Updade: I currently have a few of these for sale on Reverb:
One great reason to build your own transformer DI box is to help interface your gear into your vintage or modern recording equipment. Recording the bass guitar direct is especially appealing for a multitude of reasons, mostly to do with how loud the instrument is with an amplifier.
Doing a quick search for impedance matching will bring up a myriad of definitions, terms, and equations to help our understanding of Ohms law. It's a lot of good reading. But you need not do all that reading to make your own great solution.
However, if you have some spare gear(and time!) you can build a DI that will quickly interface most Hi-Z instruments to a Lo-Z input, most likely a mic preamp. Another reason to build your own DI box, is that a transformer will give your instrument a great "Sound" that's very hard to replicate without one!
Now, you may be thinking that this problem was solved years ago with the invention of the "Instrument" button on your mixer or computer interface. Well, that's partially true. That's one way to get your instrument into the box(or your mixing console, for that matter). But, another great sounding way is to do what the audio engineers of the 1960s and 70's did which is use an impedance matching transformer. Simple. Effective.
For this project you'll need:
A suitable enclosure, preferably aluminum
An input transformer (primary side approx 600 ohms, Secondary approx 20k ohms)
A mono input jack
A male XLR jack (and wire if you are making a cable end as I did)
Optional Noval Tube socket (if you choose to make the transformer removeable, I assume you won't.)
Soldering Iron and Solder
You could make the box more or less complex or useful by adding an attenuator of sorts. However, it does add to the complexity of the circuit including a switch, potentiometer and resistors. It also requires more drilling, which is a pain.
The video above shows the input and output wiring in detail.
Basically, you're going to figure out the primary or secondary of each winding of your transformer. No multimeter readings are necessary, you can just check the taps (or pinout) of your favorite transformer and you'll have all the info you need.My transformer is wound for 200:15K. Or, another way to use the transformer would be 15K:200
For this project, our "Primary" side is going to be the Hi-Z taps. So, if you're using a mic input transformer as I did that is technically the "Grid" winding. This is the side that will be hooked up to the 1/4" input jack. Generally this is two wires- Hot and ground.
Now, we need to hook up the "Secondary," Lo-Z side of our transformer. Again, if using a mic transformer this would be the "Mic" or "Line" winding. This will consist of connecting the "hot, cold, and ground" of the winding. This will be clearly marked on your transformer taps or transformer literature.
Once these connections are made, and your connectors are fitted to your chassis, you're done! Give it a test and record your dream motown tones!
Best of luck!
