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.