Grateful Dead Music Forum

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 #124987  by GratefulMets
 Thu Feb 28, 2013 12:34 pm
Hi Folks,

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 300

10 Space Effects Rack:
Furman Power Strip (older model)
Lexicon MPX-1
GCX Switcher
Rack Shelf with:
VoodooLabs Pedal Power2
Boss OD-2
Boss OD-1
Boss GE-7
Boss OC-2
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! :hail: :hail: Maybe it will make all my problems disappear! :-)
 #125039  by redeyedjim
 Fri Mar 01, 2013 12:28 pm
I am by no means an expert on these things, but when I had a similar situation with my rack setup, I was able to resolve it by lifting the ground on the offending component (in my case, a Radial HeadBone amp switcher). It had a ground lift switch, which made this easy. A quick look at the Lexicon site's images of the MPX1 ( ) doesn't show a ground lift option, but it does seem like you could power the MPX1 via MIDI phantom power (i.e., the "remote power in" jack), and that would be the next thing I'd try. No guarantees, but it may work and it worth trying.

I don't own any Lexicon gear so I am a bit out of my depth here, but this is what I'd try if I were in your situation.

Good luck! Ground loop related noises are incredibly frustrating, and solving them can be a huge time sink, but you have to do what's necessary to make the gear usable.
 #125042  by redeyedjim
 Fri Mar 01, 2013 1:22 pm
Never mind about that "remote power in" thought I posted above -- I believe that's to provide phantom power to downstream MIDI accessories, like Midi switchers, etc.

If you remove the MPX1 from the loop entirely -- bypassed, not connected, not plugged in or turned on -- does the hum go away?
 #125056  by playingdead
 Sat Mar 02, 2013 9:39 am
Wherever the problem is, this will likely solve it for you ...


Try it between the preamp and the poweramp and see if that doesn't solve it. I used to have one between my preamp and power amp (Channel 1), and the other between the GCX output and the guitar (Channel 2).
 #125064  by GratefulMets
 Sat Mar 02, 2013 3:31 pm
Thanks for taking the time to reply, guys. This has been incredibly frustrating, and even through methodical troubleshooting, I can't seem to get a clear determination of the problem. I used a ground lift adapter on the MPX-1 and it does reduce the noise, but not entirely, and from what I read, using those adapters is strongly advised against.

In any case... I was considering the Hum Eliminator, but I also saw their HumX ( product. I'm not sure which would be more appropriate in my case. I'll probably just get both, but I am curious.

If I have absorbed what I've been reading on the inter-tubes, It sounds like the Hum Eliminator accomplishes the same thing as the recommended disconnecting and isolating of the shield braid on the connecting audio cables, to eliminate noise from the audio lines, whereas the HumX seems to be sort of a mini power conditioner for the ground line? I wish I had a better understanding, but I missed the boat on an electrical engineering degree.

Anyway.. thanks again for pitching in. I'm gonna order both, and see what happens.
 #125065  by playingdead
 Sat Mar 02, 2013 3:56 pm
You want the one I posted for your application ... it gets involved at the audio signal level, which is where your problem is cropping up. Be interested to know if the other one does anything, though!
 #125077  by Jon S.
 Sun Mar 03, 2013 6:49 am
GratefulMets wrote:I just picked up a Velocity 300 to replace my Crown XLS-1000 power amp.


P.S. While composing this note, I received word from Brad that my SMS Classic is shipping today!
You are gonna love how that SMS sounds into your new RV300 (works wonderfully for me!).
 #125079  by redeyedjim
 Sun Mar 03, 2013 9:48 am
Not hum, but related: will the Hum Eliminator (or something similar) do anything for RF noise? I've been fighting that recently with one of my fuzz boxes, and it's annoying. I already use a Furman power conditioner, but this is RF not line noise. I can only take so much AM radio...
 #125133  by milobender
 Mon Mar 04, 2013 8:23 am
I had a similar problem with our rack containing PA equipment... I was able to eliminate it by isolating each unit from the rack. I saw in your initial post that you used washers, but the ears still touched... that's not going to do any good... I used the specially designed washers that go down into the ear slot, and one on the back side too; no screw touching, no ears touching, no cases touching... it's a pain in the butt, but after getting each unit truly isolated it worked... no more noise. I don't know how the 'hum' eliminators work, but I would suspect they must be some sort of filter; and if they filter that frequency for the hum, they filter that frequency for your instrument too... it seems it would have to disturb your tone... I think it would be much better to solve the problem instead.
 #125142  by mkaufman
 Mon Mar 04, 2013 11:21 am
To my ears, it seems that the hum eliminators I tried relied on phase reversal technology, which may have reduced hum, however, it also affected tone, introducing a nasal-like phase reversal artifact.

Not my cup of tea.