[Elecraft] [K3] Additional Gain Stage for FP Mic?

[Wright, Robert]

In the K3 manual (version D10) under the MAIN:MIC SEL menu is the following note:

	"For the front-panel mic only, tap 3 to turn on an additional gain stage. Use this only
	with very low-output mics. An apostrophe will appear after the H, e.g. H' ."

(At first it didn't work, but I discovered by searching this list that tapping "7" instead is required)

Anyways, when I am monitoring my transmit audio, I can get a much better S/N by using the additional gain stage and changing the main gain from H to L.  So instead of FP.H, using FP.L'.  Mostly, the reduction in noise seems to be to the K3's internally generated noise.

(I'm still studying the schematics, but the pertinent gain stage seems to be U5A and U4 on the KS DSP:CODECS page (29 of 56).  I see three different gains available at this stage.)

Is there any drawback to setting up my gains using the additional gain stage?

And this leads to the question of where is the switchable gain for the rear panel mic connector.  I see no hardware to support RP.L vs RP.H.  So is the only gain change for the RP in the U7 codec?  If so, my tendency would be to drop the value of R15 on the KIO3 board (KIO3 Main:Sheet 2, page 12 of 56) to improve the S/N from the rear panel mic (assuming most of the noise is introduced on the lines going into the codec or by the codec itself).

Of course, one wonders whether any of this can be heard over normal HF propagation noise.  But it could make a difference on the higher bands and with FM.

73, Bob, N7ZO