In defense of 'star' grounding

[okcrum]

Instrumentation grounding is unique in that the impedances are typically much higher than even passive pickup circuits, in the hundreds or thousands of megohms in some cases instead of a few tens of kohms. One bass humming for the OP suggests a poor or missing connection to ground in the bass. Or one of the pickups is wired backwards so there is no humbucking action.

[SGD Lutherie]

Originally Posted by 1958Bassman

...a ground wire from one place to another ended up acting as a great antenna for noise.

See, this is the problem. You are making the assumption that one end a grounded wire is not grounded, and it's leading to ground. This is wrong. The whole wire is at ground potential. On a car the whole cassis is also ground, which is why ground connections are made to it. When those connections get flakey, as on an old car (like the 1980 BMW I used to have), then current will find the next lowest potential, which is often not ground.

So if a ground wire is grounded, how is it an antenna for noise? It that were true, then your grounded shields (like on your coax cable) and strings would also pick up noise. But they don't when they are grounded. That's why they are shields. If you unground them, they will be an antenna for noise.

Once again, in tube amps and similar situations, you have a lot of high voltages which are causing ground loops. The star grounding is to eliminate ground loops. But this discussion is mixing a lot of metaphors on what star grounding is for. It's not to cut down on RF/EMI pickup. That's what shields are for.

What I'm waiting for is a controlled test with a before and after demonstration showing how star grounding is cutting down on noise in a bass. In the test nothing else would be changed but the ground paths. This has not been done yet. All we have is anecdotal stories about two different basses, or people's experiences in fields that have nothing to do with bass guitars, or incomplete electronics theory.

I've been wiring up guitars and basses for about 41 years. I have tried all different ways of doing it, including star grounding. In my opinion based on my own tests, I find no benefit in doing it. I got the exact same results with a fully shielded control cavity and pickups, and the control cavity grounds connected to the closet point, with a single or sometimes dual ground wire running to the output jack. This has created basses so immune from noise that I often disconnected the bridge ground wire.

[SGD Lutherie]

Originally Posted by okcrum

Instrumentation grounding is unique in that the impedances are typically much higher than even passive pickup circuits, in the hundreds or thousands of megohms in some cases instead of a few tens of kohms. One bass humming for the OP suggests a poor or missing connection to ground in the bass. Or one of the pickups is wired backwards so there is no humbucking action.

Right, and that 60 cycle hum is not from RFI pickup, which is electrical field noise like you sometimes hear when you aren't touching the string.

Single coil pickups are susceptible to magnetic field noise, which is a low pitched hum. Humbuckers eliminate that, but they don't fix electrical field noise; that high pitched buzz. Shielding helps with that. Star grounding does not, nor does it get rid of the magnetic field noise picked up by single coil pickups.

The problem is that electric instruments pick up different types of interference. But in these threads these interferences are being clumped together, and then star grounding is said to be the cure. But star grounding is for yet another problem, and not the above listed problems.

[DiabolusInMusic]

Originally Posted by frits51

I posted this in reply to a reply to a question I put up in another thread. Thought the subject deserved its own post (not that this is the first one on the topic!)

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...

I can offer no valid opinion on star grounding but...

If you have a noise that gets louder when you touch the metal/strings/bridge you have a grounding issue.

If you have a noise that goes away when you touch metal/strings/bridge you have a shielding issue.

Your bass needs some proper shielding, the wiring is probably fine. If you shield the bass properly the buzz will go away.

[SGD Lutherie]

Originally Posted by frits51

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.

Now how exactly would you star ground a P bass? It's a very simple circuit.

On the other hand your Jazz is not star grounded because the pots are sitting on a metal control plate. So they are already all connected together, just as if you put a buss wire across the back of each pot. Just because you chose to not solder the ground wires there doesn't mean a thing because they are already connected.

You can also see that the P bass doesn't have the metal plate for the controls. You are comparing two totally different setups.

[pkstone]

I'm about to shield the control and pickup cavities on my Jazz. While I am dubious as to the benefits of star grounding for noise reduction, it looks like a good way to physically wire the electronics. Soldering to the back of a pot seems like a bad idea to me, i.e., very prone to cold solder joints on such a big heat sink.

What do those of you with experience in bass wiring think of star grounding strictly from a quality-of-wiring perspective?

[SGD Lutherie]

Originally Posted by pkstone

I'm about to shield the control and pickup cavities on my Jazz. While I am dubious as to the benefits of star grounding for noise reduction, it looks like a good way to physically wire the electronics. Soldering to the back of a pot seems like a bad idea to me, i.e., very prone to cold solder joints on such a big heat sink.

Not anymore than any other solder joint. You need a nice size iron. On a Jazz bass you don't have to solder to the backs of the pots because everything is mounted on the metal plate. You will even notice that fender doesn't run a ground wire to the jack.

However, since it's a mechanical connection, if a pot or your jack where to loosen up, you will lose your ground connection, and your sound will be intermittent.

Soldering a wire is always better than a mechanical connection. Mechanical connection are also prone to oxidizing.

And once again, because everything is mounted to a conductive metal plate, you wont really be star grounding anything. You will just have multiple ground paths. It's redundant.

Get a 30 watt iron and solder to the back of the pot. It only takes a second or so, and it's not a heatsink at all. The cans get hot very quickly.

What do those of you with experience in bass wiring think of star grounding strictly from a quality-of-wiring perspective?

No difference, and it introduces a lot of extra wire you have to deal with. So instead of a nice neat simple wiring job, you have a whole bunch of ground wires trying to solder to one small lug on your output jack.

This is the proper way to do it, and has been working fine for a long time:



On a Jazz bass, make sure the start wire on the pickups goes to ground. That will prevent the buzzing you hear when you touch the magnets. It's usually the black wire, but not always. If you get buzzing when touching the magnets reverse the wires on both pickups.

This is also true of P basses, but only affects one coil, since they are in series. But it's helpful to have the coil under the E-A to not hum if you touch it.

[joelb79]

Originally Posted by frits51

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...

On a P bass one set of series coils floats it's ground (probably the wrong term). Ground the poles on the pickup, and make sure the wires leading to it are shielded top to bottom.

[Munjibunga]

Originally Posted by Cadfael

This is the reason why I don't use the star wiring:

SdKfz 250, for control of motorized formations, equipped with FuG12 radio which used a 2 meter star aerial.

They must have had a reason - and using an antenna form isn't the best shielding solution to me ...

BUT I AM NO EXPERT!

You sure aren't. That antenna isn't shorted to ground, is it? There's no need to defend star grounding - it works fine. But it's really not necessary as long as you ground everything one way or another.

[SGD Lutherie]

Originally Posted by Munjibunga

You sure aren't. That antenna isn't shorted to ground, is it? There's no need to defend star grounding - it works fine. But it's really not necessary as long as you ground everything one way or another.

+1

I have a test anyone here can perform to see if a grounded wire will introduce noise into your signal. Get about 10 feet of unshielded wire. Something manageable like 28 gauge.

Now solder one end to the ground at your output jack, or any ground in the bass. Back of a pot, doesn't matter. Do you hear any noise? Of course not. Any signals being picked up by that wire are now shunted to ground.

If you connect that wire to the hot side of the jack, you will hear all the noise it's picking up.

[Bassamatic]

You star ground a P bass the same as everything else. Every wire and terminal that is to go to ground has a wire to a central spot - preferably in the center of your shielded control cavity.

[SGD Lutherie]

Originally Posted by Bassamatic

You star ground a P bass the same as everything else. Every wire and terminal that is to go to ground has a wire to a central spot - preferably in the center of your shielded control cavity.

But it's still not really star grounding because the controls are sitting on foil on the pickguard along with the output jack. So you have multiple ground points, not just at that screw.

But the question is, what did that do for you?

[1958Bassman]

Originally Posted by SGD Lutherie

See, this is the problem. You are making the assumption that one end a grounded wire is not grounded, and it's leading to ground. This is wrong. The whole wire is at ground potential. On a car the whole cassis is also ground, which is why ground connections are made to it. When those connections get flakey, as on an old car (like the 1980 BMW I used to have), then current will find the next lowest potential, which is often not ground.

So if a ground wire is grounded, how is it an antenna for noise? It that were true, then your grounded shields (like on your coax cable) and strings would also pick up noise. But they don't when they are grounded. That's why they are shields. If you unground them, they will be an antenna for noise.

Once again, in tube amps and similar situations, you have a lot of high voltages which are causing ground loops. The star grounding is to eliminate ground loops. But this discussion is mixing a lot of metaphors on what star grounding is for. It's not to cut down on RF/EMI pickup. That's what shields are for.

What I'm waiting for is a controlled test with a before and after demonstration showing how star grounding is cutting down on noise in a bass. In the test nothing else would be changed but the ground paths. This has not been done yet. All we have is anecdotal stories about two different basses, or people's experiences in fields that have nothing to do with bass guitars, or incomplete electronics theory.

I've been wiring up guitars and basses for about 41 years. I have tried all different ways of doing it, including star grounding. In my opinion based on my own tests, I find no benefit in doing it. I got the exact same results with a fully shielded control cavity and pickups, and the control cavity grounds connected to the closet point, with a single or sometimes dual ground wire running to the output jack. This has created basses so immune from noise that I often disconnected the bridge ground wire.

I wasn't making assumptions- using a ground wire and attaching it at several points along the way was the "conventional wisdom" in the late-70s but the problem was, it didn't always work, most of the people writing articles didn't have a clue and that's how most car stereo installers got whatever "training" they received. Most store owners just asked if someone wanted to do it and someone always said "Sure, why not?".

The real problem with cars is that the belly pan isn't one piece of steel- it's usually made up of several and this means it's really NOT at ground potential. The battery grounded either to the inner fender or the block, which was bonded (not grounded) to the body to keep RFI from escaping and messing with anything outside of the engine compartment. The resistance I measured between the amp's location and the battery - post ranged from about an Ohm to many Ohms, sometimes tens of Ohms. At 12VDC, that's a big problem. When the difference between the ground wire in the dash and the trunk was more than about .1 Ohms, we got noise and back then, either it was a matter of using a noise filter or living with it because car stereo hadn't developed to any level of A) quality, B) training or C) filled with easy-to-find info like the internet. I can name one site that would have been sufficient for anyone to learn to do car audio and never have a problem, but this didn't exist.

BTW- all of the ground connections on a car (especially of that vintage) work great for lights and motors but that's hardly sufficient for an audio system. Re: finding the next best path- I ran into that so many times it amazed me those cars actually ran and the stereo worked. If you had a 320i, did you have a loud click when you down-shifted or let off of the throttle? I had to put a cap on a lot of BMW kick-down solenoids. Also, the belly pan is looked at as a good ground but when you have lights, rear window heaters, motors and trunk release solenoids sharing the path with audio equipment, noise gets in. Even a loose air cleaner wing nut will cause it to act as a secondary radiator if the engine block's bonding conductor is loose/missing/corroded. I saw that one, too.

How can a "ground wire" act like an antenna? When it's not truly grounded. If it's slightly above ground potential, the noise can still affect the signal because the amp's power supply ground and signal ground aren't completely separate by virtue of the lifted ground. The difference is made up by the current using the audio cable(s) as the path, which is a classic ground loop. Also, other things that are supposed to be grounded but aren't, can act as radiating antennae for noise, like the mid-'80s Cavalier hood bolts that were spec'd with one part and a different one was subbed, causing a lot of noise complaints and a tech bulletin put out by GM for all of their service shops. I got one from my friend, who worked for GM at the time and he knew I did car audio, so he would send bulletins with anything that pertained. Another one was for the Celebrity, which caused EMI to be picked up by the tape head in certain head units (Pioneer was the worst) because the head didn't have a good EMI shield and the wire carried enough current that it's path and proximity to the head unit made it a problem. The official "fix" was to re-route the wire back into the engine compartment, across the firewall and back into the passenger compartment to the heater motor, covered by high-temperature wire loom. The alternative (since most people didn't have a connection at GM) was to use a piece of Mu-metal, made into a shield and tested for its effect before mounting. That remedy was in a trade publication series of articles by Dave Navone.

Electricians usually make that comment about having many grounds when someone tells them that they want a dedicated circuit for all of the AV/security/network equipment, or several circuits that are on the same phase and all should be bonded.

The points I have learned by having done car audio are:

1) Don't assume a ground is good.
2) Find out what is causing the noise. Adding a ground wire or braided shielding for RF doesn't do much/anything when the problem is EMI and an EMI shield doesn't help either an RF problem or a ground loop.
3) Ground things to one main conductor, looking toward the real ground from the end of the chain and that means from the strings through the instrument's circuits to the next device, through that device to the next, etc and on to the receptacle and building's ground. Everything that's grounded in the guitar needs to connect to the amp's chassis with as little resistance as possible.

I'm surprised the practice of shielded, twisted pair hasn't become a well-used method to separate the signal path from the shield and combine it only when it gets to the amp.

If it's a simple rig and only has a guitar and amp, this should be easy but I have seen this fail because the manufacturer or person working on the instrument didn't pay particular attention to this and left a wire off, a control or jack, an un-grounded/cold solder joint or the wire had most of it's strands broken, especially on the braid.

[1958Bassman]

Originally Posted by pkstone

I'm about to shield the control and pickup cavities on my Jazz. While I am dubious as to the benefits of star grounding for noise reduction, it looks like a good way to physically wire the electronics. Soldering to the back of a pot seems like a bad idea to me, i.e., very prone to cold solder joints on such a big heat sink.

What do those of you with experience in bass wiring think of star grounding strictly from a quality-of-wiring perspective?

OK, but did you consider the fact that the cover if a pot is supposed to be grounded, to allow blocking noise? It's convenient to solder a lug to the cover and then solder a wire to the back and carry that form one pot to another, then to the jack. It doesn't hurt anything if the iron isn't too hot/cold or in contact for too long/short a time and because it works. Ever wonder why a Strat has un-shielded wires from the pickups? It's because twisted pair wires reject certain kinds of noise well enough that braid isn't needed. It's the same reason phone and network cabling uses twisted pair- it's not used for instrument cables because they're handled frequently and this could un-do the twist, rendering any benefits useless.

[1958Bassman]

Originally Posted by SGD Lutherie

+1

I have a test anyone here can perform to see if a grounded wire will introduce noise into your signal. Get about 10 feet of unshielded wire. Something manageable like 28 gauge.

Now solder one end to the ground at your output jack, or any ground in the bass. Back of a pot, doesn't matter. Do you hear any noise? Of course not. Any signals being picked up by that wire are now shunted to ground.

If you connect that wire to the hot side of the jack, you will hear all the noise it's picking up.

Since you're a builder, have you ever seen/used Mu-metal? It's copper-nickel alloy. Stiff, very sharp when cut (meaning, watch the edges) but as a magnetic shield, I wonder if there's much merit in testing it for cavity shielding. It would be grounded by soldering onto it or using the pot/jack mating surface as the connection. Another material recommended at the time was copper foil tape with an attached copper screen. The foil had adhesive on the plain side and the foil was on the outside. Since copper screen is an effective RF barrier, the combination was very effective in certain situations.

[azureblue]

star grounding is used in a recording studio to reduce ground loop potential, which causes hum, due to the long electrical lines that have to be run to the console, power amps, FX rack,etc. The solution for signal level runs is to float the ground at one end of the balanced cable. I found I got the lowest noise floor with this, if I used a twisted two pair, ala Mogami, that is the white pair for +and the blue pair for-. I tried it for my bass's bass to amp cable and it helped. Inside a bass, it may work with a 2 conductor / shield cable, if the collective shields and the - were all connected to the jack's ground, and no shields connected within the body cavity. And you used a standard hot plus shield guitar cable.

Really, though, the entire bass rig, from the pickup to the amp's AC cable, has to be looked at as a whole, when it comes to reducing the noise floor.

[1958Bassman]

Originally Posted by azureblue

star grounding is used in a recording studio to reduce ground loop potential, which causes hum, due to the long electrical lines that have to be run to the console, power amps, FX rack,etc. The solution for signal level runs is to float the ground at one end of the balanced cable. I found I got the lowest noise floor with this, if I used a twisted two pair, ala Mogami, that is the white pair for +and the blue pair for-. I tried it for my bass's bass to amp cable and it helped. Inside a bass, it may work with a 2 conductor / shield cable, if the collective shields and the - were all connected to the jack's ground, and no shields connected within the body cavity. My bass has a foil shield inside the backplate, which is grounded to the jack.

True balanced audio has a pair for the signal, floated above chassis ground potential, often about 50 Ohms above. I suspect that the twist is reducing the noise. Look into 'common-mode noise'- this is exactly why twisted pair works and it has to do with the magnetic field caused by the current in the wires. The field from one wire cancels the other.

[Troph]

Originally Posted by Eight_Stringer

Which is it, a signal carrying conductor or a shield, cannot be both can it? Conundrum one.

Perhaps you were just posting these to be sarcastic, but I' not sure. In any case, I'd say, this isn't a conundrum, is it? It is both. It's a shield because the shield conductor is hard-wired to ground potential, and therefore it is much more difficult (although not impossible) to induce a current there. The inner "hot" conductor, by contrast, is essentially at "floating" potential, driven primarily by the weak inductive field from the pickup. It's much easier for stray fields to induce unwanted currents in the inner conductor, because nothing else is holding it at particular potential.

The shielding is also a "signal carrying conductor", although in reality it is only carrying current. Because it is held at ground potential, the signal on that side of the circuit is basically useless.

It would be great if there was a "shield" that did not carry signal current, lets try a two core inner conductor with an outer conductive braid. Wonder why that is not used all the time? On everything?

Obviously balanced cables with differential signal conductors are used in pretty much everything (with respect to pro audio equipment), including microphones.

I think there are 3 reasons why they're not more popular on instrument signals:

1. The noise floor on guitars is already pretty high to begin with (i.e., they're noisy), since the pickup system in guitars already introduces tons of noise not present in the majority of microphones. The noise coming from EMI into a shielded unbalanced cable is likely to be smaller than the noise coming from the guitar itself by an order of magnitude or more, so for most applications, reducing noise coming from the instrument cable would not significantly impact the overall signal to noise ratio. (There are some exceptions, obviously.)

2. Balanced runs are usually done out of necessity due to really long distances, where interference has more opportunity to cause problems, and differences in ground potential can introduce noise. But instruments are typically played near the corresponding amplifier with much shorter runs.

3. Unbalanced 1/4 inch TS jacks are ubiquitous for instrument amplifiers, so for historical compatibility reasons, they continue to be popular.

In any case, it's not hard to mod your guitar/bass to produce a balanced output, if that's your thing. It's a little harder to mod your favorite amp to accept a balanced input signal, but you can always use microphone preamps.

[SGD Lutherie]

Originally Posted by 1958Bassman

How can a "ground wire" act like an antenna? When it's not truly grounded. If it's slightly above ground potential...

A situation that will never arise on a bass. You have one ground point at the output jack.

[okcrum]

Originally Posted by SGD Lutherie

Right, and that 60 cycle hum is not from RFI pickup, which is electrical field noise like you sometimes hear when you aren't touching the string.

Single coil pickups are susceptible to magnetic field noise, which is a low pitched hum. Humbuckers eliminate that, but they don't fix electrical field noise; that high pitched buzz. Shielding helps with that. Star grounding does not, nor does it get rid of the magnetic field noise picked up by single coil pickups.

The problem is that electric instruments pick up different types of interference. But in these threads these interferences are being clumped together, and then star grounding is said to be the cure. But star grounding is for yet another problem, and not the above listed problems.

True. The shielding in a typical (let's say P) bass is an electrostatic shield, so it does not particularly affect electromagnetic field coupling, mostly to the pickups, because of transformer action. Think of the pickups as the secondary of an air coupled step up transformer. The "primary" is nearby power wiring or a nearby power transformer whose magnetic field intersects the pickup winding. This isn't always simply hum, though, as the field from a typical amp power transformer is pulsed because it's in a rectifier circuit, not driving a motor or some other constant AC current load.