Shielding - How do you know it will help?

[Old Garage-Bander]

AM radios and old Walkman players are great for this. If someone wants to check for EMI and the motor has stopped working, it's a good time to remove the tape head, solder it to a piece of Belden 8451 (or equivalent) and solder the other end to the Walkman. Press the Play button and move the head around- it will pick up any nearby EMI sources that aren't picked up by the AM radio and even if the radio does pick it up, the tape head is more directional. This is a troubleshooting tool that was recommended when I did car audio.

Oh man! Wish I had a gizmo like that when I was doing car audio.
I did make a "clicker" in my auto daze.
Basically just a battery and a "dirty" switch.
I'd click it to generate a little noise and move it around to see where the noise was loudest.

Yahoo! 8451. If I had a nickel for every foot of that stuff I've used...

 

[jamro217]

It's not so much whether you need it or not, but rather a case of "will it hurt". The answer to that is no, but since it's so inexpensive, why not do it? Even if it only helps you once, it's worth it.

 

[somebrains]

Here's a valuable clip:

 

[DrummerwStrings]

Some components, and some faulty wiring can act as a diode and rectify the signal. In extream cases you might even hear a radio station.

Reminds me of when I was little, being surprised that a toy's speaker is playing a radio station & I would open it up and look at the wiring. I was stupid enough to ignore the Warnings on the back - found out about capacitors years later - but I was smart enough not to poke around with a rod or try to remove components, & I was definitely Lucky. It's been so long since I took Physics I forgot my grades, so I'm brushing up before I start modding my bass & amp - I'm a few chapters in on the US Navy's NEETS Basic Electronics book/course.

Learning a lot from this thread, filling in some gaps, thanks all.

Doesn't the UPS have a power line conditioner?

+1 (KbD beat me to it again) I'm going to dig out my UPS manuals, I could have swore they had conditioners - I thought they basically each had a battery & my electronics were avoiding spikes by running off the battery.

 

[Killed_by_Death]

Ah yes, the Naval Electricity & Electronics Training Series. At one time I had every volume, but apparently they're available online now.

 

[DrummerwStrings]

Ah yes, the Naval Electricity & Electronics Training Series. At one time I had every volume, but apparently they're available online now.

Your tax $ at work - the whole course is available as a free download - thanks DoD.

While searching for NEETS I found some good info on using tubes (from RCA?) - don't remember the title & I saved it on a computer that crashed, will be faster to google it than to load a backup (& off-topic anyway).

 

[Bruce Johnson]

Yeah, to answer Killed By Death's overall question, the combination of humbucking pickups and a careful job of shielding all the wiring inside the bass is really the best you can do, to the bass itself, in preventing stray electrical noise. I don't know what else you could do to the bass. Is humbucking and shielding absolutely necessary? Well, no. You may be playing it in a location where there isn't much electrical noise. Or you may use a noise gate in the signal path. Or the hum may not bother you.

Will having a fully humbucking and shielded bass guarantee that you won't have hum out on a gig? Well, no. The whole amp system has its own vulnerabilities to electrical noise, which are much greater and more complicated than in the bass. The amp system can easily have big hum problems that have nothing to do at all with the bass. Having a silent bass won't stop hum that's being induced directly into the amp through the air or through the power lines.

The bass is just one of the antennas that can pick up noise. Making your bass as quiet as you practically can, eliminates it as a noise problem, but doesn't necessarily eliminate noise overall.

 

[Barrytone]

FWIW, this is a 60 Hz tone:

That is not the tone I'm hearing with my single coils.

You would be able to hear it if you had a PEQ capable of boosting a notch at 60Hz up very high, however the first order harmonic at 120Hz is what may be most audible, because the magnetic flux density is strongest twice every electrical cycle, so the fundamental "hum" frequency will be twice the electrical frequency. Additional harmonics above 120 Hz may also be present, so what you hear is the typical mains "buzz".

I'm speculating a little here as I'm no electronics expert but, from what I can gather, It may be that your amp has a conventional power supply (i.e. not a SMPS) so all the ambient 60Hz induction interference in your electronics are going to mains earth, so touching the strings makes no difference, but you still get the noise anyway because it is so difficult to fully shield single coil pickups from ambient induction. My bass amp is a modern class D with SMPS so has no contact with mains earth. Even though the electronics of my bass are well grounded and shielded I get a little noise but I get a reduction in that noise when I touch the strings because I am acting as an earth which is not present in my bass amp. I think - would be interested to know what the electricians make of that.

 

[Rockin John]

You would be able to hear it if you had a PEQ capable of boosting a notch at 60Hz up very high, however the first order harmonic at 120Hz is what may be most audible, because the magnetic flux density is strongest twice every electrical cycle, so the fundamental "hum" frequency will be twice the electrical frequency. Additional harmonics above 120 Hz may also be present, so what you hear is the typical mains "buzz".

I'm speculating a little here as I'm no electronics expert but, from what I can gather, It may be that your amp has a conventional power supply (i.e. not a SMPS) so all the ambient 60Hz induction interference in your electronics are going to mains earth, so touching the strings makes no difference, but you still get the noise anyway because it is so difficult to fully shield single coil pickups from ambient induction. My bass amp is a modern class D with SMPS so has no contact with mains earth. Even though the electronics of my bass are well grounded and shielded I get a little noise but I get a reduction in that noise when I touch the strings because I am acting as an earth which is not present in my bass amp. I think - would be interested to know what the electricians make of that.

With all due respect, Barrytone, the first harmonic - usually called the fundamental - is 60Hz in this case. The second harmonic is 120Hz. In theory, the power line should be a clean sine wave. But it won't be due to back EMFs from switched, inductive loads elsewhere on the system. (This is why there are strong moves to use electronic filtering to mains powered equipment). It could of course be the case that 120Hz is strongly radiating from somewhere and that is what the bass picks up.

As to the SMPS, it is almost certain that some components will be referenced to mains earth somewhere in the PSU. These would normally be filtering components (capacitors) and could be on the incoming AC side, the DC side or both. Sometimes a DC path to earth is provided by connecting the 0V side of the DC to earth or referencing it to earth with (say) resistors.

You cannot be acting as an earth as such. Your body resistance - depending on various factors - is likely to be in the K Ohms region: not an effective earth at all, and that doesn't take account of the added resistances of your shoes, your floor, and so on. The capacitance of your body may be a different thing, tho....

The whole area is a really difficult topic, to be honest, and there could be lots of things causing very similar symptoms. Shielding the bass with copper tape or these sprays and keeping as far away as possible from the **sources** of interfering signals is probably the best way to stay hum free!

 

[1958Bassman]

OK on the amplifier stage. My mistake.

I still don't see how you are using this to find noise on a performing stage.
They shouldn't be used on a hot circuit, audio or otherwise, and if you disconnect both ends of a cable to use the device as it was intended, you're not testing under actual conditions are you?

Maybe I need pictures, because I don't see in my head what exactly you are testing and how you are testing it with this particular device. Are you only using the sniffer without the generator? That kind if makes sense to me.

Sorry. I'd like to understand though.

Yes, using the inductive pickup without the tone generator. It will pick up hum from AC lines. You never know if they stapled Romex to the bottom of the stage, drum riser, etc- electricians don't think in terms of causing noise and they often tend to being opportunistic and lazy, like the guys I sometimes work around and I'm tired of them.

 

[megafiddle]

IME, if the noise is being induced in the atmosphere (electromagnetic waves), then shielding will help, but...

All electrical noise phenomena (except DC) is electromagnetic in nature. While this is technically true, it is not always practically useful.

If you look at shielding products, you will see three different values for the shielding's effectiveness: one value for magnetic interference, one value for electrical interference, and one value for plane wave (electromagnetic) interference.

A plane wave is an electromagnetic (EM) wave in free space, and this condition only exists several wavelengths away from the EM source. This distant location is known as the far field. For radio frequencies, the wavelengths are relatively short, and so much of the radio frequency interference (RFI) that we have to deal with exists as plane waves in the far field. Thin layers of copper are very effective in shielding against RFI.

But unless your bass is picking up a ham radio communication or the top 40, we are not dealing with RFI.

So that leaves magnetic and electrical noise. The frequencies involved here are often 60 Hz and it's harmonics, 120, 240, etc. But why don't we simply treat this as a 60 Hz radio wave? And the answer is simply because the wavelength at 60 Hz is enormous. If we were far enough away from the source, we would be dealing with a plane wave; but we are not picking up a 60 Hz signal from the opposite coast of the country, but from locally right here in the room. This close location to the source is known as the near field, and in this case it is the extreme near field.

It is in this near field that we can (and should) treat magnetic and electrical noise separately. In the far field, the plane wave has a very definite relationship between the electric and magnetic fields; in the extreme near field, the relationship is extremely lopsided. So there can be a very strong 60 Hz magnetic field accompanied by a very weak electric field. Or a very strong electric field accompanied by a very weak magnetic field.

So back to the noise problem:

Single coil hum

Single coil hum is caused by strong 60 Hz (or 60 Hz harmonic) magnetic fields intersecting the pickup coil windings. Power transformers and high current power conductors are common sources of these magnetic fields. This type of noise is characterized as being highly directional. There is normally an orientation that eliminates it completely. It is also unaffected by grounding yourself by contacting the strings (or other grounded hardware).

Single coil hum cannot be shielded against without also making the pickup inoperable; anything that blocked the 60 Hz field would also block the magnetic signal from the vibrating string.

Electrical noise

This type of noise is also known as capacitively coupled or electrostaticly coupled noise. The originating source of this noise is from the various electrical wiring and devices in the room, but the most prominant source of this noise in relation to the instrument is often your own body. That is why grounding yourself by contacting the strings eliminates the noise. It eliminates the main source.

There is capacitance between yourself and the instrument wiring. You are one plate, and the wiring is the other plate. And so the noise makes it's way into the wiring by capacitive coupling. Capacitors operate on electrostatic principles, and so this is also known as electrostatic coupling.

This type of noise can be effectively shielded against with thin conductive foil or coating.

Just one more note about near field phenomenon: Imagine a short length of power ground conductor carring a heavy 60 Hz current. The magnetic field is quite large due to the large current; but the electric field is very small; there is virtually no voltage present along the length of the conductor. This is an example of the very lopsided electric/magnetic relationship I referred to. This conductor makes an extremely poor antenna, and radiates an extremely weak plane wave.

-

 

[Killed_by_Death]

Wow, thanks for writing that all out. Do you work for a TeleComm company?

 

[megafiddle]

Wow, thanks for writing that all out. Do you work for a TeleComm company?

Hope it's useful.

Most recently I was working with industrial process control. Very noisy environment, mainly from several 3 phase motor speed controls. If you can imagine a 20,000 watt light dimmer, that's the kind of noise we had to deal with.

Before that, was working in an engineering lab on instumentation for high energy physics research (atom smashers). The problem there was with the extremely tiny signals being measured, like femto coulombs. One femto coulomb is about 6250 electrons.

-

 

[Killed_by_Death]

Gas Turbine Generators put out a lot of noise also. These days fiberoptics are what people are trying to use, completely oblivious to noise, but also more expensive to initially install.

 

[DrummerwStrings]

All electrical noise phenomena (except DC) is electromagnetic in nature. While this is technically true, it is not always practically useful.

If you look at shielding products, you will see three different values for the shielding's effectiveness: one value for magnetic interference, one value for electrical interference, and one value for plane wave (electromagnetic) interference.

A plane wave is an electromagnetic (EM) wave in free space, and this condition only exists several wavelengths away from the EM source. This distant location is known as the far field. For radio frequencies, the wavelengths are relatively short, and so much of the radio frequency interference (RFI) that we have to deal with exists as plane waves in the far field. Thin layers of copper are very effective in shielding against RFI.

But unless your bass is picking up a ham radio communication or the top 40, we are not dealing with RFI.

So that leaves magnetic and electrical noise. The frequencies involved here are often 60 Hz and it's harmonics, 120, 240, etc. But why don't we simply treat this as a 60 Hz radio wave? And the answer is simply because the wavelength at 60 Hz is enormous. If we were far enough away from the source, we would be dealing with a plane wave; but we are not picking up a 60 Hz signal from the opposite coast of the country, but from locally right here in the room. This close location to the source is known as the near field, and in this case it is the extreme near field.

It is in this near field that we can (and should) treat magnetic and electrical noise separately. In the far field, the plane wave has a very definite relationship between the electric and magnetic fields; in the extreme near field, the relationship is extremely lopsided. So there can be a very strong 60 Hz magnetic field accompanied by a very weak electric field. Or a very strong electric field accompanied by a very weak magnetic field.

So back to the noise problem:

Single coil hum

Single coil hum is caused by strong 60 Hz (or 60 Hz harmonic) magnetic fields intersecting the pickup coil windings. Power transformers and high current power conductors are common sources of these magnetic fields. This type of noise is characterized as being highly directional. There is normally an orientation that eliminates it completely. It is also unaffected by grounding yourself by contacting the strings (or other grounded hardware).

Single coil hum cannot be shielded against without also making the pickup inoperable; anything that blocked the 60 Hz field would also block the magnetic signal from the vibrating string.

Electrical noise

This type of noise is also known as capacitively coupled or electrostaticly coupled noise. The originating source of this noise is from the various electrical wiring and devices in the room, but the most prominant source of this noise in relation to the instrument is often your own body. That is why grounding yourself by contacting the strings eliminates the noise. It eliminates the main source.

There is capacitance between yourself and the instrument wiring. You are one plate, and the wiring is the other plate. And so the noise makes it's way into the wiring by capacitive coupling. Capacitors operate on electrostatic principles, and so this is also known as electrostatic coupling.

This type of noise can be effectively shielded against with thin conductive foil or coating.

Just one more note about near field phenomenon: Imagine a short length of power ground conductor carring a heavy 60 Hz current. The magnetic field is quite large due to the large current; but the electric field is very small; there is virtually no voltage present along the length of the conductor. This is an example of the very lopsided electric/magnetic relationship I referred to. This conductor makes an extremely poor antenna, and radiates an extremely weak plane wave.

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Thank you Megafiddle for a well-written, entertaining post. I learned something today.

I wish I could say the same about more academic papers & work memos.

 

[megafiddle]

This is exactly why I started this thread. Isn't 60 Hz hum a thing of that past that came in through the power on amps that weren't properly isolating the "ripple" from the instrument?
Isn't all the noise today electromagnetic interference & not actually caused by the 60 Hz of the power grid?

I believe you are referring to the deep toned hum often caused by improperly grounded amps?

Line voltage is a fairly good sine wave, and has a strong fundamental tone when made audible. On the older amps, which used ungrounded 2 wire power cords, it was possible for some of this line voltage to find it's way onto the circuit common (the amp chassis), and into the signal.

There is always some leakage between the power line poles and the circuitry, and this problem was solved by making the grounded neutral conductor the main "leaker". This held the chassis at ground more or less, and was typically done with a grounding capacitor between the neutral wire and chassis.

The path for this leakage is typically through very thin layers of insulation in the power transformer or through the grounding cap, if the poles are swapped. So much of the fundamendal 60 Hz frequency makes it through. Grounded power cords on modern amps have pretty much eliminated this type of noise.

There are magnetic sources that also produce a deep 60 Hz hum across space. AC motors, incandescent lights, heaters, anything with a non-switched fairly high current. Coils of wire greatly intensify the magnetic field, but these are typically wound on a permeable (soft iron) core, which concentrates the field within the assembly. Still some magnetic flux leaks out, and can cause hum in a nearby pickup or circuit.

Instrument amp power transformers would seem to be strong source of 60 Hz hum, due to it's being powered by the AC line. But the important factor here is not the 60 Hz sine wave voltage, but the current. It is the current that determines the magnetic field. And the current in a typical power transformer is not a sine wave, but a series of current pulses, as the current charges the filter caps through the rectifiers. This has the form of a weak 60 Hz fundamental with strong harmonics and is more of a 60 Hz buzz.

So single coil hum can be in the form of a deep 60 Hz hum, but due to the myriad of electronic devices around us, the current, and hence the hum, consists mainly of harmonics.

Electrically coupled noise is much more likely to be a buzz due to the frequency selective nature of capacitive coupling. Just as a capacitor passes higher frequencies more easily, capacitive coupling also passes the higher frequency components of the noise more easily.

-

 

[megafiddle]

When I was using copper tape, I'd always cover it with a layer of clear packaging tape. That would help keep the overlaps stuck together, and provide a layer of insulation so that nothing could short against it.

Fish paper. The old phenolic electrical insulator. Highly puncture resistant.

The thinner stuff can be cut and folded like card stock. A piece with folded flaps can be cut to sit in the cavity.

-

 

[megafiddle]

While I can't explain it, I have seen shielding help to a limited degree with 60 cycle hum.

As shielding thickness increases, it's effectivenss at lower frequencies also increases. Also, 60 Hz hum often contains harmonics that are more easily shielded against, relative to the 60 Hz fundamental. Not sure just how thick it would have to be make a noticable difference on single coil hum though. It would also undoubtetly have an effect on the string tone.

Pickups, whether single coil or humbuckers, are also suseptible to electrically coupled noise. So shielding would reduce this portion of the noise if present, and reduce the overall noise.

-

 

[Jim C]

As shielding thickness increases, it's effectivenss at lower frequencies also increases. Also, 60 Hz hum often contains harmonics that are more easily shielded against, relative to the 60 Hz fundamental.
-

I think we need to define frequency range to make this 100% true.
MIL-STD285 requires 100 dB shielding from 1K to 10gHz which can be easily accomplished with about 20 gauge CRS.
I forget what the H shield low freq. requirement but am certain it's in the K range.
When you get down to very low (hZ range) thick ferrous material becomes your friend.
Not that it matters to this discussion but use a general home RF meter.
Put it up next to your kids wal wart for whatever game.
It will be high; pull it back 6" to a foot and it drops like a rock.

Very good point on harmonics.
Never thought about it but what frequency do you hear with say a Tele and a high gain tube amp?

 

[Rich Fiscus]

I used an actual UPS to power my amp to prove that the noise wasn't coming directly from the power.

It's been a few years since I was in IT, so I haven't had to buy a UPS for a while. Some of this could be a little outdated, but I doubt it's changed too much in the last decade.

Unless you're using a very expensive UPS, until the power goes out, all you're really getting out of it is a surge suppressor. Most of the devices labeled UPS are actual Standby Power Supplies (SPS), meaning you're only running off the battery if you lose power. If it doesn't have an indicator light showing that the surge suppression is working, that may even be optimistic.

Even a unit marketed as a real (online) UPS likely isn't. In many (most?) cases, it's actually a SPS that can handle a certain amount of over or under voltage without switching to battery power. Generally speaking, if it doesn't say double conversion, it's probably not a real online UPS.