Fixing low-frequency hum in an undersaddle acoustic guitar pickup

Question

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:

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?

Answer 1

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.

Answer 2

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.

Answer 3

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 http://www.fishman.com/files/advanced_undersaddle_pickup_installation.pdf

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.

Answer 4

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.

Answer 5

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

Answer 6

After reading all the solutions submitted, I didn't see the one that worked for me. The copper shielding tape one almost matched mine; however, mine is an even easier fix. When I replaced my saddle I inadvertently scratched the protective foil coating on the piezo itself creating the hum described above. I pulled the piezo out of the slot, cut a thin strip of copper shielding tape the length of the pickup and a little wider than the pickup. I then pressed the tape over the entire pickup and wrapped the excess around the sides, replaced it in te slot, and re-inserted the saddle. I strung it back up and "presto", no hum.

Answer 7

Just to say that the advice in the posts by Simon Bosley (Aug 2/17) and Paul Smith (Jul 25/17) above seemed to have worked for me.

I've a Martin 000c-16GTE with inbuilt electronics by Fishman (which allows amongst other EQ/phase settings the option to balance between the internal mic and the undersaddl pickup fitted to the guitar).

Noticed recently that the guitar had developed a bad earth hum present when using the under saddle pickup. Removed the strings to have a look - nothing awry, but noticed that when I carefully removed the piezo rod (to inspect the red foil for damage etc - it was OK) the hum stopped.

So with no strings on, and the piezo transducer hanging out in mid-air the guitar was quiet as a whisper. Re-seating it, back comes the hum. Repeated, this time jiggling the wire and also making sure the ends of the transducer weren't touching the ends of the bridge slot. Bingo - no noise. Saddle back in slot on top of piezo - still good.

Re-strung the guitar, 2 strings in and back comes the hum. A small wiggle of the saddle and its gone again. Carefully re-strung the rest of the strings and I'm still good - so my conclusion is that such devices are very fussy about how they sit, what they touch etc.

Saved me some money and was worth the hour or so faffing about to get back to clean guitar land. Hope it helps someone too.

Cheers David Y

Answer 8

I had pretty serious hum in my Ibanez AEG20E a $400 guitar- you would think Ibanez would not ship these with such hum-- but.... Anyway, I shielded EVERYTHING electronic inside with sticky metal tape from the hardware store, the lines running from the control box to the output jacks, around the output jack parts inside (after using electrical tape over any soldered points to avoid shorting), and around the pickup wire all the way up to the copper braid on the piezo pickup. The saddle was also pretty tight, and I seeing some other posts about this I also filed it just a tad thinner so it would slide in and out of the slot easier- snug, but not overly tight. This would also allow better and even contact with the piezo pickup, which I noticed was not completely balanced in all of the strings (now fixed). This completely eliminated all of the hum. A bit of work, but now the guitar is quiet as it should be.

Answer 9

I have a similar problem but have discovered one common issue: It seems that is the power is 50 cycles versus 60 cycles it creates the hum. When I plug my Fishman amplifier into a non-grounded or 50 cycle circuit the guitar hums like crazy unless I hold the plug shield. If I'm on a good 60 cycle circuit it works perfectly.... The microphone is not impacted. I've had my guitar in the shop a couple of times and they could not find anything wrong (the greater Chicagoland), but I was playing in Arkansas and the hum occured at three different locations. When I returned home it all works perfectly.

Answer 10

I had the same problem only on an acoustic 12 string, which I installed the under saddle pick up about 15 years ago. The hum started just about a week ago and got worse. It was so bad that I needed to do something. The hum would also almost all go away when I touched the metal around the plug in. Just as was described above.

In fact I just got done working on it and now there is no hum at all. What I did is simple and I want to share it with you all. I just took all the strings off. Removed the under-saddle pick-up then I removed the pre- amp. I looked for signs of bad wires and/or connections. However, everything looked perfectly good. Mind you this pickup was first installed 15 years ago and it has seen a lot of the road and has been to Belize for 3 years.

So I just cleaned up the slot in the bridge where the saddle lays. I used a flat head screw driver and cloth to do this so the bottom of the slot would stay perfectly flat. That is a must for the pick-up to work right. I also cleaned the pick-up it self and then reinstalled it. Before placing the saddle in the slot over the pick-up I cleaned it too. Then I put he strings back on and brought it up to one step below concert pitch and bingo.... no hum! No hum what-so-ever.

I don't really know why this worked in this case. The slot where the saddle sits is slightly larger then the saddle so the saddle can easily be moved up and down. I'm sure that over the years dirt had gotten in there. Perhaps the dirt interfered with the grounding of the pick-up resulting in the hum. I don't know. At any rate... I just "cleaned it" and then I got a "clean sound".

Hope this helps yours!

Answer 11

If the end of the piezo element, or the wire attached to the piezo element is tight against the guitar body at any point, the guitar will hum and feedback.

Usually the wire should be held away from the guitar body with plastic spacers, and the hole the pickup passes through should be considerably wider than the pickup.

Answer 12

I have an old martin matrix undersaddle pickup that was getting worse and worse with a low frequency hum. I took the advice here and looked at the pickup element and saw that the shielding had rubbed off in several places. I put copper tape from a guitar shielding project (took very little) and covered the exposed areas. Restrung and all hum was gone! Very cheap and easy fix, didn't affect the sound of the pickup at all.