Midian's NFBF-10 is a single channel 600 ohm balanced line audio amplifier with a tunable notch filter and AGC as well as an adjustable gain output stage. It can be used in several ways. It may serve as a simple single channel line audio amplifier, an AGC amplifier, a tunable notch filter, or a combination of these functions. It's most common use it to remove the 2175 Hz tone remote keying tone from voice recording systems and 1950 Hz Pilot Tone from voting systems. A high pass, low pass, bandpass filter can be effected through the use of jumpers.