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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.

SjoerdvdBelt
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    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. – JRE Aug 25 '16 at 14:27
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    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. – JRE Aug 25 '16 at 14:29
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    Batteries or are you using a power supply? – JRE Aug 25 '16 at 14:33
  • I agree and I have looked intohow all of the parts individually work, and what role all parts of the circuit have. – SjoerdvdBelt Aug 25 '16 at 14:33
  • I am using two 9V batteries – SjoerdvdBelt Aug 25 '16 at 14:34
  • Do you really have an electret microphone? It isn't a dynamic mic instead? – JRE Aug 25 '16 at 14:39
  • I am very certain, it looks like an electret and it also says it on the bag in which it was delivered. – SjoerdvdBelt Aug 25 '16 at 14:40
  • Just checking. Sometimes people use whatever is at hand, and it turns out to be something other than what is needed. – JRE Aug 25 '16 at 14:41
  • GND is the mid point of the two batteries, right? They are in series, and you have a plus and a minus tied together? – JRE Aug 25 '16 at 14:42
  • Yes, I have connected the negative side of one battery with the positive side of the other. Then I have connected the other two wires, the ones that form the 9V and -9V, to the junction of the wires through capacitors. The junction of these two caps and two wires is what I use as GND. – SjoerdvdBelt Aug 25 '16 at 14:45
  • Try disconnecting the microphone and see what you get. – Andy aka Aug 25 '16 at 14:56
  • How much noise are you seeing? I don't know the LM324 but perhaps it's 1/f noise. – George Herold Aug 25 '16 at 15:04
  • It looks like an oscillation is being sustained in your mic-cap-100k chain. Please change that 100k attached to the cap to a lower value and tell us what happen. I'd try 10k as a start. – PDuarte Aug 25 '16 at 15:28
  • I changed to a 10K resistor and the noise dropped but so did the actial sound coming from the mic. The amount they dropped seemed to be the same. – SjoerdvdBelt Aug 25 '16 at 17:35
  • Interestingly enough, when i unplugged the mic, the noise did not go away, rather it only dropped quite a lot, but the same noise pattern was still visible. So I think that it is not (only) the mic causing noise issues. – SjoerdvdBelt Aug 25 '16 at 17:37
  • Could it be that there is still small amounts of noise on either my GND or 9V line (even though it has caps on both lines) which leads to the op amp through the coupling cap which is then amplified by the amp? – SjoerdvdBelt Aug 25 '16 at 17:40
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    Batteries shouldn't produce any mentionable amount of noise. That's why I asked before. Using batteries excludes certain noise sources. – JRE Aug 25 '16 at 18:07
  • What does the noise sound like? There's possibility that you are picking up hum from the power lines in your house. – JRE Aug 25 '16 at 18:08
  • It sounds like a low, electric hum, it is very stable and doesnt change over time. What so you mean with hum from my power lines? – SjoerdvdBelt Aug 25 '16 at 18:14
  • Tomorrow I will have access to an oscilloscope and I will try to measure exactly at which point in the circuit the noise starts. – SjoerdvdBelt Aug 25 '16 at 19:36

1 Answers1

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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)
Pranav
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  • 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. – SjoerdvdBelt Aug 25 '16 at 21:24
  • 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. – user2943160 Aug 26 '16 at 00:46
  • 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! – SjoerdvdBelt Aug 26 '16 at 10:00
  • 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? – SjoerdvdBelt Aug 26 '16 at 11:20
  • @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. – Pranav Aug 27 '16 at 05:56