I am designing a system which contains an intercom i.e. 2 simple audio amplifiers back to back. The problem is because the intercom is operated remotely I cannot put push button switches, which would eliminate feedback. Therefore need ideas for filter to minimize feedback noise.

 

This may not be a practical system--the remote location may be talk back, if there is a push to talk switch at the local station--making both continuously live may not work.

But to help the situation, at least you can linearize the frequency response to control unreasonable resonances (peaks in freq response) in the system--feedback tends to occur at these resonance points.

 

Try making one of the amplifiers inverted.

 

Sounds like you need a full duplex intercom circuit such as this.

 

You cannot filter out feedback frequencies because they are the same frequencies as the sounds from speech.

The full-duplex intercom still might cause feedback if the mic and speaker at each end are close together. A sound into mic 1 goes to the other end 2 and comes out speaker 2. Then mic 2 picks up the speaker 2 sound and sends it back to the original end where its mic 1 picks up the speaker 1 sound and it goes around and around.

High quality full-duplex speakerphones like Polycom use a digital echo canceller that makes a model of the feedback and the room acoustics. Then it cancels what comes out of the speaker and picked up by local mics from being transmitted.