Fishman Prefix Plus 60 Hz Noise
 

I retired my 1997 Taylor 314-CE from gigging over a year ago but have continued to play it at home in rotation with my other guitars. I was recently going to record with it and noticed a bad 60 Hz ground hum.

I tried different amps/preamps and guitars and confirmed 100% that the problem is with the Taylor. If I touch the endpin jack all the noise goes away.

Can anyone offer suggestions on debugging this problem? Thanks.

It’s a grounding issue. Check the pickup element for damage would be my first suggestion. Other options are to make sure the system you are plugging into is properly grounded; I used to have similar issues with my Guild and even my ES2 Taylor which is why I grounded all the electrical outlets in my home - problem went away!

Given this is the only guitar with a problem, after a new battery I would inspect the Matrix UST.

Since the Matrix is installed from the top, it is wired with screw connectors to the preamp or uses a miniature plug. I would check that connection for a loose screw or oxidized plug (pull the plug and reseat it).

If that is all good, then gently move the wire from the UST in the guitar and see if that stops the hum or makes it worse. The solder joint at the UST is fragile and can crack.

Finally, I would gently lift the UST out of the saddle slot and check for a ripped shield.

The Matrix sounds better than a coax UST but its shield and solder connection are fragile.

Replacements are plentiful on-line and you might find a very cheap gray market or counterfeit good enough if a US vendor's real thing is out of your budget. There are not that many options in making a UST and the cheap stuff is likely the same piezo polymer.

Thanks for this great detailed information. I will start looking at this today.

Now, I'm starting to think I had this issue for some time. What masked and eliminated the problem for my live performances was my use of the Prefix's notch filter. It is/was very effective for that but eliminating 60 Hz ground noise isn't it's primary function.

During sound check, I always assumed it was a low frequency resonance. But the direct recording process (with no acoustic sound) revealed the real problem.

Still, I may be able to use the notch filter to finish my demo before taking things apart. Thanks again for your help!

Even if the UST looks physically fine and shows no sign of a cracked solder joint, try disconnecting it and see if the hum stops.

Success! Thanks so much for all your helpful messages. I finished recording my demo last night with the Aura notch filter completely attenuating the hum (the tone was great). Then, this morning I started the debug following your advice. Below are the details for others seeking help.

After visually inspecting the cables and connections I removed the strings with the jack plugged into my amp.

I noticed that the hum stopped completely once the string tension was off the saddle. I removed the saddle and completely inspected the pickup. There were some superficial abrasions in the red foil but no cuts or tears on top or bottom. I took a small paint brush and cleaned the saddle slot but it was already pretty clean.

I started to reinstall the strings and noticed that once I put tension on the G string the hum started again. I inspected the saddle - perfectly flat.
I noticed that pushing the saddle back towards the bridge pins would cause the hum to stop.

After removing the saddle again. I carefully repositioned the pickup so that it was right in the center, neither touching the front or back of the slot.

Then I started reinstalling the strings. I tuned each one partially, trying to balance the tension across the saddle and listening for the hum.

When all the strings were finally tuned to pitch the hum was still gone and I started celebrating!

I thought for sure that I would need a new Matrix pickup but that wasn't the case. These pickup elements are definitely sensitive to their position in the saddle slot and I suspect a lot of players will experience this problem.

Excellent! Glad it worked out!

Placement certainly is important. If there are abrasions on the pickup it could mean that the shielding has been damaged - if the issue returns try covering the pickup with a very thin slice of kitchen foil - that’s what I did to my Matrix pickup and it solved the problem. A more permanent solution can be to get electrical insulating paint (the kind you can shield pickup cavities with) and spread a very thin layer where the scratches (if any) are.

If it is not a pickup you need to work at a gig with no back up, then Pietrh suggestions make sense.

But, if it was mine I would replace the UST.

https://www.bluestarmusic.com/Replac...NPU_p_631.html

And a counterfeit...

https://www.ebay.com/itm/Acoustic-Gu...QAAOSwgGle5k74

Thanks for these suggestions.

Thanks again for your advice and the dealer info. Just before I achieved success yesterday I was looking at the Fishman website for the narrow profile Matrix and I saw they had removed that page!

But these dealers seem to have that one in stock.

I just retired in March and am not gigging (trying to self-distance). And my main acoustic guitar is a Godin Nylon string. So I may wait until the Fishman acts up again and try Pietrh's suggestions before replacing it. But now (thanks to your guidance) I feel pretty comfortable doing the job if necessary.

I started to make another recording yesterday and the 60Hz hum was back with a vengeance. No amount of filtering would help and even light tension on the saddle caused the hum.

Microscopic cracks must develop in the shielding wrap over time and the string pressure on the saddle causes them to separate, resulting in an open circuit and the ground hum.

I took your advice and carefully put just over one wrap of aluminum foil snuggly around the entire element and it worked like a charm. Afterwards there was no sensitivity to placement or tension and the hum was 100% gone. The foil is so thin that I could not perceive any change in action or tone.

Conductive copper tape might work also but it could be thicker than the aluminum foil. And unlike my wrap, it might cause damage to the element if it needed to be adjusted or reapplied in the future.

I plan to do some more recording today so I'll see if the shield is still working. Thanks again for your suggestions.

I would also be wary of the copper tape; partly because it is relatively thick, as you say, partly because of the adhesive layer. This is why I went for the kitchen foil solution and not copper tape insulation even though I had some lying around.

Copper shielding foil is awesome stuff for shielding a MiM telecaster though!