Very loud buzz coming from my rack!

[Steven Latch]

Hello,

My rack has been stable for over a year now. It's a 12U and features the following:

Furman
G Major 2
2 X RM4
Decimator pro rack G
GCX guitar switcher
A couple of analog pedals
Mesa Boogie 2:50 power amp

It's been working fine so far in terms of noise as I use humfrees on each rack unit, and isolated each unit from one another with tape.

A few days ago, I plugged the rack and it was fine as usual. Today, same place, same conditions and it hasn't moved since, and a HUGE buzzing noise parasites my tone. I have been at it for two hours and couldn't find the source. Hoping somebody here will have an idea.

Here are some things I did check:

- Power amp tubes are fine (tried another preamp with it)

- One of the RM4's is fine when bypassing all other units and plugged in the power amp

- The second RM4 is NOT fine when bypassing all units and plugged into the power amp. Severe, abnormal noise. It is not the tube preamps as I checked them one by one

- When running the rack altogether, even when completely removing the "faulty" RM4 from the circuit (power cord and audio cables removed), the RM4 that sounds right on its own also gets some severe buzzing.

So... it sounds as is a huge ground loop has landed on me unexpectedly. Any idea what to check next?

Thanks to those who will take the time to read this!

 

[ncsouthpaw]

Does the RM4 have a V1 preamp tube? Solved a static/hiss/sqealing noise that was bothering me by replacing V1 on my RM100.

(EDIT - I mean on the head not in a preamp module).

 

[tuna141]

Couple simple things to check:

Any ground lift switches on any of the rack devices, that may have gotten bumped?

Maybe one of your cords are bad?

Good luck.

 

[Steven Latch]

Thanks all.

I did try all of the above with no results. To put it simply, things were working fine and then not anymore. Lots of A/B testing, plugging, unplugging, unmonting, mounting ahead it seems!

 

[rhequiem]

A process of elimination to determine if any specific part of your rack is causing the issue might be a good approach, too. Maybe start with just your power amp and one RM4 and see if the buzz is still occurring there. If so, maybe try the other RM4. If it's still occurring, maybe try a different power amp? Etc etc? If you can isolate the problem, it will be much easier to address, it would seem. Good luck, dude!

 

[m0jo]

Perhaps a stupid question: did anything else in the room or perhaps the building change??

Also: tried running only both preamps + poweramp? No other gear.

It does seem like a groundloop, that means it is probably not caused by one unit but by the combination of multiple. So don't "rule out" one part of the setup. It's all about finding what combo causes it.

 

[Steven Latch]

Well I did many tests today but couldn't pinpoint something so far.

To my knowlege, nothing has changed in the building's electrical setup but I will move the rack somwhere else for a checkup.

If that fails, I'm good for some more elements per element testing...