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I've designed a very simple microphone pre-amp to use with my headphones. The thing is I get this 100Hz hum in the output.

Is it because I should've used a voltage regulator?

Here's what the VRipple in the Op-Amp's output looks like:

enter image description here

Circuit:

enter image description here

http://no4nwo.com/images/2133891163.png

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    \$\begingroup\$ You have access to a scope, check the power supply ripple. Make sure the scope is AC coupled. Also check the inputs of the opamps to find where in the circuit the hum is introduced. \$\endgroup\$ – jippie Sep 30 '13 at 19:14
  • \$\begingroup\$ This is the amount of ripple with two pairs of headphones connected: no4nwo.com/images/1305615104.bmp When I disconnect the circuit from AC and it's running from capacitors' power there is absolutely no humming! \$\endgroup\$ – Bartek Szablowski Sep 30 '13 at 19:19
  • \$\begingroup\$ I think the problem comes from the microphone being powered directly from V+, OpAmp amplifies that little ripple from the V+ via the microphone. And the gain is 100 ! \$\endgroup\$ – Bartek Szablowski Sep 30 '13 at 19:22
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Your ground connection is adequately decoupled by C1 and C2 but there appears to be nothing on it to set the mid-rail DC voltage to be half V+. However, if the centre tap of the transformer is connected to gnd then this will be OK.

If you are not using the centre tap you'll need a pull-up and a pull-down from ground to either voltage rail to set gnd at mid-point.

The next thing to consider is that the V+ supply will have a small amount of main AC ripple voltage on it. Because you are seeing 100 Hz I can presume your supply is a 50 Hz waveform. This will still get through a little bit and you have to find a way of killing this off. The way it affects things is by superimposing itself on the microphone signal via R3.

The type of mike you have is an electret which relies on a dc bias but it can't perfectly cope and eradicate ripple so you may have to use a small voltage regulator such as an LM78L05 feeding R3.

I'd also put 100 nF capacitors across C1, C2 and the power pins to the LM324

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    \$\begingroup\$ Did you not notice the 3-terminal connection to the transformer, one of which is grounded? Presumably, that's the center-tap. \$\endgroup\$ – Dave Tweed Sep 30 '13 at 20:59
  • \$\begingroup\$ @DaveTweed No I didn't notice that Dave - I'll correct my answer. \$\endgroup\$ – Andy aka Sep 30 '13 at 21:04
  • \$\begingroup\$ Adding a 100 nF capacitor helped reduce noise a bit. Thanks ! \$\endgroup\$ – Bartek Szablowski Sep 30 '13 at 21:33
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I've fixed the issue very easily. By adding a 7808 regulator to V+. Thanks for the answers anyways !

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  • \$\begingroup\$ You may think you have fixed it but what about the midpoint gnd connection - it has no resistors to set the midpoint voltage - see my answer. \$\endgroup\$ – Andy aka Sep 30 '13 at 20:01

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