Finding gain in series of amps and attenuators

[eng_stud]

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?

 

[KataKoniK]

Hi there! Your solution for A looks correct. As for B, you can calculate the gain of the second amplifier by using the same formula for voltage gain in dB. You would just need to know the input and output voltages for the second amplifier to plug into the formula. Once you have the gain for the second amplifier, you can use it to calculate the level at B using the formula for dBu. Hope that helps!