Guitar ground buzz problem

Guitar ground buzz problem - help!!

Was all set to shoot a video tonight and a nasty buzz put an end to that. Here is the setup:

1/4" Guitar ----> into 1/4" PreSonus BlueTube 1/4" output -----> into 1/4" Alesis Multimix 6FX 1/4" out ------> 1/4" (left and right) Rolls Matchbox DB25 (set to GND) XLR output -----> XLR input on Panasonic DVC80 camera

Problem is that I'm hearing a loud buzz/humm. As soon as the guitarist touches the metal part of the 1/4" plug going into the guitar, the buzz disappears completely...

What's the solution here?

If you can, try turning the plug around that goes into in the wall socket
sounds as if somethings not grounded properly...

A lot of guitar amplifiers have a switch on them to switch around the power to kill hum problems like that. But I have also seen a guitar player shocked silly by grabbing a mic stand that was grounded when his guitar wasn't. :( If you do switch the plug around, I would try another socket first. Most amp plugs are probably 3 wire plugs anyway. I found it useful to check the guitar ground to a known good ground with a voltmeter to avoid nasty surprises. :)

But it sounds like you are going from the guitar to a preamp. It could be a loose ground or reversed ground at the guitar. You have probably already tried a different guitar cable. Have you tried adjusting the guitar's level controls? You would probably have to reduce or adjust your preamp gain.

Been too long since I was involved with that, but hum like that was a common problem with some of the messed up stage wiring the bands had to deal with.

yeah tried different cables, tried plugging the three prong plug from the system into an adapter thing that converts it to two prong, tried different outlets.. Tried killing as many possible sources of interference as I could..

Does he have single coils or humbuckers for pickups ?

have you tried a different guitar?

Yes, tried two guitars, both are high end jazz guitar instruments.. Going to troubleshoot again tomorrow..

If the guitarist touching the outer metal shell of the plug stops it, then that shell and likely the whole shield in that cable is floating and not grounded, letting it pick up the hum from other sources. When they touch it, their body's capacitance acts as a ground, so mutes the hum.

If you find a good ground somewhere, say the screw between sockets or the (in US anyway) round ground blade of an electrical plug, it's easy to stop. Hook up one end of say a 100K ohm resistor to the ground, and a wire to the other end of the resistor. Run the wire to the 1/4" plug and that will stop the hum instead of touching it. Really hook it to the shield of the plug at the other end of that same cable will do it without having an extra wire at the guitar, the ground of that plug connects to the cable shield and over to the other plug, either end will ground the cable's shield.

Somewhere that cable's shield is isolated from ground, and is able to pick up the hum. Could be that cable is broken too somewhere, either way it's shield is floating and needs to be grounded. You don't want a direct ground for safety though, probably why there is isolation in the first place, that's why you want a 100K or 1M ohm resistor between the real hard ground and the wire to ground the plug. That way if you hit something live and also the grounded plug, the resistor will keep the current low and you won't get electrocuted, but it is still low enough value to stop the hum. Plus it helps keep you from dying if you accidently hook to a live connection at the other end instead of ground.

Really should be some sort of resistor or capacitor inside that last piece of equipment to do this, but maybe it's broken etc or they just didn't put it in, happens a lot and you have many pieces of equipment. Again though, could be the shield connection in that cable is broken and not connecting both plug shields..

For sure it's this or something directly related if can touch that plug shield and it goes away.

Alan

You can reduce a lot it in post (yeah, I know, the wrong place to fix it) by cutting frequencies at 60hz, 120, 180, 240, 300, 360, 420, 480, 540, 600

http://en.wikipedia.org/wiki/Notch_filter (this article doesn't mention the harmonics that need to be muted as well, hence the 60hz increments noted above)

You can even test it on this 60hz sound clip of grounding hum

http://en.wikipedia.org/wiki/Mains_hum

This VST filter will provide notch filtering, you'll probably need to load it several times, or run it on all of the frequencies above.

http://www.betabugsaudio.com/dl.php?id=11

Quote:

Originally Posted by sdsumike619

Was all set to shoot a video tonight and a nasty buzz put an end to that. Here is the setup:

1/4" Guitar ----> into 1/4" PreSonus BlueTube 1/4" output -----> into 1/4" Alesis Multimix 6FX 1/4" out ------> 1/4" (left and right) Rolls Matchbox DB25 (set to GND) XLR output -----> XLR input on Panasonic DVC80 camera

Problem is that I'm hearing a loud buzz/humm. As soon as the guitarist touches the metal part of the 1/4" plug going into the guitar, the buzz disappears completely...

What's the solution here?

Are all your A/C electrical items connected to the same circuit?

Often, if you have, say, the mixer plugged in one outlet and the camera on a different outlet that happens to be a different circuit from the main distribution point (i.e., breaker box), you can get a ground loop.

The other causes can be dimmer switches, monitors, fluorescent lights....

I'd revisit the XLR set to GND bit, too.

Alan69, you totally lost me with all your electrical talk =)

Quote:

Are all your A/C electrical items connected to the same circuit?

Often, if you have, say, the mixer plugged in one outlet and the camera on a different outlet that happens to be a different circuit from the main distribution point (i.e., breaker box), you can get a ground loop.

The other causes can be dimmer switches, monitors, fluorescent lights....

I'd revisit the XLR set to GND bit, too.

Well the audio equipment is in a small rack, and there's a 6 outlet power strip mounted inside the rack and the equipment is all plugged into that strip. Then the powerstrip power cable comes out the back of the rack and connects to a heavy duty extension cord which then goes into the wall. The camera equipment is plugged into a different power strip which plugs into the wall. There are other items in the rack besides the mixer and the bluetube, but they are not on. [/quote][/code]

Quote:

Originally Posted by sdsumike619

Well the audio equipment is in a small rack, and there's a 6 outlet power strip mounted inside the rack and the equipment is all plugged into that strip. Then the powerstrip power cable comes out the back of the rack and connects to a heavy duty extension cord which then goes into the wall. The camera equipment is plugged into a different power strip which plugs into the wall.

Try plugging the camera equipment into the same strip (or plug its strip into the other one temporarily) and see if the buzz goes away.

If not, I'd try this:

Connect the guitar to the mixer. Disconnect everything else. Have only the guitar and the mixer hooked up (no BlueTube). Can you monitor the mixer output with headphones? If so, see if the buzz has gone. If it has, add the BlueTube back to the signal path. Does the buzz come back?

If the buzz has gone in the previous cases, it most likely means something is out-of-balance between the mixer and the rest of the path - i.e., the Rolls Matchbox DB25 and, finally, the camcorder.

I suspect the Matchbox because you have it "set to GND". According to http://www.rolls.com/new/db25.html , the switch lets you "lift" the signal from ground to prevent hum. So, if you haven't tried it yet, just set the switch to "lift".

Buzz is completely gone. Here's what I changed:

Before, we were using two small yamaha powered speakers for monitors. I removed those (they used electricity) and brought in some old Ramsa 8" boxes and powered them with a Peavey DPC 1000 powerhead. I also pulled the rack as far away from the small room we're shooting in as I could (only moved it about 3 feet back). I plugged the camera into one wall outlet, and the rack into a different wall outlet and removed use of that second power strip.

End result, no buzz. All we have now is hiss, which I'm pretty sure I can't get rid of, but it's a lot better than a buzzz.. Also, the Rolls DB25 is still set to GND (I never understood this thing, someone just said to use it so I am)

Thanks for the help!
Mike