Model 705 Microphone Low Output Mod

From the Ten-Tec Reflector December 22, 2000

The reason you are having low output from your TT 705 desk microphone is due to a design flaw in its bias circuit.

The Primo EM-60 ( EM-100 in the 1201 kit ) electret cartridge used in the 705 requires 1.5 volts DC minimum to bias the JFET inside the electret well into the constant current region. Actually, 3 to 4 volts is much better.

The existing TT design has way too much resistance (24K) in line with the electret such that it's bias voltage never rises much above .5 ~.6 volts DC; and the JFET never gets out of the constant resistance region, and consequently cannot develop adequate gain.

To fix your 705 mic, tack solder a 3300 ohm resistor across R11 ( which is 22K now ).

You will find that you will have plenty of output after this mod .

If you are interested in enhancing the low end freq response as well, increase C2 to .47 uf or larger.

Also you should verify that the 705 is being run on at least 7 volts by measuring its operating voltage at the M+ pad on the 705's printed circuit board. Some older Corsairs have 10K behind the mic connector pin 4, which should be reduced to less than 68 ohms.

73, - Phil KF6ZZ