RF Puzzle

[3guk]

So again comes the time when I need help,

 

This time its regarding RF interferance along a 60m mic cable, the mic cable is good quality van damme stuff, so I have no worries about that.

 

I have tried two different mics on the end, both of which require phantom power, these are :

 

AKG CK91's

AKG C747s

 

The moment I put a non phantom powered mic on the line, the signal is crystal clear.

 

We don't think it is phatom as we still get the RF even when the phatom is off.

 

For those that are interested, we are managing to get Capital Gold, in crystal clear quality.

 

We have tried sticking a capacitor in the line, this was suggested to use by another technican and this has had no effect what so ever.

 

Anyone got any suggestions about how to get rid of it ?

[Network_Josh]

I have seen this before when the Shield and the Hot were wired up wrong at both ends on the signal cable - the longer the signal run the better the radio reception you get.

 

We had Five Live though not Capital Gold

[3guk]

I forgot to mention, the cables are perfect, they work fine with any mic that does not have phantom powered circuitry in !!

 

Metered accross the cable, and checked for shorts, nothing wrong what so ever !!

[bruce]

But is the shield definitely on pin1 ?

[Tomo]

Do the mics do funny things when attached to a different cable?

 

My guess is that shield isn't wired to pin 1.

[Bobbsy]

First, a long shot: what mixer are you using? Some Mackie models from a few years back have a known fault causing RFI when mic cables are plugged in.

 

Second, assuming it's not the Mackie, you should indeed be checking the connections on the screen (shield) of your cable.

 

Bob

[3guk]

Shield is connected at both ends.

 

Mixer is a little soundcraft one, it does the same thing on yamaha's too !!

[p.k.roberts]

Just a couple of thoughts - dry or dirty joints are pretty good at rectifying AM radio, so it might be worth having the connectors off of your long cable and re-soldering them and whilst there, checking the general condition/cleanliness of the pins/sockets. The CK91 is actually the removable capsule on the AKG mic (on an SE300 body) so again, removing and re-seating the capsule a few times in case of a dirty connection might help. The '747' is a pre polarised back electret, so the cable between the 'head' and the 'power end' is probably unbalanced. You could well have a couple of these faults simultaneously occuring which is why the problem is proving so difficult to solve. I would guess, as you have, that the 'phantom power issue' is a red herring.

 

Otherwise, learn to love Capital Gold :-)

 

Let us know if you manage to solve it.

[bruce]

Metered accross the cable, and checked for shorts, nothing wrong what so ever !!

.

(snip)

.

Shield is connected at both ends.

 

 

Sorry to be pedantic, but have you just done a pin-to-pin continuity test between each end, or have you actually opened up the connectors to check that the shield is definitely on pin 1 of the XLR? If the XLR was wired with the shield on pin 2 or 3, it would look Ok when you metered it, but it could conceivably exhibit the behaviour you describe.

 

Have you tried a different cable?

[dbuckley]

Good old AM radio breakthrough.

 

The issues you have are that something is picking up the station at a significant signal level, and that there is some non-linear element in there that is rectifying the RF to get AF out, much like an old crystal radio.

 

First, find the rectifier. Through a process of substitution, identify which element contains the rectifier, desk, cable, mic. As well as badly soldered joints and corrosion, electronic elements in the mic or desk could be the source of the rectification. Phantom powered mics have electronics, as do desks. The Phantom power path may have diodes in it, in the desk, the mic, or both, and these could be contributing. In this case the presence of the diodes is a design feature, so swapping the mic or desk for an identical one may not be helpful.

 

The other approach is trying to reduce the interfering RF level. Phantom uses the screen of the mic cable in a way that makes it an unbalanced leg, what you generally would call an aerial. The ground leg needs to be bonded solid, with great connectos and low imedence joints.

 

There is one thing you could do to try and reduce the interference, which is to custom wire the starquad. Normally you have two pairs of twisted pairs in starquad, straight paralleled. So use one of the two of the outer twists as signals, as normal, and then use the two remaining conductors as grounds. At the desk end, wire the two new ground conductors and the cable screen to pin 1. At the mic end leave the screen unconnected, and wire the two ground cores to pin 1. Obviously this is an odd cable, has to be in one piece, so label. The rationale behind this is that the unbalanced ground is now contained within a faraday cage, and so hopefully will not act as an antenna.

 

AM breakthrough - always good fun.

 

Capacitors will help, but only if placed so as to bypass the interfering RF, and to do that you need to know where its getting in.

[i_hate_fisicks]

I had this in a small theatre once, and the amplifier was the root of the problem... Don't know if that might be applicable to you.