Is this a ground loop ? - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: July 27, 2018, 11:07:54 pm
content: 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.

author: Brumby
author_rank: Unknown
timestamp: July 28, 2018, 07:38:44 am
content: 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 seeexactlywhat you are doing.Just disconnecting an earth wire makes me (and I am sure many others here)verynervous- 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 haveDIEDbecause an earth lead had been cut.

author: borghese
author_rank: Regular Contributor
timestamp: July 28, 2018, 02:52:55 pm
content: 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)

author: tggzzz
author_rank: Super Contributor
timestamp: July 28, 2018, 03:42:17 pm
content: Quote from: Brumby on July 28, 2018, 07:38:44 amYes. You didn't use enough emphasis.

author: lordvader88
author_rank: Frequent Contributor
timestamp: July 28, 2018, 05:04:12 pm
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: July 28, 2018, 05:15:37 pm
content: Quote from: lordvader88 on July 28, 2018, 05:04:12 pmThat's scarcely a schematic.You may find that, as was traditional with rock bands in the 60s, that annoying hum is replaced by an annoying chattering sound. Of teeth, as someone is electrocuted.But that's only a joke. Electrocution usually locks muscles solid.Please take Brumby's sensible advice.

author: lordvader88
author_rank: Frequent Contributor
timestamp: July 28, 2018, 05:25:33 pm
content: No it's as simple as I've stated. The wire is 3" long and 1" from the transformer on the right. The TDA2003 is about 1" from the trans. on the left side.The entire negative rail is completely 1 piece of copper on the PCB. This is a super cheap/simple 10W practice amp. The PCB is the size of a TV remote control

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: July 28, 2018, 08:27:28 pm
content: The usual reason you get induced hum is because the earth plane (typically the front panel or chassis, or maybe a PCB track) has the return path of the (unfiltered) DC power supply running through it. The panel might only be a few milliohms resistance but if the current draw of the output stage is an amp, that's enough to induce a significant hum into a sensitive input. to Good design principles are that a high current supply and its load are earthed to the SAME point on the earth plane.(This is the reason that 'star' earth tags are used on high quality equipment.)Disconnecting the input socket earth stops the hum but is not a safe way to tackle it.

author: tggzzz
author_rank: Super Contributor
timestamp: July 28, 2018, 08:36:13 pm
content: Quote from: IanMacdonald on July 28, 2018, 08:27:28 pmI suspect he will find a "reason" (and I use that word loosely) not to follow that advice.Oh well, we've done the best we can.

author: lordvader88
author_rank: Frequent Contributor
timestamp: July 29, 2018, 12:56:44 am
content: Quote from: tggzzz on July 28, 2018, 08:36:13 pmNo relax, I only disconnected that since I don't have an isolation transformer, but wanted to probe it.That's back on of course.

author: Brumby
author_rank: Unknown
timestamp: July 29, 2018, 02:40:33 pm
content: Quote from: lordvader88 on July 28, 2018, 05:25:33 pmA worded description is not nearly good enough for us to know what's going on. If you aren't comfortable putting together a schematic, then a couple of clear, well lit and in focus photographs are better than what you've offered so far. Also, expect questions arising from the photos.Our concern is safety - and that is why we are being very particular.Quote from: tggzzz on July 28, 2018, 03:42:17 pmYou are right - but this member has been around for a while and has asked a number of questions, so I didn't think they would appreciate being howled at straight up.lordvader88- please understand we want to be sure on any comments we offer when it comes to earth connections - based on others' mistakes and our experiences with hum and ground loops.We can't look over your shoulder and warn you of a potentially fatal action on your part - which is why we are being so particular. Please do not dismiss this.

author: tggzzz
author_rank: Super Contributor
timestamp: July 29, 2018, 03:20:39 pm
content: Quote from: Brumby on July 29, 2018, 02:40:33 pmAll good points.On reflection I think my initial post was a little abrupt, probably based on inferring too much from his moniker. I suspect the "88" implies he is too young to remember the 60s/70s!

author: Brumby
author_rank: Unknown
timestamp: July 29, 2018, 04:08:47 pm
content: I don't think your initial post was too bad. It came across as a strong reinforcement of my warning - which, in the case of a potential safety issue (as this is), is not inappropriate.