Capacitor to stop RFI......i think

[Tidy Max]

hi folks, today ive found myself at a job where there is a humming on a sound system.

The humming is coming from a mic line that has 4 mics on it wired in series and the line is left open, each mic has a switch which switches in the mic capsule.

I think the hum is rfi, i have tried a resistor across the end of the line to close the mic line but it has made no difference.

My question is, what capacitor do i need to stop the rfi, there is an abundance of flurries through the building which i assume is the issue. I know assuming is dangerous.

Ive googled this topic and the general answer is 4700pf, which is for a type x use (between hot and cold).

Any help would be much appreciated!

Max



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[MarkieSparkie]

The humming is more likely to be caused by an earth loop or loops, or poorly decoupled power supplies rather than RFI.

 

[Tidy Max]

Cheers markie, its deffo some sort of rfi the mic its self seems to be picking it up. When i get home il post up and explain better but basically there is an office on one side of the building with a single 2 core (and screen) but not shielded all the way to the other side of the building and in series feeding 4 mics.

Each mic has its hot leg switched.

The buzz off one mic is tolerable but with all 4 on its 4 x the amount and pretty annoying.

If i disconnect the mics there is no buzz just off the cable.

With the mics connected and the screen DISCONNECTED the buzz is worse.

If i connect the mics directly to the amp in the office.... no buzz at all not even with all the pots turned right up.

The mics are a metal gooseneck mic, i think the mics themselves are picking up the interference. If i had our sound meter i would measure the frequency of the hum. If im correct a ground loop would be giving off a 50hz hum would it not?

If you are correct and it is a ground loop what would you do to suggest to fix it?

Im now swapping the switches for double pole switches. So both the hot and cold are switched at the same time as there is no interference with the load completely d/cd. So when they use a mic they will still get a hum however it will only be from one mic at a time and also only be a quarter of what they are dealing with at the moment.

Thanks for the reply Markie

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[Rockingit]

Assume if they're goosenecks that they're also condensors of some flavour and therefore need phantom power. 4 in a line in series is NEVER going to give decent results! You need 1 mic per pre-amp, always. Once you've done that, THEN you can go chasing the earth loop buzz that you've almost certainly got, not RF interference.

 

[Tidy Max]

Nope, just regular cheapo mics.

Wiring mic points in series is pretty common imo. The amp being used only has vox on one mic input and even if all the points were wired seperately i don't think they would fork out for a pre amp/mic mixer, its just so the women at the tills can shout for help of they end up swamped.

There is only one amp for the equipment fed off one socket, with one cable going out to the mics and one speaker line (100v). It is not an earth loop fault.

Ive attached a pic of the mics i have made and fitted.

The job was to replace there existing mics which were some war of the worlds looking bad boys, i kept one somewhere so il upload a mic of that!

Its a hain of shops and this arrangement has worked fine at the other 6 however they were wired in 8723 (2 pair screened) because those amps had an access rather than vox.

Any replies appreciated as always and thanks for taking the time to read and help.



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[RobWansbeck]

I've never seen this system but if it's just push to talk and the mics are normally left switched off I would assume that they should be wired in parallel.

 

[DieselShunter]

You described the cable as 2-wires plus screen - which sounds like a balanced audio system. But then you say the mics are switched in the hot leg - which sounds like an unbalanced audio system. You can't mix the two arrangements correctly without using balancing transformers. Otherwise you will get lots of hum, which I think you've proved.

Capacitors won't fix it.

You need to get proper balanced microphones with switches on. And then wire them in parallel, not series.

 

[static zap]

Wondering if old system was moving coil in series but shorted out ones NOT in use !

Just guessing no practical experience with retail/banking

 

[Rockingit]

If you think about how a mic capsule actually works - using a diaphragm to move a coil to make a current - then it's easy to see that you can't wire them in series anyway - otherwise the energy produced in one capsule would disipate instantly across the others, plus losses. What I *think* the OP is actually describing is a parallel-in-series system. And as others have stated, you need a variety of other components to make this work, depending on exactly what impedance etc the amplifier is expecting to receive.

May I be the first to suggest the very un-manly principle of RTFM?!

 

[Tidy Max]

Here comes the apology...

I meant parallel, sorry i just was not thinking. The microphones are balanced and it is a balanced system, i take the term 'hot' and 'cold' does not apply to balanced audio?

Cheers for the replies, its my own fault the thread has digressed. There was a PSU for the receipt printer under the counter next to one of the mic back boxes when the printer was switched off the humming stopped.

I'm afraid i don't get the "no practical experience with retail/banking comment though"? Please explain

 

[grantr37]

buy a few ferrite bobbin beads and fit them on the lines to provide screening

 

[Tidy Max]

Cheers grant, tried a couple of ferrite rings which didnt appear to do anything, however they were huge, potentially could have been sold in ann summers lol

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[grantr37]

quite a lot of noise off of a power supply....

 

[BigAndyB]

You have to wrap the cable at lest twice round the ferrite and cable tie it to keep it secure.
This way will work best.

 

[static zap]

If you think about how a mic capsule actually works - using a diaphragm to move a coil to make a current - then it's easy to see that you can't wire them in series anyway - otherwise the energy produced in one capsule would disipate instantly across the others

But in a freaky design day , a switch shorts each one out ,
Good conditions -- all shorted --or 3 of 4 shorted --Shorted ones work as wiring !
Bad condition 2 pressed simultaneously .. now 2 in series



( Retail-banking comment -more to do with not having ever stripped a setup to see how wired ! )
( Series does sound nutty but not impossiible for 3 - 4 mikes ! )
( Plenty of cheap mic setups Shorted out when off )