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Astatic D-104 microphones from back in the tube radio days is considered a high impedance microphone. An old D-104 with a crystal voice element checks in at around _4000 ohms_* which is considered pretty high impedance. Later the D-104 went to an updated ceramic microphone element which remained high impedance. Over time there were changes in the ceramic element used in manufacture. When the amplified D-104 was introduced the impedance changed. If you desire using an old hi-Z microphone on a modern solid state transceiver then amplification is necessary. One approach is the use of a Field Effect Transistor. Astatic D-104 microphones from back in the tube radio days is considered a high impedance microphone. An old D-104 with a crystal voice element checks in over '''[http://www.internetwork.com/radio/kb1nfs/D104Matcher.htm 1M ohms]''' which is considered pretty high impedance. Later the D-104 went to an updated ceramic microphone element which remained high impedance. Over time there were changes in the ceramic element used in manufacture. When the amplified D-104 was introduced the impedance changed. If you desire using an old hi-Z microphone on a modern solid state transceiver then amplification is necessary. One approach is the use of a Field Effect Transistor.