Ground Loops : Please Explain It To Me

[avdekan]

I recently read a post here that had groung loops discussed in it, and since I didn't want to hijack it i'm starting this one.

I have several problems with understanding this phenomenon. here are the issues I'm having trouble with:

1. I don't think I ever heard a guitar with ground loop problems. how do I diagnose a gound loop (by listening to the guitar)?

2. with all the strats and les pauls in the world that are both grounded via the pickguard shielding and star grounded working fine, how can guitar "ground loop" be a problem?

3. I understand how grond loops can happen in electricity networks when there is actually a ground, but to my understanding the guitar "ground" is actually the negative current that is ultimately connected to the other end of the speaker coil. how can ground loops occure in an ungrounded circuit?

disclaimer:

I know very little about electronics. I know the guitar wiring stuff and not much beyond it. I am in no way responsible for people suffering signal loss or being electricuted to death as a resault of me doubting the 11th commandment: "thou shalt not ground loop your guitar".

[psw]

Hmmm...there probably is a bit of voodoo and misunderstanding...most ground loops are small enough not to have too many bad effects.

Generally though...a single coil pickup is susceptible to noise as it "loops" of wire in the coil will form an anntena to passing radio frequency waves and electromagnetic "noise" about the place.

Now, suppose you have a guitar as quiet as it is going to be...say hb pickups that cancel out the noise inherit in the SC pickup construction...poor wiring can result in creating little "loops" that act as antenna to pick up signals and so create noise. So, that should be avoided in general by grounding everything with the minimum wire to a central point (as in star grounding)...but often such things are of such a small length that it makes little difference I suspect and are blamed for other bad wiring problems.

There are some traps though...I often use shielded cable, but it is only necessary for it to be grounded at one end which avoids a loop and the longer grounding from a pickup to a selector can be of enough length to have qute a bit of effect to allow noise back into the system. Plus, ground looped systems are almost always simply badly planned and executed and that is where a lot of the problems arise as much as any ground loops themselves.

Sorry...perhaps not the best or fullest explanation...

pete

Hmmm...well perhaps checking up on myself...Wiki...my explanation is a bit poor. Components like pickups and pots can vary the resistance and so the ground potentials between grounding wires...therefore unless everything is going to a fixed point there can be differing potentials (some grounds hotter than others) within a control cavity.

As such, noise can be introduced as currents can flow as some grounding points may be "less" ground than others.

[WezV]

my wiring was shockingly bad when i started doing this so i got ground loop issues all the time.

if you want to see the effect then the easiest way is to do it on purpose - have a look at some info on star grounding and then do the exact opposite!!

also, the amp used to make a difference on mine. Some would amplify the ground loop really clearly, some would not... i could never decide if the electrical socket i was plugging into made a difference too

[Paul Marossy]

You can easily get ground loops in power supply grounds where several devices are sharing a common ground, or if you connect the inputs of two different amplifiers together thru a common ground.

While I have star grounded at least one tube amplifier and it made a very noticeable difference, in an electric guitar, I do not consider that to be ground loop territory. Personally, I do not see it happening in a guitar. The weak link in a guitar is the pickups and no amount of sheilding and star grounding is going to fix cheap pickups that are just flat out noisy and like to pick up any noise around them (ask me how I know). Shielding does help to a point, but it will only do so much. The shielding does help to shunt EMI & RFI to ground, though, that's the purpose of it. But then you have metal strings that act like a big antenna, and that's part of the reason why the bridge is also grounded.

Here's a page that might be a good read for you. While based on tube amp design, the same principles apply to any audio circuit. http://www.geofex.com/Article_Folders/lead...n_tube_amps.htm

This is a good page on shielding a guitar: http://www.guitarnuts.com/wiring/shielding/shield3.php