Shielding - How do you know it will help?

[megafiddle]

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.

We never got quite that advanced in the signal transmission. Fiberoptics are already showing up in consumer products though. One disadvantage is that a power supply is required at both ends.

Much of the equipment in the plant I worked in dated back to the 70's. A lot of the controllers operated on old technology that was inherently immune to noise. The problems really started when we had to use newer microprocessor based controllers as the old stuff was being phased out.

The thermocouples we used were virtually immune to noise due their extremely low impedance, just a few ohms. The entire probe and lead wire was practically grounded. Tough to drive noise into something like that. In contrast, a passive bass circuitry is pretty high impedance. It's easy for a noise source to drive it.

Some of the other old tech signal sources like tachometers and frequency signals operated at relatively high voltages, and so were well above the noise level.

So there wasn't much trouble with the signals themselves. It was mainly power line noise from the motor speed controllers getting into our new "high tech" microcontrollers. These microcontrollors were basically tiny dedicated computers, and would just freeze up. I finally found some power line filters that greatly reduced the frequency of failure, but never quite eliminated it.

As for the high energy physics instrumentation that I worked on, I can only imagine the shielding problems they must have had at the experiment site. We only produced the instrumentation. The actual experiments that used them were performed at sites like Fermilab and CERN. The envoronment there had voltages up towards the billion electron volts, and who knows how much current.

We had physisists that I worked with that described the magnetic field used. They said you could stand a lead brick on end and tip it over; it would drop very slowly, like it was moving through molasses.

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[megafiddle]

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!).

Just a technical point, but CMRR is a characteristic of a differential amp or device receiving the signal. CMMR is the ratio between the common mode signal present on the balanced pair input, and the signal present on the output. So if there is a 1 volt common mode signal present on the input, and 0.01 volt signal on the output, the CMMR is 100:1 or 40 db.

What you described is the signal to noise ratio.

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[megafiddle]

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"?

Ethernet wire pairs are also transmission lines, with a definite characteristic impedance. If you untwist them, you can form an impedance discontinuity with resultant reflections and signal degredation.

The frequencies here are several orders of magnitude higher than audio. So it's more than just common mode rejection.

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[1958Bassman]

Ethernet wire pairs are also transmission lines, with a definite characteristic impedance. If you untwist them, you can form an impedance discontinuity with resultant reflections and signal degredation.

The frequencies here are several orders of magnitude higher than audio. So it's more than just common mode rejection.

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That's true, but Category cable can be used for a lot more than network applications. Many companies offer AV baluns that are made for simple distribution of stereo audio, S/PDIF, composite/component video and HDMI. Specific to our discussion, this cabling works great for audio with just one twisted pair for each channel as long as no grounding issues exist (ground loops). If the baluns are made with quality parts, the sound when using these is good enough that nobody will pay attention to how it was done or what kind of cables was used. Tweaks can debate these cable issues, they're used for AV distribution because they work, without noise.

 

[1958Bassman]

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.

I'm lucky enough to live very close to a few antenna farms (about a half mile from the closest) and at various times, I have had the pleasure of listening to a Country Music station, CB/Ham radio transmissions, cell phone emissions and occasionally, the analog audio from Channel 6. It was a PITA, but a good way to make sure my grounding and shields were doing their job. Once I re-soldered some connections in my amp, the problems were gone.

Monster Cables has/had a device called 'Dr Noise', which was to be used for demonstrating how well their power strips filtered. I borrowed one from the store where I had worked and plugged it in, pressed the button that bypassed the filtering and was listening to a Milwaukee Brewers baseball game. I plugged it into their power strip and it made little difference but the cheapo power strip I had from OfficeMax left the background silent.

Most of the noises I have heard that came from AC power would be called 'buzz', rather than hum. Often, it was from cheap dimmers, power supplies and the inverter in UPS devices.

 

[1958Bassman]

AC motors, incandescent lights, heaters, anything with a non-switched fairly high current.

I met with a friend yesterday, to verify the repairs to the generator for his boat and at one point, the guy who repaired it held his cell phone near the solenoid, which is also close to the stator. When he opened the phone app, I saw that the middle of the display had a white square and as he moved it close to the stator while the engine ran, I could see a pattern rotating at the center.

What is your take on the safety of cell phone radiation? When I used a CRT computer monitor, the screen would go nuts when my phone was close to it.

 

[1958Bassman]

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.

'UPS' is an improperly used general term for surge filtered power strip, too.

 

[Rich Fiscus]

'UPS' is an improperly used general term for surge filtered power strip, too.

As is power conditioner.

 

[megafiddle]

I met with a friend yesterday, to verify the repairs to the generator for his boat and at one point, the guy who repaired it held his cell phone near the solenoid, which is also close to the stator. When he opened the phone app, I saw that the middle of the display had a white square and as he moved it close to the stator while the engine ran, I could see a pattern rotating at the center.

What is your take on the safety of cell phone radiation? When I used a CRT computer monitor, the screen would go nuts when my phone was close to it.

Having never owned a cell phone, I never followed any of the reports or studies since the concerns were first raised years ago. Anything back then would have been premature, as things were inconclusive, and it looks like this would be more about long term exposure.

There doesn't seem to be much new definitive information coming out yet. I suspect some studies of long term effects will be out sooner or later.

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[BlueTalon]

Having never owned a cell phone, I never followed any of the reports or studies since the concerns were first raised years ago. Anything back then would have been premature, as things were inconclusive, and it looks like this would be more about long term exposure.

There doesn't seem to be much new definitive information coming out yet. I suspect some studies of long term effects will be out sooner or later.

Straying off topic a bit here, but I have a personal theory that holding a transmitter right next to your head that can communicate with cell towers a distance away is probably not good for your long-term health. A few famous people have had brain cancer recently, and it may or may not be related, but I try to use earbuds when I talk on the phone with anyone for longer than a minute or two. That way, the transmitter is a couple feet (60-80cm) away from my head.

 

[DrummerwStrings]

Straying off topic a bit here, but I have a personal theory that holding a transmitter right next to your head that can communicate with cell towers a distance away is probably not good for your long-term health. A few famous people have had brain cancer recently, and it may or may not be related, but I try to use earbuds when I talk on the phone with anyone for longer than a minute or two. That way, the transmitter is a couple feet (60-80cm) away from my head.

...Pick your poison - I find earbuds too loud, even when my phone's volume is at its lowest. Don't want my tinnitus to get any worse.

I would like to see some studies on Wi-Fi networks, tho.

 

[BlueTalon]

I find earbuds too loud, even when my phone's volume is at its lowest.

Yeah, that could be a problem.

 

[somebrains]

Asbestos clothing and frying your meals in an aluminum pan used to mean you were a modern person enjoying life.
Either way the zombie apocalypse will probably start from cell phone induced brain tumors.

 

[dustklose]

I came in wanting advice on shielding my bass... I left worried about Cell Phone Zombies...

 

[BruceWane]

A good FYI I thought I'd drop in here...........

A while back, in house fixer-upper mode, I undertook the task of repairing a damaged leaded glass transom panel for my entry way.

Turns out that making your own stained/leaded glass panels is done with a decently powerful soldering iron (which I have, a Hakko FX601, just needed a big tip)......... and it's a pretty popular hobby, and I found a shop that holds classes and sells supplies less than 10 miles away - who knew?

So, went by to get a piece of glue-chip glass to repair my transom, and noticed that they had 12"x12" adhesive-backed copper sheets that are used in one method of making leaded glass. And they're only a few bucks for a pack of three.

And man, do they ever make shielding your guitar a breeze.............. no soldering all those strips of tape together.

Like I said, making stained glass panels is a pretty popular hobby, so you will likely find a shop in your area.

Of course, you can order this stuff online as well.

These folks appreciate good soldering irons, too, so if you're in dire need of something they usually have parts for Weller, Hakko, etc.

 

[DiabolusInMusic]

A good FYI I thought I'd drop in here...........

A while back, in house fixer-upper mode, I undertook the task of repairing a damaged leaded glass transom panel for my entry way.

Turns out that making your own stained/leaded glass panels is done with a decently powerful soldering iron (which I have, a Hakko FX601, just needed a big tip)......... and it's a pretty popular hobby, and I found a shop that holds classes and sells supplies less than 10 miles away - who knew?

So, went by to get a piece of glue-chip glass to repair my transom, and noticed that they had 12"x12" adhesive-backed copper sheets that are used in one method of making leaded glass. And they're only a few bucks for a pack of three.

And man, do they ever make shielding your guitar a breeze.............. no soldering all those strips of tape together.

Like I said, making stained glass panels is a pretty popular hobby, so you will likely find a shop in your area.

Of course, you can order this stuff online as well.

These folks appreciate good soldering irons, too, so if you're in dire need of something they usually have parts for Weller, Hakko, etc.

I actually first found copper tape at stained glass supply shops. One of the fellows at my local electronics supply shop suggested it when I asked for some copper tape. I guess his wife was into the craft. For what it is worth, you could also find slug tape at any given hardware store and it would perform the same. Neither the slug tape nor the stained glass stuff has a conductive adhesive though. You are better off getting your tape online. It will also be much cheaper than either of the aforementioned options. I actually use paint now as it is a far better option.

 

[Bruce Johnson]

Here's a shielding paint that I haven't tried yet:

https://www.amazon.com/gp/product/B00A6FO6G8/ref=ox_sc_sfl_title_3?ie=UTF8&psc=1&smid=A2YTLWSIIIEHLL

It's copper in a latex base, thinned with water. It's sold by Caswell, the home-shop electroplating company. It's manufactured for electromagnetic shielding, and is sold by Caswell to be used for coating non-conductive materials, so they can be electroplated. It can be brushed or sprayed, dries quickly, and is supposed to be fully conductive. Not dirt cheap; $42 for 4.8 oz, but less expensive per ounce than SuperShield.

When my current cans of SuperShield run out, I may try it. I do shielding often enough that I'd rather have a jar of liquid, and buy a cheap little touchup spray gun to dedicate to that job. Less wasted paint, no more problems with clogged spray cans.

 

[TH63]

It can only help!

View attachment 2837526

Is that a Farrah Fawcett-Day cage?

 

[Killed_by_Death]

It's probably Goldie Hawn, from her paid-to-dance days at the Whisky a go-go.

 

[Rockin John]

Just a technical point, but CMRR is a characteristic of a differential amp or device receiving the signal. CMMR is the ratio between the common mode signal present on the balanced pair input, and the signal present on the output. So if there is a 1 volt common mode signal present on the input, and 0.01 volt signal on the output, the CMMR is 100:1 or 40 db.

What you described is the signal to noise ratio.

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Your are correct. I did indeed. My apologies (I think I'm going round the twist!!). Sorry.