Need help analyzing circuit!!! HELP

Thread Starter


Joined Nov 27, 2012
Hey all!

I am a newbie and was wondering if anyone could help me solve this circuit. I need to know what kind of filter it is and the gain on the amplifier. Relatively simple questions but im having a hard time of where to start. To make it easier all I need to know is the characteristics with a 10K resister instead of the potentiometer.

It is a circuit to a electric stethoscope.

Any help is appreciated!



Joined Mar 31, 2012
Try redrawing the schematic in a more logical way, using amplifier symbols and not IC pinout symbols, and then identify what you think are the different building blocks of the circuit.


Joined Dec 20, 2007
The piezo transducer might pickup only high pitched squeaks.
The 0.047uF capacitor has a value so small that it passes frequencies above 262Hz. Then heartbeats will barely be heard.

The 220uf capacitor cuts low frequencies depending on the impedance of the headphones.
There are no other filters.

The Jfet has a gain (it actually has a small loss) of 0.8 times and the datasheet for the LM386 shows a gain of about 47. Then the total gain with the 10k volume control set to max is only about 37.6.

The 10k volume control can be removed then the 0.047uF capacitor can be connected directly to pin 2 because inside the IC pin 2 already has a 50k resistor to ground. Then the capacitor passes frequencies above 64Hz.

My heart stethoscope circuit uses an electret mic and two 9V batteries (one for +9V and the other for an extra -9V to the preamp) then the preamp directly passes frequencies down to DC (no coupling capacitor) to the power amp.


Joined Dec 20, 2007
My electronic Stethoscope is not my design, instead it is a design from somebody else that didn't work and I fixed it.
Its electret mic feeds a preamp with a gain of about 14 and a very low output impedance. It drives a second-order Butterworth lowpass filter that cuts high frequencies above 103Hz but its resistors can be increased to hear higher frequency resperation or car engine sounds. It resonds to very low heartbeat or rumbling sounds. The lowpass filter feeds an LM386 little power amp that drives over-the-ear headphones.
It has an opamp that lights an LED when there is a sound.