... high-grade unidirectional electret condenser micSpecial Circuit DSP de-noise, AGC circuitProtect Circuit Over-loading protection, Lightning proofAudio ... , reliably protecting the back-end audio surveillance equipment* Low noise,low echo and clear speech,suit for the big demand ... -in over-loading protection and lightning proof circuit.*Pre-amplifier, directly drive DVR, VCR, headphone, loudspeaker etc.* ...