I am designing a device that is similar to a megaphone to amplify voice. I have a feedback problem due to the close proximity of the mic to the speaker. This is a hand held battery powered device. The microphone is in an enclosure, which almost prevents feedback, but doesn't always. I've read about notch filters and frequency shifting as possible solutions. I'm using a dynamic mic with an op amp preamp feeding into a class D 30W amplifier, all running at 24-28V. I need a circuit to reduce or eliminate feedback. Can anyone help point me to a circuit that would accomplish this while being compact and efficient? Absolute fidelity of the audio is not critical, understandability of the speech is what is needed.