The input stage is the same circuit as the mixer amplifier we've used in several past Labs. Its three inputs will be the carbon microphone, the dynamic microphone, and the function generator.
If your mixer amp is in good shape, you can use it as the starting point for your system, but be sure it meets the construction guidelines set out above.
Verify that the output signal is 1 V peak-to-peak when speaking into either microphone at a normal level.
This circuit performs two functions: It converts the input signal voltage and it establishes a constant bias current of 10 mA in the diode so that it does not cut off during positive excursions of the input. This eliminates the distortion caused by trying to drive the LED directly from a voltage source as we have done in previous labs.
Since the current in the LED also flows in the 10 
resistor, and since the negative input to the opamp is at
(virtual) ground, the voltage 
is proportional to the
current in the LED.
Connect the output of the input stage (
) to the input of the LED
driver. Set the function generator to produce a sine wave
of about 1 kHz and adjust the amplitude until
begins
to distort. This should correspond to a value of
of about
2 V p-p.
Set the function generator frequency to about 1 Hz and
verify that the LED blinks.