Electronic – How to design pre amp for a condenser studio microphone

It is possible that the circuit may simply not have enough gain for the microphone, that's all.

You have a gain of 10X in the first stage. A LM386 without any connection between pin 1 and 8 has a gain of 20X. Together, these multiply out to 200X which may seem like like a lot. But, expressed in decibels, it is only:

$$20\times\log_{10}{200} \approx 46\text{dB}$$

It could simply be that this is not quite enough gain for the given microphone.

The datasheet for your RadioShack mic gives a nominal sensitivity of -65 dB, which means that for that given reference sound pressure level, the signal would have to be amplified by 65 dB just to reach 1 Volt RMS.

If this sensitivity figure is referenced to the standard 1 Pa = 94 dBSPL, it is rather low in comparison to some other microphones. For instance, the widely used dynamic microphone Shure SM58 is quoted as -54.5 dB, which is more than 10 dB "louder".

Because mic sensitivities can be so low, microphone preamplifiers usually have gains that begin at around 60 dB and can get as high as 80 dB or more. And note that these gains are just for the microphone pre-amp to bring a signal to "line level"; they do not include any additional gain in power amplifiers for driving speakers or headphones. In your circuit, we are including the power amp's gain.

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 ;)