RF signals in PA equipment

[funkmeister]

I've been dealing with some really strong AM radio signals invading a local church's PA system. All the normal remedies, RF filters, shorter cable lengths etc seem to help, but it keeps cropping up in different areas. First it was in the choir mics, then the pulpit mic (all condensors w/phantom power). Now a new wrinkle has emerged - I was recording a performance (not in their system, but in my Roland VS1680) and AM radio invaded a track channel that was fed by an SM57! RF filters did nothing to help. Any ideas? The church has the tallest steeple in town and sits a block from a the radio station. The folks at the station (engineer, manager) claim that NOTHING has changed since this problem started back in early September of this year.

[JackANSI]

I'm pretty sure the radio station is going to be the best people to work with aside from running to the FCC. Have their engineer come over and see if he has a few tricks to try.

If the station has been there longer than the problem, its either going to be a ground/shield issue at the church (might need a new electrical ground or the current ground system checked) or an equipment problem at the station (parts do age and go out of spec without showing on the often minimal readouts some of this equipment can have).

You could also try your local ham radio association. They usually love this kind of stuff and would probably be quite eager to help.

[Craven Sound]

Have you checked the shielding on all the cabling?

[lonewolf]

The 1st thing I'd do is try powering everything from a high-end multi-stage power conditioning unit with a ground quality detection circuit. Even if it doesn't solve the problem, its still something that a permanent install should have.

[Jasaoke]

Try ground-lifting EVERYTHING. I worked at a studio nearly relatively close (several miles) from an airport and if anything was grounded, we'd get all kinds of noise. FCC said that the airport had "right of way" and we had to groundlift everything.

[LHSL]

Try ground-lifting EVERYTHING. I worked at a studio nearly relatively close (several miles) from an airport and if anything was grounded, we'd get all kinds of noise. FCC said that the airport had "right of way" and we had to groundlift everything.

Do not follow this advice. It's dangerous and might land you in jail if someone gets electrocuted.

[Jasaoke]

Equipment grounding is supposed to be a safety device but it is really nothing more than a secondary neutral. Only in special installations is the ground truly isolated; it's usually bonded to neutral inside the panel box. That way, even if the neutral is interrupted, a short circuit will trip the breaker. Electrocusion would be possible in the event of a double (actually triple) failure (Neutral fail + ground fail + actually equipment fail). And I've never heard of anyone going to jail or even getting a ticket for using "those little grey 3-to-two-prong adapter things".

[LHSL]

Equipment grounding is supposed to be a safety device but it is really nothing more than a secondary neutral. Only in special installations is the ground truly isolated; it's usually bonded to neutral inside the panel box. That way, even if the neutral is interrupted, a short circuit will trip the breaker. Electrocusion would be possible in the event of a double (actually triple) failure (Neutral fail + ground fail + actually equipment fail). And I've never heard of anyone going to jail or even getting a ticket for using "those little grey 3-to-two-prong adapter things".

When no ground is used, here are the things that must happen that could cause someone to receive an electrical shock.

1) Phase conductor shorts to some metal part that should be grounded
2) you or a friend touches the metal part

Done.

Do not give electrical advice on an internet forum unless you are qualified too. I'm sure Phil would back me up on this one but I haven't seen him posting on here in quite some time. If you are an electrician and do not agree with my advice, you need to go back and study some more.

As for going to jail... pretty simple. You remove the ground on a device, steps 1 and 2 happen above when a friend or some guy off the street is using the equipment you modified. Said person dies or is badly injured from the shock. You are charged with involuntary manslaughter or willful negligence. Get a good lawyer. After the criminal case, the family or the person involved (if they survive) brings a wrongful death suit or sues for lost wages and pain and suffering.

Good luck bud.

[lonewolf]

Equipment grounding is supposed to be a safety device but it is really nothing more than a secondary neutral. Only in special installations is the ground truly isolated; it's usually bonded to neutral inside the panel box. That way, even if the neutral is interrupted, a short circuit will trip the breaker. Electrocusion would be possible in the event of a double (actually triple) failure (Neutral fail + ground fail + actually equipment fail). And I've never heard of anyone going to jail or even getting a ticket for using "those little grey 3-to-two-prong adapter things".

When connected to properly designed equipment, neutral carries the current from/to the breaker box and ground does not...EXCEPT when it is carrying out its safety function to draw off voltage differentials in the event of equipment malfunction or poor wiring. Ground loops also cause a small current to flow thru ground, but that is usually very minimal and cause more problems with noise than serious danger from shock.

That is why I recommend using a good power conditioner that maintains ground quality thru a detection/correction circuit.

[LHSL]

Equipment grounding is supposed to be a safety device but it is really nothing more than a secondary neutral. Only in special installations is the ground truly isolated; it's usually bonded to neutral inside the panel box. That way, even if the neutral is interrupted, a short circuit will trip the breaker. Electrocusion would be possible in the event of a double (actually triple) failure (Neutral fail + ground fail + actually equipment fail). And I've never heard of anyone going to jail or even getting a ticket for using "those little grey 3-to-two-prong adapter things".

When connected to properly designed equipment, neutral carries the current from/to the breaker box and ground does not...EXCEPT when it is carrying out its safety function to draw off voltage differentials in the event of equipment malfunction or poor wiring. Ground loops also cause a small current to flow thru ground, but that is usually very minimal and cause more problems with noise than serious danger from shock.

That is why I recommend using a good power conditioner that maintains ground quality thru a detection/correction circuit.

A power conditioner will not remove ground loops. They will rarely fix RF problems either.

The absolute best and completely safest way to fix ground loops and other interference is to use audio isolation transformers between the pieces of gear that are causing the problem on the audio portion of the connections. The best one that I can think of is the the Radial Twin-Iso

http://www.radialeng.com/di-twiniso.htm

If there are any connections to the cable company one simple way to fix them is to use a 75 to 300 ohm matching transformer and then a 300 to 75 ohm balun. (note these won't work for satellite equipment).

Lastly, if you are operating a real recording studio or have a "real" install you should probably have an isolation transformer near the service entrance for the electrical system. All audio and only audio equipment should should be connected to the electrical panel supplied by this isolation transformer. This will derive a new ground for the electrical system and completely filter out anything it isn't designed to pass through.

[songsmith]

I dare any of you to say you've never lifted a ground. I'd be willing to bet many of you have a ground-lift or two in your gig-boxes. I don't like to use them, and often warn against them, but at gig time, you do what you have to.
And iso-transformers are nice, but I've seen RFI/EFI get by them any number of times, they're better at ground-loop reduction. Shielding is how to stop radio interference, or eliminating the RF signal altogether, often by telling the church across the street to change the channel on the pastor's wireless lav mic. For a radio station nearby, you should make certain all your cables and component-cases are shielded well enough. Lately, a big culprit is plastic casings. Most electronic PA gear used to have a sheet-metal case. For good Faraday-cage reason. The Chinese do not care about your RFI problems, and prefer to knock 5 bucks off the price. --->JMS

[lonewolf]

Equipment grounding is supposed to be a safety device but it is really nothing more than a secondary neutral. Only in special installations is the ground truly isolated; it's usually bonded to neutral inside the panel box. That way, even if the neutral is interrupted, a short circuit will trip the breaker. Electrocusion would be possible in the event of a double (actually triple) failure (Neutral fail + ground fail + actually equipment fail). And I've never heard of anyone going to jail or even getting a ticket for using "those little grey 3-to-two-prong adapter things".

When connected to properly designed equipment, neutral carries the current from/to the breaker box and ground does not...EXCEPT when it is carrying out its safety function to draw off voltage differentials in the event of equipment malfunction or poor wiring. Ground loops also cause a small current to flow thru ground, but that is usually very minimal and cause more problems with noise than serious danger from shock.

That is why I recommend using a good power conditioner that maintains ground quality thru a detection/correction circuit.

A power conditioner will not remove ground loops.

1st of all, I never said they did, I was merely explaining another condition under which ground can carry a current...however, these do...these are power xfm isolated:

http://www.furmansound.com/product.php? ... d=P-2400IT

[lonewolf]

What brand or type of cables are you using and are they all pretty much the same quality?

Neglex/quad dual twisted pair with 98% shielding might help a lot. There are some custom cable makers on eBay that use high quality neglex/quad cable and sell the cables for little more than the retail cost of the parts. This is what I use for most of my PA cables and it is drop dead quiet to the point you don't even know its on, lol.

If shortening the cables did some good, these might do the trick.

Process of elimination.....

[LHSL]

I dare any of you to say you've never lifted a ground. I'd be willing to bet many of you have a ground-lift or two in your gig-boxes. I don't like to use them, and often warn against them, but at gig time, you do what you have to.
And iso-transformers are nice, but I've seen RFI/EFI get by them any number of times, they're better at ground-loop reduction. Shielding is how to stop radio interference, or eliminating the RF signal altogether, often by telling the church across the street to change the channel on the pastor's wireless lav mic. For a radio station nearby, you should make certain all your cables and component-cases are shielded well enough. Lately, a big culprit is plastic casings. Most electronic PA gear used to have a sheet-metal case. For good Faraday-cage reason. The Chinese do not care about your RFI problems, and prefer to knock 5 bucks off the price. --->JMS

I honestly do not have a 2 to 3 prong electrical ground lift stored with any of my gig gear. We play in rain, wind, and all kinds of adverse conditions... I take the safety of the musicians I run sound for very seriously when it comes to electricity.

About the only time I've needed to run without a ground is when the venue does not provide a tie in with a ground.

[songsmith]

Like I said, I don't make it a habit, but I certainly have used them.
I also carry a receptacle tester, because I'm just flat-out amazed at how poorly most venues are wired, especially bars. The most common culprit is reversed hot and neutral, which you'd think would be rare. I've also gotten numerous open grounds, and on one occasion at a backyard show, somebody hooked up 2 legs of 220V and no neutral. That sucked, and cost us a Peavey CS-800. --->JMS

[Craven Sound]

The main cause of RFI into electronics is due to poor shielding. When the Blair County Convention center was built, all of the Cat 5 cabling had to be shielded Cat 5 because of all of the AM and FM stations right across the road at Forever Broadcasting. It was a lot more expensive to shield all of the data cabling, but a necessary evil.

I think some of the confusion here is AC grounding vs. audio shielding, which are related but serve different purposes. Sure you can get rid of ground loop with either an AC ground lift or audio ground (shield) lift; lifting the shield on a piece of equipment MAY eliminate RFI, but it's not always going to work. So to the original post, I would:

- turn the equipment on one piece at a time to see if you can isolate where in the audio chain the interference is happening.
- check all interconnecting cables, make sure they have proper shielding
- keep us posted!

[LHSL]

Some more info. Here's a great PDF on RFI in audio systems.

Check out the biggest myths...

http://www.audiosystemsgroup.com/AES-RFI-SF08.pdf

[lonewolf]

Put simply, RF interference happens when a signal source ends up doubling as an "antenna". This can happen anywhere in the signal chain.

The solution is to find all these "antennae" and shield them.

Of course, you can't properly shield jack (pun intended) if you don't have a proper ground. That's why I suggested that you should start there.

Once you have established that you have a good ground, then you can continue with the process of elimination. Unfortunately, you have had various gear get RF at different times. You might want to buy some of those super-shielded, dual twisted pair mic cables and see if that eliminates some of your mics from the problem...because...

A cheap microphone cable can act like a cheap full-wave FM antenna. Kinda like a person whose bowling and golf scores are about the same: in the lower 100's (that would be me, lol). Being that close to the transmitter, I'd bet some of those mic cable runs are getting hit with the full wave of the transmission frequency.

[jai]

maybe you could use the lead caning in the stained glass windows to make a faraday cage! lol

[Ron]

On the subject of lifting grounds, I'm in the NEVER do it club.
The main reason is tube amps. With a tube amp, your guitar strings are directly tied to the neutral side of the AC power. With solid state equipment there is usually a transformer or switching power supply which isolates the guitar from AC power.
That said, there are too many situations (as Johnny pointed out), where the neutral and hot are switched on a receptacle. See where I'm going?
A tube amp + lifted ground + switched neutral/hot = possible death. Just hope that you aren't well grounded the first time you touch the strings.

As far as RF or any other electromagnetic interference, it is mostly black magic. We have an isolated EMI chamber at my workplace. It is used to test AV equipment for use on airplanes, in order to keep any interference out of the band where the communication and guidance system signals are located. You can hook the same devices up with the same cables in the same way (and even eat the same thing for breakfast), but you can never fully predict what interference signature may pop up or what combination of shielding/electronics may be needed to stop it.
I agree with calling the station and seeing if their engineer will take a look. After all, it is his job to generate the signal you're picking up, so he should know it best.

[floodcitybrass]

About the only time I've needed to run without a ground is when the venue does not provide a tie in with a ground.

You can always rely on a bohemoth truck driver to pound in a ground stake for you.

[BloodyFingers]

I ran sound for bands for 30 years. What I did to get rid of unwanted noise was I made up a 220 cable and ran the pa and the bands all from the 220. I split the 220 into two 120 outlets. 6 per side. This got rid of noise and the singers didnot get shocked from the mics anymore.... On the 1 side of the 120 I ran the whole pa (mixer, effects, 4 power amps. The other side of the 120 I powered the guitar amps, bass amps, and all the other instruments. This got rid of radio interference and hums and shocking mics.......

[BloodyFingers]

A lot of the sound i did was in old church's. It could be out of date electrical wiring. I don't know the church your having problems with. but look at their wiring to ???

[Hannibal]

There are many things that can cause your RFI issues. Shielding, grounding, etc have all been covered. You should NEVER lift an electrical ground. I do not carry ground lifts in my PA gear. If you are not sure what your are looking for, perhaps a visit from a more experienced sound and/or electrical technician would be of assistance to you and the church. Sometimes, a particular piece of lower priced audio equipment does not have the ability to do the RFI filtering that it should. There may also, it appears, be more than one cause of the problem. If you only fix one or two, there may be still more in the system. That may be the reason you think you are chasing your tail around on this one. Sometimes, a new piece of equpment is added to a sound system, and you now have a problem that you never had before. Help is available, but an in person visit from a good tech will sort out a lot of things.

[funkmeister]

Many thanks for all the advice on eliminating my RF problem. It's still there, but we're now checking on the church's wiring. last summer a new YMCA went up (4 stories) just about the same time the problem reared its ugly head. That, coupled with the HUGE steeple (tallest structure in town by a lot!) may be a big part of the problem. Since there are no plans to tear those things down, I guess I'll keep looking. So far I've utilized 4 Shure RF filters on mics and line feeds. Those helped immensely but I still get a problem dead center in front of the altar - only with wired mics!