I've designed an amplifier to receive ELF-VLF (3hz to 30Khz) signals through a loop antenna and want to connect it to Sound Card mic input to analyze received signal.

The problem is that the maximum output of the circuit sould be limited to protect sound card from damages and avoid cutoffs in waveform because of saturation to prevent data loss.

I'm going to use Mic input over laptop and it should not be confused with Line In.

What is the maximum and nominal voltage values for microphone input? How much is the input impedance of Mic jack?

**Edit: ** It's going to be introduced to a group of enthusiast people with limited knowledge about electronics and it's definitely NOT a precise solution but a cheap and easy-to-use one. The aim is to keep it simple and safe. So please do not think of limited frequency response of sound cards or the exact maximum voltage suitable for every specific card. Just a safe voltage range should be enough.


closed as off-topic by Leon Heller, Fizz, Brian Drummond, Olin Lathrop, PeterJ Nov 7 '15 at 12:54

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    \$\begingroup\$ Answers should be in the sound card spec. Question should be closed. \$\endgroup\$ – Leon Heller Nov 7 '15 at 10:23
  • \$\begingroup\$ It's not going to be used on a specific sound card. Nominal values can be a good assumption for the design. \$\endgroup\$ – zxcmehran Nov 7 '15 at 10:27
  • \$\begingroup\$ And, I'm not looking for the maximum possible value. At least a safe zone would be enough. \$\endgroup\$ – zxcmehran Nov 7 '15 at 10:28
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    \$\begingroup\$ 30 kHz = VLF? If it's not going to be used on a specific sound card then looking up the data on any soundcard will give you the answer you need. \$\endgroup\$ – Andy aka Nov 7 '15 at 10:28
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    \$\begingroup\$ -1 for the stupid and lazy question. You are asking us to speculate, when the only real answer is in the datasheet of the sound card you have. RTFM. \$\endgroup\$ – Olin Lathrop Nov 7 '15 at 12:43

The mic-in of consumer sound cards is designed to power (DC bias) and amplify the signal of an electret condenser mic. Alas there's no real standard how to do this.

Ignoring the [usually 1.5-2.5V] bias voltage which you can just block with a capacitor (for your application), the input impedance also varies a fair bit between products, but you can expect it around a couple of kiloohms or even lower, but probably no less than 600ohms. The default amplification will be somewhere around 100x (40dB), but some cards offer a boost function of around 20dB (another 10x) or even more, although I think the latter is typically done in software.


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