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Thread: Ground Buzz

  1. #1
    Registered User Mandobart's Avatar
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    Default Ground Buzz

    On my Eastman mandocello recently installed this magnetic pickup along with this volume and tone pot combo. All wired up per directions with a shielded pair. The hot + lead from the pu goes to the pot combo + in, then from the pot combo + out to the tip connection on the TRS output jack. The "-" lead from the pu goes to the pot combo "-" in which is also the "-" out. The "-" out goes to the sleeve or ground of the TRS output jack. The shield, per the directions, is not connected to the pu. It connects to the "-" in/out of the pot combo and on to the sleeve/ground of the output jack.

    The pickup and pots work fine and sound good. However, when plugged in I get a hum (more of a quiet buzz) coming from the amp (Carvin AG300) if I touch the pickup. When I connected a ground wire from the "-" in/out of the pot combo to the body of the pickup (not to either of the output connections) the signal is completely killed (although it no longer buzzes when I touch it). I've never had this happen when wiring up a magnetic pickup in a solid body electric. This is the first mag pu I've installed on an acoustic instrument. The location of the the pu is such that I don't touch it unless I want to (not in the way of normal playing). I'll see if I can get a legible pic of the wiring directions here later when I get back from work. Any thoughts or suggestions? Thanks.

  2. #2
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    Default Re: Ground Buzz

    Did you ground the case of the pickup to the case of the pot to the ground of the jack? I usually run a solid bare ground wire to the above.
    Last edited by pops1; Feb-15-2017 at 11:33am.
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  3. #3
    Registered User Mandobart's Avatar
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    Default Re: Ground Buzz

    The Schatten pots don't have a metal case. They're on a small PCB that has a tab to wire the ground to. When I jumpered that ground to the mag pickup case it killed the signal regardless of pot settings. The pot ground goes to the "-" pickup lead, shield and output jack sleeve/shield.

  4. #4
    coprolite mandroid's Avatar
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    Default Re: Ground Buzz

    Amp have a Ground Lift? sure the plug in the wall is grounded?

    Stew Mac has wiring diagrams.. the Pot has to have a ground lug even if the case is plastic.

    there a schematic in the users manual,? Or should be.. Contact Shatten online Yet?



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  5. #5
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    Default Re: Ground Buzz

    Post a schematic of your wiring if you can, it would help to see it.
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  6. #6
    Registered User Mandobart's Avatar
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    Default Re: Ground Buzz

    Wall outlet is grounded, hot and neutral are correctly connected as well. I've never had this noise with my solid body magnetic pickup electric instruments using the same amp. The pickup and the pot set came with wiring diagrams. There are no users manuals for either item. Yes there is a ground connection on the pot set as stated in post #3.

    Here is a pic of the wiring directions for the pickup. It is right side up in my image window but sometimes shows sideways or upside down when I post it here:

    Click image for larger version. 

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    Fig. 4 (4-a) is closest to my configuration.

    Here is a pic of the diagram that came with the Schatten pot set. Again I've tried this post a few times and its anyone's guess if it will be oriented correctly:

    Click image for larger version. 

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    The connection labeled "1" is the "-" or ground. Thanks again for trying to help.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by Mandobart; Feb-15-2017 at 9:06pm.

  7. #7
    Registered User Mandobart's Avatar
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    Default Re: Ground Buzz

    Just plugged it into my old Peavey 130, with a 3-position ground switch (-, + and center ground lift), plugged into a different outlet on a different circuit that I just installed a few years back as part of a remodel. Buzzes just the same in all three, only when I touch the pickup. Tried out three different instrument cords as well to see if was a bad cord. Same noise on all three cords.

  8. #8
    Dave Sheets
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    Default Re: Ground Buzz

    Quote Originally Posted by Mandobart View Post
    On my Eastman mandocello recently installed

    However, when plugged in I get a hum (more of a quiet buzz) coming from the amp (Carvin AG300) if I touch the pickup. When I connected a ground wire from the "-" in/out of the pot combo to the body of the pickup (not to either of the output connections) the signal is completely killed (although it no longer buzzes when I touch it). .
    Just to make sure I understand what is happening here:

    -When you touch the pickup housing, you get a buzz
    -If you contact a ground wire to the pickup housing, the buzzing stops, but the output of the pickup no longer appears in the amp.

    Hmm- Here's what I think is happening. For some reason the positive output of the pickup (+) is connected to the pickup housing or cover. So when you touch the pickup housing, your body acts like an antenna and feeds random noise (buzz) into the amp input.

    If you ground the amp housing, it grounds the pickup positive side as well, and you get no sound at all, no buzz, nothing from the pickup.

    Here's how to check to see if this hypothesis, check the resistance (with a meter) from the pickup housing to the positive output of the pickup. It should be at least several thousand ohms. If it's pretty close to zero, that's the problem. Also, check to see if the ground (-) connection wire is connected to the pickup cover (using the meter again). I'm suspecting the ground to housing resistance will be high.

    Fortunately, the fix may be easy if the above hypothesis is correct, simply swap the pickup wires, positive to negative. The pickup polarity will be reversed, but that's no big deal if you only have one pickup installed.
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    Registered User rockies's Avatar
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    Default Re: Ground Buzz

    I didn't read through all the answers completely (vision problem). when using a magnetic pickup part of the circuit is the strings. Not often thought of but the strings must be connected to the common ground or they will cause a 60hz buzz. On mandolins and archtop guitars I would usually make a small groove in the end pin and install a thin wire from the electronics common ground and connect under one of the tailpiece screws. To find out if it's the problem connect a piece of wire from your electronics ground and touch it to or wrap it around one of your strings behind the bridge.
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    Default Re: Ground Buzz

    Usually the buzz stops when you touch the pickup not starts. Did you wire both the transducer and the magnetic pickup or just the magnetic. You could always not touch the pickup housing, sorry poor humor. If it only buzzes when you touch the housing I don't think it's the strings, but if you touch both the strings and housing does it buzz. This is odd.
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  11. #11
    Registered User Tom Wright's Avatar
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    Default Re: Ground Buzz

    Sheets has it, the pickup case is "grounded" to the ungrounded lead. I had this situation after swapping the phase at the pots on a twin-pickup setup. I was able to de-solder the connection at the pickup and re-do it to the real ground to solve the problem.

    With a single pickup, you can try switching out the "hot" lead at the pot, if the two leads run separately from the shield. If one is tied to the shield at the pickup, you need to change it there.
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  12. #12
    Registered User Mandobart's Avatar
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    Default Re: Ground Buzz

    Thanks everyone for the suggestions. I grounded the tailpiece to the output jack and there was no change in the buzz. Then tried grounding each string individually which also had no effect. With the instrument unplugged I checked resistance from the + lead pickup tab to the pu housing and the meter did not deflect away from infinity even on the highest scale (RX1k). Definitely not grounded. The resistance from the - lead side to the pu housing is a short, 0 ohms. This kind of surprises me as the shield is not connected to the pu at all. The shield and the negative lead are both connected to ground (output jack sleeve) at the pot as shown in the second pic in post #6. The pickup screws into the wooden neck of the instrument, so the only way the housing can be grounded is with a ground jumper (not installed) or the - output terminal is internally shorted to the pu housing. But the instructions state that either terminal can be used for the + and - leads, and that wouldn't be true if one terminal were intentionally connected to the housing. Also, if the - terminal is connected to the housing, then I should read coil resistance when I measure between the +terminal and the housing (not infinity). That would indicate an open coil. But the pickup works so the coil can't be open. And, if the housing is connected to the - terminal which is connected to the shield, why would providing a second ground connection to the housing change the signal when its already grounded? I guess I'll have to disconnect the signal leads to check each terminal resistance to the housing.

  13. #13
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    Default Re: Ground Buzz

    It is possible your hot and ground are reversed somewhere in the circuit.

  14. #14
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    Default Re: Ground Buzz

    If you measure between the + and - do you read coil resistance? I am surprised that a short from the - to the case wouldn't measure coil resistance from the + to case or -, really both. I am also confused that it only has a buzz when you touch the housing. You could always as they say not touch the housing. Sorry couldn't resist, and haven't had first cup of coffee yet.

    Have you contacted Armstrong about this, seems like a weird problem to me.
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  15. #15
    Registered User Mike Conner's Avatar
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    Default Re: Ground Buzz

    Mandobart, As the others have said, it is confusing that the pickup hums when touched - usually it is the opposite. It does seem like the leads are reversed somewhere and that the hot lead is connected to ground.

    Here are some details from my recent build, and maybe this will help or give you some ideas:

    Essentially I have the same wiring scheme as you; humbucker to Shatten volume and tone pots, then connected to the tip connection on the stereo endpin jack. A JJB piezo connects to the ring tab on the endpin jack, and both are grounded together (the "-" wires are all connected).

    In a prior installation, and especially at our church with dodgy grounding, there can be some proximity hum and it can disappear when touching the pickup housing (metal shell) or the strings. To address this on the latest guitar build, I did the following:

    The Shatten volume and tone pots (StewMac kit) do not have a housing to shield them. I ordered some copper shielding tape and modified a plastic box to hold the PCB that the pots are mounted to and short length of wire connects the copper tape to ground. I also connected a wire from the humbucker ground to the ground lead going to the endpin rather than just trusting the PCB connection. Instead of just 3M taping the pots inside the f hole, I assembled everything in the box, with the PCB stuck to the lid, and the box case stuck inside the f hole. Essentialy, the pots are in a shielded box. I didn't take enough photos while building all this, but maybe this shows enough:
    Click image for larger version. 

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    You have to really be careful with the connections to the Shatten pots - it is easy to have a stray copper strand from the pickup leads touch the wrong place, etc.

    The pots inside the f hole look like this:
    Click image for larger version. 

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    I managed to set up a string ground this way:

    My tailpiece has an ebony anchor block with a hollow to cover the ball ends of the strings. I used some of the copper tape to line the hollow and connect to the metal tailpiece harp:
    Click image for larger version. 

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    A hole drilled at an angle allows a wire to connect the metal tailpiece bracket to the shell of the endpin jack. This completes the string ground connection:
    Click image for larger version. 

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    Maybe this is way too much information, but I hope this gives you some ideas!
    //mike
    Last edited by Mike Conner; Feb-16-2017 at 1:02pm.

  16. The following members say thank you to Mike Conner for this post:

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  17. #16
    Registered User Mandobart's Avatar
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    Default Re: Ground Buzz

    Thanks Mike. Looks like I tried everything you did except the shielded box. Its worth a try, but there's not a lot of room between the pickguard/finger rest and the top of the 'cello...

    Pops, as I said in post #12 it has me stumped how the case can be connected to one pickup terminal, isolated from the other pu terminal, and yet NOT read coil resistance between the case and isolated terminal. And the couple of suggestions to just not touch the case really aren't out of line. I only touch the pu case when I try to.

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    Default Re: Ground Buzz

    If you have a continuity tester you can check if there is continuity between the hot lead and the case . Start at the jack. If there is continuity, work back through the circuit to find the source.

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    Default Re: Ground Buzz

    Bart have you contacted Armstrong. Sometimes a safety cap is used and would screw up your measurements. And there may be a possibility there is something wrong with this pickup. I have used these, different volume control, and not had this problem. What if you jumper the volume control or remove it from the circuit, does it still buzz?
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  20. #19
    Registered User Mandobart's Avatar
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    Default Re: Ground Buzz

    Quote Originally Posted by Nevin View Post
    If you have a continuity tester you can check if there is continuity between the hot lead and the case . Start at the jack. If there is continuity, work back through the circuit to find the source.
    Already done, as described with results in post #12. I used a multimeter on resistance setting, not a "continuity tester."

  21. #20
    Registered User Mandobart's Avatar
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    Default Re: Ground Buzz

    Quote Originally Posted by pops1 View Post
    Bart have you contacted Armstrong. Sometimes a safety cap is used and would screw up your measurements. And there may be a possibility there is something wrong with this pickup. I have used these, different volume control, and not had this problem. What if you jumper the volume control or remove it from the circuit, does it still buzz?
    I did send an email to Stew-Mac as that is where I bought all the parts. Haven't received a response yet, but I know they are pretty good at responding.

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