Hissing preamp

[geoham]

A few months ago, I upgraded the Bartolini MK-1 preamp in my Lakland 5501 to a Bartolini NTMB - a pre-wired loom from Bass Direct. On the whole, it's been working fine - but since moving to in-ears, I've noticed a high pitched hiss that's only present when the pre-amp is engaged. It disappears entirely when the pre-amp is bypassed.

It sounds a bit like white noise, and Audacity indicates it's around the 3kHz frequency. This is with all controls set flat. If I turn the treble down, it does go down a bit. 

I can't really hear it at all when I am playing, and I've got a noise-gate to deal with it when I'm not. But I'd still like to get to the bottom of it!

Any tips / obvious things to check?

Thanks

[Jonse]

Flat battery? I've forgotten that before. 

[geoham]

40 minutes ago, Jonse said:

Flat battery? I've forgotten that before. 

I've checked the battery with a multi-meter - getting a touch over 9v, so presumably okay. 

Edited November 4, 2019 by geoham

[geoham]

I did a little work on this today and I think I improved it somewhat. I shielded the control and pickup cavities with copper tape and gave the wiring a general tidy up too while I had the circuit out. I also adjusted the gain-trim to have the passive and active volumes equal. I reckon this is the most important part.

It seems successful. There’s definitely less hiss in my in-ears, and engaging the active circuit doesn’t cause it to get louder. Any hiss I hear is present with my other passive basses too. Damn in-ears highlighting imperfections! 

George