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Jim's 470K resistor solution is a good one where the transceiver limits
audio response to what we're normally accustomed to for SSB. I use a 470K
resistor in the base of a D104 when using my Drake TR5 and Kenwood TS-830S
transceivers. For AM and ESSB modes, the JFET source follow helps to extend
the low-end range by completely unloading the crystal mic element.

Another reason to use a JFET as a source follower isn't always apparent: by
keeping the source Z low, the attenuation effect from mic cable capacitance
is minimized. True, it's really only needed on longer than normal mic cable
runs but it does help with physical microphonics on a cable with a high
source Z. For example, kick a mic cable with a high source and termination
Z and you'll hear it. But sure, we don't kick our mic cables. With a
capacitive source, which the crystal element is, cable capacitance creates a
voltage divider. The level at the mic jack is reduced but the frequency
response is unaffected with a capacitive voltage divider.

The worst case occurs when a mic with a highly resistive source Z feeds a
long cable. In that case, the parallel capacitance of the cable in series
with the high source resistance creates a low-pass filter. The JFET as a
source follower *placed at the source end* solves either problem.

Unrelated but I recently rewired the 4-pin mic jacks on my TR5 and TS-830S
transceivers. In both instances, the mic ground pin isn't bonded to the
chassis at the mic connector. Instead, and despite what the TR5 schematic
shows, the ground pin is attached to the audio shield and the shield is
connected to a 0.1" header on a PC board some considerable distance away
from the mic jack. That's a classic example of the so-called "Pin 1
problem."


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20into%20the%20audio%20circuitry.

Paul, W9AC




Paul, W9AC