ground loop??

[scrappydogg36]

i built a decent 200 watt amp for just music and whatnot. however, when i hook it up to an audio source that is also plugged into the wall, it creates a buzzing sound and blows the fuses. For example, if i have the amp plugged into my laptop for music it works great, but once i plug my laptop into the wall to charge, the fuses blow... so any ideas what it could be or how i could fix this?? 

[Enzo]

Yes, check your mains wiring.  make sure hot and neutral are wired correctly and that the third prong goes to chassis and neither of the other two prongs go there.

[joecool85]

Sounds to me like you might have a short onto your chassis from the mains power.  I'd check it out before using it at all anymore, could be dangerous.

[scrappydogg36]

the mains wiring is pretty simple and there is nothing that could be wrong that i see... the mains fuse doesn't blow and its half the size it could be. Its the dc fuses that blow that are at the input of the amp chip... could it still be the mains if it does this??? (since it is totally separate)  i dont mean to sound like a know-at-all but im stumped...

i tried using a different power cord and for some reason it works better :loco.... the sound stays until past half volume, then the buzz and fuse goes...
i tested the mains and there doesn't show to be any short, all wires are right. HOWEVER, the amp is a  V+  0  V-  design and the 0 volt line shows that it is connected to the chassis. It is not anywhere and this confuses me. could that be the problem?

[joecool85]

the mains wiring is pretty simple and there is nothing that could be wrong that i see... the mains fuse doesn't blow and its half the size it could be. Its the dc fuses that blow that are at the input of the amp chip... could it still be the mains if it does this??? (since it is totally separate)  i dont mean to sound like a know-at-all but im stumped...

i tried using a different power cord and for some reason it works better :loco.... the sound stays until past half volume, then the buzz and fuse goes...
i tested the mains and there doesn't show to be any short, all wires are right. HOWEVER, the amp is a  V+  0  V-  design and the 0 volt line shows that it is connected to the chassis. It is not anywhere and this confuses me. could that be the problem?

I'm pretty sure that is normal for the "0v" to be tied to ground.  Someone may correct me though on that.

Are the input, output and power grounds all tied together?

[scrappydogg36]

yeah all the grounds go to the 0V line.  i just found it odd how 0v can be connected to the chassis when it goes straight out of the transformer (through the filter caps) to the amp...

[teemuk]

Simple guess: One or several of your input or output jacks are not "isolated" type and the "sleeve" terminal connects the chassis.

[scrappydogg36]

checked that too....  i might just bring it into school with me and make my instructor figure it out ha

[scrappydogg36]

ok so completely disconnecting the ground from the outlet solves it, but is this safe??

[joecool85]

ok so completely disconnecting the ground from the outlet solves it, but is this safe??

I wouldn't consider this safe.  But, this does show you it is a ground/short issue that is happening somewhere.  Track it down and you're good to go.

[J M Fahey]

Hi Scrappy.
Unfortunately you have an unstable amp, because of a bad grounding scheme inside itself.
When the input source is "floating", it apparently does not matter much;but when *both* your amp and your laptop are plugged into the wall, you have *two * ground points, the original one and the signal one, and the problem appears.
Take a good hard look at your amp and re-wire the grounds, referring every ground connection to a common ground point, located at the "ground" point at your power supply, using individual insulated wires.
That´s called "star grounding".
Do *not* rely on using any part of your chassis as a ground connection except that single one, which should be sturdily wired to your power cord grounding lug.
We use green/yellow for that, but check your local norms.
Be aware that sometimes you have unwanted ground connections, specially thanks to uninsulated connectors and such.
Good luck.