Ultra Sensative Amp

Hi Addohm,
Your mic preamp and LM386 power amplifier won't work.
The preamp has the input to the TLC072 biased at only 0.089V but the datasheet shows that it must be at least 0.5V. Make it 4.5V.
The mic (dynamic) shorts the input to 0V anyway because there is no input coupling capacitor.

My Electronic Stethoscope was fixed by me, not designed by me so I didn't change its input to be a high input impedance non-inverting type.
Your preamp is a inverting type with a very low input impedance.

The datasheet for the TLC072 explains that it has Cmos inputs that have a fairly high capacitance that causes the opamp to oscillate when its feedback resistor has a high value. It also oscillates when the load capacitance is more than about 100pF. The opamp should have a coupling capacitor feeding the volume control to block DC.

Your LM386 power amplifier is missing the RC network at its output as shown on the datasheet. It will oscillate without them.

Both ICs need a supply bypass capacitor.

Please post your schematic here instead of at ImageShack.

audioguru said:

I used a thick plastic lid from a jar of peanut butter. I drilled a hole in its center, mounted a rubber grommet in the hole then mounted an electret mic in the grommet. The mic didn't touch the skin and the flange of the lid sealed out external noise and prevented acoustical feedback to headphones. A speaker might cause acoustical feedback.
I used a lowpass filter to attenuate background noises but pass heartbeats. I will post a pcb layout if you ask.

Click to expand...

Hi Audioguru, Not that I need it, But it Looks like a Good design for this purpose.
Gary

Space Varmint said:

I would suggest putting a Darlington amp configuration on the mic input.

Click to expand...

No.
A dynamic mic is 600 ohms and an electret mic is about 3.3k ohms.
Opamps already have a darlington input and most low noise opamps have FET inputs so extra transistors used as a darlington are not needed.

A single 2N3904 transistor used as an emitter-follower with a 4.7k emitter resistor has an input impedance of about 611k which is high enough for almost anything. Adding a second transitor to make them a darlington increases the input impedance to about 79.4M which is rediculous.