Subject:Radio Station Interference

Posted by: captn_spalding

Date:9/17/2003 10:40:25 AM

This is a bit OT, but here goes. I'm doing some work transcribing some LP and 78's for a client. For reasons of his own it needs to be done at his location (I move my equpment, the recordings can't leave his premisies) To make a long story short, my RIAA preamp is picking up a local radio station. The 50k watt transmitter is just down the road. I can lessen but not eliminate the problem by changing the orientation of the equipment. Any ideas would be appreciated. The client's equipment has the same issue, he's just grown use to it!

..spalding

Subject:RE: Radio Station Interference

Reply by: RiRo

Date:9/17/2003 12:34:36 PM

Depends on exactly where in the process it comes in, and how much you want to cut into your preamp.

1. make sure the cables from the TT to the preamp are shielded. Even if they are, try new ones.

2. A capacitor from each center lead ground on the cables will most likely take care of the problem. You have to get the impedience of the preamp and do the math to find out the value. I used to have that stuff memorized, but wouldn't even know where to find it now. Amature radio (ham) sites on the web would likely have the values. The capacitor will let the RF go to ground without touching the signal in the audio range.

3. These can be done in the cables (my suggestion) or in the preamp itself. If it works in the cables... do it there. If it needs to be in the preamp, you will have to find out exactly where the noise is coming from and kill it there. It may not be possible to do it that way.

I would try the cables first.

Next I would rent or buy another preamp and try that.

cut into the box is last resort.

RiRo

Subject:RE: Radio Station Interference

Reply by: fishtank

Date:9/17/2003 3:21:10 PM

Before you add the capacitors, I would suggest adding some clamp-on ferrite cores over the cables just before they enter the preamp. Many times the RF will enter on the shield and using a capacitor from center conductor to shield may not help. I assume the 50 KW transmitter is probably in the AM band and you will most likely need some specialized material ferrite to get decent results at that low of frequency. Unfortunately, Radio Shack ferrite products are not of this type. You should also wrap the power cord around a ferrite core just before it enter the preamp as well.

There is a company called "CWS Bytemark" that carries these products and another called "FairRite" I believe. They could probably advise you as to which products may work best for your application. The bad thing is that it is very difficult to determine exactly where and how the RF is getting in - you may spend time and money on this and get little to no improvement. The fact that you can move the preamp around and reduce the interference is a good sign, though.

Subject:RE: Radio Station Interference

Reply by: Rednroll

Date:9/17/2003 5:32:42 PM

Those are some pretty good recommendations. In addition to those, I have a hack work around that might be simpler and cheaper. Go buy some Reynolds alluminum foil. Lay sheets of foil across all the equipment (or wrap it around cardboard boxes and place the box over the equipment) and rap it around all the wires, making sure each foil sheet is touching one another. Get a piece of wire or alligator clip and connect one end to the foil and the other end to a water pipe or the 3rd pin of an AC outlet. It won't look pretty, and you might get laughed at doing it and it might not work, but they'll think you're a genious mad scientist if it does. In doing this you're effectively creating a RF shield, which will take the majority of the RF signal to ground instead of into your equipment. Mcguiver at work!!!

Subject:RE: Radio Station Interference

Reply by: MJhig

Date:9/17/2003 7:07:19 PM

And when the job's done, be sure not to waste that foil....

CLICK :-} ME

MJ

Subject:RE: Radio Station Interference

Reply by: Frenchy

Date:9/18/2003 9:50:58 AM

Red:

Does it really HAVE to be REYNOLDS aluminum foil?... ;)

Subject:RE: Radio Station Interference

Reply by: Rednroll

Date:9/18/2003 10:39:00 AM

Anything of good thickness, don't be going to the dollar store and buying that stuff. I've never seen aluminum foil that you could see through, until my wife picked up a roll of this before.

Subject:RE: Radio Station Interference

Reply by: DKeenum

Date:9/18/2003 12:04:17 PM

My guess is that the cheap foil wouldn't be the best for the mind-control-bolcking beanie either.

Subject:RE: Radio Station Interference

Reply by: Rednroll

Date:9/18/2003 12:53:42 PM

"My guess is that the cheap foil wouldn't be the best for the mind-control-bolcking beanie either."

LOL!!! now damn it...you got me wondering...what movie was that? Was that "Signs"?

Subject:RE: Radio Station Interference

Reply by: MJhig

Date:9/18/2003 1:02:23 PM

LOL DKeenum, good call...

Quote from the site posted above:

>>>>>
Foil Thickness:
How thick is your aluminum foil? Find out using Zapato Productions Intradimensional's Aluminum Foil Thickness Calculator (pops up into separate window, requires Javascript). If you do not have Javascript, use this formula (with grams and centimeters):

((mass / 2.702) / (width * length)) * 10000 = thickness in microns
If your foil is less than 16 microns, increase the number of layers used in construction.

<<<<<<<

MJ