Remove 1170 KHz AM radio station from microphones

[getcwd]

Hi. I work for a security company and we currently have a challenge.

How do I remove a 1170 KHz AM radio station from my 8 microphones that are installed with 2 Conductor shielded, 22 gauge with a 24 gauge drain wire with lengths ranging from 15 - 250 feet from my server room?

The radio station is KCBQ 1170 AM located 5 miles away and transmits at 50,000 watts daytime; 2,900 watts nighttime:

http://en.wikipedia.org/wiki/KCBQ

I have 8 Louroe Verifact-E microphones:

http://www.louroe.com/mics.asp?model=emic

The microphones are connected to a Louroe IF-8 Audio Interface Adapter:

http://www.louroe.com/audiointerface.asp?model=if8

The adapter outputs are plugged into an 8 channel PCI audio card for my PC based video surveillance system.

I built a simple low pass filter using an LM324 op-amp, 2 resistors, and a capacitor that only cleared up one of the microphones that is only 15 feet from the server room.

My low-pass filter is similar to this image:

http://www.roboticsindia.com/public/images/op/lowpass.png

When the low pass filter is applied to the 15 ft distance microphone the static is removed and it works properly. The low pass filter applied to the other microphones removes most of the static although the radio station comes in more than any sounds in the environment.

I heard that the LM324 only operates up to 1 MHz and the frequency I need to remove is 1.17 MHz AM. Do I just need to use a higher frequency op-amp for an effective low-pass filter? What type of filter setup do you recommend?

I have been working to solve this challenge for over 6 months. Any help is appreciated.

 

[berkeman]

How are you terminating the shield at the microphones and at the main accumulator/amp module? Are the microphones being powered by the same twisted pair that is carrying the audio back to the main amp module, or do they get their power locally from the AC Mains at each microphone location?

 

[getcwd]

We ground the microphone cable at the server room and have tried grounding at a microphone end also with no noticable improvement. The cable has a foil wrapped red and black wire with a bare ground wire outside of the foil wrapped pair of wires all within a main insulation jacket. The wire is straight and not twisted pair. It has both power on the red wire (12 VDC) and microphone signal on the black wire and they are foil wrapped together.

 

[berkeman]

We ground the microphone cable at the server room and have tried grounding at a microphone end also with no noticable improvement. The cable has a foil wrapped red and black wire with a bare ground wire outside of the foil wrapped pair of wires all within a main insulation jacket. The wire is straight and not twisted pair. It has both power on the red wire (12 VDC) and microphone signal on the black wire and they are foil wrapped together.

I'm not quite understanding the grounding thing. Is the shield & drain wire used as power ground for the microphones? With the red wire 12Vdc and the black wire the microphone return signal? That could be part of the problem if I'm understanding that correctly. You want the shield to be separate from the microphone ground. Can you try 3-conductor shielded cable, where there are 3 wires (12Vdc, uPhone GND, uPhone signal) inside the shielded cable? Ground the shield at the amp end, and terminate the shield to Earth ground at the microphone end in about 150 Ohms (or whatever the common-mode Zo is from the shield to Earth ground for most of the run of cable). If you are using the shield for the power return, then yes, the RF signal from the AM station will induce ground currents that will add into your uPhone signal...