When my Keurig coffee maker stopped working in late 2018, I decided to take it apart like I do with most things that I own. Once I was inside of the machine, I got to thinking about how to connect it
The preamp is now in a case. The last part that I would like to get worked out is which potentiometer works best for this use. While working on this, I managed to blow up the transformer that provides power to the circuit. I have a new one on order, but the preamp will not operate without one. Below is a picture of the preamp (mostly) in the case.
After realizing that I had blown the transformer, I dug one out that I had laying around that I thought would work. Unfortunately, this transformer (pictured below) puts out too high a voltage for my use.
Today I got the preamp project to work. I used the input and output stages located at diyrecordingequipment.com with the THAT1200 for input and the DRV134 for the balanced output. The power supply was adapted from eleccircuit.com. Initially I was only able to get fuzz noise, but that was caused by a grounding issue that I was able to track down. The noise floor is still relatively high for a preamp, but I will be troubleshooting that as/after I design a case for my preamp. Below is a picture of the bottom of my design as I was testing.
This week was mostly focused on building the power supply stage. I got my parts in and put together a schematic that I thought would put out +15v, 0v, and -15v needed for my preamp design. When I finished the design and took my multi-meter to the circuit, I was only pulling +11.5v, 0v, and -11.5v, which is not sufficient to power my design. I have ordered and should soon receive a beefier transformer that will give me the power that I need. Once this is in and I can verify that I have sufficient power, I will connect it with the rest of my circuit, which I am building in the meantime.
This week has been mostly research and testing designs/circuits. I did a lot of research into how opamps are used in circuits and studied datasheets for a few different opamps, including the INA217, which I think will be the one I end up using. The biggest breakthrough I had in researching this week is beginning to understand the power staging necessary for this type of circuit. Most opamps require a positive voltage as well as a negative voltage and ground to operate. My understanding at the beginning of this project was that the negative voltage and ground were the same thing, but my research this week is showing otherwise. I found a few designs that give both positive and negative voltages, as well as a 0v ground that I will be building this week. I would like to be able to incorporate 48v phantom power into my design if possible, so I have been looking for ways to have both 48vdc and 15vdc (required input for most opamps). This part I have not figured out yet.
This week I will plan to build in stages. In a conversation with my boss at work, we determined that the four main stages that I will need built are the power stage, a balanced input stage, the amplification stage, and a balanced output stage. Building these separately and combining should work. I am placing an order for components this afternoon so that I can hopefully have them in hand late this week or early next.
The other main thing that I did this week is building and testing some circuits that I found online to see how opamps function in a circuit. While I was not able to find any that worked for my application as I thought they would, I did learn quite a bit about the functionality of opamps, including how gain control works. Below is a picture of a few of the circuits that I tried (there were more. Some were not nearby for the picture and more still were taken apart for the components).
This Sunday I started tracking for this project. I am recording the band The Plebeians. We tracked most of the first song in studio F this Sunday. Attached are some photos from the session.
This week I researched more about op amps and how they work in a circuit. To get some hands-on learning, I started putting together a few test setups. I built the ‘famous $5 preamp’ but didn’t have much luck probing things out.
I also bought a pre-built kit by Velleman (similar to this one) that I got put together and probed through a bit this week with my multimeter. In the coming week, I plan to finish probing through the different op amps that I have on hand to figure out exactly which I need to use for this application.
If I get that done early enough in the week, I will start to add other components to my design to help with signal regulation and filtering.
This past week I have been researching designs and components used in preamp design and building. I’ve found that the most essential component in a preamp is the Operational Amplifier. This component does the step up in level from mic level to line level. Other components in preamp designs serve functions such as filtering, padding, and peak protection. Other features often included are +48v and optional pad. These require additions to the circuit that can be simple depending on how involved their integration is. For mine, I would like to have both options in a simple form.
The first thing I would like to build is the “famous $5 preamp” or something similar. This is a cheap way to have a functional design that I can experiment with. Attached is a picture of the schematic. I have the parts on order or in hand.
The goal of this project is to learn more about the circuity involved in audio gear, specifically in microphone preamplifiers. I will be researching preamp designs and building my own. My end goal is to have a functional preamp that I can use in real-life scenarios.
In order to accomplish this, I will likely build multiple versions before I find one that works reliably. I will consult with coworkers and family who know more about circuity and electrical theory than I do to learn how to best go about designing my project. I will order individual components from sites online, as well as buying what I can at local shops. This particular project will require the ability to read schematics as well as the ability to solder. The idea stuck out to me because I have always been naturally curious about how things work, and this will afford me the opportunity to dive into the inner workings of gear I often take for granted.
