Andrew Harmon

Preamp Build Project

For my senior capstone project in Capital University's Music Technology program, I decided to learn about how microphone preamplifiers work, and to build one of my own. I chose this project because I've always been curious to learn how things work, and I thought that it may be helpful to understand the inner workings of some basic audio gear, given that things like mic pre's are the basic hardware elements that make the field of Music Technology possible.

The design that I chose to research and build was an op-amp centered design. I purchased and experimented with many different op-amps before I really understood what they do and how they do it (and what I was doing).

Part of the reason that I had trouble learning about op-amps and their functionality was that I did not understand the power requirements for them. I did not have a power supply that would supply +12-15v and -12-15v with a center tap, and all of the op-amps that I had required something meeting this spec. Since I had never worked with audio circuitry before, it took me longer than I'd like to think it should have to realize what I was missing. After realizing what I needed, I found a schematic online for a basic audio power supply that would provide +15v, 0v ground reference, and -15v, ordered the parts, and built one. I had to eventually rebild this as I ended up blowing up some of the components, but the power supply was the first step in the right direction.

The next step to figure out was the audio balancing circuitry. Because I wanted to be able to use my preamp with other audio gear, I needed to have a balanced input and output for optimal usage. For this, I found another schematic online that used the INA217 IC to convert a balanced mic signal to unbalanced. Paired with the DRV134 to exactly the opposite, I had figured out the last of how I wanted to build my project. The final project ended up being a combination of the three schematics I had found (and modified) to give me a 2 channel balanced microphone preamp.