Shielding - How do you know it will help?

[Rockin John]

One interesting way to look at the issue of electrical noise, is the work of Guglielmo Marconi. By the use of a spark transmitter he transmitted signals across the Atlantic many moons ago. To generate a spark he used an electrical coil (of wire). By supplying a coil with power, then breaking it again it’s possible to generate huge voltages that will jump across a gap to make a spark. The spark contains every electromagnetic frequency imaginable ... from audio - you can hear the ‘click’ of the spark - up to visible light - you can see the spark - and well beyond that. And, of course, there are all frequencies in between. Sparks are very, very dirty!!

This is exactly how automobile ignition systems work and how gas discharge starter circuits work. It’s what happens very time the electricity is interrupted to a coil of wire. So the modern world is rammed full of electrical devices kicking out all sorts of noise and rubbish. No wonder it’s difficult to keep noise out of an electric bass! Nowadays its possible to suppress sparks but older installations may still cause trouble, as folk on this thread have discovered.

 

[Bruce Johnson]

Jim C made the comment a few pages back that spray-on shielding is nowhere near as good as copper tape shielding.

I completely disagree with that. Sprayed on nickel or copper, properly grounded, provides RFI shielding that is just as good as what you get with the normally-used copper tape.

If you measure copper shielding tape, it's about 2.5 mils thick (0.0025"), including the adhesive on the back. The copper itself is around 1.5 mils thick (0.0015").

Sprayed on nickel or copper shielding is more variable in thickness, but it's usually in the range of 2 to 5 mils thick. Some of that volume is the acrylic/polyester base paint, but the layer of metal particles will be several mils thick. They are packed in tight enough that they are conductive on the front and back surfaces and all to each other. The paint is almost as dense as a solid sheet of nickel or copper.

In DC resistance, a cavity shielded by spraying and one shielded with copper tape both measure very low, like less than one ohm across the farthest points. Like a solid metal shell.

About eight years ago, I personally tested the effectiveness of the SuperShield Nickel spray shielding, before I switched to it. I had two of my new Scroll Basses side-by-side, one done in copper tape and one in SuperShield Nickel.

My electronics assembly bench area is actually a very electrically noisy environment. I've got a florescent lamp with an old-style ballast hanging low, various amps and power supplies, and the main power panel for my shop less than 10 feet away. Large 3 phase machines running upstairs, directly above. There's even a museum of antique neon signs here in the building. I kid you not. This is a great place for testing for noise. Basses without humbucking and shielding will hum badly every time, particularly when I hold them up close under the florescent lamp. If it's quiet here, then it's ready to go out into the world of clubs and studios.

Anyway, I'd been doing all my shielding in my basses with copper tape, carefully, for about ten years. It worked; they were quiet. The test instrument done in SuperShield was equally quiet. I tested them side-by-side and couldn't hear any difference between them. Identical instruments except for the color and the shielding. That was good enough for me to switch. I ended up pulling the copper tape out of that one and redoing it with SuperShield. I've been using SuperShield on all of the basses I've built since then, and I test every one of them for noise.

My complaints with copper tape are that it's too much labor to install, and I've had reliability problems with it. When installed correctly, copper tape works very well as shielding. That was never a question. But it's wrong to claim that copper tape does a better job of shielding than a good sprayed on shielding paint.

 

[somebrains]

I picked up a 5-string Jazz with Fralins in it yesterday after avoiding J hum for years.
They sound good, so I'll deal with it.
You do what you can, but not all of the 60-cycle hum will be eliminated.
The bass still sounds really good, and that's what matters until you go buy a noise gate or compressor.

 

[twelvetrombones]

This is the one I'm having difficulty with. Is it because there's more of a magnetic component to the interference?
or is it simply because of the frequency?, which of course every electrical device is powered from

I think that most electrical conductors inside your bass can have electrical currents induced to varying degrees, including the coils in the pickups, wires, pots, what have you. How well each component picks up interference depends on how well they interact as antenna with the frequency content and the strength of the electromagnetic fields in the environment.

Shielding simply adds a Faraday cage that if perfect external electromagnetic radiation cannot penetrate (in reality it will be imperfect because the conductivity of metal is not infinite and you typically have gaps but shielding will reduce the strength of the electromagnetic noise).

Pickups are intended to pick up changes in magnetic field with a frequency of the vibrating strings, which is not too different than the frequencies in mains hum, so I expect that if they have no features to subtract out noise they are very good at picking up mains hum as well.

 

[1958Bassman]

The whole subject of electromagnetic interference is extremely complicated and falls well outside of most bassists, I would think. There are, tho, I believe, lay-mens’ guides to the subject, but bass practice is probably more profitable in the long run! Ha ha.

The Common Mode Rejection Ratio is the ratio of the wanted signal through a system and the unwanted signals (the noise). As noted the CMRR can be improved by twisting signal hot and cold wires together and running them inside the instrument. However, the cable runs are so short that it hardly makes much difference whether they are twisted or not (tho it doesn’t harm to twist them!).

Ever twist the wires in a guitar with single coil pickups? Ever see Ethernet cables that weren't twisted? Did you know that the recommended maximum untwisted wire length in a punchdown block or a keystone connection is 1/2"?

 

[1958Bassman]

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?

Has anyone ever effectively shielded a single coil Precision bass?

Shielding worked for the noise I was experiencing here at home, but on a humbucker-equipped bass.
I'm not sure if it would do any good for the Big Singes in my current Ibanez.

I just cranked up the gain on my amp & soloed the bridge pickup.
Touching strings or bridge did nothing to quiet the noise.

If the amp makes this sound with nothing connected and no cord plugged in, it's a problem with the amp but with an instrument and cable, it's from RF/EMI or bad shielding.

All "grounds" need a great path to the Earth, in order to do their job. You can shield the crap out of something but if the resistance on the path to ground is high, it's like the work was never done.

If you crank the gain, you're amplifying everything- signal AND noise. If it's a big problem and only happens when you're not playing, I would recommend using a noise gate.

 

[1958Bassman]

As a sort-of side issue.....
Without sounding too obtuse, clearly the best way to reduce noise pickup is for players to keep as far away from the noise as is possible! I suppose this is not always thought of when on stage. And, whoever lays the cables for the band should be aware of the need to route the mains supply away from the musicians as possible.

A very inexpensive way to test the stage for noise is to get a cable toner- it comes with an inductive pickup wand and using this would save a lot of people a lot of time.

Cable Finder Tone Generator - Probe Kit

 

[1958Bassman]

So you're saying the 60 Hz hum is magnetically induced?
How is this different from the electrostatic noise that can be shielded against?

I've been trying to get my head around this literally for years now, but recently (since I got a bass with single coil pickups) it's more relevant for me.
Humbuckers solve my noise problem at home, with one exception, a Mexican Fender Blacktop. I'm not sure if it was because the humbuckers were overwound or what...

The humbuckers in my SRX (SR eXtreme) & my guitars are nice & quiet, but the noise doesn't seem to be exclusively 60 Hz hum.

I did some experimenting with my Ibanez Premium (with the Big Singles).
First I tried quieting the single coil noise by touching the strings, doesn't work, nor does touching a grounded switch on the bass. I did discover that there's 200 Ohms of resistance between my bridge & the output jack, and that the coating on Elixir strings is definitely not conductive. Thoe strings are surprisingly well insulated.
That's another issue that I'm going to try and figure out tomorrow, the 200 Ohms of resistance

At least I know the switches are grounded, because if I have both pickups selected & touch a switch there's definitely a drop in any noise.

If I face the bass to the South or North the noise is at its pinnacle, but East or West & it at its minimum. Luckily I play facing East, Feng Shui & all
To try and see if any of the noise is from the power, I disconnected the UPS that feeds my amp from the wall, after I turned off my computer, speakers, & monitor.

Flipped on the amp & tried the same touchy feely stuff, no change. The directional bit still remains the same as well.
Now, it did occur to me that the battery power of the UPS is being inverted to the same 60 Hz that's coming from the wall outlet, so...
how could the noise be directional if it's coming from the power?

The strange thing about running on UPS power was this,
if I put my hand over a pickup, even with both selected, there was extra noise.
It was picking up static electricity from my body, but oddly (& luckily) doesn't do that under normal power.

I'll assume you use 'UPS' to mean 'power filter', but that's not what a UPS is- that stands for 'Uninterruptible Power Supply'.

As I posted, resistance on the ground path and/or neutral makes all the difference- if you have a Multi-meter, measure the resistance from Neutral to Ground at the outlet, then plug in the amp, cables and effects- measure from the sleeve on the cable that you use for your bass to the metal junction box or the ground wire if the junction box is plastic. You WILL see a difference and if it's more than about 1 Ohm, you need to fix that.

If the noise increases when you touch grounded instrument parts, it's because you're acting as an antenna. With WiFi, cordless phones, baby monitors, wireless alarms, cell phones and everything else, it's a wonder it all works at all- most of this stiff works on the 2.4 GHz band. If you have light dimmers, make sure they're not the cause.

 

[Killed_by_Death]

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

 

[1958Bassman]

You're talking about a power conditioner? What exactly do those things do? Can they help with under-voltage situations?

Not unless you get something with power regeneration and it's heavily regulated. The vast majority of these are for surges, although some maintain good grounding- they (Furman) make a lot of equipment for live rigs and some of their design engineers are musicians, so they're aware of the problems that can come with different venues.

Some club owners will do everything in their power to avoid spending money on maintenance, so they grab some schmuck who knows how to twist a few wires together to add/change circuits. That doesn't mean it's done right and it definitely doesn't mean the ground and neutral are close to free of resistance.

 

[1958Bassman]

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

Was it plugged into an outlet? What was the result?

 

[Old Garage-Bander]

A very inexpensive way to test the stage for noise is to get a cable toner- it comes with an inductive pickup wand and using this would save a lot of people a lot of time.

Cable Finder Tone Generator - Probe Kit

Those are intended to be used to find a single cable in a group of cables by placing the generator on one end and "sniffing" the generated signal at the other. Having used these in my work, I would suspect that it would generate too overwhelming of a signal to be useful for testing noise in an amplifier stage. They don't produce a very clean signal. That signal can crosstalk into other cables in the bundle. The cable you are looking for is the one with the loudest signal. The impedances involved in cable tracing are very low compared to those that might be found in an amplifier, especially the pre-amp section. The higher impedances involved in the amp can make isolating the type of signal from one of these tracers difficult. There is other test equipment that I would use to trace signals through an amp. YMMV.

 

[Killed_by_Death]

Was it plugged into an outlet? What was the result?

My Experiment

 

[1958Bassman]

Those are intended to be used to find a single cable in a group of cables by placing the generator on one end and "sniffing" the generated signal at the other. Having used these in my work, I would suspect that it would generate too overwhelming of a signal to be useful for testing noise in an amplifier stage. They don't produce a very clean signal. That signal can crosstalk into other cables in the bundle. The cable you are looking for is the one with the loudest signal. The impedances involved in cable tracing are very low compared to those that might be found in an amplifier, especially the pre-amp section. The higher impedances involved in the amp can make isolating the type of signal from one of these tracers difficult. There is other test equipment that I would use to trace signals through an amp. YMMV.

Not in an amplifier stage, on or near the stage where equipment is set up, to find noise sources. IN an amplifier, I would use a signal tracer with a very thin wire connected to a coil or just hover the lead near a suspected source location.

These can be made more sensitive by NOT connecting the second lead of the signal generator (toner) to another wire in the bundle- it come in handy when wiring has been buried by overzealous drywallers. I sometimes clip it to the steel brace on my ladder- it really makes finding wires easier. Probably 10dB louder than if both clips are on the same wire pair/coax.

 

[1958Bassman]

60 cycle hum can be from the magnetic or H field
An example could be an amp that normally doesn't hum unless it's very close to another amp or wall wart.
This magnetic field interference drops exponentially with distance

I think the majority of single coil noise is due to RFI emanations which can be a harmonic of the power line, or from a variety of sources to include devices in the house.
I thought the UPS was a brilliant idea but now see that they can produce RF in the DC to AC conversion process

Here's one link although not the one I was looking for:
Identifying Sources of Radio Frequency Interference Around the Home

I do low voltage installations (audio/video, home theater, distributed audio, etc) and in one job, I wanted to use the receiver in the rack in a basement to feed an amplifier in an out-building. This is something I have done many, many times and I also send audio from one part of houses to other locations that may be well over 100' away. As soon as I plugged a Y cord into the record out jack, I had noise in the system on the main floor of the house- it was so loud I could hear it in the basement, which is not a direct audio path. Didn't matter what kind of cable I inserted, it made noise. The HVAC was running, so it was difficult to be certain of the source, so I plugged a set of on-ear headphones into the jack on the probe and moved it in the area of the equipment and found that it was coming from one end of the UPS that I installed to prevent temporary power outages from disabling the security camera DVR, modem and router. As soon as I unplugged any cables other than the ones I need, it was silent. I tried using a 1:1 transformer in case it was a ground loop, but that didn't work with/without the far end being terminated. In that case, it's just radiated noise.

The ground integrity issue was reinforced, in my case, when my Tweed Bassman developed some intermittent hum and I found that I could eliminate it by pressing on a corner of the tag board. At times, it was quiet, but when I touched the strings/bridge, etc, it hummed. I resoldered the connections and measured the resistance to the chassis- big difference. I thought that maybe one or more resistors had failed (it was made in early '58) and found that when I referenced the resistors to ground from the far end, I saw values that were way off, so I bypassed the resistors in that area and found high resistance between the side that was supposed to be connected to the chassis through solder joints and wires. Once I was done, I re-measured and tested- no hum, no buzz and all of the resistance measurements showed that the paths were very good. The hum/buzz when I touched the strings was gone, too.

 

[Old Garage-Bander]

So you're saying the 60 Hz hum is magnetically induced?
How is this different from the electrostatic noise that can be shielded against?

I've been trying to get my head around this literally for years now, but recently (since I got a bass with single coil pickups) it's more relevant for me.
Humbuckers solve my noise problem at home, with one exception, a Mexican Fender Blacktop. I'm not sure if it was because the humbuckers were overwound or what...

The humbuckers in my SRX (SR eXtreme) & my guitars are nice & quiet, but the noise doesn't seem to be exclusively 60 Hz hum.

I did some experimenting with my Ibanez Premium (with the Big Singles).
First I tried quieting the single coil noise by touching the strings, doesn't work, nor does touching a grounded switch on the bass (this is with only one pickup selected). I did discover that there's 200 Ohms of resistance between my bridge & the output jack, and that the coating on Elixir strings is definitely not conductive. Thoe strings are surprisingly well insulated.
That's another issue that I'm going to try and figure out tomorrow, the 200 Ohms of resistance

At least I know the switches are grounded, because if I have both pickups selected & touch a switch there's definitely a drop in noise.

If I face the bass to the South or North the noise is at its pinnacle, but East or West & it at its minimum. Luckily I play facing East, Feng Shui & all
To try and see if any of the noise is from the power, I disconnected the UPS that feeds my amp from the wall, after I turned off my computer, speakers, & monitor.

Flipped on the amp & tried the same touchy feely stuff, no change. The directional bit still remains the same as well.
Now, it did occur to me that the battery power of the UPS is being inverted to the same 60 Hz that's coming from the wall outlet, so...
how could the noise be directional if it's coming from the power?

The strange thing about running on UPS power was this,
if I put my hand over a pickup, even with both selected, there was extra noise.
It was picking up static electricity from my body, but oddly (& luckily) doesn't do that under normal power.

Best crap in the power lines tracing tool I've ever come up with is one of those old 9volt transistor AM radios. Tune it to a place between stations. You'll hear noise. Move around and follow where the noise gets louder. That gets you closer to the noise source, or at least the conductors in the wall that are carrying the noise source.

Power companies can put a lot of hashy sounding signals on power lines in addition to those that occur naturally as a result if the power lines being big, long antennas. I knew a guy who worked for the power company and also was a ham radio operator. He did a lot of testing of power line noise back when the power companies thought they could do internet over power lines. They may still do some forms of signaling over lines, but it's mostly just other crap being carried into your house or being passed from one part of your house to another one the power lines. The tests showed that those signals radiated out and put a lot of extra noise in the Radio High Frequency bands.

If you are familiar with the old X-10 devices, that is one form of signaling over power lines. It all ads to the noise floor. Now, wall warts and other electronic devices such as TVs and computers are a bigger part of the problem.
The little old AM radio can help find that stuff in your house.

 

[1958Bassman]

My Experiment

The magnetic field is directional- ever look at an amplifier that has an output transformer or a speaker crossover? The transformers and coils are placed at right angles because they don't pick up hum and noise as effectively. Lay your instrument cables parallel to some power cords, then increase the distance between them and if they need to cross, make them cross at a right angle- the noise should be reduced. Signal cables should always be placed at least 12" away from any power cords/wiring, preferably more than 12". If they're parallel, it acts like a pickup.

 

[1958Bassman]

Best crap in the power lines tracing tool I've ever come up with is one of those old 9volt transistor AM radios. Tune it to a place between stations. You'll hear noise. Move around and follow where the noise gets louder. That gets you closer to the noise source, or at least the conductors in the wall that are carrying the noise source.

Power companies can put a lot of hashy sounding signals on power lines in addition to those that occur naturally as a result if the power lines being big, long antennas. I knew a guy who worked for the power company and also was a ham radio operator. He did a lot of testing of power line noise back when the power companies thought they could do internet over power lines. They may still do some forms of signaling over lines, but it's mostly just other crap being carried into your house or being passed from one part of your house to another one the power lines. The tests showed that those signals radiated out and put a lot of extra noise in the Radio High Frequency bands.

If you are familiar with the old X-10 devices, that is one form of signaling over power lines. It all ads to the noise floor. Now, wall warts and other electronic devices such as TVs and computers are a bigger part of the problem.
The little old AM radio can help find that stuff in your house.

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.

 

[Old Garage-Bander]

Not in an amplifier stage, on or near the stage where equipment is set up, to find noise sources. IN an amplifier, I would use a signal tracer with a very thin wire connected to a coil or just hover the lead near a suspected source location.

These can be made more sensitive by NOT connecting the second lead of the signal generator (toner) to another wire in the bundle- it come in handy when wiring has been buried by overzealous drywallers. I sometimes clip it to the steel brace on my ladder- it really makes finding wires easier. Probably 10dB louder than if both clips are on the same wire pair/coax.

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.

 

[jamro217]

It can only help!

View attachment 2837526

Dr. Goldfoot is at it again.