I have made a circuit like described at this site

enter image description here but the sound I receive through the mic sensor is not very high. Can I add some Opamp circuits to increase the analog data input in android phone?? ( Mine is a Samsung galaxy note 1 )

What is the input voltage limit and power for the smart phone 3.5mm plug?? does it depend upon different models of the phone i.e do they have different max input voltages ?

  • \$\begingroup\$ I searched for pre-amp circuits and found this Instructable: instructables.com/id/… This is intended for oscilloscope apps, but I'm wondering if the "minimal circuit" in Step 3 can get the job done. \$\endgroup\$ – Shabab Oct 16 '13 at 15:02

You should understand how the microphone input of the phone works, first. Here's a good article:


Note that the microphone port provides a tiny bit of power to the microphone. Honestly this isn't a lot of current, and it's meant simply to operate the JFET inside the microphone capsule.

If you can, you should install a battery in your project and use a typical amplifier setup, which then leads to the microphone port of the phone.

Beyond adding a battery, what I'm suggesting is that you may be able to subvert this current and use it to power an amplifier circuit. If you measure the current available under different loads (just measure the voltage and current across a variety of resistors, graph it, and it'll tell you how much power is actually available for your use.

Once you know the voltage and current you'll be running at, you can attach a regulator to the microphone port, filter the output of that, and power a simple opamp electret microphone amplifier with it. The output of the opamp then goes back through a capacitor to the microphone port (which is the opamp's power supply source).

You're going to have a lot of side effects doing it this way, but it may give you enough headroom in your signal to succeed in your project.

I still think a stethoscope style pickup is a better option, though.

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  • \$\begingroup\$ openmusiclabs.com/projects/bootlegmic For this page, seems a cool device, but i havent try to build it \$\endgroup\$ – Akien Cheung Oct 18 '13 at 6:45
  • \$\begingroup\$ @AkienCheung Neat idea. It decreases the signal output, opposite of what you want, but is a nice way of performing such a modification. \$\endgroup\$ – Adam Davis Oct 18 '13 at 15:06
  • \$\begingroup\$ youtube.com/watch?v=TQB1VlLBgJE I find this youtube video talk more amplifier, but i wonder this output voltage or current will or will not broken my phone, as too high voltage or current \$\endgroup\$ – Akien Cheung Oct 18 '13 at 18:38
  • \$\begingroup\$ @AkienCheung I think you might need to find a consultant. You have a good chance of damaging your phone or tablet if you aren't careful with your design, and the questions you're asking suggest you need a complete design, rather than simple guidance. \$\endgroup\$ – Adam Davis Oct 19 '13 at 16:30

It may be that you can add an op-amp, but it's a bit of trouble. The easier solution is to use a more sensitive electret microphone. They are readily available with -87dB to -32dB.

You might try using one from a cellphone headset first to compare with the electret you're already using.

The article "Understanding Microphone Sensitivity" from Analog Devices discusses microphone sensitivity and the tradeoffs involved when choosing a microphone for an application such as this.

The electret microphone already contains an amplifier, so switching out the microphone element should be sufficient, but be sure to inspect the rest of your wiring and assembly to make certain the problem of low gain isn't due to wiring/soldering/connection losses along the way.

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  • \$\begingroup\$ As i am using the headphone to record human heart beat, it just put near our heart area, so i would like to enlarge the analog signal recorded by headphone \$\endgroup\$ – Akien Cheung Oct 17 '13 at 7:10
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    \$\begingroup\$ You should have more luck, then, if you use a stethoscope head or create your own. In effect you're enlarging the microphone diaphragm. Take a very short cylinder, make sure the back is sealed and stiff, and the front has a flexible membrane. Cut a hole in the side and install the microphone with an airtight seal. The cylinder can be a few mm thick, so it doesn't have to be noticeable, but it should be 3-5cm in diameter. Trying to capture heart sounds with a tiny microphone doesn't sound like an easy task, but if you can't use a larger diaphragm, then you can build an amp. \$\endgroup\$ – Adam Davis Oct 17 '13 at 11:31
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    \$\begingroup\$ I'll see about posting some info later if no one else does. Generally there's a DC power signal on the microphone wire which you can separate using an inductor, then filter and regulate before you power your opamp. The input of the opamp goes to the microphone (along with a pull up resister to give it power and capacitor to block the power to the opamp) and the output of the opamp goes back to the microphone input through another capacitor, again to block the power DC signal. You'll need to use ultra low power design and components, and it'll need to run at voltages below 3V. \$\endgroup\$ – Adam Davis Oct 17 '13 at 11:35
  • \$\begingroup\$ Adam Davis.. U mean below 3 V can inject to the phone by 3.5mm plug?? \$\endgroup\$ – Akien Cheung Oct 17 '13 at 17:09

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