You were sort of convincing me until this point:
So if you have to set the dial to "3", don`t worry. If the mic gain switch is set to high, you could set it to low, and turn up the dial to a value that is more comfortable for you ;-)
As I understand it, the mic lo/high switch is just optional attenuation - so if you have it set to "lo" you are actually attenuating the signal to avoid overloading the input. That being the case, why not just plug it into the line input where it wouldn't need attenuating?
yousef,
I do not have a schematic of the M10, but of the D50. In engineering, you try to re-use proven solutions. The mic preamp of the D50 has variable gain, and switching between Mic hi and Mic low also changes the gain of the preamp. This ensures maximum headroom and best noise figure. My measurements of the M10 hint at the same solution. This makes sense for battery powered devices where you simply cannot increase the preamp supply voltage to increase headroom. Using an attenuator makes sense when you have a real mixer with +-15V power supplies and symmetric inputs, where you can expect 1..2V worth of audio from a phantom powered condenser mic. Noise figure is not an issue then...
On the other hand, attenuating the weak signal from a battery powered electret mic with maybe 8mm diaphragm diameter reduces precious signal-to-noise ratio. Setting the gain of the mic preamp to just the value you need is the better solution in this case.
I did some measurements of the mic and line inputs:
Mic low, meter at -6dB, 1khz Sine wave:
Gain 10: 4.5mV
Gain 5: 29.4mV
Gain 3: 129.5mV
Mic high, meter at -6dB, 1khz Sine wave:
Gain 10: 0.4mV
Gain 5: 3.3mV
Gain 3: 18.4mV
Line in, meter at -6dB, 1khz Sine wave:
Gain 10: 260mV
Gain 5: 835mV
Gain 3: >2000mV, limited by signal generator
As you can see, we have 3 more or less overlapping gain ranges - nice! And the line input can take pro levels without problems.
Please take my measurements with a grain of salt, because the signal generator/meter combination I use is well worn (30 years old RIM PG-100) and not lab grade.
Hope that helps,
Rainer