Question about DI ground lift

[One_Dude]

Not sure this is the correct place for this question, but it is in regard to a live sound situation.

We play about 40 gigs a year at about 12 different venues. Yesterday was at a venue we've played several times in the last two years without any problems. But this time we had a noticeable hum when plugging my bass into our mixer. We proved our cords were good, and nothing we checked had an impact on the hum. We rehearsed the day before this gig with the same bass plugged into a bass amp and had no hum problems. I used a different bass the last time we played this venue and had no hum issue.

I use a DI between my bass and the mixer, but this time the hum was there whether my DI was in the signal chain our not. In retrospect, I suspect the problem could have been cleared by using my ground lift switch, but in the confusion of setup and the last minute problem, I forgot to try that. So my question is, what actually causes a ground loop problem; is it usually a wiring issue at the venue, or is it more related to the type of pickups in the bass. And why is there no problem when plugging into a bass amp.

Thanks in advance for your responses.

Thump on,

One_Dude

 

[31HZ]

If the noise is induced from electromagnetic interference via the pickups, then a ground lift won't help.

If the noise is due to a voltage difference across two different ground sources (at either end of the signal cable), then lifting the ground can help.

I've never seen a ground lift help with hum from a bass plugged directly into a DI, but I suppose it could help with the kind of hum that goes away when you touch the strings.

 

[Wasnex]

Sometimes lifting the ground helps, and sometimes it makes matters worse. There is no set it and forget if you are going to play in different venues. When you have hum, flip the switch and hope for the best.

I remember one gig where my amp was humming so I flipped the ground on my DI. The hum from my amp stopped but the PA to start humming. Guess who got to live with it.

Sometimes the trick is for some of the band gear to take power from a different circuit...or to run an extension from the stage out to front of house.

A few years back I went to see a concert by my old band. I was there for sound check and the keyboard rig was humming and the ground lift didn't fix it. I think the tech was using a Radial JDI Duplex which is usually great for keys. He said he was going to drive back to the shop to get a pair of Countryman Type 85s. I thought he was nuts...but it worked .

Lot's of potential causes for hum and I don't claim to fully understand every possibility. Do the best you gain.

 

[eniac]

A few years back I went to see a concert by my old band. I was there for sound check and the keyboard rig was humming and the ground lift didn't fix it. I think the tech was using a Radial JDI Duplex which is usually great for keys. He said he was going to drive back to the shop to get a pair of Countryman Type 85s. I thought he was nuts...but it worked .

Yikes. Stories like that give me migraines because they make no sense electrically. Unless I'm missing something really obvious.

 

[Wasnex]

Yikes. Stories like that give me migraines because they make no sense electrically. Unless I'm missing something really obvious.

Yep...It appears to be random magic.

 

[s0c9]

Ok.. Are we talking about lifting the ground on the power soutce or flipping the ground switch on the DI?
Two very different things, with different impacts. One of which - lifting the power ground - might be dangerous to ones health!

If the hum existed without the DI, then you've got a ground loop. Try a different outlet, or try to find the source by removing other devices plugged into the same outlet.

 

[One_Dude]

The hum was present both with the DI in the signal path and without it in the signal path. Having played at the same venue and using the same outlet a month or two earlier there was no hum. The only noticeable difference in our setup was that I was using a different bass. However, I used that same bass in our rehearsal room the day before our play date in to a bass amp and had no hum.

In a rush to address the last minute problem, I focused on checking for bad cords, along with removing the DI from the signal path. I totally forgot to flip the ground lift switch on my DI. Everything we tried seemed to point to the bass as the problem; I was able to balance the pickups some and that slightly reduced the hum. We play at a different venue this coming week, so I'll take two basses along; the same one from the last venue, and a second one I've never had a problem with. I don't have access to our PA before our next playdate, so I'll report back after that time.

Thanks for your responses so far.

Thump on,

One_Dude

 

[Mike Caldwell]

As mentioned if the hum/noise is from the bass itself a DI ground lift is most likly not going to help.

I remember one time with a keyboard ground noise issue, ground switch position on the DI just made the noise sound different, Radial
or Whirlwind DI did basically the same thing, last minute fix was adding a 1 to 1 iso transformer inline after the DI!
Before anyone ask phantom power was not turned on that channel.
Both of the DI's were passive, looking back maybe I should have tried an active DI.

 

[One_Dude]

Here's an update; due to bad weather our final two gigs for this year have been cancelled. They were scheduled for today and tomorrow, which doesn't allow enough time to clear the roads. All schools in the area are closed for the same reason. We normally take January and February off to work on new material and to avoid cancellations like this week. So it will be some time before I can try to do anymore testing with our main PA. We have a small PA in our rehearsal room, and it's not worth setting up our large PA just to test for this issue.

When we start our 2026 schedule I'll report back on the hum problem.

Thanks for your input,

Thump on,

One_Dude

 

[pbass2]

I remember one gig where my amp was humming so I flipped the ground on my DI. The hum from my amp stopped but the PA to start humming. Guess who got to live with it.

This has happened to me recently, and (see my recent thread start in this forum) it appears that flipping the ground lift on any DI (with an XLR connection going out) can introduce hum/buzz at the amp (I'm no electronics expert but I've tested as exhaustively as I can with different amps, active and passive DI's, and done all the usual trouble shooting of cables, process of elimination, etc).
I wonder if something like this placed between the DI and the amp could help?: LEHLE P-ISO – LEHLE Support

 

[Wasnex]

This has happened to me recently, and (see my recent thread start in this forum) it appears that flipping the ground lift on any DI (with an XLR connection going out) can introduce hum/buzz at the amp (I'm no electronics expert but I've tested as exhaustively as I can with different amps, active and passive DI's, and done all the usual trouble shooting of cables, process of elimination, etc).
I wonder if something like this placed between the DI and the amp could help?: LEHLE P-ISO – LEHLE Support

Unfortunately, there is only one way to find out if the P-ISO will rectify your specific problem. Try it.

Solutions can appear totally random since we frequently don't have any way of knowing why we are getting the noise. Can the P-ISO help? Maybe, but it depends on the specific problem causing the noise. So it might help in one situation and not help in another.

I have an older version of this:

The Morley Hum Eliminator is basically two channels of transformer isolation. You can patch using any combination of XLR, 1/4" TS, or 1/4" TRS. So it's sort of like two of the P-ISOs in one unit. IME, sometimes the Morley helps reduce hum and sometimes it doesn't.

As I mentioned in an earlier, changing to a different model of DI can sometimes help. At soundcheck for a concert I attended: Radial JDI Duplex hummed with keys. A pair of Countryman Type 85s did not hum. I have no idea why the Radial had noise and the Countryman did not. I generally prefer the Radial for keys, but I will take a quiet Type 85 over a noisy JDI every time.

In a perfect world you would understand and identify the exact problem and choose the best solution. Unfortunately, the best most of us can do with hum problems is use a shotgun approach...try everything available (within reason).

Sometimes lifting a power ground works, but I don't recommend it as IT IS UNSAFE.

Lifting the power ground is actually what this device does:

The ground connection goes through a couple of diodes connected in antiparallel. Diodes need a small voltage potential across them in order to conduct. So if the voltage is small, the diodes remain reverse biased and break the ground connection. If there is a problem, the voltage across the diodes exceeds the forward bias threshold, so the ground connection is made. A problem with this idea is diodes can burn out or short.

The Morley Hum Exterminator is intended for guitar amps as it can not handle enough current to support a high power bass amp. Also, AFAIK it is not UL certified; so use at your own risk.

If your curious, here is a video that explains how it works in more detail:


If the problem is with the power ground, try plugging both devices into the same AC circuit. As an audio tech, I frequently took power for the mix station from on stage. IME, this reduced the occurrence of unresolvable hum problems.

 

[pbass2]

Unfortunately, there is only one way to find out if the P-ISO will rectify your specific problem. Try it.

Solutions can appear totally random since we frequently don't have any way of knowing why we are getting the noise. Can the P-ISO help? Maybe, but it depends on the specific problem causing the noise. So it might help in one situation and not help in another.

I have an older version of this:
View attachment 7392407
The Morley Hum Eliminator is basically two channels of transformer isolation. You can patch using any combination of XLR, 1/4" TS, or 1/4" TRS. So it's sort of like two of the P-ISOs in one unit. IME, sometimes the Morley helps reduce hum and sometimes it doesn't.

As I mentioned in an earlier, changing to a different model of DI can sometimes help. At soundcheck for a concert I attended: Radial JDI Duplex hummed with keys. A pair of Countryman Type 85s did not hum. I have no idea why the Radial had noise and the Countryman did not. I generally prefer the Radial for keys, but I will take a quiet Type 85 over a noisy JDI every time.

In a perfect world you would understand and identify the exact problem and choose the best solution. Unfortunately, the best most of us can do with hum problems is use a shotgun approach...try everything available (within reason).

Sometimes lifting a power ground works, but I don't recommend it as IT IS UNSAFE.

Lifting the power ground is actually what this device does:
View attachment 7392412

The ground connection goes through a couple of diodes connected in antiparallel. Diodes need a small voltage potential across them in order to conduct. So if the voltage is small, the diodes remain reverse biased and break the ground connection. If there is a problem, the voltage across the diodes exceeds the forward bias threshold, so the ground connection is made. A problem with this idea is diodes can burn out or short.

The Morley Hum Exterminator is intended for guitar amps as it can not handle enough current to support a high power bass amp. Also, AFAIK it is not UL certified; so use at your own risk.

If your curious, here is a video that explains how it works in more detail:


If the problem is with the power ground, try plugging both devices into the same AC circuit. As an audio tech, I frequently took power for the mix station from on stage. IME, this reduced the occurrence of unresolvable hum problems.

Good stuff. Yeah, I'm not down with lifting the power ground. I remember as a teenager way back in the 70's playing in our garage bands, people pulling the pin out of extension cords.
My recent experience I mention in this long-winded (but thorough!) thread: https://www.sweetwater.com/store/detail/HumElim--morley-mhe-2-channel-stereo-hum-eliminator
In a nutshell it would appear that flipping the ground lift on a DI (any DI-active or passive)that is going XLR out to FOH, with 1/4" out to your amp will quite possibly (perhaps likely?) cause ground buzz at the amp. Different amps, different DI's, different venues, different you-name-it.
I may try something like the P-iso, or the Morley. If it works even some of the time, would be worth it.
EDIT: on further research, The Morley looks like what I will try. The Lehle sure is sexy but the Morley looks more versatile: https://www.talkbass.com/threads/di-amp-ground-issue.1681082/

 

[pbass2]

Well, I got the P-iso, and it is a keeper. I did not go for the Morley because it says it's not for instrument level, which is what the "thru" on most DI's is to my understanding. Only tested it in my studio so far, but for example, using a JDI with XLR out to my recording rig and 1/4" through going to a bass amp and trying different power sources I was able to get it where the bass amp buzzed unless I lifted the ground on the JDI. Putting the P-iso in between the JDI and the amp made it dead quiet no matter how the JDI ground lift was set. So, that's pretty cool. It did not work on every variation of stuff I tried but I think it's gonna be good to have on hand.

 

[s0c9]

Yeah, NEVER lift the ground on power to an amp, especially a tube amp! Doing so might have fatal consequences.

If there's hum in the system it can be a PITA to track down, and is usually bad venue power related. Far safer to fix the problem then lift power grounds.

Back in the day I used to carry a star-ground drop. Basically a poor mans distro with dual 4-gang boxes with 2 x 50' power cables wired across a common ground. One end plugged into one power outlet, the other into a second (usually different circuit). Cured most hum..