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I am trying to build a circuit that amplifies an electret microphone. I have made it based of a youtube video.

The microphone sort of works at this point, however there is huge amounts of noise around 0 to 100 Hz.

I have connected the mic to my PC and with some software analysed the frequencies and it gives the following graph: Frequency analysis when no sound is noticable

The graph is what the microphone puts out when it is in a mute environment.

I have used capacitors to remove any noise from the power supply, and I have tries using a high pass filter but that doesn't seem to really solve anything as I would need to use a very high cutoff frequency.

I should also note that when I increase the gain, the noise also increases, so I assume the noise comes before the signal is amplified.

Circuit

This is the circuit as shown in the video, I have made mine exactly alike, also using the same op-amp.

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    \$\begingroup\$ Please extract the relevant circuit diagram from the video and post it here. Also post a copy the circuit diagram as you have built it. \$\endgroup\$ – JRE Aug 25 '16 at 14:27
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    \$\begingroup\$ Be aware the Youtube videos are a poor source of information for building circuits. Try to find written tutorials that include proper circuit diagrams and written explanations of the circuit function. It isn't very effective to try and clip fuzzy pictures of diagrams and part lists from a heavily compressed video. \$\endgroup\$ – JRE Aug 25 '16 at 14:29
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    \$\begingroup\$ Batteries or are you using a power supply? \$\endgroup\$ – JRE Aug 25 '16 at 14:33
  • \$\begingroup\$ I agree and I have looked intohow all of the parts individually work, and what role all parts of the circuit have. \$\endgroup\$ – SjoerdvdBelt Aug 25 '16 at 14:33
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    \$\begingroup\$ Batteries shouldn't produce any mentionable amount of noise. That's why I asked before. Using batteries excludes certain noise sources. \$\endgroup\$ – JRE Aug 25 '16 at 18:07
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Low frequency noise (~50/60Hz) is mostly caused due to induction of electrical noise into the traces on PCB (or wires, if you're not using a PCB). This is particularly bad with breadboard setups due to the nature of their construction.

Since you have figured out that the source of noise inductance is between your microphone and the amplifier, you could try a few techniques to reduce it:

  1. Use shorter wires between mic and amp
  2. Use electromagnetically shielded cable (wrap some grounded aluminium foil around it maybe?)
  3. Use a twisted pair cable (keep them differential)
  4. Keep the ground connection of mic close to that of amp, and use equal length wires (adds to the above point)
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  • \$\begingroup\$ I have ordered and some wire that i can use tomorrow, as off now i was using jumper wires which are way to long for their purpose. I will try the aluminum foil thing to see if it has any effects. I will report to my findings soon. \$\endgroup\$ – SjoerdvdBelt Aug 25 '16 at 21:24
  • \$\begingroup\$ Using commercially-made shielded wire would probably be a lot easier, but testing with short, twisted wires should be a helpful starting point. @Pranav Your response stands as an answer, so you might want to remove the first sentence. \$\endgroup\$ – user2943160 Aug 26 '16 at 0:46
  • \$\begingroup\$ So this is really interesting, for the sake of testing i wrapped the entire system in aluminum foil and connected it to ground. The result was that the noise dropped by about a factor of 3. So I will try to get my hands on some real shielded wire and that will certainly solve it. Thanks for the answer! \$\endgroup\$ – SjoerdvdBelt Aug 26 '16 at 10:00
  • \$\begingroup\$ One more small question, wheb using twisted pair cables, how do i create the differentiaal signal? Do I use an op amp to invert the signal? \$\endgroup\$ – SjoerdvdBelt Aug 26 '16 at 11:20
  • \$\begingroup\$ @SjoerdvdBelt Glad to know that the aluminium foil trick works! Regarding differential, keep your setup single ended, but let the mic ground be connected to amplifier ground somewhere close to the point where you attach the amplifier input. Keeping the wires an equal length, and twisting them together ensures that whatever noise enters into the signal wire, affects the gnd wire equally. Due to this, noise on signal wire, relative to ground, is reduced as equal noise (in amplitude and phase) is affecting both. \$\endgroup\$ – Pranav Aug 27 '16 at 5:56

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