Passive Acoustic 50hz hum

Bl4mm · #1 07-31-2023, 04:10 PM

Hey

Trying to eliminate a 50hz hum on my passive acoustic. It's a Epiphone AJ15 with a piezo under the bridge. No preamp. It happens regardless of what's between the guitar and amp (DI preamp only makes it worse) and only gets quieter when I touch the jack socket on the body. I presume this is not a ground loop issue as there is no preamp in the guitar, the pickup is wired direct to the output jack. So assume it's a shielding issue. Am I right?

Mandobart · #2 07-31-2023, 07:31 PM

Do you mean you're hearing an audible tone in the G1 to G#1 range due to some strange resonance (the lowest note playable on a standard tuned guitar is ~168 Hz)
or are you really saying its an AC power hum (which is actually 60 Hz in most of the western hemisphere, 55 Hz in some other places)?

chasapple · #3 07-31-2023, 07:51 PM

If it gets quieter when you touch the jack, I would open up the jack and see if the ground connection has come loose from its soldering point.

zeeway · #4 07-31-2023, 08:11 PM

If it gets quieter when you touch the jack, likely you are becoming the ground. So, yep, the output jack is not grounded, IMHO.

Gordon Currie · #5 07-31-2023, 10:18 PM

When that happened to me 15 years ago, it was the UST (undersaddle piezo) that apparently was separating internally. If so, you'll have to remove it and replace it.

Mandobart · #6 07-31-2023, 11:29 PM

Quote:

Originally Posted by Gordon Currie

When that happened to me 15 years ago, it was the UST (undersaddle piezo) that apparently was separating internally. If so, you'll have to remove it and replace it.

Or lose the UST and replace it with an SBT.....

Bl4mm · #7 08-01-2023, 01:52 AM

Quote:

Originally Posted by Mandobart

Do you mean you're hearing an audible tone in the G1 to G#1 range due to some strange resonance (the lowest note playable on a standard tuned guitar is ~168 Hz)
or are you really saying its an AC power hum (which is actually 60 Hz in most of the western hemisphere, 55 Hz in some other places)?

Hi. Am in the UK, mains is 50hz here. Its mainly a 50hz hum (approx g half sharp

, with some very high overtones.

Bl4mm · #8 08-01-2023, 01:59 AM

Quote:

Originally Posted by chasapple

If it gets quieter when you touch the jack, I would open up the jack and see if the ground connection has come loose from its soldering point.

Quote:

Originally Posted by zeeway

If it gets quieter when you touch the jack, likely you are becoming the ground. So, yep, the output jack is not grounded, IMHO.

Thought it might be that so resoldered the jack. No difference. It a cheap unshielded one so might try replace it. Also noticed that the hum gets way quieter when I touch the wire inside from the pickup to the jack, presumably thats the same as touching the jack?

Also also noticed that when I took the saddle out the bottom of it was not flat at all (clearly not finished properly) so sanded it down.

Bl4mm · #9 08-01-2023, 02:14 AM

Quote:

Originally Posted by Gordon Currie

When that happened to me 15 years ago, it was the UST (undersaddle piezo) that apparently was separating internally. If so, you'll have to remove it and replace it.

Ta

How could you tell that was happening. Was it obvious physically?

Mandobart · #10 08-01-2023, 06:22 AM

Quote:

Originally Posted by Bl4mm

Thought it might be that so resoldered the jack. No difference. It a cheap unshielded one so might try replace it. Also noticed that the hum gets way quieter when I touch the wire inside from the pickup to the jack, presumably thats the same as touching the jack?

Also also noticed that when I took the saddle out the bottom of it was not flat at all (clearly not finished properly) so sanded it down.

The bolded statement indicates to me (USN nuclear sub electronics tech in my wasted youth) that your signal lead is not adequately shielded, or the shield is not adequately grounded ("ground" in the US = "earth" in the UK). It's also possible your instrument cable shield is not correctly grounded, or your amp is not.

I'd rule out a problem with the instrument cable or amplifier ground first, then take out the under saddle POS and replace it with a good soundboard transducer like K&K, JJB or Dazzo.

Bl4mm · #11 08-01-2023, 08:49 AM

Quote:

Originally Posted by Mandobart

The bolded statement indicates to me (USN nuclear sub electronics tech in my wasted youth) that your signal lead is not adequately shielded, or the shield is not adequately grounded ("ground" in the US = "earth" in the UK). It's also possible your instrument cable shield is not correctly grounded, or your amp is not.

I'd rule out a problem with the instrument cable or amplifier ground first, then take out the under saddle POS and replace it with a good soundboard transducer like K&K, JJB or Dazzo.

Thanks. Will look into that.

MarkF_48 · #12 08-01-2023, 09:44 AM

I've had an ongoing problem with a Takamine having hum (touching the jack eliminates it) occasionally which I think is related to humidity causing the body/neck wood to expand and contract which causes the saddle to tilt in the bridge slot. When the saddle tilts it doesn't make good physical contact to the piezo element and seems to be when the hum occurs. If I loosen the strings and re-tighten/tune while ensuring the saddle stays flatly seated, I can usually eliminate the hum. This is easy to try and may remedy the hum you have (or not). A proper fitting saddle should be snug, but not real tight and not have much if looseness.

At about 1:50 in the video he shows what may happen.....
https://www.youtube.com/watch?v=tIwo_Nj1pxE

TedBPhx · #13 08-01-2023, 10:19 AM

Quote:

Originally Posted by Mandobart

The bolded statement indicates to me (USN nuclear sub electronics tech in my wasted youth) that your signal lead is not adequately shielded, or the shield is not adequately grounded ("ground" in the US = "earth" in the UK). It's also possible your instrument cable shield is not correctly grounded, or your amp is not.

I'd rule out a problem with the instrument cable or amplifier ground first, then take out the under saddle POS and replace it with a good soundboard transducer like K&K, JJB or Dazzo.

EM2 SS SSBN 611 many decades ago. Used to keep my guitar in forward torpedo. Occasionally the Sonar Techs would call down and tell me to knock it off.

Gordon Currie · #14 08-01-2023, 02:51 PM

Quote:

Originally Posted by Bl4mm

Ta

How could you tell that was happening. Was it obvious physically?

No, but I took off the strings and saddle and reseated it, and it worked for another week before crapping out again.

I replaced it with my first K&K (have since moved on to Dazzo).

I have zero tolerance for flaky/intermittent electronics.

Mandobart · #15 08-01-2023, 10:38 PM

Quote:

Originally Posted by TedBPhx

EM2 SS SSBN 611 many decades ago. Used to keep my guitar in forward torpedo. Occasionally the Sonar Techs would call down and tell me to knock it off.

Cool! ET1(SS) (nuc before they had different nuc rates) SSN 596 out of Ballast Point. I also kept my old Applause in the torpedo room. Didn't have much time to pick between port n starboard watches, training, maintenance, drills, battle stations and of course field day.

I don't miss it even a little.