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Help Quieten my Precision !

Discussion in 'Pickups & Electronics [BG]' started by The Great Rober, Dec 9, 2003.

  1. The Great Rober

    The Great Rober

    Dec 4, 2003
    Ok, so I've read the article on getting a strat quiet on the "guitarnuts" website.
    A few question on how to do it on a P
    The system ground ( PUP return,cap.ground etc.)must be isolated from the shielding ?If so then you must isolate/insulate the pots from the shielding (foil) on the pick guard?
    Also,what do you do with the ground from the bridge ? Is this connected to the shielding foil, OR the system ground ?
    What is the purpose of the cap that goes between the system ground and the shielding ?
  2. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Hi Great Rober, the guitarnuts website is "good", but it doesn't tell the whole story. Also there's some pure BS up there, like all that stuff about 400 volts DC mysteriously appearing on the ground plane in your guitar. What a crock o' bologna.

    Here's the deal: most manufacturers ground one side of the pickup (whether by a "star grounding" method like the one advocated by guitarnuts, or by some other method). This is absolutely the worst possible way to wire a pickup, and you're just begging for ground loops if you do it that way. Ground loops are insidious and difficult to troubleshoot, because the root cause can be in the amp even if the symptoms only appear in the guitar.

    The "correct" way to wire a pickup is ground ONLY the housing. The housing or casing "must" be grounded because that's what gives the "shield" its meaning. Both wires from the pickup should be active, and neither should be grounded. This means one of two things: either you're running a "balanced" cable (an XLR or TRS cable, kind of like a mic cable) from the guitar to the amp, OR, you have a differential input active preamp in your instrument that's converting your balanced pickup signals into a single-ended low impedance signals that's suitable for driving long cables.

    If you remove the ground from one side of the pickups, you won't have to worry about any of the stuff they're talking about up at guitarnuts, in terms of ground loops inside the guitar and the like. A ground loop arises because "ground" at point A is not the same as "ground" at point B. This is most likely to occur when your amp is point A and your guitar is point B. In a perfect theoretical world both grounds would be the same, 'cause they're connected by the shielding of your instrument cable. But in the real world, that cable has resistance, capacitance, and inductance, and behaves more like a "transmission line" than a wire. That's why you're far more likely to experience hum and noise with a long cable.

    The "best" way to address these problems is to install a differential input active preamp directly into your bass. A good preamp won't change your tone "at all" (except for whatever EQ you decide to use). But be aware, that the vast majority of COTS preamps are not DIAP's, they're single ended "gain stages" with some bells and whistles.

    One of my favorite stories relates to the very first F bass I bought some 10 years ago. It had a horrible hum problem. After tearing my hair out trying to troubleshoot it, I finally gave up and installed a DIAP (actually a dual, one for each pickup, which was necessary because of the other electronics that were already in the bass). Well, that cured the problem. I can take that bass, plug it into my most powerful two kilowatt amp, turn all the volume controls all the way up, and put the pickups no more than three inches away from a fluorescent light fixture, and it's dead quiet. Nary a peep. Quiet as a church mouse. Quite amazing really. YRMV!
  3. The Great Rober

    The Great Rober

    Dec 4, 2003

    Thanks for the info.I'm not sure I want to go the XLR or DIAP route (yet)
    I'm sure a stock 73 P would have been quieter than mine. As soon as you touch the strings or another metal part AND are holding it the instrument is quiet.I just want to get that quietness when not touching the strings!

    So from what you are saying the the shielding needs to be grounded ?

    Any more ideas ? I want to exhaust the non XLR/DIAP route first ! In Europe these 30 year old P basses are starting to fetch silly money, as long as they are resonably "original"
  4. pjbasser


    Sep 26, 2003
    Here´s my 2 cents...:)

    The thing here is about stargrounding just like it´s been said at the guitarnuts site.

    What u have to do is this:

    1 collect all "signal-earths" (that is from the jack,pu/pu´s,the -pot´s right lug seen from behind-) and there should not be any connection between the housing of the pot and that lug...

    2 Now solder these cables together and connect this to one end of a capacitor (I used 0.33uF/400v)

    3 Take the other end of the capacitor and connect it together with the earth-cable from the bridge
    and mount it tight some where in the shielded cavity.

    This works great!
    And of course the cavity must be properly shielded.


    The reason for using a capacitor is to prevent urself from getting shocks from the amp or other electrical appliances in cases where something has gone wrong.

    Hope this helps!
  5. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Yeah, try the star grounding. Hopefully that'll work.

    That part about the capacitor though, just doesn't hold water. That cap won't do anything to protect you from the types of electrical shocks you're most likely to get from an amp (as a result of bad grounding, which is mainly related to 115 volts AC appearing on the chassis ground via a bad ground switch cap coupled with improper or outdated AC wiring). The only possible way 400 volts is going to appear on your guitar is if there's a dead short in the preamp tube between the grid and the plate. And in that case it's going to appear on your "hot" and not your "ground" (which means your pickups will fry long before you will), and there are no "hot" exposed parts on a guitar (or shouldn't be). And it wouldn't be 400 volts anyway, more like about 165.

    Also, the cap may make a ground loop problem even worse, if it's one of those problem that originates in the amp. That's because it can affect the transmission line properties of the instrument cable. But, the only way to find out for sure is to try it.
  6. The Great Rober

    The Great Rober

    Dec 4, 2003
    So here it is.The 73 P is now a quiet as she is goin' to get.

    Fully shielded with baco-foil.Cut to shape and stuck in position.Nice looking job,even if I say so myself.

    All 4 ( pup return,cap ground,bridge ground wire and jack socket ground )grounds commoned at one point.The brass plate that sits below the PUPs was insulated from the shield.The shield is only connected to the ground side at at the output jack ( the foil on the pick guard contacts the ground ot the jack here).

    All hot signels are taped up with insulating tape to ensure that they do not accidently touch the shield anh hence ground.

    It not only looked good, it did work !

    So thanks to all who helped me quieten precision !! All you get now is a bit of hiss-I can live with that
  7. pjbasser


    Sep 26, 2003
    Glad that it worked out for U :)


    I do agree with u partially, since a cap doesn´t stop any AC from getting through, it only stops DC..
    But IMHO it doesn´t hurt to prevent urself from shocks if all it takes is a cap..does it? :)
    (even if these cases are rare)

    Keep the groove...
  8. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Excellent. Yeah, glad it worked. Nice to have a quiet bass, right? The star grounding stuff works pretty good most of the time, as long as you have a good (and fairly short) instrument cable you should be fine.

    That's actually how the Alembic company started out. Interesting story. When the Grateful Dead started playing larger venues early on in their career, and they'd have to stand maybe thirty or forty feet in front of the amps, Phil Lesh was having all kinds of hum problems with his passive pickups. So Ron Wickersham at Alembic built an active preamp into his bass, with a low impedance output so he could drive very long cables with no signal loss and very little hum and noise. And that relationship blossomed, and eventually led to the Wall of Sound, and etc.
  9. The Great Rober

    The Great Rober

    Dec 4, 2003
    Just to "fine tune" this further.Why not make the ground on the jack socket your "star" point ?

    You could have a long single wire from the PUP ground ( brass plate on an older P ) to the jack.There, you could attach the other 3 grounds !

    Antway, It would make little difference anyway, as the only noise I get now is from the amp !! Yep ,turn the amp gain upto 10 and you get the same hiss as if the lead and bass are not plugged in.

    One interesting observation is that when you put your hand near (within and inch) of the PUPs you do get some hum/noise. Not a real issue though
  10. ampegloud

    ampegloud Guest

    Oct 14, 2002
    kansas city mo
    be carefull on what you read some of these so called guitar repairman think they are the wholly grail of guitar repair like that idiotout in las vegas that guys an ass
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