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My Yamaha silent guitar SLG-100 Steel string with undersaddle acoustic guitar pickup has serious hum when a 9V power adapter is used. No noise if using the 9V battery to power the guitar. I play enough that using batteries makes no sense.

How can I test if it is the pickup or the pickup circuit board?

I have the service manual and some limited knowledge of using a multimeter. Any directions would be appreciated before I go broke buying 9V batteries (PS. tested with multiple 9V adapter same results. pretty sure its a 60 cycle AC hum. Also tried a attenuator on cable.

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  • Having dealt with this issue on multiple piezo pre-amplified classical guitars, it's my belief that the shielding of the pickup itself is culprit here. I have resorted to using rechargable batteries for guitars and other gear that's using them. It fixed the problem for me, and find it more efficient since it's way cheaper than buying new ones and the cable from AC source in no longer in the way.
    – Dalv Olan
    Commented Jul 9, 2017 at 20:55
  • What 9V power adapter is that? Model?
    – Jason Han
    Commented Aug 8, 2017 at 9:33
  • Are you by any chance using a Switching Mode Power Supply (SMPS)? Changing to a linear mode (regulated) might solve the problem. This will work only if your problem is caused by ripple in the power supply. You normally need an oscilloscope to check this and the power supply needs to be connected to the device so it sees a load. But you might be able to get a reading using the AC setting on a multimeter.
    – Schizomorph
    Commented Sep 7, 2017 at 9:36

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Could be any of the three (or most likely a combination):

  • power supply
  • UST (under saddle transducer pickup)
  • preamp circuitry

You already tried out different power supplies, but still that's where I'm thinking the problem starts as you say there is no hum with the battery supply.

The circuitry and transducer could amplify even the slightest ripple from the power supply (and surroundings), but if it were a grounding problem with the transducer and/or preamp, I'd expect you to hear some hum even with the battery.

However the combination of bad transducer grounding and bad grounding of the power supply and the ground of whatever you hook it up to, may create some sort of ground loop.

Take a closer look at the transducer and consider a replacement. Here is a quote from a guy that had luck with this (with his SLG100N):

I decided to yank out all the circuit boards and replace it with B-Band's aftermarket UST Preamp/Pickup System. When I called B-Band to get some technical info, their customer service person told to me to try their "replacement" under-saddle ribbon transducer, made specifically for the Yamaha Silent Nylon String Guitar first. He told me other people have had the same problem. For $39.00, it was a lot cheaper than than buying a whole new pickup/preamp system. The B-Band site offers detailed instructions on installing their under-saddle ribbon transducer pickup, if you buy one of these, take the time to read it. It arrived in the mail, I installed it in 10 minutes and WOW! This guitar now sounds absolutely amazing! This new version of the ribbon transducer is very impressive. Touch sensitive, very clean dynamics, amazing amount of headroom through the headphones or plugged into an amp, AND it is totally quiet. As in NO 60-cycle hum or interference of any sort regardless if I'm using the AC power converter or a 9volt battery.

(source: "Lacewoodstrat1" review at harmony central)

Also take a look at this related question:

Fixing low-frequency hum in an undersaddle acoustic guitar pickup

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