Stethoscope amplifier

So my stethoscope is broken and unrepairable, according to the manufacturer.
I was wondering if there were a small, battery-powered microphone/amplifier/speaker I could use with the chest piece, instead of using the earpieces (which is the broken end).
The inside diam of the tube where I imagine the microphone would go is 4.2mm.
Any thoughts?
Thanks in advance


Hi Andrew
There are lots of mics and amps out there. Core stock an electret mic 4.4mm diameter. The amp would depend on whether you intend to use headphones or speaker.
Cheers Bob


Hey Andrew,

As Bob suggested a small electret mic and amp should do the trick, but what’re you planning to hook it up to? Should give us a better idea of the ratings that you’d be after for it.


I was thinking to either hook it up to a small speaker, of similar quality to those found in phones, or bluetooth it to headphones.

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I’m hoping that there won’t be feedback with the speaker because the microphone is insulated inside the tubing

see if you can find one of those old telephone cup stick on mikes dicky`s used to sell them…
or takr the scope to a machine shop and fit an electret mic insert on the sethoscope outt put tube
modd it machine it down to fit

Hi Andrew.
Don’t forget an electret mic has to be powered.
I am going to make a suggestion here to hopefully simplify things.
Use a small breakout board where all the power injection and pre amplifying is done for you. My suggestion is the Sparkfun Core SKU: BOB12758. This is a simple little board with am OP Amp and only 3 connections.
The electret mic is probably too large for you so remove it and replace with Core SKU: ADA1935. OBSERVE POLARITY.
You will then need another small power amp to drive a speaker or headphones.
Cheers Bob


Thanks, Bob.
Do you have any recommendations for the small power amp and speaker?

Hi Andrew
I don’t think I can “recommend” anything.Particularly if this pertains to the medical field. All I can and will do is try to point you in a direction and final choices have to be yours. I don’t know at this stage what you are going to use it for.

It is a bit hard anyway without having the bits and measuring. The specs for some of this type of thing are very thin on the ground. For instance the output of one of these amps might be quoted as 2.5 Watts. Is this RMS Watts or peak (sometimes called peak music power) Watts. There is quite a difference. The peak is important as it indicates the level you can go to before clipping of the peaks start. When this occurs it can sound very ordinary.

Anyway back to the present problem. Core have a number of small audio amps listed. Search “audio amp” There are 4 which may interest you. SKU numbers quoted. All Mono.

REF0064. Analog Amp based on LM386. Gain of 20 - 200 which is settable. You will have to read the instruction information to find out how this is achieved.
BOB11044. Sparkfun Class D amp.
ADA2130. Adafruit Class D amp.
CEO7850 Class D amp with a tiny speaker on board.

All Class D amps MUST have a speaker connected (4Ω to 8Ω) as the speaker inductance forms part of the audio recovery system. They will NOT work properly with a resistor as a load.

There are lots of small amps out there. The LM386 is a popular analog chip but the class D amps will be a bit more economical on battery.

Check them out. You may have to get more than one to try but they are not too costly.
Cheers Bob

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Have you considered using 3D printing (filament or resin) to re-create the broken piece?

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Thanks I’m always so impressed with the generosity of people taking the time to help out with little projects!
I will buy in a few of those low cost items and see how they fit together.
Jeff105671 - I like the way you are thinking. The bit that is broken is some spring steel, which I’ve tried unsuccessfully to resurrect with a spot weld - see pic. I’m not sure how fruitful it would be going down this path, though I’ve considered sourcing some more spring steel, bolting it on, etc…

Again, many thanks - you guys are really great!

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The reason that I would recommend this path is that I suspect you will be very disappointed with the results from a microphone and earpieces. In any case, I would advise trying out any microphone in a prototype before making changes to the existing piece, because you might end up with a device that doesn’t work and cannot be restored to original.

It appears from the image that there is enough length left in the two parts to cut them both off short and square and fit a new intermediate piece. It could be bolted or sleeved in place. A suitable source for that piece might be some old headphones. One pair I have here are mounted with a two-piece clamp that could likely be modified to clamp around the stub of the existing spring. If you hunt around for some old parts there is a good chance that you will find something that can be adapted.


Hi Andrew

A piece of steel to overlap the broken area and heat shrink maybe ???. If you use 4:1 heat shrink it will finish up with a quite thick wall thickness and will probably be strong enough.
Cheers Bob


??? spot weld it together…???