D-104 microphones come as either Crystal or Ceramic models, which are very high impedance elements. They have an equivalent circuit of a very small capacitor and are intended to be loaded by a resistive impedance in the vicinity of 1 megohm. The tradeoff is that the lower the load resistance, the more low frequencies will be rolled off in the audio. 10K might leave you with a response quite a bit on the bright side. Some people recommend sticking the D-104 into the 50K load common on older gear intended for Hi-Z dynamic microphones, to create a "peaky" intelligible sound, but this has always seemed excessively bright to my ear. Astatic has already built a "communications" sound into the microphone working into the nominal load. You don't have to go to any extra trouble to achieve "intelligible" sound. The best universal answer to the problem of plugging a crystal microphone into a solid-state input is to use some sort of active buffer amplifier such as an op-amp, or an FET to provide a very high impedance load to the microphone and a very low impedance drive to the following transmitter. This circuit would also be compatible with any legacy VT gear. As has been suggested, if your D-104 has the Astatic preamp in the base already, these issues have already been dealt with and you don't need to worry about matching; it's Just a question of setting the gain of the amplifier so you are not driving the input to the radio too hard. Someone was offering a buffer amplifier kit for general purpose interfacing between hi-Z microphones and Lo-Z radios a couple of years ago, but my saved files don't go back that far anymore. If you have a "directbox" either active or passive, for matching high impedance musical instruments to lo-Z microphone inputs, that might be an effective way to go as well. Behringer makes active direct boxes that sell through Internet music dealers for as little as $19.95, which is competitive with the cost of a good quality transformer.
Bob Bruner W9TAJ _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
