[Elecraft] Getting higher gain from input of line in or rear panel mic on K3

[David F. Reed]

I am trying to get set up for JT9 in Data A mode; I am not getting 
enough drive from the sound card to adequately drive the K3.

I am wondering (but cannot figure out from the manual) how to achieve 
this using either Line In or the rear mic jack when in Data A, and get 
more gain, but not use the front mic (leave it connected but not get 
audio from it).

At the same time, the front mic is working fine (correct bias etc.) and 
I don't want to loose this.

Surely someone must have solved this before me, and could help me figure 
it out.

Thanks & 73 de Dave, W5SV