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Tim Stanley's FAQ for rewiring/grounding questions
Also see my shielding FAQ since shielding and grounding are related issues.
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Single coils pickup noise. Period. Grounding problems can be subtle.
If you a a beginner at wiring your pickups etc, you may have made some
subtle mistakes. I have made wiring mistakes myself. Loud humming is
certainly a wiring mistake, and likely to be related to grounding.
Who knows?
Here are a few hints:
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Shielding
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Shield your guitar with some of that paint you can get from (say)
Stewart MacDonald. They also sell tape and foil for this application.
It will almost certainly make a difference. Take your time, do a good
job, they recommend at least 2 coats, go for 3. I did this to a stock
Strat Squier and it cleaned things up quite a bit. IMHO - whenever
someone goes to the trouble to open up their guitar for custom work -
the first task (particularly with single coils) is to shield the
cavity. Do it do it do it.
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Hum
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Are your pickups single coil or humbuckers? If you are getting noise
out of the humbucker - you have a real problem. Talk to an
experienced friend. I would bet on a ground problem of some kind, or
a bad soldering connection.
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String ground
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Also, read a book that discusses the use of a string ground. I have
seen two types - an actual wire from the cavity/electronics ground to
the bridge/strings. And sometimes I have seen a small capacitor
between the cavity/electronics and the bridge strings which would
provide you protection from getting inadvertently shocked. In any
case - check out that, too.
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Rewiring
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Your wiring should be short and shielded. Short means no longer than
necessary - but not even close to mechanically pulling. Shielded
means any long run is shielded (especially pickups to switches/pots,
and the wire to the output jack). At least one end of the shield
braid must be grounded. If you ground both ends, be careful of ground
loops. Be *SURE* that all your connections (especially ground) are
solid. Sure Sure Sure. If this was you first project with a
soldering iron - really double check things.
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Ground loops
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Are you a EE or do you have electronic tech experience? Do you know
what a ground loop is? Ground loops are easy to get into, not too
common inside of a control cavity but definitely possible. Craig
Anderton's Electronic Projects For Musicians goes lightly over the
practical ways one gets into these loops. Basically, there can be one
and only one path to ground; and in spite of what you learn in school,
all grounds are not equal.
One practical matter to remember, even for a college educated EE. All
grounds are not alike. Be *very* careful not to create a ground loop.
Follow these simple rules:
+ All grounds should come together at one and only one point
(usually the back of a pot case).
+ Don't use the shield as ground, rather, use it to conduct
noise to ground. The sheet resistance of the paint is at least
a few ohms - not a good ground conductor, but low enought to
conduct the noise to ground. Do you see the difference?
+ Thusly, there will *never* be two paths to ground.
Consider this ground loop (as an example of a loop - this specific
case may or may not cause you problems - that is the nature of ground
loops!):
a) The potentiometer case of the tone control pickup is used as the
ground lug of your system.
+-------+ +-------+ +-------+
| Tone | | Vol-B | | Vol-N |
| | | | | |
| | | | | |
+-------+ +-------+ +-------+
| | | | | | | | |
0 0 0 0 0 0 0 0 0
b) One leg of your *bridge* pickup volume must be attached to ground.
So, you attach that leg of volume control to a wire which is also
attached to the case of the ground lug pot.
+-------+ +-------+ +-------+
| Tone | | Vol-B | | Vol-N |
| | | | | |
/----| | | | | |
| +-------+ +-------+ +-------+
| | | | | | | | | |
| 0 0 0 0 0 0 0 0 0
| |
\ |
---------------------/
c) One leg of your *neck* pickup volume must be attached to ground.
So, you attach that leg of volume control to a wire which is also
attached to the case of the ground lug pot.
+-------+ +-------+ +-------+
| Tone | | Vol-B | | Vol-N |
/------| | | | | |
| /----| | | | | |
| | +-------+ +-------+ +-------+
| | | | | | | | | | |
| | 0 0 0 0 0 0 0 0 0
| | | |
| \ | |
\ ---------------------/ /
\ /
------------------------------------
d) For some reason, you decide to also attach a wire between the
ground connections of these two volume controls.
+-------+ +-------+ +-------+
| Tone | | Vol-B | | Vol-N |
/------| | | | | |
| /----| | | | | |
| | +-------+ +-------+ +-------+
| | | | | | | | | | |
| | 0 0 0 0 0 0 0 0 0
| | |\ /|
| \ | \-----------/ |
\ ---------------------/ /
\ /
------------------------------------
Now, from the either volume pot's grounded lug - there are two paths
to ground:
1: to the other volume control, to the pot case, to the shielded
surface of the pickguard, through whatever connection(s) the pickguard
has to the output jack.
2: directly to the pot case to the shielded surface of the pickguard,
through whatever connection(s) the pickguard has to the output jack.
All paths have some resistance, there are two paths here, this is
known as a ground loop. Which of the paths is real ground? In fact,
both are paths to ground and each has a different resistance. Using
shielded wire, etc., it is not uncommon to inadvertently create this
situation.
The best way to deal with this as far as I know is called
star-grounding. Have one point inside the cavity where *all*
connections to ground are made. Each and every ground connection
comes directly to this point: no side stops allowed, no shortcuts
allowed, no other connections allowed. It is probably easy to use a
large solder lug of some kind for all of the wires. The back of a
potentiometer is typically used for this purpose. Any grounding
method that prevents loops is good!
Wires that are shielded (and the shield is not used to carry ground),
get one and only one end soldered to this lug also. With shields, it
may be handy to solder a wire to the shield (careful that you don't
melt the insulation on the wires inside the shield), and run that wire
directly to the common ground lug rather than trying to drag the
shield to the lug, but leaving the shielded wire somewhere else across
the cavity. Finally, connect the shielded cavity to this lug once and
only once (use some sort of wire mechanically screwed down to the
timber or pickguard surface or something).
Run a single wire from this common lug to the ground lug on your
output jack. You now have no ground loops. In other words, for every
desired connection to ground, there is one and only one path to get to
the ground point on the output jack. One and only on is the key.
Also, IMHO, don't use the shielded cavity as a path to ground - it is
not such a good conductor. It entire purpose it to conduct noise to
ground. Use it as a shield *only*. Use real wires to connect
directly to the ground lug for all cases. I put three coats of
shielding paint down - it was nice and thick. I ohm-ed out some long
paths on the pickguard and it is definitely a few ohms. This is fine
for a shield, but distinctly un-fine for a connection to ground that
you must count on. Remember, it is paint, not wire!
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Environment
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After all of these things are squared away, you axe is as good as
possible. If your cables are all good, you are left with only
environmental causes. Motors, refridgerators, AC, dimmers,
televisions, computers, etc.
Good luck,
Tim Stanley
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String Grounds
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In my experience, a proper string ground, rather than shielding, will
take care of noise that goes away when you touch the strings. It is
independent of the shielding issue. Orthogonal issues, you might say.
If you shield, but neglect the string ground, you will still get the
touching-the-strings-noise-reduction situation. Both are important
and if you are in there fixing one, you may as well fix the other, no
doubt.
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Digest of grounding articles
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From dambik@fnalo.fnal.gov Thu Oct 7 12:58:10 1993
From: dambik@fnalo.fnal.gov (Ed Dambik)
Newsgroups: alt.guitar,rec.music.makers.guitar
Subject: Re: Hummmm and buzzzzz
Date: 6 Oct 93 12:00:28 -0600
Organization: Fermi National Accelerator Lab
NNTP-Posting-Host: fnalo.fnal.gov
In article <CEH8sA.4Goq@austin.ibm.com>, rg@futserv.austin.ibm.com (R.G. Keen) writes:
> Something I picked up from Dan Erlewine's guitar repair column:
> The hardwired connection to the strings puts you at risk of
> electrocution if another piece of equipment, say the PA or mixer
> has a leak to its ground. You're grounded through the strings
> and bridge, and the leakage from the other piece of equipment
> flows through *you* if you are holding strings and grab a mike.
> It is less risky to connect the bridge to the signal ground
> through a 100K-200k resistor in parallel with a .001 capacitor.
> This establishes you at RF ground through the cap and spaces
> you 200K ohms off 60 Hz ground so the leakage doesn't zap you.
Very good advice. I had an old tube amp which, if plugged in
the wrong way, zapped you when you touched the guitar strings.
Turns out cement floors aren't the best insulators either. Used
to be able to light up a neon bulb by holding one lead and touching
the other to the strings. IN those days, I played an Electrified
guitar.
> While we're on grounding - if youever take the time and effort
> to tinker in your amp, replace the two wire line cord with a
> three wire cord and ground the chassis to safety ground. This
> protects you from a fault in your equipment, and dramatically
> reduces the hum and buzzing from your setup. It can cause
> problems with ground loops in complicated racks, but that is
> another problem.
This was the solution I used. Worked great.
Ed
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From rg@futserv.austin.ibm.com Thu Oct 7 13:09:31 1993
Newsgroups: alt.guitar
From: rg@futserv.austin.ibm.com (R.G. Keen)
Subject: Re: Hummmm and buzzzzz
Date: Thu, 7 Oct 1993 15:00:10 GMT
Reply-To: ...futserv.austin.ibm.com!rg
Organization: IBM Coporation - Advanced Workstations and Systems.
I looked up Erlewine's article, and he attributes it to Adrian Legg.
The circuit values are 220K and .001 uF. Since this resistor never
has any significant current flow in it (that is the point!) it does
not need any significant power rating. Beyond being long lasting and
not mechanically fragile, any 220K will do. I used 1/4 watt carbon
film. The .001 cap should withstand at least the line voltage peak
where you are. For USA, with 120VAC, a 250 volt cap is good enough.
for UK, make it at least 500 V. I used a ceramic disk cap. If you
want to use premium, high power and high reliability components,
you should go ahead. I decided that since there is no significant
electrical stress on them other than perhaps voltage, I would use
parts that fit easily in the cavity of the Strat.
Your mileage may vary.
R.G.
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From backstge@aol.com Sat Oct 22 15:14:05 1994
From: backstge@aol.com (Backstge)
Newsgroups: alt.guitar
Subject: Re: HELP - Guitar SHOCKS me!
Date: 22 Oct 1994 03:04:11 -0400
Organization: America Online, Inc. (1-800-827-6364)
NNTP-Posting-Host: newsbf01.news.aol.com
In article <388ted$4qr@hermes.unt.edu>, sed0001@jove.acs.unt.edu (Scott
Everett Douglas) writes:
This excerpt from Adrian Legg's "Customizing Your Electric Guitar" may
also help:
"On any good quality guitar, you will find a wire running from the bridge
to the earth side of the guitar's wiring. This is the string earth, and is
essential for cutting down string hum and noise, and you can add this item
to replace it altogether.
Wire together in parallel, a 220k ohm resistor (red, red, yellow and
silver or gold) and a .001 capacitor with a minimum voltage rating of 500
volts. Twist the wires and then solder them together on both ends.
Wire this unit inside the guitar between the bridge or tailpiece and the
earth side of the circuit, replacing the string earth wire.
In the event of the chassis going live, it will only send about 40 volts
through the strings, enough to give you a warning tingle without hurting
you. It will allow enough leakage for the string earth to operate
normally. "
-Richard
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