A little while back(2 years) the guys at record-producer decided to do another equipment test; this time with mic pre-amps. They chose three different ones ranging from $5 to over $1500. They recorded samples and allowed people to have a listen. Here is the link:
http://www.record-producer.com/learn.cfm?a=3062[Edit: this link is no longer available]

I was able to find another website that hosts the sounds tests:

https://www.audiomasterclass.com/blog/the-famous-5...

Today I will be writing an Instructable on how to build the $5 pre-amp.

Mine cost a bit more then $5 but not that much since the majority of it was samples :D

Step 1: Prepare the Encloure

This time, I was able to borrow a drill for long enough so I could take picture of the project. Since the XLR jacks were not perfectly circular, i found i used my dremel a carve out most of it after a hole had been made.

Since the enclosure is made of plastic, I was able to use my wood drill bits. These bits are sharper and cut faster through the plastic.

Step 2: Planning

Planning is an important part of any project one builds. As for what I did was:
1. I found all the technical data for the semi-conductors and figured out what I could use for this design. I found three chips from Texas instruments that would fit the application nicely. These are the OPA137, INA217 and the TLE 2426.
2. Using the data sheet for each figure out how they will work together and make a schematic for the project. In my case the schematic was already made from TI, so all I needed to do was fit the third chip in.
3. Take the schematic and convert it to something that allows you to build.
4. Obtain the materials that you will need for this project.
5. Start building, double check everything as you build the project. This allows you to fix errors as they happen rather then unsolding connections after.

So there are still a number of thing that iâm not sure what I should so with.
The first is that I used two electrolytic caps for the input, what other types of polar caps can be used?
Secondly, for off board wiring, I connected pin 2 and 3 to positive and negative inputs, however for the output, I only have one out. Does this mean I no longer have a balanced signal and pin 3 and pin 1 are both connected to the ground?

Step 3: Off Board Wiring

For this step what has happened is the three chips are mounted to the enclosure and wires are connected inside. I would draw you a plan but everyone is going to be different.

If you are not using shielded wire, it is good to twist wires together; this protects from electromagnetic interference by cancelling out.

I also stacked the two boards on top of each other for a neater appearance and so that one is not randomly floating. It is the second picture.

In the end I didn't use the 4 diodes for clipping protection as it was just to much of a hassle to connect it all up.

Step 4: Power Supply

The original Mic pre-amp used all batteries for both the chip power and for the phantom power. I figured that if I could get the power clean and stable enough from a wall jack that is what I will do.
The first thing is to get clean AC power from the wall. What I use is the Monster HT800, which is just a line filter and surge protection. From there I use an old laptop power supply to give me a DC current. The last step since there is still some rippling of the DC current I built a voltage regulator from the LM317. This is an adjustable voltage regulator which allows me to get the exact voltage I need.
There are no pictures for this step as I will be writing another instructable on how to build the voltage regulator.

UPDATE

I decided to change the power supply a little bit but for the most part, it's the same. I am just using a laptop charger which just connects to the TI's rail spliter(TLE2426). i'm thinking in the future of actually using the LM317 but for now i just have the rail spilter. the schematic for the TLE2426 is found on http://tangentsoft.net/elec/vgrounds.html
it's about half way down the page.