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First time on the platform, I will try to be as clear as possible, and electronics is a hobby for me, so I'm open to any welcommed criticism.

I'm trying to build a little project, recording the sound coming from a stethoscope, and then transmit that sound to various devices (laptops, smartphones, desktop PC) via the jack input of said devices.

I bought multiple electret microphones (list below), and did some tests by simply wiring my mics to a jack cable as follow : simple connection between an electret mic and a jack (TS) input

Technically, it's working, I do get sounds, I can tap on the stethoscope and get a feedback (on Audacity or something similar) ; sadly, it's VERY low, and the sounds I'm trying to get (heartbeat, lungs sounds, ...) are very faint, and those results are across all mics I have.

I tried various schematic I found, but most either require an external PSU/Battery, or don't work (as far as I tried and didn't wrongly made those).

So my question is : How can I amplify / add gain to my mic signal without sticking a PSU on it ?

Or maybe, I'm going in the wrong direction wanting to use an electret mic ?

Thanks in advance to anybody trying to help !

Mics list :

724-3138

KECG2738PBJ-A

KECG2742WBL-25L

KEEG1538WB-100LB

KECG2742TBL-A

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    \$\begingroup\$ Try read this ti.com/lit/ug/tidu765/tidu765.pdf \$\endgroup\$
    – G36
    Commented Feb 20, 2019 at 14:03
  • \$\begingroup\$ You should not need an amplifier for your microphone. Connect to the microphone input of the device. Use the mixer and input settings to get as much gain as possible. Apply extra gain in software if needed. A typical electret microphone and a typical sound card have way more than enough dynamic range to record heart sounds. Use a shell around the microphone to reduce noises from the environment. I know this will work because I do it often in my experiments. \$\endgroup\$
    – JRE
    Commented Feb 20, 2019 at 14:39

3 Answers 3

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On this Wikipedia page it is explained how an Electret microphone works.

What you call the "microphone" is actually the microphone diaphragm itself and a JFET. The JFET is needed to amplify the small signal from the microphone diaphragm such that it can be used. Without the JFET the signal would simply be too weak and the microphone would not be useful.

To make the JFET work, it needs a little bit of DC current. This current can be provided through a resistor (usually a couple of kilo ohms, 5.6 kohm to 10 kohm), see the schematic below.

enter image description here

That V+ is the supply voltage, usually a DC voltage between 5 V and 12 V will work just fine. The JFET outputs the signal as a current which will result in a signal voltage across that same biasing resistor. It is then easy to tap-off just the signal (while blocking the DC biasing voltage) by using a capacitor.

What you can maybe do is place the resistor and the capacitor inside the device to which you're connecting the microphone. Then you can connect the electret microphone directly to the jack connector as you would like. Usually there will be DC supply voltage available which you can use for the biasing voltage V+.

If this is not a solution, for example you cannot modify the device's microphone input, then you will need to construct some box that sits in between the microphone and the device. Fact is that electret microphones simply need this DC supply voltage to work. As the current is quite low you do not immediately need a PSU, it can be operated from a 9 V battery for a very long time as well.

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  • \$\begingroup\$ Thanks for your answer ! It's very informative. I have a follow up question, you're saying that a 9V battery could work for a very long time, could you elaborate on this a little ? And just to be sure about your schematic, the - of the battery would go to GND then ? \$\endgroup\$
    – Pymous
    Commented Feb 20, 2019 at 14:02
  • \$\begingroup\$ A typical Alkaline 9 V battery has a capacity of about 500 mAh (source: techlib.com/reference/batteries.html ). With a 5.6 kohm resistor the current would be around 1 mA so that gives 500 hours of battery life. That's 20 days if you leave it on 24/7. If you just switch on/off when needed it should last much longer (but still 500 hours in total). Yes the - of the battery should be connected to Ground. \$\endgroup\$
    – Bimpelrekkie
    Commented Feb 20, 2019 at 14:18
  • \$\begingroup\$ Once again, thanks for your answer, a small battery is manageable for me, in the project ! So adding a 1mA LED would lower that to 250 hours, but it's still plenty ! Sorry to bother you with yet another question, but maybe you can point me in the good direction ; would it be possible to have, instead of a manual switch, using something like a door, to only use the battery when a jack is connected (and the circuit closed) ? That would be more efficient for the usage ! \$\endgroup\$
    – Pymous
    Commented Feb 20, 2019 at 14:32
  • \$\begingroup\$ Yes, there are 3.5 mm jack plugs which have a switch that opens or closes when you insert / remove the plug. You will have to figure out how to connect that yourself, the switch probably needs to be in series with the - connection of the battery as the switch connects to Ground as well. \$\endgroup\$
    – Bimpelrekkie
    Commented Feb 20, 2019 at 14:39
  • \$\begingroup\$ It's me again ! I don't know if I'm doing something wrong, but I tried this schematic : i.imgur.com/0qIl5Q3.png And all I get is a loud high-pitched noise, and the microphone doesn't work at all. I know USB is only 5v, but it was in order to test your schematic, am I doing something wrong ? \$\endgroup\$
    – Pymous
    Commented Feb 21, 2019 at 14:02
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So, basically, you have two problems. The first one is that you need a preamp (something like this link) to amplify the microphone signal to get it to a voltage level a soundcard can use. The preamp will deliver voltage and current to the mic capsule. The second problem is that the cable from the microphone to the preamp will add noise to the signal, long cables tend to pick up electromagnetic noise, if not balanced.

The first problem is solved by buying a preamp. The second problem, in your case, is a little more complicated to solve, because you only have the mic capsules. Here is the link explaining the problem.

The setup will work if you do not solve the second problem, but the noise picked up by the cables may make the whole setup useless.

So, connect the mic to the preamp with an unbalanced cable, 3.5mm jack, and then connect the preamp output to the PC soundcard input. This will solve your problem of having too little volume, but as for the noise, I can't say.

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The signal microphones pick up is always pretty weak. This is why every microphone needs an amplifier to increase the signal's amplitude. But to increase the amplitude requires energy. That is why you will have to use a circuit with additional power supply (e.g. a battery).

The typical inputs of soundcards in PCs/Laptops are designed to receive a maximum voltage amplitude (so the signal strength for maximum volume) of 1V. You should measure what amplitude your mic is delivering and then you can design / adapt your amplifier.

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