This video shows how I modified the Astatic D104 microphone and added a JFET source follower circuit to it with a 9V battery. Its a pretty easy mod. I also use a phase switch. I highly recommend using this circuit for the D104. I prefer the source follower rather than a common source type circuit as the D104 mic already has a high audio output and presents a high Z .to the TX speech amp. I have used this circuit in this mic and it has been running for about 5 years with no issues on various transmitters.
Schematic is here at the bottom of the page.
https://fibrdesign.com/frp-downloads-and-links/
D104 Mic Source Follower
Here is a Field Effect Transistor (JFET) source follower for the ubiquitous D104 microphone. The D104 element is a high output mic and a source follower is all that is needed. This allows the operator to use almost any transmitter input impedance for a transmitter first speech amp stage. I mike to use a 47K ohm input on the first stage of a speech amp (typically a 12AX7 on vintage transmitters) to reduce susceptibility to 60 Hz AC hum. Also it is a good idea to not run the mic signal through the shield. I have used this circuit for about 5 years with no failures and with various vacuum tube transmitters. The circuit is powered from a 9V battery, but a regulated DC supply with work too.
Note: Bear WB2GCR pointed out that the 47uF cap is a little large for a battery input. You could use just a 1-2uF cap in that case. However, I wanted maximum protection from transients with vacuum tube transmitters, so keep that in mind.
D104 FET Follower N1PS
https://fibrdesign.com/wp-content/uploads/2020/03/D104-FET-Follower-N1PS.pdf
No comments:
Post a Comment