RF Interference on Homeade Patch Panel

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acanthusarch

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I have just installed a combo-cable with 12 individually shielded cables (3 conductors each) to carry 12 channels of audio from a Studio Room to a Control Room. I have 10 XLR type and 2 1/4" Type wall connecters at each end installed in a wall plate. I soldered all connections, and when testing get signal accross the cable, but I also get a lot of RF interference in the form of AM/FM radio. I have tried doing some experimenting with some aluminum foil to shield the connectors to see if this affects the noise, but it doesn't. Is there any way to shield the connectors from the RF?

Also, I am wondering if the issue has nothing to do with shielding, but grounding. The cables are of course connected to the wall plugs which are connected to the metal wall plate, but the wall plate is not grounded at either end. Is this the cause of my problem, and if so what is an easy way to ground it.

Thanks for any help you could give.
Jason
 
Hmmmmmmmmmm...

First, are your 1/4 connectors TS (Tip, Sleeve) or TRS (Tip, Ring, Sleeve)? If they are TS, then that is more then likely your problem.

Second. How did you discover the RF interference? Did you hear it through your monitors? This could very well be a problem with your speaker cables picking up the RF, rather then the shielded cables, as speaker cables do not have a shield.

You need not worry about grounding the wall plate. It's purpose in being some sort of metal is to provide shielding to the connectors. You may although have a AC power cable nearby that may be contributing to the problem, but I really think that your problem lies in your monitor path, not the audio cables.

In the case of balanced cables (TRS, or XLR), the purpose of the two hot leads is to eliminate RF. This does this through the use of the cables shielding as well as the two out of phase hot leads. TS cables only provide shielding to protect against RF, but, you would need to be running a longer then 25 foot run of TS cable for RF to really start playing a part.

Good luck.

Ed
 
Where's the RF problem. . .

Ed,
Thanks for the input. I'm not quite sure whether the 1/4" connectors are TS or TRS. They aren't like the ones found on a guitar amp, you have to push a small tab (like on an XLR connector) to release the cable. They also have 3 connectors on the back, not 2.

I know it's not the monitors because all other sources into the board are very clean at full gain. Any signal I run through the homemade patch bay however has the RF interference.

As far as the out of phase leads on the XLR connectors goes, Does this mean that the two hot leads should have reverse polarity? In other words, If I have a cable with Black, Red and ground, should I connect 1>Black 2>ground 3>Red on one end and 1>Red 2>ground 3>Black on the other? Right now I have the cables connected straight across and get signal, but with the RF noise.

Thanks for troubleshooting this with me. I appreciate the help.
Jason
 
Nooooooooooooo.......!

Just wire up the cables with the ground on pin 1, and just stay consistent with pins 2 and 3 having either red or black leads. The circuitry on your equipment will take care of the phasing at their output.

If you have three connectors, then it is a TRS 1/4" jack.

The way a TRS 1/4" jack should work is that you have the ground wire to the Sleeve, Hot (+, usually the red wire) to the Tip, and Cold (-, usually the black wire) to the Ring.

If you are making a XLR to TRS cable for some reason, it would be from the XLR to the TRS pin 1 = sleeve, pin 2 = tip, pin 3 = ring.

If you were to say have the ground wire on the tip of a TRS jack, or on pin 2 or 3 on a XLR, you would of course have the audio running through a unshielded wire, which would not be good....:) So make sure you don't have that going on anywhere.

If you are using balanced (3 wire) cable in a balanced configuration, you should not have ANY RF problems that would show up also in your monitor path too, unless you have your wires crossed (literally...:) ).

Check all your wiring on the patch panel to make sure it is wire appropriately.

Ed
 
I'm starting to understand

Ed,
Let me clear up the situation a bit. In the Control room I have 10 XLR type wall connectors that correspond directly to 10 XLR type wall connectors in the Studio room. I also have 2 TRS 1/4" jacks in the Control Room connected to 2 TRS 1/4" jacks in the Studio room (For headphones).

Right now the way I have them wired is like such:
Black>1 Braided-Ground>2 Red>3 on both ends.

So my error is in assigning the braided ground wire to Pin 2 instead of Pin 1?

Does it matter that about 1/2" of wire is exposed on each connector where it is soldered to the post? The Wall plate with all the connectors in it is also close (18") to an AC power cable and outlet on both ends. I figured the shielding on the combo cable would be enough to block any interference from the AC. I hope this isn't the source, because relocating the cable away from the AC wiring will be nearly impossible. How do I know if this is the main cause of the RF interference?

Again, thank you immensely for helping a beginner get his act together.
Jason
 
I see....

Well, if you think about it, 1/2" is not a very good attenna. Not quite big enough. But, it may be enough.

Also, 18" away from AC power SHOULD be enough distance in any case.

In either case though, either of these thing, of both could be the problem. It is not impossible, but, I find it almost incredible that it is. If you seen my patchbay in my mobile rig, you would understand! :) I have many connections with at least 1/2" of exposed (meaning, not shielded) wire on one of my patchbays, but have no problems with this. I have also wired a friends studio in much the same manner as yours and had no problems with it.

One way you could test to see if the AC wire is the culprit is to shut off the breaker that feeds that wire. With no AC going down the line, it would not carry RF. Try that. If your problem goes away, well, you will need to find a solution......

If it is still there, then it COULD be the unshielded part of the XLR cable in the wall picking up RF. A way to see if this is the problem would be to use some tin foil, and make sure that it encases all the exposed wire, but also make sure that it is in contact with the wall plates on both sides. This will look funny, but you only need to do it for a test. It is important to make sure that there are no gaps in the foil to wall plate or anywhere around the connections for this test, as RF shielding requires a solid wall around what you are trying to shield.

If this doesn't do the trick, I am going to assume that you have some cabling somewhere in the path that does not actually have a shield around the two hot wires for the audio. I have seen 2 wire with a ground soak that looks a whole lot like shielded wire, but is not. Verify that ALL of your wire is actually shielded wire. The shielding should surround the other two wires.

Another possibility is that the AC circuit you have your microphone preamps on (this could mean your mixer if that is your microphone preamps) is not grounded properly. The way to see if this is the case is to try a power conditioner on the AC circuit between the wall outlet and the gear. A power conditioner on all your equipement is a good idea no matter what, so if you are not using one, get one. There are many acceptable brands and models out there that work well, and you shouldn't have to pay more the around $75 for one. Make sure all of your audio equipment is running off of the power conditioner.

Try that stuff out and let me know.

Ed
 
So my error is in assigning the braided ground wire to Pin 2 instead of Pin 1?


Yes. You should definitely wire the braided shield to pin 1.
That's industry standard.

Not to second guess sonusman,but I would be surprised if the interference was getting in on the speaker leads. The RF is undoubtedly very low level (unless you live in the shadow of a radio tower),and would need amplification to be heard. Unless your monitors are powered, then it would be possible.

I would first address the pin 1/shield issue, and if you still have a problem, systematically break the signal path, working from one end to the other, to see if you can determine the point where it's getting into your system. Pay attention to any High gain devices such as mic preamps. Their inputs are capable of amplifying extremely small signals.

Good Luck!

Twist
 
I should read the details better!!!

My god, you have the braided shield to pin 2? I misread that!

Well, there is your problem!!!

What is happening is that the RF is hitting your shield, but that shield isn't going to ground like it should. Instead, pin 2 is the + hot lead for the audio, so when it gets to the microphone preamp, it is being amplifies a lot!!!

Good eye twist! I missed that. And of course you are quite correct about the RF in the speaker lines, mostly. I have experienced problems with this in the past which is why I suggested checking it out. But yeah, it would have to be a very hot signal....:)

I gave the way you should wire the XLR's and the TRS, and the TS cables, and also the XLR to TRS cables. Follow that and your problems will go away.

Ed
 
INDUSTRY STANDARD

Thanks a bunch guys,
I looked everywhere on the 'net for an "industry standard" pin-out guide for wiring XLR Cables but came up with nada. So I just went for it and guessed wrong. Now everything makes perfect sense. The pre-amp (Mackie 16ch Mixer) is amplifying the RF signal on the ground wire, which I incorrectly assigned to a carrier wire instead of the ground. Duh! What a relief, I was starting to think I was going to have to rerout the entire cable.
I'm not going to have a chance to fix this until Friday, but y'all will be the first to know (and thanked!) if it works.
Again, the help is much appreciated.
 
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