Ground loop hum? How to identify?

B

bigrigg

Member
Hello,

ch #1 is silent, 2 & 3 have a hum all the time. If I plug straight into the front of the amp, no hum. Didn't have the hum before the RG. Same pedals and power. I can unplug the power supplies, still a hum. I can disconnect the fx loop, still a hum.
If I unplug the ch. switch cable from the RG the hum is a little less, but still noticeable, maybe 40% less.
If I leave everything the same and simply take out the cable to the front of the amp and replace with straight from the guitar it's gone. Ch. 2 or 3 still on, RG on etc. Simply change to direct from guitar.

Would this be considered ground loop hum from the RG? If I perform the ground lift internally (with Ron's guidance) will it matter afterward if it's not the solution?

Thanks very much
 
I had the same issue when I got my effect gizmo. are you using a head or a preamp/poweramp combo? try lifting the audio ground on a cable and see if it goes away. I solved the issue by floating the ground on the preamp- so the send to the preamp input and the preamp send back to the effect gizmo had lifted grounds. that solved it. I accomplished this by making cables with the tip connected but no shield. make sure you label them so they don't get used for another purpose and cause a ton of hum!

anyway, that may solve your problem.

N
 
Hello, thanks very much for the info.
If I'm understanding correctly, you are suggesting to make two cables for the fx loop of the amp head that ONLY have the tip connected. No shield. This would cause the ground to find another path correct?
Couldn't this cause shock if I touch a ground etc?
Also, would the ground lift fix mentioned by Ron internally work just as well?
Finally, what about using an Ebtech HE-2 between the loop cables and the RG?
I would rather not spend the $ on the Ebtech and have to add more cables and gear if possible.
The ground lift option from Ron or cables without ground I suppose.
Any input is greatly appreciated.
Thanks.
 
First things to check:

Is the RG-16 in a rack with other gear? (Especially a power conditioner)?
Have you tried using the RG-16's buffer (or tried bypassing the buffer if you were already using the buffer)?

The RG-16 buffer is one path to ground, the RG-16 chassis is another. If you have a ground through the buffer and also have a ground through the chassis (because it's connecting through the rack rails to other gear), then you'll definitely have a ground loop. Breaking the loop by bypassing the buffer or flipping the ground lift switch (which lifts the chassis ground) will fix.

On the other hand, if you don't have a ground through the rack rails and you're not getting any other ground (via the buffer or the like), then you have a floating ground, which sounds more like buzz than hum.

Anyway, try the buffer first - the results of that test will let us know what the next step (if any) should be.
 
Hello,

I tried both bypass and thru the buffer and the noise in ch 2 & 3 is the same. I also took the RG out of the rack, so it was not touching the rails, or any rack gear. Just the cables hooked up. Still the same noise.

Thanks very much for all your help.
Please advise.
 
Hmm... interesting...

So, what exactly do you have plugged into the RG-16?

You might also want to open up the RG-16 and check the ground lift switch. It may be lifted, and, from what you've told me, you need it to be connected, not lifted. RG-16s are supposed to have the ground connected when we ship them from our shop, but sometimes one sneaks out with the ground lift in the wrong position. (Or, if you bought it used the switch could be in either position, of course).
 
I have opened the RG, but don't know how to identify the ground switch?

I have connected this:

guitar to RG input / 8 out to front of amp
amp switcher connected

RG is not touching any rack gear, it is literally sitting on wood, no chassis ground possible.

There is no fx loop connected. Nothing but into RG and into amp. No effects connected to RG.

Ch 2 & 3 major buzz. Even with volume rolled completely off with guitar.

I can't imagine anything else BUT the RG in this diagram. Is the ground located at U5 and U9 on the circuit board area? If so, they are not connected visually, physically other than the circuit board.

Note* If I hold the plug of the amp switch cable (steel) and touch the chassis of the RG it almost completely kills the hum. I believe I'm completing the ground telling me the switch is lifted. So perhaps, it just needs to be connected.

Anticipation is killing me and hopefully the hum. ;)

Thanks again.
 
The ground lift switch is a small slide switch near the power jack. It's labeled GND LIFT.

Unless, that is, this is an old RG16. Then it doesn't have a ground lift switch. The ground can be lifted, but only by cutting a trace on the circuit board (the trace is right next to the screw nearest the power jack on the PCB). The trace would not be cut unless someone intentionally did it.

From the results you're seeing, the RG-16 is definitely not grounded.
 
I have exactly the same issue, did lifting the ground inside fix the issue? lol i'm just trying to see if i need to disassemble my rack to do this

it's the last thing to try really, as the hum is pretty bad, even after i've bypassed the buffer, swapped cables.

Can any of the items in the loops make it noisy even if they aren't active?

crikey just noticed this was over a year ago, might have to reach for that screwdriver
 
There's a quick test to see if the ground lift switch will help: Unscrew the RG-16 from the rack and pull it out just enough so that it's not touching the rack rails or any metal in the rack. If the hum goes away, then flipping the ground lift switch will help. If the hum persists, the problem is elsewhere.

Anything in any of your loops can cause a ground loop problem, and it will typically cause it regardless of whether the loop is on or off. Other things to look at are your cables - a bad cable can cause a lot of noise that sounds a lot like a ground loop. I just fixed a rig with that exact problem recently.

Things like this have to be debugged one step at a time - removing connected effects until the hum goes away, then addressing the problem with the last device you removed.
 
thanks, i'll step through the cabling one by one this weekend.

and find the offending little bugger.

do you recommend the use of balanced or unbalanced cables?
 
You must log in or register to reply here.
Back
Top