USB Interface earth buzz

[Muppet]

I have a few preamp/DI stomp box things that are powered with anything from 9v to 18v chargers. I have two USB interfaces connected to a PC, not both at the same time. (a Focurite Scarlett 2i2 and a Presonus Audiobox). If I use the DI out from the preamp in to either interface there's a earth buzz.  After a process of elimination I have found that this earth buzz goes away when I disconnect the laptop power and have it running on battery only. 

Any ideas why this might be and whether it can be resolved? 

thanks

Steve 

 

[Owen]

Other people more qualified will be along to explain this better soon. But it is to do with your power supply. Mac Books tend not to do so. I am not getting all "Macs are the best". They should be quiet for the kind of money they cost. Is it possible to get a better Power Supply? I would imagine so, but I am not actually qualified to have a definitive opinion. 

[paul_c2]

You can get a thing which plugs in between the power supply and the mains, I can't remember what its called but essentially it "cleans" the ground signal, which will have some noise on it due to the way the power supply works.

I can't find a UK one but this is the kind of thing: https://www.amazon.co.uk/Ebtech-Hum-Voltage-Filter/dp/B0002E4YI8

[TwoTimesBass]

This is a common problem with laptops and other kit that has a non-earthed power supply as part of the audio chain. Some laptops are better than others for PSU design. What cable are you using between the DI output and the soundcard? XLR-XLR?

Each bit of kit in your pedalboard chain that has a PSU (and the laptop) offers a potential route to ground for the audio signal, and multiple grounds create ground-loops and hence buzz/hum. This is why unplugging the laptop cures the problem, it breaks the chain.

You could try running from a line out rather than a DI (ie. connect the soundcard to where you'd normally connect to your amp using a regular cable), ground lift the XLR (either with a ground lift adaptor or with an XLR with pin 1 disconnected), or bypass each item on the pedalboard to find the likely culprit.

It's not an issue with 'dirty' power as such, more about bad grounding.

[Owen]

 I told you one of the grown ups would be along soon.

[TwoTimesBass]

1 minute ago, owen said:

 I told you one of the grown ups would be along soon.

Hardly... But sound engineering is the day job and this comes up ALL the time 😫

[Muppet]

6 minutes ago, TwoTimesBass said:

Some laptops are better than others for PSU design. What cable are you using between the DI output and the soundcard? XLR-XLR?

It's a Surface Pro so not the ideal laptop for this sort of thing I would imagine - it has a very specific supply and a magnetic latching connector. Yes I was using XLR-XLR.  

7 minutes ago, TwoTimesBass said:

You could try running from a line out rather than a DI

This is clearly an obvious answer, one I did not even try and now that I have, it works perfectly!! I'll explore the ground lift adaptor too as I reckon that's maybe what @owen was suggesting.

 

[Owen]

2 minutes ago, Muppet said:

it works perfectly!!

 

Basschat group hug!!!!!!

[TwoTimesBass]

Great news @Muppet glad it's working for you.

To be honest i'd stick with this going forward. Unless your soundcard is miles away from the pedalboard, the balanced DI out isn't really an advantage, and by not using the XLR you've broken the ground-loop.

Good work team!