Finding gain in series of amps and attenuators

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Homework Statement


Please see the attached drawing. Find levels at A and B.


Homework Equations



voltage gain in dB = [tex]20log(V_out/V_in)[/tex]
dBu defined: [tex]20log(V/0.775V)[/tex]

The Attempt at a Solution


First question, since I know the level before the first amp, can I disregard the impedance of the source? So the gain and dBu at pos A would be:
[tex]V_{1-in}: -70dBu=20log(V_{1-in}/0.775V) \implies 0.775*10^{-70/20}=2.45*10^{-4} [/tex]
Gain through first amp: [tex] 20log(100/2.45*10^{-4}) \approx 112 dB[/tex]
Level at pos A: [tex]-70dBu+112dB=42dBu[/tex].

is this right or have I done something terribly wrong?

I'm not quite sure how to get B. How can I figure out the gain of the second apmlifier, and thus the level at B before it turns to 0 dBu?
 

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