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Pot cases - to ground... or not to ground...

Discussion in 'Pickups & Electronics [BG]' started by frits51, Jan 26, 2013.

  1. frits51

    frits51 Supporting Member

    Apr 30, 2011
    Heath, Texas
    That is the question.

    I've left a blend pot case ungrounded before with no problem.

    And, at other times, I've grounded all of the pot cases.

    I've always used the 'star' configuration - separate ground wire from every component and case, brought together in one joint.

    Just read an article on Best Bass Gear about leaving the pot cases ungrounded, so I removed the pot case grounds on a P-bass type Yamaha (RBX250F). I still have the buzz which is most noticeable with tone knob all the way to treble. Goes away when I touch any metal part.

    I've checked ohms everywhere. Everything is grounded to 0.2 ohms or less. Still got the buzz.

    Is this an issue with the stock Yamaha P-type pickup?
  2. They need to be grounded. Doesn't matter how, just as long as they are.
  3. abemo


    Feb 27, 2012
    Arvada, co
    They do have to be grounded, but not necessarily to the signal chain. If your control cavity (or pg, whatever they're mounted to) is properly shielded, the pot casings will be connectedto the shielding, so as long as that's grounded you're fine. What I do, as a variation on star grounding, is tie all the pot casings, shielding, and bridge ground to the ring of the output, and all the signal grounds (pups, pot lugs, switches, tone caps, etc) to the sleeve. When the bass is plugged in the two grounds connect through the cable shield, but they are separate on the bass.
  4. frits51

    frits51 Supporting Member

    Apr 30, 2011
    Heath, Texas
    abemo, I like your approach.

    My cavity is not shielded.

    I've seen plenty of posts about trying to get rid of stubborn treble buzz.

    Seems like the answer may be shielding. I don't feel like doing it, but...

    Thanks for your reply. It will go into my personal archive.
  5. +1

    What I started doing is making sure the cavity is completely inclosed in shielding, with all controls making solid contact even the jack, ground the pickupshield and bridge wire to the shield, if you have a seperate pickup cavity shield that too and tie those wires to one single place(I use a ring terminal and a screw). When this is done correctly you only need one wire going to the jack(the positive) I get the cleanest, strongest signal ever!

    I think it's bad when you have multiple paths to ground. All your jack/pots/switches should be connected to the cavity shield anyway just by securly tighting the locknut on ea. If not then your gonna have a weak shield system anyway. Just remember +positive should have shortest path to jack+, and -negative should have the shortest path to -ground, but not multiple paths.
  6. SGD Lutherie

    SGD Lutherie Banned

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    There's no reason to do that, and you are introducing a lot of extra wire to the ground plain.

    Just daisy chain them together. The pot cans act as shielding, so you want to ground them.
  7. Eight_Stringer


    Feb 22, 2009
    Star point earth or return system is a good approach, well proven and helps circumvent the chance of circulating currents in a wiring system. Well done Fritz51. "Ground plane" is a type of decoupling system, not a shield, perhaps the following will help explain alternate concepts.

    Of "earthed" situation on a stage, this is also well understood by the people whom have a grasp of first principles. Why instruments are earthed is a complex subject, will do another topic today to limit any irrelevance to this thread or topic.

    In essence you have "earthed" metal on an instrument, one of the most misunderstood concepts is the misnomer "Shielding". What you are really trying to accomplish is "decoupling", that is not allowing an unwanted signal to be induced into a wanted signal. Imagine this, your right hand is an unwanted signal your left a wanted signal, the closer you right gets to your left then the signals will combine. So you put something in between them like a "shield" say copper foil, now this copper cannot at all reduce the induced effects between the hands, unless it can take away this interaction, and that is where many try to "earth" the copper shield. To what? Another earth point with voltage gradients on it, and by implication circulating currents ( misused term "ground loop", it is the current that does the damage not the voltage, big concept for some). In fact, what you may well do is increase the coupling effect of this "shield" to cross induce the wanted and unwanted signals by virtue of increasing the surface area of mutual coupling. The oft heard, "i shielded it and it is worse", or you touch your bass strings and the interference is worse! No mystery why. What is an earth?

    You want to decouple the signal sources and that is a long topic, maybe too long for this forum. HTH.
  8. frits51

    frits51 Supporting Member

    Apr 30, 2011
    Heath, Texas
    I'm an instrumentation and electrical tech by trade with minimal experience in guitar electronics. Nevertheless, electricity is subject to the same rules in my bass as it is in an instrumentation circuit.

    My reason for 'star' grounding is this - The voltage we use in guitars is low. Thus, that voltage's ability to push through a resistance without being diminished is also comparatively low.

    I do not want to make any signal, ground or otherwise, have to push its way through a steel pot case. I would rather have it zoom through a copper wire and back to the amp.

    That is why I personally have chosen to employ 'star' grounding which provides each component in the bass with its own 100% copper path back to the amp.

    That being said, my MIM P bass still has a buzz that goes away when I touch a knob, the bridge, the strings, the jack nut, etc.

    My MIJ Jazz, on the other hand, is 'star' grounded (and has a Bourns M/N blend pot) and is quiet.

    Interesting creatures, these basses...
  9. I don't think you are understanding the concept very well.

    First off, it's current that matters. Resistance becomes significant when the current is high. You are dealing with currents in the microAmp range in a bass, and these tiny fractions of an Ohm that oppose the flow of current in a bass are completely insignificant. This is probably the first time I have ever heard anyone express concern.

    Second, star grounding makes the equivalent series resistance problem worse. You are adding extra wire, rather than allowing the current to find the fastest path to ground.
  10. SGD Lutherie

    SGD Lutherie Banned

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    Are you really a tech? No rules of electricity are being broken here.

    If you are trained in electronics you would know that pot cases are usually not steel, and that electrons "flow" on the surface of wires. And even if they were steel, so what? It's a conductor. Most amos have a steel or aluminum chassis. They don't have copper chassis. Current doesn't care unless the conductor has high resistance. And even then, the conductive paint used for shielding has higher resistance than metal conductors like copper or aluminum foil.

    Electricity is not being pushed though a pot can. The cans are connected together with copper wires. The wires lead to ground. So the whole assembly is at ground potential.

    The pot cans form a ground plane. You can connect to any point on a ground plain, just as on a PCB.

    Lets take the chassis of your amp or an effects unit. Same thing. Plus no one uses star grounding on printed circuit boards. Think about that for a moment. Please tell me what instrumentation you use that has star grounding.

    Also, nothing is "zooming" through a wire. Electronics don't even flow though a wire. They wiggle back and forth. It's the potential charge changes. Once an electron is introduced on one end of the conductor, another one pops off the other end. Like marbles in a tube. You really have this all mixed up. Also we are talking about ground. not a current source.

    As far as "zooming" the sea charge is instantaneous, and is mediated by virtual photons. So we are taking the speed of light here.

    The size of the conductor is not going to make current flow faster. It flows at the same speed. But you can fit more electrons because the surface is larger. So larger wire can handle more current. Now the pot cans have more surface area than a dinky little piece of wire, don't they? If you really wanted a good ground path, you would have a solid large conductor, not some wires.

    Plus star grounding introduces more wire into the circuit. Star grounding is used on AC powered tube amps because they have a large differences in the voltages being used. You have the high plate voltages (600V), and the lower voltage filaments (6-12V) and other parts of the circuit. They all have to share a common ground. Since some of the voltages are higher, this is where the problem arises. None of this is present in a bass, passive or active.

    And it's not "voltage's ability to push through a resistance" it's current. Voltage doesn't push anything. And it's not pushing through anything.

    Lastly, the noise that goes away when you touch the strings is from lack of shielding. When you ground your body via the strings, you are shunting the noise to ground. The fact that it gets quieter shows your grounds are just fine. But that's electrical field noise.

    Star grounding has nothing to do with electrical field noise. Star grounding is to eliminate ground loops in AC powered gear. The bass is not AC powered, only has one ground, and that noise has nothing to do with ground loop hum, which is 60/120 Hz noise. You are hearing high frequency buzzing, from electrical field interference. Not ground loops or anything like that.

    If i might ask, what is your formal training in?

  11. Eight_Stringer


    Feb 22, 2009
    Hello David,
    Can see by the volume of your posting in this topic, you are passionate about musical instrument electronics and electrical wiring. Use what you know to attain the results that work for you. No problem there, having said that, there are first principles in electricity ( and by implication electronics ) that apply each and every time. Read you complete reply and find it a mix of basics and extrapolation from aspects that are not well understood. Did you resolve the function of the gate to return ( zero volts ) resistor on the single fet pre amp you use? Being self biased circuit configuration, it requires this resistor to establish the gate negative bias in a single supply application. At that time your pickups were performing the function of this Gate to return resistor, should this important resistor be omitted.

    Can discuss circuit conductivity in conventional current flow or Hole flow as you desire, both are valid and much so in semiconductor theory.

    Of what constitutes an effective interconnection system on say, a bass. Imagine a wire going from one most extreme points of a bass, body to machine head, perhaps. Is this wire going to have exactly the same EMF all along the entire length, in a circular circuit or open ended, in the presence of for instance, a stage? All along its length there will be magnetic fields inducing an EMF of varying magnitudes. These EMF's will allow a differential current to flow when a path is formed, that could be another ( connected ) conductor of any material, ferrous or non ferrous or it could be even free air if frequencies are high enough. It can even be your surface high impedance body. Voltage gradients can be induced or generated by current flow, ohms law, E=IR. If you break current paths you break the IR generated voltage gradients, why the term ground loop intimates current by implication and not the obvious "interference voltage" ( unwanted EMF ). Star point wiring is a good technique to avoid current paths. Deltafred is correct.

    If i understand you correctly, ( point me out if not so ), your concerned all this extra wire length will degrade the bass wiring scheme to decouple the wanted from the unwanted signals, can see how that would occur to you, as it is based on more wire will yield more unwanted coupling, you could also imagine how current behaves in place of a voltage only thought process. Will give you a lift in circuit understanding no end. Taught Electrical Engineering for 10 years, the look of complete mystery on students faces when the unity gain amplifier they constructed oscillated with ease, input single or no present. Their circuit had a voltage gain of one ( unity ) yet it needed gain greater ( much greater ) than that for parasitic oscillations to occur. Clue is your correct that a voltage gain of one will not oscillate, what is the function of this amplifier ( buffer amplifier )? It has a power gain, if the voltage has not changed then what is the other element of the equation P=E*I that must be greater than one, much greater, to give unwanted oscillations the condition to be present.

    Have worked on systems from 330,000 Volts AC to scientific instruments sensing femtoamp currents, to Radar for more than 40 years, can be as technical as this forum will allow, yet we need useful outcomes as opposed to theory detail, obscuring the many useful techniques for fellow bass players to improve their instrument performance, witness your good work and press of the pickups designed and fabricated by yourself, many find helpful and then some. Your effective in your field. Others can be that also, in their fields of expertise.

  12. calebbarton


    Aug 25, 2007
    Manufacturing: Pedals, Cables, Instruments.
    Blerrr....I know tunz of stuffs tew.
  13. SGD Lutherie

    SGD Lutherie Banned

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    You are not going to find those paths in a bass. We are talking about the ground plane. All your shielding, which is also the backs of the pots form a continuous connection to ground. There are no points in that different from the rest.

    Feel free to test for it. Any digital multimeter should have adequate resolution.

    As far as a wire going from the machine heads etc... you have the strings. What happens with those strings? They certainly pick up noise like an antenna and it's induced into the pickup coils. So what do we do to combat that? You ground them. Problem solved.

    No one would have that length of unshielded signal wire. Long runs of unshelded wire will pick up noise. That has nothing to do with star grounding.

    All metal parts on the bass should be grounded. They only need to be part of the ground plane that is made up by either the shielding in the control cavity, which also connects to the pot cans, or on unshielded instruments by the ground buss made up in the usual way.

    And still, none of that has anything to do with ground loops.

    If this were truly a problem, every manufacturer would be using a star ground path. Yet, no one does because it's not needed. You will also see quite a number of people with noisier basses after doing this. Why? Good question. Probably because they have introduced an un-nneded level of complexity to the wiring.

    And even on amplifiers. Lots of tube amps use start grounding. Lots of tube amps don't, and they are still very quiet. So even here, the point is not proven.

    All your effects pedals, and your amp, assuming it has a PCB, don't use star grounding. In fact most are very similar to your bass, if you bass had a shielded control cavity. Look at an effects unit. It has a metal box, usually aluminum, but sometimes steel. The jacks ground to the enclosure. The circuit also grounds to those jacks. There is no star grounding, and in fact, all the grounds form a box, which it is. That doesn't cause any noise, and any interference that "hits" the box gets shunted to ground.

    That's the point you are missing. We are talking about a shielded system. Interference hitting the shields is prevented from getting into the signal path because the shields are grounded.

    Regarding extra wire; I said for people who cite the resistance of the ground wires as a problem, you are introducing extra lengths of wire, i.e. more resistance. Isn't that what you are trying to get away from? Obviously parallel runs of wire will reduce this resistance, however, this is the point in which people mistakenly believe that some kind of ground noise is being introduced into the wiring (and keep in mind that al these wires are ground wires, not signal wires), and that it will create a ground loop.

    Yet no one has been successful in demonstrating a ground loop in a bass, or guitar etc. The noise the poster I replied to was referring to had nothing to do with ground loops, and is nothing that star grounding will cure. Ground noise is low frequency noise. RFI noise is high frequency noise. Shielding fixes that. Most factory basses, i.e. Fenders are woefully inadequate when it comes to shielding. They might have been fine 50 years ago when we had less electronic gizmos. And when they do introduce shielding, it's not executed well.

    Feel free to demonstrate otherwise.

    Regarding current and magnetic fields. They are essentially two sides of the same thing. Any conductor in the presence of a magnetic field will has current flowing in it, and vice versa. That's how the pickups work (variable reluctance transducers) and also how eddy currents are formed. All the metallic parts of your pickups have these eddy currents. But they do not produce noise. What they do is produce a magnetic field that opposes the permanent magnetic's field. This increases the AC resistance and you lose some high end. But it does not cause ground plane noise.

    So please demonstrate some real world situations where anything you wrote has any impact on a bass.

    BTW, when I worked for a defense contractor in the 70s we didn't use star grounding on any of the communications gear we were making. All the prototypes were wire wrapped by hand, and the "production" pieces used PCBs with standard ground planes. And lot of redundant ground wires to the case. Can't have things fail in the field.
  14. Eight_Stringer


    Feb 22, 2009
    Glad to demonstrate any and all of 40 years electrical and electronic experiences. Do that in a controlled lab environment, even in the field, dont have enough years remaining in my life to do that, typing on a forum, am an old bloke! For me, am happy to leave it at, you have your position and i have mine. My noise free basses await to be played. Regards.
  15. You seem to have two contradictory statements here. Let me
    paraphrase your statements:
    1) Resistance is insignificant.
    2) Star grounding is bad because it adds resistance.

    So, I take it that you have never heard a noisy guitar.
    I guarantee you that they are out there. I've heard plenty of
    them. They hum, they pick up radio broadcasts, they make
    all kinds of noise. If these noise currents are insignificant,
    why are all these guitars making noise?

    When you're dealing with high gain amplification, small noises
    become big noises.

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