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Mitigating RF interference?
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plurgid
gettin' sounds


Joined: 24 Mar 2010
Posts: 135
Location: Huntsville, AL

PostPosted: Mon Dec 19, 2011 12:39 pm    Post subject: Mitigating RF interference? Reply with quote

I'm kinda going nuts.
For years, my studio was in a downstairs home office, while I worked on getting my real studio space (a renovated upstairs spare bedroom) outfitted.

Now I've moved my stuff in and the space sounds great, but ... *everything* with a wire has become an antenna. It's insane.

My first clue was plugging in my powered monitors. On their own without any cables plugged in, they're quiet, but plug *any* cable into them ... 1/4 in, XLR, does not matter ... with *anything* on the other end ... audio interface, amp, laptop, guitar ... anything ... and I'm picking up the local country & western station loud and clear.

I thought at first it was that I had dirty power, and that maybe I had a bad power conditioner. I ruled that out by plugging a phantom powered pre-amp into a bus-powered firewire interface on my laptop and running off the battery ... crank up the gain, and there's the radio station again.

Any suggestions on how to mitigate this? Line the walls with tinfoil? Build a Faraday cage around my room? How do they handle this in pro studio construction?
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Gregg Juke
mixes from purgatory


Joined: 12 Jun 2010
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Location: Buffalo, NY, USA

PostPosted: Mon Dec 19, 2011 2:42 pm    Post subject: Re: Mitigating RF interference? Reply with quote

First silly question might be-- "Are you running everything possible with balanced, rather than unbalanced cable?"

GJ
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plurgid
gettin' sounds


Joined: 24 Mar 2010
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Location: Huntsville, AL

PostPosted: Mon Dec 19, 2011 4:45 pm    Post subject: Re: Mitigating RF interference? Reply with quote

it's definitely true that unbalanced connections catch the interference much more loudly than balanced ones ... both types are affected though.
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cavemusic
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Joined: 13 Jul 2010
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Location: Canada

PostPosted: Mon Dec 19, 2011 5:50 pm    Post subject: Re: Mitigating RF interference? Reply with quote

plurgid wrote:
I'm kinda going nuts.
For years, my studio was in a downstairs home office, while I worked on getting my real studio space (a renovated upstairs spare bedroom) outfitted.

Now I've moved my stuff in and the space sounds great, but ... *everything* with a wire has become an antenna. It's insane.

My first clue was plugging in my powered monitors. On their own without any cables plugged in, they're quiet, but plug *any* cable into them ... 1/4 in, XLR, does not matter ... with *anything* on the other end ... audio interface, amp, laptop, guitar ... anything ... and I'm picking up the local country & western station loud and clear.

I thought at first it was that I had dirty power, and that maybe I had a bad power conditioner. I ruled that out by plugging a phantom powered pre-amp into a bus-powered firewire interface on my laptop and running off the battery ... crank up the gain, and there's the radio station again.

Any suggestions on how to mitigate this? Line the walls with tinfoil? Build a Faraday cage around my room? How do they handle this in pro studio construction?



How close is the station? If it's within a mile or so and transmitting thousands of watts it could be tough. Does the house have modern electrical wiring with a ground or old wiring with only 2 prong outlets? Checking the house ground might help. However, grounding for radio frequencies is not an easy task.
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Snarl 12/8
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PostPosted: Wed Dec 21, 2011 3:21 am    Post subject: Re: Mitigating RF interference? Reply with quote

I read this

Quote:
misc - I am picking up radio stations, how can I stop this?

From Eric Hensel: A 47pf cap to,ground, after the input cap will probably do it --you can adjust this up or down --higher values will start to cut treble. use the lowest value that works.

From Zachary Vex:

Mount the pc board as close as you can physically to the metal box. this will serve as a ground plane to reduce the heterodyning that can pick up rf. don't let the input and output wires cross... make sure the layout doesn't allow input and output connections to get too close to each other. if you can, solder in ground connections around (near) the sensitive input circuitry. if rf is being picked up by rectification, these fixes won't help, but if it is being picked up due to beat frequencies generated by heterodyning (internal rf oscillation in the circuit beating against rf from radio stations) you can get rid of it through the use of careful layout and grounding to reduce oscillation.

You might try using miniature shielded cable inside of the enclosure for the input to the board too.


on this site

http://www.diystompboxes.com/wiki/index.php?title=Frequently_Asked_Questions_%28DIY_FAQ%29

And thought of this thread. I have no freakin' idea if something like this might solve your problem. My first question before trying any of this would be does other gear have this same problem in your space, (like anything that can drive some headphones or speakers) or just your interface. If it's just your interface maybe some simple mods will help.
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plurgid
gettin' sounds


Joined: 24 Mar 2010
Posts: 135
Location: Huntsville, AL

PostPosted: Wed Dec 21, 2011 10:42 am    Post subject: Re: Mitigating RF interference? Reply with quote

it seems like anything with a gain knob is affected ... so as I mentioned ... powered monitors, preamps, my interface, my mixing board, etc.

My physics knowledge is weak. Is there an way to build an RF signal blocking device? The thought had occurred to hit the home depot and try to find something like an aluminum mesh for a screen door, and ground it ... could something like that work (like an anti-antenna?)

To answer some of the questions further up the thread:

* distance to transmitter
I honestly have no idea. I'll have to set up and listen until these guys do a station identification then hit google I guess ... though even armed with this knowledge I sort of doubt there's anything I can do about it ... as in "hey WASS, you need to move your transmitter because I built a home studio and you're totally fucking with it". Yeah ... LOL.

* modern wiring / home age
the house is relatively new ... about 10 years old or so. Haven't got any reason to believe the wiring isn't up to code ... all the sockets are 3 prong, etc.
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rhythm ranch
mixes from purgatory


Joined: 07 May 2003
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PostPosted: Wed Dec 21, 2011 10:50 am    Post subject: Re: Mitigating RF interference? Reply with quote

plurgid wrote:
Is there an way to build an RF signal blocking device? The thought had occurred to hit the home depot and try to find something like an aluminum mesh for a screen door, and ground it ... could something like that work (like an anti-antenna?)

http://en.wikipedia.org/wiki/Faraday_cage
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The Scum
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Joined: 04 Jul 2003
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Location: Denver, CO

PostPosted: Thu Dec 22, 2011 11:43 pm    Post subject: Re: Mitigating RF interference? Reply with quote

Prefab faraday:

http://ets-lindgren.com/page/?i=5411

However, form the description, it sounds like you're ground isn't working. RF should find it's way to ground...but of the ground is missing, or has some resistance, the RF will go seeking other paths...one of them being the audio wiring.

It could be as simple as a loose or corroded ground connection in the main panel, or an outlet in a daisy chain. Some electricians have a fancy device that can measure the ground impedance for troubleshooting problems like this.

Quote:
I'll have to set up and listen until these guys do a station identification then hit google I guess


They're obligated to the FCC to do a station ID +/- 5 minutes of the hour, so it should take 10 minutes, max.
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Gregg Juke
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PostPosted: Fri Dec 23, 2011 1:24 am    Post subject: Re: Mitigating RF interference? Reply with quote

The simple stuff that audio/electrical simpletons such as myself might try first (before thinking about tearing apart gear or building giant cages). Admittedly, this is way basic, but I tried to post this the other night and something happened and the post went South (or North, but it disappeared), so in the interest of thoroughness:

* All/as many balanced cables as possible

* Shortest possible cable runs

* 3-prong/grounded plugs for all extensions, the shortest possible lengths of cord

* Electrical cables separated as much as possible from audio cables

* Spacing-out any wall-wart transformers, and keeping them away from audio
cables if possible

* Using devices like the Hum-X or the Hum-Eliminator and/or the EBTech Line-
Level Shifter

* Accessing clean, consistent power, with something like a Furman power conditioner &/or voltage regulator &/or some type of UPS (all of which are supposed to provide at least some basic circuitry protection from intrusive RF, if I'm
not mistaken)

* Pain-staking trouble-shooting (ears and eyes around the room) to determine
_exactly_ which pieces of gear are causing/being effected by the problem

* Being real thorough with TSing procedures as referenced above, and especially being open to seemingly silly, but workable solutions (for instance, we had a buzzing noisy piece of gear hooked-up to our patch-bay, but discovered by accident
that if a dummy-plug is inserted in one side of the devices' outputs on the p-bay,
the buzz is eliminated !! )...

* Avoiding grounding issues as much as possible (multiple electrical lines?)

* Trying ground-lifting (or grounding!) any guitar amplification equipment or other offending devices if possible...

Out of ideas, and over my head with the physics/electrical background, but I'd try all of this first before expensive solutions (re-modeling, building a giant Farady cage, on-site room consultation, moving, bribing the radio-station personell)...

Hope it works out,

GJ
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egr
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Joined: 17 Feb 2010
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Location: Kentucky, USA

PostPosted: Fri Dec 23, 2011 2:38 am    Post subject: Re: Mitigating RF interference? Reply with quote

Since you've moved upstairs maybe take a look at the roof and/or attic space above you. Is there an actual antenna nearby from an old TV setup, for example. Maybe even lightning rods or somesuch.
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goose134
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Joined: 22 Mar 2009
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PostPosted: Fri Dec 23, 2011 11:34 pm    Post subject: Re: Mitigating RF interference? Reply with quote

As an electrician, I've done a few installs where we've had to deal with minimizing RF interference. One was at a radio station where there was a copper screen installed between layers of drywall that was then fixed to ground. This creates a Farraday cage, effectively creating a shield around the workspace and "draining" the offending frequencies to ground. While this is not always practicable, it is effective. Aluminum may be a more cost effective choice, but I'm unsure as to the types of screens available in that material. (The copper we used was extremely fine screening, almost like a fabric).

The other environment we installed was an MRI room. In this, a steel structure resembling a clubhouse is erected and EVERY wire is passed into the room through an RF filter (someone posted the Lindgren link). Obviously, you're not going to build a welded steel clubhouse in your attic. The problem with using the filters without the cage is that the wires are still out there acting as antenna. You could try renting a gauss meter and see exactly HOW bad the signals are in the space and see if a screen will do the trick.
If you get the chance to do some electrical work in the space, I'd recommend installing in conduit as it helps transmitting the noise into the line voltage portion of your system.
The real key here is that if you do build a cage, you have to ground it effectively.

Here is a link that may help.
http://www.compliance-club.com/archive/old_archive/020918.htm
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The Scum
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PostPosted: Sat Dec 24, 2011 4:13 am    Post subject: Re: Mitigating RF interference? Reply with quote

And the old rule of Faraday cages: they're only as good as their largest opening. The doors on those ETS-Lindgren chambers have multiple layers of gasketing, and a latch that provides a very positive seal. It's not something that you can casually retrofit into a spare bedroom.

If this really is an RF pickup issue, look to grounds first.

The way it usually works is that radio signals are picked up by the shields and metal enclosures, which are grounded, and the noise simply finds it's way into the ground.

But of there's no easy path to ground, the induced signals will go hunting for it...they can ride on the AC lines, or signal interconnects, hunting for a path to ground.

If the grounds aren't good, they can cause problems all over the place. Although many companies would like to market their wares as a magic cure all for noise, it's largely hype. If the ground isn't connected, no expensive box with blue LEDs is going to be able to solve it.

Here's a good, accurate & truthful paper about induced noise in audio systems. Section 5 is particularly relevant.
http://www.jensen-transformers.com/an/generic%20seminar.pdf

It also describes some simple tools and procedures for sorting out where the noise is coming from.
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goose134
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PostPosted: Sat Dec 24, 2011 4:46 am    Post subject: Re: Mitigating RF interference? Reply with quote

Very true. The doors are vulnerable to RF. When we were building the MRI rooms, I had the cover of the load side of the RF filter (a 3/4" conduit opening into the room) off on the outside of the room and the guy taking readings said the meter was "off the chart".

So imagine an unshielded door or window.
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Scodiddly
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PostPosted: Sat Dec 24, 2011 10:01 am    Post subject: Re: Mitigating RF interference? Reply with quote

So if you can reproduce the problem with a battery powered rig, try dragging that around the house to see if the problem follows it. Maybe it was always there, and you hadn't looked for it with that rig?

Also, it's old connections, oxidized pins and such, where RF tends to most easily leak into the audio path. Oxides can sometimes form semiconductors and work like one of those old crystal radios. This is where you (or your qualified and licensed electrician) might need to go through all the electrical junctions (outlets, switches, lights, breaker boxes) and make sure everything is nice and tight. And yeah, how good *is* that ground? It's got further to go to actually get to real dirt than it did in your basement.

EDIT: Also maybe just borrow a nice heavy long extension cable so you can test powering it from your old basement studio outlets. If that works, then you've found your problem.

Maybe unplug gear and try building test systems up to see when you start getting noise - does your console do it with nothing else connected? How about borrowing another mixer to test with?

Those AC power cables that look like they swallowed a mouse - you'd often find them sold with computer monitors, they help reduce RF. The lump is a ferrite core, and you can buy those separately. Might be worth slapping those in a few places while building your test system to see if they help.


Last edited by Scodiddly on Sat Dec 24, 2011 10:09 am; edited 1 time in total
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Scodiddly
ghost haunting audio students


Joined: 10 Dec 2003
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PostPosted: Sat Dec 24, 2011 10:07 am    Post subject: Re: Mitigating RF interference? Reply with quote

Here is what I was talking about with the ferrite choke:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=110-454

No promises it'll work in your situation, though one reviewer said that it made his HD TV look better. Rolling Eyes
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