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Trying to Understand Full-Duplex Intercom Circuit

Stevens R. Miller

May 19, 2016
13
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May 19, 2016
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With the pot set to 80% attenuation, the system gain is 2000. With 5 mV input, that's 10 V out with a 9 V battery.
Could you walk me through that? I'm thinking that the inverting amp has a gain of 60 dB, as the ratio of the two resistors (R7 and R6) is 1000. Attenuate 80% and you're at 200. The LM386 is in its default configuration, for a factor of 20. That would be 4000, wouldn't it? (And still too high.)

Now, if the mic contributes another 2000 ohms, then I think that takes us down from a factor of 1000 on the op-amp to 500 (from 60 dB to about 54 dB), and that gets us a system gain of 2,000 when attenuated 80%, doesn't it?
 
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