Well the rca inputs have their own gain control, as do the mic inputs. I know VERY little about audio circuitry, but I would guess that you can't have two seperate gain controls for one analog gain stage. It would seem logical that they each have their own gain stage. Once again, I don't know squat about this kinda stuff.
ps, I know that on top of the ua-5 there is a simple schematic of the ua-5. Perhaps our answer is there (I'm too lazy).
I've traced the UA5
.
I don't know about the Oade mods, but I have upgraded the RCA ins on my unit.
Here is the rough gain structure:
Line In L ----***----amp3a------
\
L (hot +) - amp1a\ \
L (gnd) ----------&&& - amp2a ----- amp2b ---- ADCL (unbalanced)
L (cold -) - amp1b/
Dual opamps, eg., 1a and 1b are on same chip.
Right side uses two new chips (four opamps) and shares amp3b .
***: gain on *input* of audio (simple attenuator)
&&&: gain on mic input is via "instrumentation amplifer" involving amp1a,1b and 2a. That is, there is real gain variation, not just attenuator.
Poorlymodded version: AD712 for amp1a,1b. AD8620 for others. Should improve AD712 though.
Richard