U1a operates as a
low-noise microphone preamp. Its gain is only about 3.9 because the high
output impedance of the drain of the FET inside the electret microphone
causes U1as effective input resistor to be about 12.2K. C2 has a fairly high
value in order to pass very low frequency (about 20 to 30Hz) heartbeat
U1b operates as a low-noise Sallen and Key, Butterworth low-pass-filter with
a cutoff frequency of about 103Hz. R7 and R8 provide a gain of about 1.6 and
allow the use of equal values for C3 and C4 but still producing a sharp
Butterworth response. The rolloff rate is 12dB/octave. C3 and C4 can be
reduced to 4.7nF to increase the cutoff frequency to 1KHz to hear
respiratory or mechanical (automobile engine) sounds.
The U4 circuit is optional and has a gain of 71 to drive the bi-colour LED.
U5 is a 1/4W power amplifier IC with built-in biasing and inputs that are
referred to ground. It has a gain of 20. It can drive any type of headphones
including low impedance (8 ohms) ones.