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I don't have much experience in circuit assembling or designing but I wanted to create a pre-amp for my microphone(Philips SBC MD110 Dynamic Microphone which I directly connect to my PC) to boost the signal levels. So I created one by refering this 2 mins video https://www.youtube.com/watch?v=1wbgbbbILc0

So this is the circuit diagram(screenshot from video): enter image description here

This is the circuit I actually made from the above diagram: enter image description here

This is the underside of the board: enter image description here

That's it. But its not working. When I connect the output of amp to pc's mic socket, all I can hear is a noise and nothing else.

The Vcc (~5V) is reaching the Vcc-pin of opamp IC as my multimeter shows that. Also I've used the 1st opamp of the IC (+ve input is 3rd pin, -ve input is second pin and Output is from 1st pin).

What I am doing wrong here?

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  • \$\begingroup\$ What kind of microphone are you connecting there? Does it produce any output on its own (e.g. dynamic or electret with battery) or does it need phantom supply voltage? Try scoping the input points. Also we have no idea how your board looks from the underside to find any possible assembly errors. \$\endgroup\$ – PlasmaHH May 13 '15 at 10:04
  • \$\begingroup\$ I am using a dynamic microphone. I will post the picture of the underside of my board in a minute. \$\endgroup\$ – DroidHeaven May 13 '15 at 10:10
  • \$\begingroup\$ Can you get it to buzz at all (touch the points with your finger and hear the AC mains pickup)? You should be able to trace back to where the signal path is being broken. If you have a scope that would be even better. \$\endgroup\$ – Jon May 13 '15 at 10:15
  • \$\begingroup\$ @Jon sorry I don't understand the first part of your question. I don't have any measuring instruments other than a mutlimeter. \$\endgroup\$ – DroidHeaven May 13 '15 at 10:32
  • \$\begingroup\$ @PlasmaHH I've updated the question with the picture of the underside of board. Please check it. \$\endgroup\$ – DroidHeaven May 13 '15 at 10:32
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Possible assembly errors aside, at timecode 0:36 the guy tries to calculate the circuits gain, and comes up with 500 (which would be ~54dBV). Given that a dynamic mics voltage range is round about 40mV (Possible peaks to 80mV or more) this should ring some alarm bells:

\$500 \cdot \pm 40mV = \pm20V\$.

Not possible with the present supply voltages from 0 to 5V.

So what we need here is a much lower gain. Assuming some useful gain within the soundcard itself and to avoid clipping of the opamp near the rails, amplifying +-40mV to 1V seems like a much better idea, which leads to a gain of 25 amounting to ~2k resistor instead of 39k.

For extra fun you can replace it by a 10k pot and play with it.

In the end where that guy shows the final assembly, I can also nowhere find any negative feedback, so either its on the back, or just a wire (making this a voltage follower then).

What to learn from it? Don't just blindly build some youtube videos circuit without understanding it, especially from a video with 0 comments and not even 200 views... At least try to understand it by doing some simulation, e.g. in ltspice. It takes 10 minutes and gives you valuable information, in this case it could also give you the information that the frequency response of this circuit is nowhere near hifi ;)

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  • \$\begingroup\$ another day, another lesson learned :D \$\endgroup\$ – DroidHeaven May 13 '15 at 11:18
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The diagram you have is for a dynamic microphone (see note on the diagram: "Moving coil mic preamp.") If you connected a typical electret microphone to it (the sort you would normally use with a PC) then it will NOT work, it CANNOT work.

Out side of that, there could be many possible errors made in building the circuit. Check your wiring, check for bad solder joints. You may want to draw a new circuit diagram from your circuit and check that all of the connections are corret - that is, reverse engineer your circuit as built and see if the diagram you draw from the hardware is the same as the one you intended to build.


Update after comments: The info on electret microphonees turns out to not be relevant to your problem, but I'm leaving it here in case some one else has a similar problem but is using an electret mic.

This site has a diagram that will work with an electret microphone. Here is the diagram: enter image description here

Note R4 connecting the Microphone to Vcc - that is a MUST for an electret microphone.

You should be able to build this circuit and connect it to the USB-power as you did in the original. You'll need the 330Ohm resistor and the 220µF capacitor, and use that as VCC in the diagram I provided.

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  • \$\begingroup\$ My microphone model is Philips SBC MD110 - this one amazon.co.uk/Philips-SBC-MD110-Microphone/dp/B000A63IWG. It is a dynamic microphone. \$\endgroup\$ – DroidHeaven May 13 '15 at 10:40
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    \$\begingroup\$ OK, that is good to know and should have been in the question. The suggestion to trace and redraw the circuit still stands. \$\endgroup\$ – JRE May 13 '15 at 10:51
  • \$\begingroup\$ My mistake. Sorry. I will try the rest of what you suggested. \$\endgroup\$ – DroidHeaven May 13 '15 at 10:55

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