Spy microphone amplifier circuit

[re-Qest]

Hello. I need a high quality microphone amplifier circuit. Conditions from -20 to +60 Celsius, humidity up to 100%. Distance to the object from 3 (60%) to 5 meters. The distance from the amplifier to the recorder is 3 meters. Frequencies up to 1300 Hz (it is very important that there are no frequencies higher). The cost of all components except PCB is up to $ 10. The microphone must be built into the amplifier.

 

[Tony Stewart]

You don't have enough specs for your contradiction of a high quality mic amp limited to 1300 Hz

 

[re-Qest]

Good. I just need an amplifier circuit with frequencies up to 1300 Hz

 

[transconductance]

What is the input to the recorder? Is it high impedance like a 3.5mm phone jack or low impedance like an XLR connector? Balanced or unbalanced? What is the required level, i.e. "line level" which is usually hundreds of millivolts or "microphone level" which can be a few millivolts and sometimes less.

A lot of small condenser mics have a built in FET amplifier (really an impedance converter) and give you a few millivolts (from 90 dBA) without additional amplification.

If you want to ignore anything above 1300 Hz, you're going to have to explicitly filter the signal, probably with an op-amp active filter. The good news is that you can design any required overall gain into this same circuit. Something like an LM358 or LM324 plus some resistors and capacitors. You'll need more information than you have shown here to design a filter. Simple electronic filters don't have a brick-wall cut off frequency. How much attenuation do you need at and above 1300 Hz? How much attenuation can you tolerate below 1300 Hz? It sounds like there must be some very specific sound you need to capture. What is the actual frequency band of interest? What is it that's above 1300 Hz that you need to avoid?

Forget about the distances. What you need to know is the sound sound level (usually in dBA or dBC) or the sound pressure level (usually in Pascal) at the microphone.

Three meters from the amplifier to the recorder is no big deal. High-quality, low-impedance mic cables can be 100 meters long.

 

[Tony Stewart]

All microphones of any quality will produce frequencies above 1300 Hz from a pulse. Why you did not specify the attenuation that a custom LPF would be required to add to cutout these aliasing effects above Nyquist rate if digitizing, is beyond me.

Normally you would say -3dB BW from A to B and >= -40 dB at C with -6dB/octave per order of LP filter.


Design is easy, cost of design not so cheap for 1 of. Why 1300 Hz must be explained. Is the sound omnidirectional or directional. Any electret with 60 dB gain and AGC with active filter can be done for $2 without a PCB. But you didn't specify interace.

 

[gary350]

Build this you can hear people talking 600 ft away like they are standing right next to you. Use the 10mm or 3/8" mic not the smaller 1/4" mic. Be sure to use 12v nothing less.

 

[Tony Stewart]

1300 Hz LPF ? where is the RC cutoff? 2k ?

 

[audioguru]

The circuit has nothing to limit the maximum power that will destroy your hearing and earphones if a loud sound occurs nearby.