Discussion in 'Tele-Technical' started by megafiddle, May 27, 2011.

1. megafiddleFormer Member

Posts:
610
Joined:
Feb 27, 2011
Location:
VA
If you read about noise, hum and shielding, you will often see the AC wiring described
as an transmitting antenna and the guitar circuits as a receiving antenna. The AC hum
is transmitted as a 60 Hz radio wave (electromagnetic wave). Also you may see that
you yourself is an antenna, picking up 60 Hz hum.

As a simplified way of explaining hum pickup in guitars, I see no problem with that. We
tend not to be electrical engineers and physicists. But I do see a problem when an actual
problem needs to be solved, and the simplified explanation is used as a basis for a
solution.
No one really has time to study electrical engineering just to fix a hum problem. But no one
really has time to cover the whole interior of the guitar with copper tape only to find that it

I am providing some links on this subject. If you are interested in this stuff, there is some
good information there. If you are not interested in the theory behind it all, take a look anyway.
If for nothing more than to see that it's based on real science, not fringe.

THE 60 HZ ANTENNA

The effectiveness of an antenna, e.g. how well it radiates depends, for one thing, on it's length
compared to the wavelength of the signal. Antennas are usually tuned, and depending on type,
will be 1/4 wave, 1/2 wave, etc. (wave = wavelength of radio wave being transmitted). As the
length of the antenna becomes shorter compared to it's design length, a mismatch occurs in
the impedance, and the radiation falls off. Shorter you make it, the worse the mismatch becomes
and the worse the radiation falls off.
Now how long is a section of AC wiring in your house? 50 or 100 feet? That's your "antenna".
A 1/4 wave antenna for 60 Hz would be about 790 miles!
To put that in perspective, that is like having an FM radio antenna less that .001" long.
The impedance mismatch is so severe, that I could not even call that 100 ft a wire an antenna.
Those are the real numbers.

A 60 Hz receiving antenna is bound by the same rules. It's ability to receive also depends on the
length and impedance mismatch. The mismatch in this case is not nearly as large, since the
guitar circuits are in the kilohm range, while the AC mains are about zero ohms. Still very poor
though, and add to that the fact that there is not much of a 60 Hz wave to pick up to begin with.
Not to mention that guitar wiring is on the order of inches, not feet.

It gets worse.

THE ELECTROMAGNETIC NEAR FIELD

The energy radiated by an antenna does not behave like an electromagnetic wave until you get
about 2 wavelengths away. For a 60 Hz wave, this is over 6000 miles. This is the far field.
At closer distances, less than 1/6 wavelength, the energy is in the form of magnetic or electric
fields, with one or the other dominating. This is the near field, and is the one in your house.

THE ELECTRIC FIELD

You probably noticed that there is a large hum signal on yourself. I think probably everyone has
at one time or another touched the tip of a guitar cable that was plugged into an amp. That hum
was picked up by you as a consequence of being in an electrical field. The field was supplied
by the 120 VAC wiring all around you. The electric component in the near field.

Also, if you ever had ungrounded strings, you might have noticed that touching the strings caused
some hum in the guitar signal. Again, the voltage on you, now being picked up by the strings and
in turn being picked up by the wiring or pickups.

THE MAGNETIC FIELD

You probably noticed a large amount of hum if you moved the guitar close to the amp, or another
device with a power transformer. This is due to the magnetic field leakage from the transformer.
Transformer shielding helps (the steel bells on the transformer) but does not eliminate all of the
field leakage. There are also magnetic fields from power cables that can induce hum into signal
cables that are lying close by.

Both of these magnetic and electric field sources have a electromagnetic wave associated with
them. But the electromagnetic wave is so weak, that the only thing of importance is the magnetic
and electric components, the individual fields.

THE WATER WAVE

Analogies are always good. You have no doubt seen a water wave. The water does not move laterally,
it moves up and down, and propogates laterally. So imagine that an electromagnetic wave is like
a water wave. But in this case imagine that the crest of the wave is 1500 miles long, and the trough
of the wave is 1500 miles long. Looking down from outer space you would see a giant wave.
Now imagine you are sitting at a fixed point near the surface of the water. What do you see?
You see the water level rising and falling. No wave, no lateral movement, just up and down. That
is what the magnetic field from a transformer is like. No radio wave, just a magnet field increasing
and decreasing and reversing 60 times a second.

http://en.wikipedia.org/wiki/Near_and_far_field

http://en.wikipedia.org/wiki/Antennas

http://vitatech.net/emf_fundamentals.php4

http://www.ramayes.com/aluminum_foil_emi_rfi_shielding.htm

THE OTHER STUFF

Factual corrections welcome

2. Keyser SozeTele-Holic

Posts:
948
Joined:
Oct 13, 2009
Location:
Johnson City, TN
I am of the opinion that most of the hum that we hear from guitar pickups is not actually 60 cycle (or 50 cycle for you European types) but instead is higher harmonics - e.g. 120 hz. (or 100 hz.) or above.

As noted, true mains hum is generally only noticed when you stand really close to an amp, or some other large power transformer, and it rapidly diminishes if you step back a few feet. Also, that hum sounds (and feels) low. It truly is a hum.

Meanwhile the other noises we hear through non-humbucking pickups are typically more buzzy (higher frequency), and often highly directional in relation to items that are broadcasting them (cathode ray tubes, poorly shielded dimmer switches, etc.) But this is also the stuff that can be alleviated by good shielding and grounding techniques.

People like to argue that shielding does not work, which is fairly accurate for true mains hum, but mains hum is rarely the issue.

3. PinewoodRoTele-Afflicted

Posts:
1,366
Joined:
Mar 31, 2011
Location:
Carmarthenshire, Wales.
I tend to agree. And thanks for including us 50Hz types

4. BolideFriend of Leo's

Posts:
4,920
Joined:
Nov 21, 2010
Location:
Rocky Hill, CT
Your sixth harmonic is our fifth harmonic, so we are already on the same wavelength.

5. dsutton24Doctor of TeleocityAd Free Member

Posts:
10,200
Joined:
Dec 29, 2010
Location:
Illinois
Everything you say is true, but it leaves out a big factor -- gain. Lots and lots of gain. Even the least efficient antenna and coupling sysems will produce signal (in this case hum or noise) if you amplify it enough.

6. jefrsDoctor of Teleocity

Posts:
13,313
Joined:
Nov 20, 2007
Location:
Newbury, England
Missed one - induction (ok, you had magnetism)

Mainly magnetic induction. Magnetism is a "weak force" and its effect falls off rapidly with distance, the rules are erm, complicated. Most radiation falls off with inverse-square rule, electro-static fields do (the electrical field), this is how capacitors work. The magnetic field falls off far more rapidly which is why stepping back from the amp's transformer (the main culprit) is effective.

A changing magnetic field will induce a current in a conductor. Any conductor but coils are really good. Pickup coils work by having their magnetic field interrupted by the vibrating ferro-magnetic string. Nickel is ferro-magnetic, as is the steel used in strings. A nice external magnetic field drives them potty.

The magnetic field generated by the mains transformer is directional and will be at mains frequency. However the output transformer will spit out emf over a range of frequencies including RF, the valves are quite adept at producing unwanted RF signals. These magnetic fields will induce current in various places: transformer cores; chassis; pickups; guitar shielding.

The last is important because any sheet of metal can have an eddy current induced in it, the current travels around in circles. Any current induced on the grounded side at the guitar will also be seen on the hot side, especially as this is fed into a high gain input of an amp. Shielding dumps the electrostatic field but can aggravate the magnetically induced. The solution is to get rid of the magnetic induction at its source by transformer design, orientation and mounting. Bolting it straight onto a steel chassis is a bad idea. Aluminium is a better choice for chassis. Some chassis leave a hole under the transformer. Raising the transformer off the chassis on a sheet of insulator (PCB) breaks the magnetic coupling. The chassis is connected to the cold side of the guitar signal, anything induced on it will be seen on the signal to be amplified.

Electrostatic fields are of less concern because the guitar circuitry is less susceptible to them. However certain household devices to produce them: television; fluorescent lamps; passing taxi radio transmitters. With these the guitar pickup coils can form part of a tuned circuit for receiving them. This is where you and the guitar form a crude antenna, and the pedals and amp front-end the tuning circuit. It takes a fairly strong radio transmitter signal to break through but it does frequently happen. And EMF - electro-magnetic field - a changing electrical current will also form a magnetic field in a conductor, they are inseparable, so now the magnetism is short-range again...

Bit of a ramble but it's 4am here. Why am I awake...

7. megafiddleFormer Member

Posts:
610
Joined:
Feb 27, 2011
Location:
VA
I agree. That power line lying next to a guitar cable, in spite of being a horrible antenna,
is a real source of AC hum. And, a transformer, also a poor transmitter, is a serious source
of AC hum.
My point is that these are not 60 Hz radio waves, but near field phenomena.

8. aunchakiFriend of Leo's

Posts:
3,049
Joined:
Mar 12, 2006
Location:
Central New York
Not trying to change the subject, but I've been wondering something. I play in a church that has a bank of six dimmers controlling overhead lights. In my single-coil guitars it produced noticeable hum. When I'm on the "stage" I can turn my guitar, when it is exactly perpendicular to the dimmers (about 75ft away), the hum disappears.

I've though about shielding my guitars, but why can't I just shield the dimmers? Would that even work?

9. jefrsDoctor of Teleocity

Posts:
13,313
Joined:
Nov 20, 2007
Location:
Newbury, England
Shielding the dimmers may work, re-positioning them so the are not projecting at you would work better. Can they be turned through 90 degrees? Shielding the guitar may make matters worse.

Shielding works best with "electrical" noise, where a device is spitting out RF, things that spark like relay contacts and motor brushes, less well with magnetic/induction fields from motors and dimmer coils.

10. Keyser SozeTele-Holic

Posts:
948
Joined:
Oct 13, 2009
Location:
Johnson City, TN
You can shield either one. In an ideal world dimmers would be properly shielded at the factory and the problem eliminated, but economic considerations mean they are not.

Since you own the guitar, I'd suggest that is where you focus your time, effort, and financial resources.

11. BolideFriend of Leo's

Posts:
4,920
Joined:
Nov 21, 2010
Location:
Rocky Hill, CT
jefrs, aren't most dimmers these days Silicon Controlled Rectifiers which transform the (almost) sine wave of the mains power into harmonic rich pulse waves?

and it seems to me that these pulse waves will be radiated through the leads from the dimmer to the load as well as from the dimmer itself, especially if the wiring has good load balancing between phases with the result that there is little counter-current in the neutral return line to balance the current in the hot line. (code practice in America, and I imagine it's similar elsewhere, is to tie neutrals together close to load with the idea of having return currents run through other phase hot lines in preference to the neutral).

12. jefrsDoctor of Teleocity

Posts:
13,313
Joined:
Nov 20, 2007
Location:
Newbury, England
Ok, well I dissected a dead one for you. Inside is a push-push switch rheostat connected to a little PCB with a couple of components and what appears to be a thyristor tucked down the side. It incorporates a toroidal choke coil. Now the thyristor will indulge in some fast switching which is what spits out the unwanted noise but this is going to be directed through that coil which converts it in to emf for your guitar.

Our neutral is tied to 0V and the live goes 0-245V (AC) single-phase. If you have current on the live then you have to have the same current on the neutral, you have to have a circuit for a current to flow. Maybe I misunderstand what you are saying but you cannot have a current on the live and no current on the neutral. I think Micheal Faraday worked that one out

13. piece of ashFriend of Leo's

Posts:
3,287
Joined:
Dec 29, 2010
Location:
Sugar Land, TX
Light dimmers radiate noise along the entire length of wire that connects them from the source to load... that is the power station to the light bulb. The noise stems from the fact that the current is being interrupted very rapidly... very sharp edges (high frequency content) on the waveforms.

14. dsutton24Doctor of TeleocityAd Free Member

Posts:
10,200
Joined:
Dec 29, 2010
Location:
Illinois
Well, that's a distinction that really makes no difference. If you're being uber-technical, near field is defined as being less than two wavelengths. Since the wavelength of 60 Hz is about 16,400,000 feet, you'd have to be pretty much in the midle of nowhere to avoid near field induction at power line frequencies.

Either way, near field, far field or transition zone, the prinipals remain the same.

15. megafiddleFormer Member

Posts:
610
Joined:
Feb 27, 2011
Location:
VA
It makes a difference if your trying to use RF shielding to stop transformer hum.
It's not going to work if the field doesn't behave like an electromagnetic wave.
Which it doesn't at close distance.

http://vitatech.net/emf_fundamentals.php4

"Furthermore, when the distance from a sinusoidally varying source such as 60-Hz AC power is small with respect to the wavelength (known as the near field), the electric and magnetic fields are not coupled and considered separate physical entities."

16. dsutton24Doctor of TeleocityAd Free Member

Posts:
10,200
Joined:
Dec 29, 2010
Location:
Illinois
I don't know what the author of that gem might be referring to, but if electromagnetic fields didn't couple in the near field, then transformers wouldn't work.

17. megafiddleFormer Member

Posts:
610
Joined:
Feb 27, 2011
Location:
VA
Once in the far field, the electric and magnetic components have a fixed relationship,
given by the 377 ohm radiation resistance of free space. You will always see the same
elecromagnetic wave once beyond this point, and this relationship is independant of the
source that created it.

The electric and magnetic fields do not have a fixed or constant relationship in the near
field. A coiled wire source will produce a predominantly magnetic field, while a short piece
of AC line will produce a predominantly electric field. The impedance of each of these are
reactive and different from that in free space. The shielding requirements for each are also
very different. Out in the far field and beyond, the dominance of one or the other field type
dissappears.

If it was all a single phenomenon (and this at 60 Hz specifically), copper or aluminum foil
would work equally well against transformers and the electric field from your own body.
And it certainly doesn't.

I interpreted "uncoupled" fields as meaning they did not have the close interdependance
that electric and magnetic fields have in a freely propogating wave. There are no options
out there in free space. The magnetic field determines the electric field and vice versa,
as far as relative strength goes.
Not so in the near field. If free field relationships held everywhere, a moving magnet would
be a shock hazard.

More here:

http://en.wikipedia.org/wiki/Near_and_far_field

Last edited: May 31, 2011
18. TequilaCasterTele-Holic

Posts:
621
Joined:
Mar 26, 2003
Location:
Between Time and Space, in Temperance
.
What blows me away, is that electric and magnetic fields are composed of photons, as is all electro-magnetic radiation... radio waves, microwaves, light, x-rays, gamma rays, cosmic rays... etc.
.
And where do photons come from? They come from electrons. Photons are like electron farts.
.
Hey, pull my finger!
.

19. Muddy T-BoneTele-Meister

Posts:
232
Joined:
Feb 23, 2009
Location:
Folsom, CA
Megafiddle-

A 60hz wave is 18.83 feet long- where are you coming up with a 1/4 wave antenna being 790 miles long? It's about 4.5 feet long.

20. jefrsDoctor of Teleocity

Posts:
13,313
Joined:
Nov 20, 2007
Location:
Newbury, England
A straight piece of wire driven by a D.C current generates a rotating magnetic field.
A coil, which is a straight bit of wire wrapped around something, still has that rotating field but now they're all pointing in the same direction so we see poles, and it projects is its field further because it is concentrated and directional. This makes transformers a problem for us. And the choke coil in the dimmer is a greater problem than the cable to the lamp.

Best solution imo is to cure problem at source.
Remove it.
Use something else.
Protect it (re-engineer, shield the source)

wave length lambda = v/f
where for RF in air v = c, the speed of light.
NB you must get your units right
Hz is cps so c has to be in metres/second
c approx 300,000,000 m/s
so lambda = 300,000,000 / 60 =500,000 metres = 500 kilometres

IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.