hello guys

i have construced a circuit that detects and amplifies heart sounds successfully!
i wanted to add speaker or headphones to listen to the heart beat, i made a circuit of lm386 with 20 Gain that i found on the lm386 datasheet!

but whenever i switch the power ON, high noise sound (whistle) comes out from the speaker and when i place the stethoscope on my chest the noise partially disappears!

anyone can help me how to remove this noise sound?

thank you

 

Well, the first thing you need to do is to post your schematic, exactly as you have constructed your circuit. If it is exactly the same as a schematic in the LM386 datasheet, then post that. If you have made any changes, you need to indicate what is different.

Without that kind of information, it's going to be difficult to help you.

 

attached are the circuit i used as heart sounds preamplifier, the output is fed into PIC! but i made another connection for the lm386 amplifier,
however, i added 330nF coupling capacitor to 100K volum control. i used the same connection of the attached for the lm386 schematic

 

Weird things start to happen when one couples into new circuits without a lot of prior thought about it!

My guess is there is some sort of interaction (feedback) between the preamp and 386. I've had these sort of things happen, and sometimes just changing the input from "+" to "-" on the 386 helps. Maybe lower the gain of the 386 and see if you can get it working, then try upping the gain again. Could be parasitics from nearby wire runs...
those with more experience will be chiming in shortly, though, I bet! )

 

anyone can help me how to remove this noise sound?

Acoustic feedback.

Use a longer cable for the speaker and place it at a distance away from the stethoscope as far as possible.

 

thank you guys for your help!
i tried all the ways to told me but i still have the problem!
yes it was somehow better when i used longer cable and place it away from the stethescope but still it gives me very high noisy (whistle) sounds!

 

Use a cable long enough to place the speaker in another room and shut the door.

If the "noise" is still there, then it is not "acoustic feedback" but feedback due to electrical coupling between varies amplifier stages.

 

Use a cable long enough to place the speaker in another room and shut the door.

If the "noise" is still there, then it is not "acoustic feedback" but feedback due to electrical coupling between varies amplifier stages.

in both cases, how to solve this problem and remove the noise!?

 

Try to use some head phones If your noise is acoustic feedback it is not much you can do about it, other that trying to avoid it. Hence using head phones

 

HF feed back is mostly due to high gain.
the opamps starts to oscillate.

One way to avoid this is use a high gain differential amp at the stethoscope.
This way common mode noise can be canceled and avoid HF oscillation too.
The high gain and the sensitive input is causing all the trouble.
U have poor ground or input shielding problems.

 

If the microphone can hear the speaker or headphones then acousical feedback howling is produced.
To eliminate the feedback then simply turn down the volume control, use a stethoscope head that seals away outside sounds, use headphones that enclose your ears with a good seal or record the sounds and play them back later.