XLR TRS hum

[Robsco]

My interface (Scarlet Solo) takes a 1/4" jack, so I use a TRS plug (which it says it supports), to XLR which goes into my DI.

When I only have the TRS end plugged into the interface, there's a hum, that's fine/expected.

However, even when I plug it into my DI, there's still a constant hum. Touching the metal case of the female XLR plug also shows signs of interference.

When I swap this out for an XLR to simple mono 1/4" plug, there's no hum when I connect to the DI.

What could be going on here?

Should the case of the TRS cable be connected to one of the 3 wires (ie, the shield, in addition to pin 1, it's not currently, but wiring diagrams I've seen online aren't either).

Any help muchly appreciated, it's driving me mad, especially having laid and hidden this huge XLR cable, and pulled it all out again to make sure it's not other interference.

Thanks,
Rob

 

[Robsco]

Interestingly switching the interface input from INSTRUMENT to LINE kinda fixes it (but it's still nice and silent with the non-TRS cable, on either setting, so clearly not the full story.

The DI is the Whirlwind IMP 2 btw, which suggests it outputs MIC level.

 

[lokikallas]

If there are two different devices with their own power supplies, most likely the TRS is making a ground loop. In the end, balanced or bridged, whatever is the quietest is best. A ground loop isolation unit may get rid of the hum. ISO-MAX being my favorite.

 

[Robsco]

I did try the Ground Lift switch on the DI, but no change, is that the same thing?

 

[Frank Tuesday]

In most cases, if you're using a DI, you should be using XLR-XLR cable. Your DI is putting out mic level signal, not instrument or line level. You've got a very low level, low impedance signal coming from the DI, and if your interface is expecting a different signal type, you'll have impedance and level mismatch.

 

[Robsco]

Thanks, but how would that make the TS cable work perfectly, but the TRS one doesn't?

When the interface is set to Line level input, and pre-amp up to about 75%, it works "fine".

I wonder what level the interface pre-amp's should be set to normally?

I have my mic set to about 75% gain too on the other input.