Shielding on a Fender Jazzmaster

[SamBooka]

The subject of string grounds and shielding came up in another thread. I have observations and question. I am sure you all do too. I know I have discussed this in other forums and people tend to get a little quiet.. hopefully people have experiences and tips to share.

I have a fender Jazzmaster. I also live in a VERY noisy house (buzz, hum, call it what you will). Jazzmasters are REALLY bad for this. Thin, wide coils. By changing my position (rotating the guitar towards North East ) I can increase or decrease the hum (which in my mind rules out dirty mains as a major source of noise). So I decided to shield it.

Removed all the original wiring including the pickup leads. replaced them with gibsonesque wire (braided shield). I used this everywhere. (pickup to controls to output) This is in addition to factory foil and paint job. I also copper taped and grounded the pickup covers (yes.. I went THAT far) I have read numerous times that changing the leads tends to roll off high end because of increased capacitance. I can live with that. I have attached a pic of the end result . The cables I use are good quality (but a little older) belden cables. about 8ft. The amps used were a Princeton reverb clone, Deluxe Reverb, YGM-4 Studio mate set for clean, no pedals. The pickups are SD SJM-1s RW/RP

What did I notice?

1) With the volume rolled all the way off. No hum (no surprise but it rules out a noisy amp.

2) When using the neck or the bridge pickup VERY noisy. In my house basicallly unusable I didnt notice a big difference in before and after shielding

3)when I touch the strings now there is no Click and drop in hum. Almost like the strings are not grounded. (yes.. I checked.. they are). So the hum is there.. but it is consistant.

4) When both pickups are selected (humbucking mode) the guitar is dead quiet. Well.. of course, they are acting as a humbucker.

What I had concluded (resigned my self to, really) what that you can shield your guitar as much as you want. It will make a difference BUT .. well .. single coils hum. Apparently several engineers looked at this and come up with a pickup to Buck the Hum.

What I also concluded (which I keep saying over and over in the offsetguitars forum). If your using RW/RP pickups and your guitar is quite in HB mode.. your guitar is quiet. Dont go nuts sheilding the cavity (seen guys shield the tremolo cavity, a stewmac page where the show shielding and entire TELE PG (?!?!).

What I havent tried yet it going through my house and removing all the dimmer switches.

[chrisp]

Hard (more like impossible) to sort out problems from afar, but to kick it around some,...

Some irresponsible web-opinion in senseless order:

I 100% agree that shielding can only somewhat help single coil hum.

If your volume control is set up with the center wiper of the pot as the output, then turning the volume all the way down also often gets rid of cable noise as well as guitar noise. It can also get rid of noise that happens between your amps input jack and first gain stage. So 'no-hum' with the volume all the way down does not mean that the noise is 100% from the guitar.

If your amp, cable, guitar go dead quiet with two single coils set up as humbucking in the middle selector position, then it is the guitar that is the source of the single-coil noise.
.

A common source of hum is a ground loop. ALL grounded points in your entire system should have just one single path to ground. (As mentioned below, ground loops even within your guitar can be a source of hum.) If you plug into two amps in series, running an input cable from one amp to the other, AND if both amps have grounded plugs, then you have a ground loop. Your guitar has two separate paths to ground. This can often cause noise. Under particularly unfortunate circumstances, it can shock you.

Also, if your pedal board power supply has a grounded plug, then you most likely also have a ground loop.

Shielding an entire pickguard is not as silly as it sounds. This is not so much for shielding external sources of noise, but rather to keep static from building up as you play, and rub dry fingers over the plastic pickguard. It is very common for an archtop with controls mounted on a plastic pickguard to give off odd crackles while playing in the dry winter. Shielding the entire back of the pickguard helps a great deal in this case.

All in my opinion.

[Meggy]

I have a "strat-type" guitar, which I did a complete shielding job on a few months ago, plus changed the wiring and switching system on it while I was at it. I basically followed the advice on GuitarNuts(tm) - Guitar & Amp Wiring, Hum & Noise Reduction, and Forums. He also talks a lot about avoiding ground loops and using a star-grounding wiring layout, and I'm sure that's important too. On my guitar the results were impressive - the hum was markedly reduced, and other noise/interference with the signal very much reduced too. I'm not an expert, but my experience has been that if you get it right, the improvement is very worthwhile. However I realise that there could be other issues with your guitar that I'm not aware of. Hope you can get your guitar working well though!

[randyc]

Drew has experienced what anyone who has attempted to shield a Fender has noticed. The reason is fairly straightforward but not to the average person, let's try to simplify.

There are two types of interference that affect your guitar: electrical and magnetic. An example of radiated (electrical) interference would be when the police car or taxi drives by your house and you briefly hear his radio transmissions. That's an easy one to fix, usually a very small capacitor value directly across the input of your amplifier will suppress RFI (radio frequency interference) adequately. So can modest amounts of shielding in the cavity where pickups and wiring are located. Fluorescent lighting and light dimmers are another source of electrical interference.

Fortunately, amplifier manufacturers made modifications to mostly suppress RFI years ago and it's not common to experience that problem. Mostly what we experience is magnetic interference, originating from power lines. Because of the very low frequency (60 Hz) and the fact that you cannot play an electric guitar without being very CLOSE to a power line, it's virtually impossible to shield a guitar from this problem. Paint, aluminum foil, copper tape and the like won't do a thing because they are not MAGNETIC. There are only two ways to suppress magnetic interference and Drew has alluded to the only practical method: configuring pickups in humbucking mode.

The remaining technique would be to surround the pickup coils with magnetic material, any ferrous (iron alloy) material. In fact there are special alloys designed to do just that, the most popular called "mu-metal". "Mu" is a letter of the Greek alphabet that is used in engineering to describe the magnetic characteristics of a ferrous or ferrite material.

The amount of shielding required to effectively block 60 Hz hum would make the guitar an impractical personal instrument. I'm sure that everyone has wondered why the factories didn't take care of this problem decades ago and that's the reason: they didn't care to manufacture guitars that required two people to play them.

Gibson P-90 pickups, although they are single coils, like Fenders, are not as susceptible to hum as Fender pickups. This is because the pickups are well-shielded magnetically except for the tops. They are mostly made with steel stampings and brackets, with one large magnet, instead of a plastic housing with 6 individual magnets. (However they do not have that Fender sound which many of us appreciate.)

Thanks Drew, for opening the discussion. This is a topic that comes up on every guitar forum that I've seen. I'm surprised that we hadn't discussed this before (or maybe we did and I've forgotten).

cheers,
randyc

[chrisp]

Randyc,

Can you clarify something for me?

What is oscillating in the two cases you describe? Is one case an oscillating magnetic field and the other an oscillating electrical field?

Are not they both "electromagnetic radiation"? Is the difference in the frequency?

I understand the unification of electricity and magnetism, but do not understand what is actually happening in the two types of radiant energy you describe.

[SamBooka]

Oh.. I forgot to attach the pic.
Plenty of ground loops but.. I dont buy that in passive circuits. I cant see there being that much of a difference in potential.

[billkath]

Oh.. I forgot to attach the pic.
Plenty of ground loops but.. I dont buy that in passive circuits. I cant see there being that much of a difference in potential.

There doesn't need to be much of a difference. Any difference at all is enough for there to be a problem.

[randyc]

Yes to all of those questions, generally.

The 60 Hz magnetic field is obtained by generators which, as we'll recall from grammar school science classes, is a device that generates electricity by passing copper wire through a magnetic field. The generators rotate at 3600 RPM which is 60 RPS and provides us with 60 Hz alternating current (50 in many parts of the world).

Diverging slightly - recall that guitar pickups operate identically. A moving magnetic string interrupts the magnetic field of the pickup which induces a current to flow in the copper windings of the pickup. Pickups are just small generators. Thinking about that for a moment, one can start to appreciate just how SMALL an external magnetic field can be to cause serious interference with the guitar pickup.

Concerning oscillations and oscillators, the very first radio transmitters were simply generators operating at many thousands more RPM than our A.C. generators. The technology was primitive but there were no "active" electronic devices at the time, the vacuum tube was still a fleeting thought in the imagination of Lee DeForest. Once the "Audion" (the first modern vacuum tube) was developed then the REAL master innovator of electronics (Edwin Armstrong, close affiliate, friend, business partner and then deadly enemy of RCA, the world's paramount electronic corporation for many decades) applied himself to exploiting the characteristics of the new device and he consequently invented the "oscillator" (and many other circuits that are in daily use). Without this device there would be no electronic circuits of any more sophistication than the first primitive telephones.

Other radio-frequency generation equipment in those primitive days used spark-gaps to generate high frequency signals. Very poor idea because of interference and these disappeared as soon as Armstrong demonstrated his oscillator, as did all of the General Electric high frequency generators of the time.

Some differences in the behavior of magnetic and electrical properties are easily understood by considering the types of interference that we experience. I think that it's now clear why magnetic fields create problems with guitar pickups - the pickup can't differentiate between an external magnetic field and the small magnetic field of its own magnet/coil/guitar string combination so it produces both sets of signals.

When an electrical source of interference is encountered, such as a radio transmitter, a source of sparking (auto ignition, furnace ignitors), a guitar pickup is not affected significantly. The guitar pickup is a magnetic-sensing device, not an electrical sensing device - THAT would be your amplifier and loudspeaker.

BUT, the guitar is an excellent "antenna", capturing sources of electrical disturbances and transmissions and then happily sending that "information" along to the amplifier. Shielding the guitar from electrical interference is almost universally successful - the point isn't to shield the pickup from receiving those interferers, though, the point is to prevent all of the internal wiring from acting as an antenna. Generally, this won't be experienced with quality guitar cables because they are shielded as is most wiring internal to guitars.

Regarding differences in frequency, there are pros and cons in shielding and suppression of electrical interferrence. Low frequencies can be suppressed by installing a capacitor shunting the signal path. Unhappily, that also suppresses the desired guitar signal. Fortunately most sources of interference are substantially higher in frequency than the guitar so it is possible to select a capacitor value that suppresses the interferor without significant effect to the guitar signal. Mostly the amplifier manufacturer takes care of that problem.

Very high frequencies UHF and microwave frequencies cannot be suppressed that easily. Because of the shorter and shorter wavelengths involved, capacitors don't function as capacitors any more, they become inductors! Other techniques are useful for suppressing high frequency interference and most would be bored to tears if I started discussing them.

If I've made this way more complex than it needs to be and confused rather than clarified, my apologies.

[randyc]

Oh.. I forgot to attach the pic.
Plenty of ground loops but.. I dont buy that in passive circuits. I cant see there being that much of a difference in potential.

It would be very difficult to do a better shielding job than this Jazzmaster. I'd say that radio transmissions, light dimmers, fluorescent lighting and most RFI sources would be effectively suppressed.

Without installing humbucking pickups, however, one would have to live with magnetic interference.

cheers,
randyc

[chrisp]

[randyc]>>>If I've made this way more complex than it needs to be

I was going to ask if you think the search for the Higgs Boson is a waste of time, and that we should be really re-evaluating our completely wrong model of entropy and the arrow of time.

But I changed my mind.

[randyc]

That's good, you wouldn't have received a coherent answer from me. I'm no physicist - just a retired engineer with some free time.

[SamBooka]

[randyc]>>>If I've made this way more complex than it needs to be

I was going to ask if you think the search for the Higgs Boson is a waste of time, and that we should be really re-evaluating our completely wrong model of entropy and the arrow of time.

But I changed my mind.

Higgs Boson.. His Live in Rio album was AWESOME!

[SamBooka]

There doesn't need to be much of a difference. Any difference at all is enough for there to be a problem.

For me to eliminate multiple paths to ground I would just need to remove a bit of the shielding from the PG where it comes in contact with the pots.

I am willing to try it but maybe you can explain a bit better what a ground loop does inside a guitar (my example, there is a path to the ground on the output jack a) via the grounded braid on the wiring and b) via the foil on the pickguard.)

[randyc]

Ground loops are more troublesome in amplifiers. In small-signal applications (like a guitar) they are of little consequence. A ground loop occurs when there are high currents present that cause voltage drops in the very small resistance between ground connections.

Typical example would be using the chassis of an amplifier for grounding. Although the resistance between the various ground points might be fractions of an ohm, if signal currents are high - as is universally the case with power amplifiers - then appreciable potential differences can exist.

Observation of the internal wiring of production guitars will indicate that the manufacturer wasn't too concerned about star grounding.

[chrisp]

I suppose star grounding is of limited value in a guitar, but for good form it can not hurt. And with the small voltages from a PU (and the even smaller currents) maybe the micro resistances from one side of a loop to the other could matter a little.

Back to the high pasture of the magnetic vs. electrical noise source,...

I understand a PU as a transducer, changing magnetic oscillation into electrical oscillation.

Electrical sources of noise get directly to the coil and we hear the resulting hum.

In a humbucker, we build the signal from two coils that are wired in opposite directions (180 deg. out of phase). This makes electrical noise from one coil cancel the electrical noise from the other coil.

To make the magnetic signals not cancel each other, we reverse the magnet in one coil vs. the other.

So a source of ambient electrical signal will be very suppressed in a humbucker, while a magnetic signal will not be suppressed.

Now, if this is the case, I do not follow the idea that shielding does not help supress magnetic sources of noise, but a humbucking 2-coil configuration does.

I completely understand the idea that a thin metal shield has virtually zero effect on a magnetic field (otherwise how would a solid-covered humbucker work?). But, it seems to me that if a noise source was magnetic, it would not be "bucked" by the humbucker.

Maybe I'll blast my new Higgs Boson CD, "Auntie Neutrino". I love the pic of "Auntie" on the cover.

[randyc]

Chris, your understanding of phasing and magnetic coupling is correct. I would change the following statement:

"So a source of ambient electrical signal will be very suppressed in a humbucker, while a magnetic signal will not be suppressed."

to read more like:

"magnetic interference is suppressed by the humbucking pickup configuration but electrical interence probably will not be suppressed".

This is because electrical interference is usually the result of the internal guitar wiring acting as an antenna, collecting and then routing the interfering signal to the amplifier (which is where the problem is manifested).

Concerning star grounding let's see if we can provide more intuition into the behaviour by intentionally introducing a ground loop (no star ground) to the internal guitar wiring; you'll have to trust me on the math if your calculator isn't handy …

Resistance between any two ground connections estimated at 0.2 ohms maximum.

Maximum voltage output from a typical guitar pickup might be about 100 millivolts.

Maximum current (from 100 millivolts across a 500k load - volume control, for example) would be 0.2 microamperes (.0000002 amperes).

Maximum voltage drop across ground connection resistances (the ground loop) from 0.2 microamperes would be 40 nanovolts (.00000004 volts).

Typical gain of a guitar amplifier producing about 25 watts output power is 50 - 55 dB, let's call it a voltage gain of 500 for simplicity.

So the output voltage at the speaker terminals caused by the guitar ground loop would be 20 microvolts (.00002 volts).

If the speaker impedance is 8 ohms, then the resultant output power caused by the ground loop in the guitar wiring would be about 50 picowatts (.00000000005 watts).

Although this estimate is based on assumptions (and a few variables have been omitted), the margin of error is not more than +/- 10 dB, which is fairly trivial for the very low signal levels that we're discussing.

Is the ground loop noise/hum detectable? Oh yes, by very sensitive electronic instruments but NOT by human hearing.

If we repeated this exercise with the voltage, current and gain distributions within the typical AMPLIFIER there would be very different and unpleasant results.

I've built a couple of guitar amplifiers without star grounds successfully but one needs to put a LOT of thought into the grounding configuration. It's apparent from opening any old Fender amplifier that star grounds were not extensively used BUT they DID put some thought into how grounds were established. (If one hasn't an intuitive feel for these things, it's best to use star grounds in amplifiers.)


cheers,
randyc

PS: on a whim, I looked up the topic in wikipedia and this seems like a very good description:

How humbuckers work

In any magnetic pickup, a vibrating soft-magnetic guitar string induces an alternating current in its coil(s). However, magnetic coils also make excellent antennas and are therefore sensitive to electromagnetic interference caused by mains wiring (mains hum) and electrical appliances like transformers, motors, and computer screens. Guitar pickups pick up this noise, which can be quite audible, sounding like a constant hum or buzz.

A humbucker has two coils with opposing windings and polarities. The string motion induces current in both coils in the same direction, since the reverse winding and reversed phase of one coil create a signal in the same direction as the other coil. Electromagnetic interference, on the other hand, induces current in opposing directions in each coil because it is only sensitive to the winding direction, which is reversed for one coil. When the signals from both pickups are summed together, the noise is cancelled due to destructive interference, while the actual signal is increased due to constructive interference, thus dramatically improving the signal-to-noise ratio. This technique is called common-mode rejection by electrical engineers, and is also used in balanced lines in audio recording.

[billkath]

I think what you are saying is correct, Randy-but consider this-
The guitar, the cable, the amp. Your measurements are correct, individually and collectively but the circuit is only complete when those three items are all connected together. There are things that happen within guitar and amp wirings that the pure math just doesn't explain-it's the juice-the magic. You'll hate that, being an engineer, but I think it has some merit. It's why people buy Fender Strats with single coils even though they know they hum-it's the magic. It's why guitarists pick Marshall and Fender amps instead of amps designed by audiophiles. It's why copies of La2A limiting amplifiers don't sound like the originals even if they follow the specs to the letter.

The star grounding system is only used for single-coiled pickup guitars, because, as you said, humbuckers use common-mode rejection (same as balanced systems use). The small differences by using a star grounding system are, as you say, inconsequential. However-it's that small change to the circuit of amp,cable and guitar that is helping hum reduction. All of it is important-the copper foil with no breaks, the soldering over the joins,including the joint where pickguard meets cavity, the star grounding in the guitar, the use of shielded cable-it's all vital. You are right-star grounding doesn't stop single-coils from humming-it can't-it's not a faraday cage on it's own. But, when used in conjunction with tons of other little changes, it does have a marked improvement.

Now-why don't Fender do that on their guitars? Cost. Every nickel, you know, has a knock on effect on profitibility. I have a Custom ST Charvel-a very limited edition guitar-that came from the factory set up like this. It was hand built, and they charged for it being quiet. It's like a humbucker in it's quetness.

[randyc]

Hi Bill,

I own single-coil pickup guitars, both Gibsons and Fenders - not one of the four guitars uses star grounding. It's not necessary for small-signal applications. You're right that as an engineer, I don't put much credence into "juice" or "magic" and the like (I'm also an atheist, for the same reason).

I'm not sure what you mean by this comment regarding star grounding: "But, when used in conjunction with tons of other little changes, it does have a marked improvement." I suppose I'd have to know about those tons of other changes and what "marked improvement" means before I'd want to objectively comment.

But frankly this sounds suspiciously like those things that the guitar "techs" recommend (like "re-capping" or "re-tubing" or "re-biasing") that are intended primarily to make money rather than truly enhance performance.

(No offense to any guitar techs that are reading this - every industry does the same thing. Next time you bring the Ford in for an oil change, count the number of other "performance enhancements" and "mileage improvements" that are offerred to you along with that cheap oil change.)

In my opinion, Fender doesn't include those things on their guitars because they aren't important - cost has little to do with the decision, it's just labor and that's a very cheap commodity in the locales where most guitars are made.

If a noticeable improvement could be made, Fender would doubtless make it, advertise it and charge for the improvement. And people would pay because musicians are trustful and not generally knowledgeable regarding technical topics.

The Fender pickup configuration is what it is: noisy and hum-susceptible by nature. It can't be made to be something that it isn't by peripheral changes that don't address the source of the hum.

This stuff is all so relative that it's certainly not worth quibbling about. My '73 stratocaster has several thousand live-performance hours on it and the hum/noise characteristics never troubled me for a microsecond. Obviously it doesn't trouble the other hundred thousand owners that bought the product either.

If noise and hum were that important, people would buy a different guitar or replace the pickups. It's simply the price one pays to have that particular sound.

Cheers,
Randy

PS: I should have corrected this earlier but the wikipedia article that I reproduced above is incorrect when it describes humbucking pickups and balanced transmission lines as being similar. There is no relationship in common between the two items other than they are electrical conductors.

[billkath]

Sorry, Randy-I really didn't make myself clear at all. Must be this blooming volcano dust!!

What I meant to say was "Star grounding, on it's own, won't make a hoot of a difference. However-when combined with foil coating the cavity, soldering those joins,foil lining the pickguard and using shielded cable,with each sheild connected to the star, it does"

See? It IS addressing the root cause of the hum-RFI interference. You are, in essence, building a faraday cage.

I agree with you wholeheatedley-star grounding, on it's own, makes not one bit of a difference (in a guitar). Put together with the rest though, it does "quiet the beast"., which is what the original post was about. The thing he's missing is the star grounding, and maybe a capacitor, to correct the rolloff due to higher capacitance with all the shielding.

[randyc]

Man, you in Ireland are really getting plastered by that stuff !! (Reminds me of the year that I moved to Eureka, there were forest fires all over the place and I didn't stop coughing for a month !)

Getting back to shielding, those foil materials are non-magnetic and have little more effect than a sheet of plastic, that's why Fender doesn't make those changes to the basic design, although they have used stamped steel (magnetic of course) shields internal to the wiring cavity - even that is not truly effective because the material is just too thin. The foils are fine for RFI (radio-frequency interference) but not effective for blocking a magnetic field. (You might want to do a web search for "mu-metal" and similar magnetic shielding materials.)

Actually, a simple experiment should be convincing: place a small piece of ferrous material (any iron alloy) on the top of a horizontal surface of thin copper or aluminum. Place a magnet on the bottom surface and note that it is possible to easily move the ferrous object by moving the magnet underneath the non-magnetic "shield".

An effective EMI screen (electromagnetic interference), like those used for critical noise measurements, are constructed from a lamination of steel and copper - steel for magnetic shielding properties and copper for RFI shielding. The "Screen Rooms" (as we call them) that I have used even have airlocks with positive internal pressure to seat/seal the single door. The ventilation system is highly filtered with labyrinth layers of "steel wool". Grounding rods are sunk as far as forty feet deep into the earth (underneath or at the water table level).

Making an effective electro-magnetic shield is not trivial - if it was, guitars would have included these features since Gibson first stuck a pickup on one. Because of the impracticality of devising thin, lightweight, low-cost magnetic shielding, Seth Lover invented the only practical device that has been yet devised for guitars.

Cheers,
Randy

PS: Faraday shields don't block magnetic fields, only electrical fields

[billkath]

Copper foil is non-magnetic? It's expressly designed to shield EMI (electromagnetic interference). That is the ONLY foil recommended for this purpose. They didn't include it in the basic design of guitars because dimmers, refridgerator and air conditioning generators , flourecent lights and all that sort of nonsense weren't in the average house in those days. It wasn't a problem then. Why haven't Fender done anything? They have. They designed the noiseless pickup. The SCN. They can charge a premium on them. They've also changed the sheilding on their stock pickups, and upgraded the wiring to shielded on modern Strats and Teles.They are not as noisy now as then. You're not shielding against a magnetic field, Randy-you are shielding against the noise component in the EM field which includes Stray RFI and A/C radiation.

[SamBooka]

Ok.. I will play along
As of tomorrow aft. No more ground loops. I will let you know how it goes.

[billkath]

Ok.. I will play along
As of tomorrow aft. No more ground loops. I will let you know how it goes.

Remember-the shield on the shielded cable has to be connected to the star as well-on each piece of wiring. And the copper foil around the cavity must touch the coper foil on the pickguard-and the control knobs must be physically attached to the foil, as do the screws. It has to be done exactly as the article suggested to bring your noise levels to an acceptable level. Remember-it won't eliminate it-only bring it down.

[randyc]

Copper foil is non-magnetic? It's expressly designed to shield EMI (electromagnetic interference). That is the ONLY foil recommended for this purpose. They didn't include it in the basic design of guitars because dimmers, refridgerator and air conditioning generators , flourecent lights and all that sort of nonsense weren't in the average house in those days. It wasn't a problem then. Why haven't Fender done anything? They have. They designed the noiseless pickup. The SCN. They can charge a premium on them. They've also changed the sheilding on their stock pickups, and upgraded the wiring to shielded on modern Strats and Teles.They are not as noisy now as then. You're not shielding against a magnetic field, Randy-you are shielding against the noise component in the EM field which includes Stray RFI and A/C radiation.

I certainly don't want to invest time attempting to reason with someone who has their mind made up but with no science to support the mindset. This is exactly like arguing religion - cheers.

[billkath]

I have science, Randy. You are simply misunderstanding what the problem is. Take a look here-he doesn't actually go deeply into the science of it, nor does he need to, because what he is saying is very basic.
GuitarNuts.com - How Magnetic Pickups Work