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I have an acoustic guitar - a Cort MR750 dreadnought, from about 1998 - with a Fishman pickup system that came pre-installed in the guitar. I haven't been able to identify the type of pickup or preamp, although I have emailed both Fishman and Cort in an attempt to get more information.

There's a continuous hum coming from the guitar, that mostly goes away when I grab the metal end of the cable jack, making me think that this is a grounding issue. Changing cables, mixers, amps - none of this makes a difference, the hum is still present.

It's also worth mentioning that the problem came upon me quite suddenly, in-between two gigs about two weeks apart. The guitar was subjected to no unusual stresses in that time, aside from being carried in the trunk of a car in the cold (but it was very well protected, in a hard case with nothing on top of the guitar).

The preamp has no identifying marks, other than saying "Fishman Equipped". (Or, if there were such marks, they've long since been worn off.) But here are pictures of the preamp and of the pickup:

enter image description here

So I've narrowed the problem down to the guitar electronics. None of the wires inside are loose (I tightened up a cable that screws into the preamp, and that didn't make any difference), and all the soldered connections inside seem solid. I've swapped the battery and made certain it's connecting solidly, neither made a difference.

How I can go about diagnosing the problem? I'd like to replace as little of this system as I can. (I don't want to order a pickup, only to find the problem is the preamp or the wiring.)

Even better, could this simply be a shielding issue? Can the problem be corrected by shielding the electronics? How does one even do that with an acoustic guitar, by sheilding the interior cables?

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migrated from Jan 27 '14 at 15:17

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Have you checked the cable socket, where you plug the cable into the guitar? – No'am Newman Dec 25 '11 at 10:23
I unscrewed it and checked it visually, it seems okay – neilfein Dec 25 '11 at 15:27
Can't you put a simple contact mic and run it tru the preamp to see if the problem is the pickup or the preamp? I had similar problems with my electric guitar - one of the humbucker pickup had its magnet broken - but then again, I have little knowledge of acoustic guitars... – Johnny Bigoode Dec 26 '11 at 2:42
If I had a contact mic, I could do that. Will ask around, see if anyone I know has one. – neilfein Dec 26 '11 at 3:18
Do you have anything else you could plug something else into the preamp? An MP3 player might work (just turn the volume all the way down to start). – Friend Of George Dec 26 '11 at 4:39

Since it goes away when you touch the metal of the cable jack, it's almost certainly a shielding issue. It could be the preamp, the pickup, or – most likely – the interior cable from the pickup to the preamp: this connection is extremely susceptible to capacitive coupling because both the preamp input and the pickup (assuming a piëzo, which it likely is) in such a system have very high impedances, and capacitive coupling is exactly the kind of hum that's attenuated if you ground your body by touching the jack.

So chances are the hum will disappear if you exchange that cable for a good coaxial ground shielded one, or alternatively wrap it in grounded tin foil. That kind of job is always more difficult in an acoustic guitar than it is in an electric one, but you should be able to do it.

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This is something I can do quite easily, so it's certainly worth trying. Replacing the cable might be difficult (it's soldered onto the pickup directly), but I have some copper foil tape left over from some electric guitar projects. – neilfein Dec 28 '11 at 2:27
Turns out that replacing just the cable is harder than I thought. This is a very good answer, but I ended up replacing the entire pickup/wire assembly for expediency's sake. – neilfein Oct 4 '12 at 18:40
BTW, my classical seems to have caught that same cold now... I hope I won't also have to replace the entire pickup. Funny, you'd think if it's shielded properly it stays hum-free forever. — Well, maybe the problem lies somewhere completely different in my case. – leftaroundabout Oct 6 '12 at 18:17
You'd think, but this guitar is fourteen years old. Fourteen years hum-free is a pretty good run! – neilfein Oct 7 '12 at 13:17
up vote 1 down vote accepted

The problem turned out to be either the pickup or the wire to the preamp. I didn't have time to shield the wire - I can't get my hand all the way into the cavity of the guitar, and I don't have tools to work inside a guitar (outside of some skinny pliers and a small inspection mirror), so I elected to simply replace the pickup. It worked; the hum is gone.

It's entirely possible that there was some sort of shielding problem on the previous wire, as this answer suggests, but I can't say for certain. In the past, I've fixed similar problems in electric guitars by shielding the inner cavity with copper tape. (There was no shielding on the endpin jack at all, something that some web sources suggest can cause hum.)

My old pickup:


According to Fishman when I emailed them about this:

It sounds like the under saddle pickup element may be faulty. Usually if you touch the endpin jack and the hum goes away, it is a shielding problem within the pickup element.

The email went on to suggest a seller for the part I needed. At first, I was dubious. I couldn't see any breaks in the foil on my pickup, although a small enough tear could be difficult to notice. I realized that the last time I had changed strings, the bridge had popped out, and I might have damaged things by putting the bridge back in carelessly.

So I decided to take a chance and order the pickup. All I needed was to measure the width of my pickup so the replacement would fit in my saddle slot (1/8" in my case), and I was in business; I ordered this part and it arrived in a few days, despite my choosing the option for cheapskate shipping.

After a few dicey moments soldering the pickup's wire onto the mini-jack that plugs into the guitar's internal preamp - you have to do this with the pickup already installed, since you can't get the jack through the pickup hole in the bridge - the installation went smoothly. (I've done this before, although not on this guitar.) The pickup gives me an even volume on all strings, despite being about 1/32" shorter than my existing pickup.

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+1 Thanks for sharing your solution. – Friend Of George Jan 15 '12 at 4:26
It's possible that leftaroundabout's solution could have worked, but I realized that I don't have the equipment to run the shielding over the wiring. Why, or why, don't they make acoustic guitars with rear-access panels? – neilfein Jan 15 '12 at 5:40

I had the same problem. I found a simple and easy solution. I took out the bridge piece (which was very tight) and cleaned it, then gently rubbed the sides on some floor tiles (I had no sand paper). This was to remove any goop accumulated from playing (sweat gunk etc). I slipped it back in (a lot easier). I then tested, without strings, applying pressure to the bridge and touching saddle off and on to see what happened to the noise. There was a marked difference, but still intermittent noise. I restrung guitar and plugged in (a very nervous moment as I'd put new strings on). I did this after reading about low frequency hum in this article

No noise whatsoever. In fact I thought I had unmuted the wrong channel on my desk. Yippee!!!

The piezo needs to have good contact. A overly tight bridge piece may not allow the correct contact or pressure to be made.

I have had this problem before with a yamaha guitar with similar pickup system. I ended up selling guitar as I could not resolve the problem even with professional help. This latest issue was a freak out. I started looking at my entire system for ground loops etc and had begun replacing leads. It can be a costly exercise. I hope this helps someone as I was very down about this problem and I understand the frustration.

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just fixed hum issue on a prefix plus by removing saddle and pulling pickup out, dusted slot and very slightly sanded saddle thinner, was still humming badly, put back together and half way through putting strings back on the hum stopped.

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After some time hunting down the same issue I completely fixed the hum by continuing the copper shielding around it right back to the Fishman electronics. I used stick-back copper tape that I'd left over from a Strat shielding job. Wrap all along the black pickup cable and - important - make sure it connects with the copper braid which covers the pickup. Did the trick for me 100%. Deep joy. Good luck. Bevan

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