Power conditioning

[Tubetec]

Id say if your in close proximity to the main wires carrying the entire current into the building thats a good source of interference , who knows what people are running these days ,

I have a friend who lived in a rented place , one day they started to get all kinds of noise issues on the sockets upstairs ,
anything audio they plugged into it had a whine ,
In the end they asked the building owner to look into it , it took time before the owners moved on it , but they moment they did the noise stopped and never returned .
All the evidence pointed to a weed grow op in the next door neighbours attic , bad wiring on a 1000 or 2000 watt sodium lamp can cause huge RF interference . A poor or no ground might have been the issue .

 

[joeblack007]

One method that should should be very effective (if you’re 100% sure it’s a shielding problem) , but can be expensive depending on the materials used, is to build a faraday cage. There is a special paint for this application that must be grounded and it can also be done with wire mesh. The smaller the mesh spacing, the more effective it is. The wire must be grounded also. The most efficient method that works 100% without having to be grounded is by using lead boards : Lead Sheet Metal - Canada Metal North America ...but it cost about 500$can a sheet. In your case, since you live in an apartment, the method of shield painting is really the best to use especially since the other 2 methods are usually used during the construction of a room and not after

 

[Newmarket]

I think we need the OP answer on the shorted input question.

 

[ubxf]

Thank you so much for all the great suggestions. I will go back there this afternoon and report back with new infos and I will also record a new file for the noise.

 

[ccaudle]

The most efficient method that works 100% without having to be grounded is by using lead boards

I'm a little speechless. That is good advice when the problem is exposure to x-rays or gamma rays, but completely inappropriate for normal frequency range electromagnetic interference. Lining your walls with toxic heavy metals is not needed, just decently high conductivity.

 

[pucho812]

Is there a transformer on it?

Don't waste your time with shielding.
One thing to investigate is to use an electric guitar with single coil pick-ups, move the guitar till you find a maximum, that will indicate the direction of the source of magnetic interference. Or it may show that magnetic interference is not the source of your problem. Elimination process.

Reminds me of a studio build I did years ago for a known guitar player. They installed some fancy lighting system in their spot instead of the traditional variac dimmers one finds in a professional studio. The reason was they were sold on the idea of being able to program the system to do different things at different times. Sure enough when dimming, the guitar picked up all kinds of noise. The quick solution was lights on at full brightness or off. Long term solution was replace with the standard variac dimmers.

 

[joeblack007]

I'm a little speechless. That is good advice when the problem is exposure to x-rays or gamma rays, but completely inappropriate for normal frequency range electromagnetic interference. Lining your walls with toxic heavy metals is not needed, just decently high conductivity.

Lol! I understand what you mean and I agree 100% with you CCaudle! No grounding needed if using the Lead method but totally overkill, expensive and dangerous if used the wrong way : it should never be exposed, never be sanded or drilled, no nails or screws in the wall which could reach the lead plate, etc. And you have to keep the building details for the future buyers... Yeah, I should have just suggested the metal sheet with a ground if you build a new room. Much less expensive and no toxicity's risk. My bad. I know some guys went for the lead solution for others reasons that I will avoid on this forum and for the same reason, if money wasn't a problem, I should also chose lead if building a new room but anyway... Thanks for your important point

 

[ccaudle]

No grounding needed if using the Lead method

That doesn't match the physics as far as I can tell, you need to explain that more.

In a general sense no true Faraday cage needs to be grounded, but the caveat being that true means absolutely nothing penetrates conductive shell. That is obviously not practical, so you have to consider capacitive coupling of common mode current between the shell and the interior circuitry, as well as conductive and radiative coupling from conductors which penetrate the shell, such as power wiring. To truly be effective you have to make sure to filter any penetrations for the frequencies of interest, so any power lines penetrating the shield would need to be filtered to the conductive shell, any audio lines in and out, etc.

I do not see how lead is any different in that regard from sheet steel, aluminum, or copper (excepting magnetic permeability differences). Possibly the lower conductivity causes higher absorption to offset the reduced reflection, but at the end metal is just metal.
The situation is of course different when the photons are of sufficient energy that they can directly interact with atom, such as x-ray and gamma ray frequencies, but that is not usually a consideration when you talk of EMI shielding.

 

[Newmarket]

"floating" metal is never a good idea. Unless you want chaotic and unpredictable behaviour.

 

[john12ax7]

"floating" metal is never a good idea. Unless you want chaotic and unpredictable behaviour.

It depends on the situation. With typical RF you just need a Faraday cage with a fine enough mesh, which would still be porous enough to allow cables and such to pass through.

Grounding is still a good idea for things like safety reasons, but not necessarily needed for effective and predictable shielding.

 

[Newmarket]

I disagree. Both from theory and experience the screen needs to be held at a fixed reference potential to be effective and predictable wrt E field shielding. H field shielding a different issue / mechanism that doesn't require 'grounding' eg using a ferrous screen, ideally (if costly) using an appropriate grade of appropriately treated 'mumetal'.
But, of course, in general 'ground' that too and you also get the E-field shielding.

Grounding / Earthing for safety is a different issue. Generally related to what 'flavour' of mains distribution you have and equipment build type / standard.

Common terminology often doesn't help here. It's not for nothing that a leading EMC consultant / author half-jokingly (?) 'bans' the term "Ground" wrt EMC.

 

[Brian Roth]

Re possible mains related noise:

I've told this story before, but here I go again <g>. Studio client reported an odd noise that was being picked up all over the place. It was a scratchy buzz that would begin slowly in level until hitting a peak, abruptly quit, then start the pattern again. Entire cycle took around 30 seconds.

I went to my truck and (with engine off) could hear the same racket on the low end of the AM radio band. It was SO bad it wiped out reception of even a local station at 640 kHz. I walked around the building and found the culprit. A mercury vapor security light was being falsely activated (it was broad daylight), and it was SRUGGLING to start....flickering like crazy....then shutting back off to begin the cycle again.

Two problems....the photocell was calling for the light to activate in the daytime AND the mercury bulb (or ballast or something) was dying.

As for lead sheets in the wall....I remember a disco club being built inside a hotel. They architect has speced lead in the walls to mitigate noise from "The BoomBoom Room".

Bri

 

[Monte McGuire]

I disagree. Both from theory and experience the screen needs to be held at a fixed reference potential to be effective and predictable wrt E field shielding.

If you tie the Faraday cage to the "ground" used by the system inside of the Faraday cage, you accomplish all of this. And still, the Faraday cage can float relative to anything outside of the Faraday cage.

A good mental exercise is to imagine grounding inside of an aircraft at 33,000 feet. The fuselage can be a great Faraday cage and there's no "ground wire" connected to anything - it is literally floating.

 

[Newmarket]

If you tie the Faraday cage to the "ground" used by the system inside of the Faraday cage, you accomplish all of this. And still, the Faraday cage can float relative to anything outside of the Faraday cage.

+1. Yes that was basically my intended meaning. The "fixed reference potential" being relative to the system/circuit being screened.

A good mental exercise is to imagine grounding inside of an aircraft at 33,000 feet. The fuselage can be a great Faraday cage and there's no "ground wire" connected to anything - it is literally floating.

Yes. The 'aeroplane' example is a go to for explaining that there is nothing special about the planet earth in all this. You can refer to boats and cars but often people will 'hang onto' a connection to Earth via the water or tyres etc ! They don't seem to do the same thing with air/space although it's as well to be ready for someone quoting the characteristic rf impedance of free space @ 377R. Then ask them "So...?"

 

[Hubbub]

The power cord is a great antenna for coupling noise into the speaker. Shielding the speaker is a waste of time if there is some antenna-ish cord hanging out through the shield. Ditto for the audio cable that feeds the speaker. One common RF noise solution is to add a ferrite core or two around the cords quite close to the powered speaker. I'm surprised no one has mentioned this. An example: Ferrite Clamshell Choke EMI Filter TIA-TECH W5 SRH

 

[MisterCMRR]

We must also remember that, below a few MHz, closed loops of wire are excellent antennas. For example, at AM radio frequencies loop antennas (as found in most radios) and whip antennas (as found in car radios) perform equally well picking up the magnetic and electric fields, respectively. Such loops are often formed unintentionally in audio systems by signal and power cables (including power wiring in the walls) when audio systems are interconnected. A lot of gear is vulnerable to RF currents that flow from cable shield to AC power connections - a slight variation of the "pin 1 problem" described by the late Neil Muncy, myself, and others in various papers. Many manufacturers (and hobbyists) just don't get it. It's one reason you should never connect pin 1 of an XLR connector to a circuit board or "signal ground." Give those noise and RF currents a way to flow through their own separate conductor (often a metal chassis) and keep it OUT of the signal path!

 

[HP200CD]

What Brian said in post #32. I have also seen this problem with bad mercury vapor lamps failing to start and putting a severe amount of RF noise on the power line. (They strike a quick arc repeatedly, buzz-buzz-buzz, then pause and repeat, until the lamp starts.) In the case I resolved, the landlord was not replacing the bulbs on the building's exterior lighting, and they were arcing all night long while trying to start up. To make matters worse, each lamp was powered by the closest branch circuit from the nearest apartment, so the RF noise was propagated into several apartments. Aside from the studio, many items of consumer electronics were affected.

First off you need to determine the noise source to know how to suppress it. Is it on the power line or is it RF in the air? Use the AM radio, a single coil guitar pickup, and but a scope on the power line. Shielding and Faraday cage won't help if the noise is coming in on the power line. And remember that if it is RF in the air, it can also be picked up on any sort of cables.

 

[HP200CD]

Also, lead is not appropriate for RF shielding. Copper is used for RF shielding on commercial radio transmitters. Lead is more appropriate for shielding from atomic radiation. Lead was also formerly used for acoustic mass loading, to reduce sound transmission thru walls and such, as mentioned above. Because lead is toxic, other similarly dense materials are now used in place of lead for acoustic mass damping.

 

[joeblack007]

That doesn't match the physics as far as I can tell, you need to explain that more.

I can’t remember where I read this about lead and grounding. Could be on the John Sawyer’s studio building site (I'd be surprised) I consulted regularly at the time, in a microwave weapons book or in documentation regarding G4 years ago.... Or it was perhaps an unfounded information that I memorized and that I should not have mentioned here... For the moment, I find nothing that can actually justify my remarks. On the other hand, the conversation that followed is very interesting

 

[ubxf]

Sorry for the long absence. I got busy and didn't get a chance to return till today. I did the suggested test of shorting pins 2 and 3 of the input of the speaker and that didn't change anything. Unfortunately I don't have a single coil guitar to investigate further. The good news is that there is an opening to move to a different apartment that is interference free. Thank you so much for all the help.