Double Bass Weird Hum trouble shooting

[Anthony White]

I have a weird signal noise problem, and wondered if anyone could help me trouble shoot it.

I have an MK Jazz EUB, with a passive piezo pickup.

I have been running it:
MK -> FDeck HPFPre (mk3) -> Source Audio EQ2 -> Reverb pedal -> Rolls Mini Mixer -> headphones.

I have an isolated power supply for the HPF-Pre, EQ2, and reverb, but I’ve also tried battery only for the HPF-Pre.

The Rolls MiniMixer has my bass, digital piano, and external audio (iPad) going into it so I can mix my levels and play along with recordings, etc.

A couple of days ago, I switched the whole arrangement on, and I had 50 Hz hum. I hadn’t changed anything, just switched it off one day without hum, switched on the next day with hum.

It appears that something in the interaction between the MK pickup and the HPF-Pre and possibly the Rolls is causing the hum. I’ve taken each device out of the signal chain, and am only getting hum when the MK goes straight into the HPF-Pre, and from there into the Rolls. My acoustic bass with Full Circle, electric bass, acoustic guitar, into the HPF-Pre do not cause hum.

If I touch the soundboard of the MK where the piezo is located, the volume of the hum increases significantly.

But it is not the MK being inherently humming - I have a Felix preamp, and the MK straight into that is silent. I’m using the amp-out on the Felix, which is unbalanced and not transformer isolated.

Weirdly, if I run the MK into the HPF-Pre, then into the Felix, I have no hum.

Any suggestions what else I can trouble shoot?

 

[Martin Spure]

I’m by no means an expert, but judging from your description the Rolls seems to be the culprit, as the hum (if I read your description correctly) is only present, when the Rolls is added to the chain. I dunno, maybe some sort of impedance ‘mismatch’ between the Rolls and the other devices, introducing the 50hz ground loop/hum ? But then, you didn’t have that before, with the same setup..

Do you also get the hum, when the digital piano and iPad is not connected to the Rolls ?

 

[DoubleMIDI]

I also assume it is a grounding issue with the other devices or the devices after the HPFpre connected to the mixer.

Just the passive piezo connected should not generate him as there is no grounding there.
The only hum with a passive piezo happens with a broken connection (piezo, internal cable, connection cable or internal amp cable or plug/jack).

 

[Anthony White]

Thanks for these replies.
So I understood that for ground hum, I need a ground.
Yet none of my devices are grounded, except for the Felix, and I’m only using that to avoid the hum.
The Rolls has a two-prong transformer to 15V DC.
The pedals have a two-prong transformer to 9V DC.
The piano has a two-prong to transformer.
???