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FDP Forum / Performer's Corner / help me diagnose hum issue

larryguitar19
Contributing Member
**

South Florida

larryguitar
Aug 9th, 2016 11:03 PM   Edit   Profile   Print Topic   Search Topic

So here is the issue:

I have my little guitar corner with a powered mixer and monitor and pedal board for acoustic guitar and mic. I do home recording there.

If I plug in my Martin HD28 with Fishman UST to the powered mixer I get a 60 cycle hum. If I touch the guitar cable at the endpin or at the input on the powered mixer it goes away.

I bypassed the pedalboard to make sure it's not coming from there Also I switched the phase on some of the equipment. So that tells me the pedal board is not the culprit. Also I plugged in my Taylor and don't get it there.

So that tells me it must be the Martin. However if I move the guitar away from the corner out into the living room the hum also goes away.

So this suggests either there is some powerful magnetic thing going on in my apartment---which is possible because there is a restaurant below---or alternatively it's my UST gone bad.

Before i take the guitar off to my local luthier for a new saddle I was wondering if there is anything else I should be checking.

Let's assume the battery is okay.

Peegoo
Contributing Member
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Eat. Sleep. Guitar.

Repeat
Aug 10th, 2016 07:15 AM   Edit   Profile   Print Topic   Search Topic

Edited: for misspellings...I typed this on my dinky phone and I have huge fingers. Which also means I have huge nostrils :0)

We know it's not a magnetic field (EMI) causing the issue because you say the noise goes away when you touch the cable's ground shield.

It's induced RF (radio frequency) noise. The guitar strings act like an antenna and the gain circuitry amplifies the noise. You have something in that corner of the room that's putting out RF.

On an electric guitar, this problem is mitigated by the 'string ground', which is a wire connected to the metal bridge structure that runs to circuit ground. This creates ground continuity to the strings, and when your fingers are on the strings, the capacitance of the human body acts like a noise filter; it kills the noise--just like when you touch the cable's plug body.

Doing something similar with an acoustic guitar is problematic because the bridges are not conductive; there's no way to add a simple ground wire.

GUITAR SECRET ALERT: years ago I ran into this exact same issue. How do I implement a simple string ground in an acoustic guitar without it affecting tone?

The solution is simple, cheap, and elegant: solder a 12" length of small-gauge wire to the end of a 4" x 1" strip of adhesive copper tape.

With the strings out of the guitar, apply the copper tape to bridge plate (inside the guitar), across the string holes. With one hand inside the guitar to hold the tape in place, use an awl or a small Phillips screwdriver to gently poke a hole through the tape for each string. Solder the free end of the wire to the ground tab of the output jack.

Reinstall the strings. The string balls will be held in place by the bridge pins, in contact with the copper tape. You now have an effective string ground and you've added no mass to the bridge.

Share this with your tech because not a lot of 'em know this one.

(This message was last edited by Peegoo at 02:54 PM, Aug 10th, 2016)

Juice Nichols
Contributing Member
*******

Panama City, FL

I'm just a dude, playing a dude...
Aug 10th, 2016 10:04 AM   Edit   Profile   Print Topic   Search Topic

Dang Peegoo! You smart!

Hammond101
Contributing Member
*********

So. Cal. USA

Aug 10th, 2016 10:16 AM   Edit   Profile   Print Topic   Search Topic

I agree with Peegoo. A thought. Your Taylor most likely has a string ground. Their later guitars have this along with a ground fuse.

You can buy the extremely low amperage fuse from Taylor and have it installed with the grounding copper tape. It would go in series with the ground wire.

Peegoo
Contributing Member
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Eat. Sleep. Guitar.

Repeat
Aug 10th, 2016 12:54 PM   Edit   Profile   Print Topic   Search Topic

Thanks Juice!

Mick Reid
Contributing Member
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Australia

American-made in Oz!!
Aug 11th, 2016 12:23 AM   Edit   Profile   Print Topic   Search Topic

"Dang Peegoo! You smart!"

Yep, taught 'im everything he knows!


No wait... *he* taught *me* everything *I* know!

;^)

Peegoo
Contributing Member
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Eat. Sleep. Guitar.

Repeat
Aug 11th, 2016 06:43 AM   Edit   Profile   Print Topic   Search Topic

I'm not really that smart. I've just made a whole lot of mistakes over the decades--and hopefully learned from a few of them.

Juice Nichols
Contributing Member
*******

Panama City, FL

I'm just a dude, playing a dude...
Aug 11th, 2016 07:58 AM   Edit   Profile   Print Topic   Search Topic

"Good judgment comes from experience, and a lot of that comes from bad judgment."

Will Rogers


tuninguitars

Indiana

lets take it apart
Sep 30th, 2016 08:36 PM   Edit   Profile   Print Topic   Search Topic

DR. Hook
I got a couple more years on me Babe that's all

Peegoo has been there and got the T shirt I would bet ..


Peegoo
Contributing Member
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Eat. Sleep. Guitar.

Repeat
Oct 9th, 2016 01:23 PM   Edit   Profile   Print Topic   Search Topic

If you're still having a hum issue, memorize the lyrics :0)

larryguitar19
Contributing Member
**

South Florida

larryguitar
Oct 27th, 2016 06:51 PM   Edit   Profile   Print Topic   Search Topic

Postscript---

I found it.

I made a poor man's RFI detector--a cheap guitar cable and kind of just waved around and probed until I found it coming from underneath the carpet and then followed the trail back to a power adapter that powered the music stand light. The adapter was a good 4 or 5 feet away. But the power supply was resting on top of another cable that ran to the mixer.

I didn't catch it earlier because the adapter was not even plugged into the same power strip. It was connected to an extension cable that ran to another outlet on another wall.

But the little thin wire under the carpet was enough to cause that much RFI.



Peegoo
Contributing Member
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Onward

Christian Slater
Oct 27th, 2016 08:35 PM   Edit   Profile   Print Topic   Search Topic

There ya go. A power line in close proximity to a signal line!

FDP Forum / Performer's Corner / help me diagnose hum issue




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