ground loops from DI

[tdbajus]

I was trying to record my guitar out of my Peterson strobe pedal tuner as a DI, as well as play through my guitar amp. As has happened several times in the past (in fact, every time) there is an ungodly amount of hum created as soon as the straight line to my MIO2882 gets plugged in.

I used a ground lift on the power strip that both my pedal board and amplifier are plugged into- while this helped, there was still too much hum to make it useable.

I'm not sure what I have done wrong- the tuner has a built in DI (loved by some on this message board) and the amp and pedals are dead quiet until they get plugged into the MIO.

I'm going to Home Depot tomorrow, and will buy a bucket full of ground lifts, but I get the feeling it is something else that I am totally overlooking.

[Dakota]

Is your DAW i/o *and* your pedalboard and amp all plugged into exactly the same wall circuit? They likely should be. Probably computer as well. Anything making a connection needs to see as close to the same ground as possible.

Guitar amp is old? Tubes? Has ground flip switch on the back (or front) of the head? Try flipping it. Amp has 2 prong power cord? Flip it.

If that peterson does not have a ground lift switch on it, it's not really a full feature DI. You may need a separate dedicated DI with a fo reals ground lift and/or transformer isolation. Whirlwind Director is an old classic, and Radial makes great stuff. Ebtech hum eliminators are also useful utility tools, they do transformer based ground isolation/breaks.

[Andy Peters]

I'm going to Home Depot tomorrow, and will buy a bucket full of ground lifts.

Don't do that, unless you want to die. Defeating the safety ground on any piece of gear is asking for trouble.

Fix the problem. You need a DI with a proper pin 1 lift.

-a

[Nick Sevilla]

...and the amp and pedals are dead quiet until they get plugged into the MIO

There could very well be a ground loop between the MIO and your guitar rig.

Knowing this :

DO NOT GET GROUND LIFTS!!!!!!!!!!!!!!!!!!!!!!

They will not solve the issue, and may get you hurt or worse.

How "Dead quiet" are your amp and effect pedals, really? I mean, really look at that first. I play guitar, and I always have guitarist being recorded who swear their rig is dead quiet. Until I record it and crank it up in a clean mix. Then they start hearing how unclean their gear is. All that lovely hissing and humming that they never ever ever hear on stage (because everything else is cranked up too). Digital is super that way. It's like a mirror... what you put in, it gives you back, warts and all.

Second, the DI on that tuner... as beloved as it may be here, should not be used. It may or may no be the issue, but it clearly is not isolating anything from your audio interface. Use a proper PASSIVE DI, which has a transformer to properly decouple the ground and other hot signals form each other. Countryman or Radial, my friend.

Cheers

[tdbajus]

Opened up the Peterson, found the ground lift switch, and now everything is quiet as a mouse.

Though, for curiosity's sake- a ground loop means there is more than one way to ground, right? so if you have two amps and lift the ground on one, then you are still grounded, yes?

[Nick Sevilla]

Opened up the Peterson, found the ground lift switch, and now everything is quiet as a mouse.

Though, for curiosity's sake- a ground loop means there is more than one way to ground, right? so if you have two amps and lift the ground on one, then you are still grounded, yes?

Not necessarily, no. check your schematics, and figure it out for your own gear setup.

Ideally you run one ground throughout the system, and terminate it on one wall outlet. But if you're using more than one wall outlet, or need more than 20 amps, thus needing a second circuit, you end up making a ground loop. Then you have to decide how to properly protect yourself and your gear, by using a DI that does have a ground lift. You got lucky that your tuner DI has one... see where it takes the ground signal though, because if it is only leaving it "open" on an unconnected switch lug, you're not really protecting yourself. A proper ground lift will have a large value Capacitor still going to ground, so in case of excessive voltage it will allow the overflow to go to ground, instead of your body.

Cheers

[ashcat_lt]

The safety issue comes when the two devices disagree as to where ground is, but are still connected through you. A proper ground lift will disconnect the chassis of one device from the other, and from your guitar strings at the same time. You might still be at risk if you actually touch your guitar and the other device, but in normal practice you at least won't be acting as a conduit for the voltage difference between the two

Lifting the safety ground on a device means that there might be a fault condition where it's easier for current to get to ground through you. Even with just one device, it might just be easier to get there through the shield of the cable>your guitar's strings>your body>your shoes>the floor than any other way.

Edit - DO NOT DEFEAT THE SAFETY GROUND ON ANY PIECE OF EQUIPMENT!!! It's called a "safety ground" for a reason. Those little adapter things you can buy are not "ground lift" plugs. They are adapters meant so you can plug a 3-prong plug into a 2-prong hole. There is always a screw tab on these things. The screw in the middle of the outlet goes through this, connects to the metal box, which connects to ground. If that screw is not in place, or (worse yet) the outlet box is not connected to ground, this adapter is no safer than just snipping off the third prong.

...well, okay, you might run a heavy guage wire from that screw terminal to some known ground point, but...

[tdbajus]

Thanks for the help. The Peterson, btw, in spite of being active, sounds really nice. I usually hate the sound of a DI, and was only doing it so that I could reamp the guitar through a couple of different amps.

[The Real MC]

If you want ground lift, not all active DIs have it. Passive DIs with transformer isolation are 100% ground isolation between two devices.