Hoping folks here might be able to help me chase down, explain, and hopefully correct some hum/buzz in my rack, which I assume is a ground loop issue of some kind. Please excuse my long explanation of where I'm at:
I just picked up a Velocity 300 to replace my Crown XLS-1000 power amp. Upon adding it to my amp rack (Rack #1), I’m suddenly experiencing a large amount of hum/buzz in my system, which was not present before, and which now only happens when my separate effects rack (Rack #2) is included in the equation. Both racks are plugged in to the same circuit.
First, here's how my racks are configured:6 space Amp Rack:
Furman M-8x2 Power Strip/Conditioner
Aviom Personal Monitor Mixer
Alembic F-2b preamp (soon to be replaced with SMS)
Real Tube Reverb (has been removed in prep for SMS)
Velocity 30010 Space Effects Rack:
Furman Power Strip (older model)
Rack Shelf with:
VoodooLabs Pedal Power2
Floor of rack:
MXR Phase 100
Petersen Strobe Tuner
When troubleshooting from back to front:
Velocity Amp>Speaker Only = no hum
preamp (no input signal )>amp>speaker = no hum
Guitar > preamp>amp>speaker= fairly normal noise from wolf guitar in single coil mode (made MUCH worse as it moves close to rack, a topic for another day, but I would like to know how to best minimize this)
So, all seems alright there in Rack 1 on its own. Which I assume means that the new amp is fine.
But as soon as I "connect" Rack 2, either through the OBEL (send to Rack #2 GCX-In, feedthru out>OBEL>mono out to Alembic preamp in Rack #1) or bypassing OBEL (mono guitar output to GCX in > FeedThru > Preamp IN) then the buzz is introduced. (I'm also picking up some radio signals, especially when cranking the preamp input gain.)
I started checking all my 7 billion patch cables in Rack#2. Mostly by just disconnecting and re-inserting them. I found that my Lexicon MPX-1 is somehow the main offending device, as disconnecting it from the GCX loop reduced the buzz ALMOST entirely. And, If I kick on the loop with my MPX-1 in it, the buzz increases in a BIG way. So, I replaced the MPX-1 patch cables to the GCX, that seemed to make the buzz to go away, but it was only temporary, and the buzz returned next time I powered up. WTF?!?
Having only changed the power amp, what could have changed so drastically to introduce so much noise from seemingly one other device? I'm assuming it is some sort of ground loop issue, but I'm truly an electrical imbecile. I've done some searching, and I find talk of ground lifting, or disconnecting ground in guitar cables, etc. along with dire warnings of possible electrocution! I did try a ground-lift adapter (3 to 2 prong adapter) on the MPX-1, and I really cant hear that it makes much if any difference. Should I try lifting ground on the entire Rack 2 power source or am i putting myself in danger? I'm getting in over my head.
A few more pieces of information:
- I currently have Rack #1 sitting directly on top of Rack#2. Never been an issue before. Now the Velocity is in the very bottom of rack#1.
- I have rubber/plastic washers between screws and rack ears of devices, but rack ears are touching the rails. I tried some Humfrees isolation tabs on the MPX-1 and the Velocity. No difference.
- I've tried separating the units in the rack by a space or two. Nada.
- I tried a different power strip in Rack #2. Nope.
- The AC circuit in my home was wired by a professional, and my other studio equipment on the circuit does not suffer from unwanted noise.
Short story long..... given all the info above, does anyone have some suggestions for helping me get to the bottom of this?
Thanks in advance!
P.S. While composing this note, I received word from Brad that my SMS Classic is shipping today! Thanks, Brad!
Maybe it will make all my problems disappear!