The Sennheiser MD 421 is a German cardioid dynamic microphone, widely used for speech in broadcasting and for music in live concerts and the recording studio. Introduced in 1960, the internal large-diaphragm transducer element of the MD 421 is still produced unchanged by Sennheiser. The MD 421 is considered a classic and an industry standard. More than 500,000 units have been sold.[1][2]

Jose Feliciano with two MD 421 microphones (for vocals and guitar) in 1970.
With a diaphragm diameter of 27 millimeters (1.1 in), the MD 421 is classified as a large-diaphragm microphone. Its wide frequency response (30–17,000 Hz ± 3 dB) exceeded the former Hi-Fi standard and came close to the performance of condenser microphones. The compact design of the MD421 transducer system was novel at the time, because directional pickup patterns had only been achieved by adding sound entry ports to the rear of the element (for instance, the Electro-Voice "variable-D system" with a row of ports in the handle.) Microphones with sound-shaping ports could only be held in the hand to a limited extent without the directional effect deteriorating. The MD 421 offered a smooth, closed plastic housing without ports, retaining its directional pickup pattern no matter how it was held.

Another special technical feature of the MD 421 is the hum compensation coil: It is wired in opposite polarity to the coil of the converter system. Low-frequency interference fields common to both coils are canceled out.
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