Low Impedence Input Modifications
By John, W4AWM

     For a true low impedance input, ground the presently floating end of R-356 (680 ohm, 1/4 watt) on the Transmit Exciter Board. Looking at the board from the component side, the resistor is on the extreme upper right hand side of the board and will be connected to a trace at one end and a pad at the other.

     Use pins 1 and 3 for the LOW IMPEDANCE Mic with pin 1 being hot. If it is later desired to use a high impedance mic, the ground must be removed, or an outboard matching transformer must be installed in the mic line. Pin 4 places a resistor in series with the mic input to allow for high output HIGH IMPEDANCE mics. Contrary to popular belief, IT IS NOT an impedance changing device, only a level changing attenuator which knocks down the input level approximately 8db.

     This could also be done with a low impedance mic instead of the internal modification.

     I have been using a Shure A-86-A inline transformer with an Electrovoice 635 broadcast dynamic mic with my 4-Lines for years and it works great. Haven't tried it with the TR-7 because I never got around to making an adapter cable, but there is no reason why it will not work just as well.