB has 1 negative rail off the FWR, it is connected to the metal heatsink/chasis/front panel in 2 places, 1 by the TDA2003 tab, the other a wire. Mains earth is also connected there.

"the other a wire" is what I don't want to hook back on, because the 60Hz hum goes away.

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