DIY DI Box. Active Direct Injection at its best.

There are so many projects out there to build, but no DIY equipment collection is complete without a decent DI box. Direct Injection is an important element to any live sound or recording rig. Whether you are recording guitar in your bedroom or in a super-studio, you will more than likely require the use of a Di Box of some description.

The DI Box is an important component for conditioning your guitars signal impedance for your mic preamp. DI Boxes provide a way of isolating the electronics from the guitar and transforms high impedance guitar signals to a much lower impedance which will provide lower noise levels and better frequency response. The DI box I am going to provide info on here is a well known circuit among the DIY audio folk and after building three of them myself, I can highly recommend this project. It is known as "BO's Di", or the 1975 active Di Box by Bo Hansen. Bo decided years ago (Almost 30 years in fact) that the DI Boxes of time were up to shit. So he went about designing and building one of his own, that would maintain low end integrity and eliminate ground loops or noisy outputs from stringed instruments. I can absolutely say that this DI beats the stuffing out of most of the commercially available prosumer boxes. And for anyone wanting to get into building their own gear Bo's DI is a must! I have replaced my old Radials with these babies. I love the way they sound, especially on lead guitar. Loads of attack, plenty of sustain and serious frequency range. Oh, and they are really quiet too!

My own disheveled self etched PCB, it doesn't look all that pretty, but it works real good! 

Bo Hansens DI box. 

Bo's Di box is a rather simple device. It is based around a 1:6.45 transformer, available options are pretty wide here, the board itself supports the OEP A262A3E or the Haufe ST 8456. But Im pretty sure you can use just about any suitable transformer for the job. The circuit has an input impedance of 1 Mohm which is suitable for just about any high impedance guitar output. The transformer steps things down to around 2oo Ohm's at the the boxes output. Which makes it very suitable for a range of mic pre's at very low noise levels. There is also a ground loop suppressor instead of a ground lift and a handy protection circuit on the input, just incase you do something stupid. There is not a lot to say about the components, the shopping list is small enough and parts are generally available from most nerd emporiums (Electronics shop)

The Schematic

Some introduction links to get you reading before building. 

Group DIY thread. (All info is here, including support)
Group DIY Bo Hansen Link

Bo's Own page on his DI box

Here is a list of what you need and where you can find it. 

PCB. 

Although this project is very doable by etching your own circuit boards, manufactured boards are available from Gustav at PCB Grinder. He will also sell you a transformer if your ok spending a bit of extra cash on a Lundahl. I have used a the OEP transformers which are a lot cheaper with great success. 

PCB Grinder Bo DI link 

Bo DI pcb at PCB Grinders site 

If your feeling sassy, and you want to melt your kitchen sink with Ferric Chloride, then you can always etch your own boards. I did this on my first one, and it worked great! And I managed to keep the kitchen sink in one piece too! 

Self etch files are available here.
Self Etch files for Bo's DI box. 

General parts list 

Parts are easy to find, I had no issue grabbing what I need for my "prototype" at my local lekky shop. Ok, better quality parts should be considered, but the box will work pretty good with standard Chinese garbage that you can pick up for a few cents a part. Although if you are serious about quality, I would recommend getting some brand names involved. The second one I built utilised WIMA and Panasonic Caps as well as Vishay 1% resistors and Neutrik connectors.  The transformer might be the only thing that takes a little bit of looking for.

The parts list can be found here. 
Bo's DI parts list

Transformer 
OEP A262A3E (This is the one I used and it sounds great) 
Believe it or not you can buy these from Amazon - Click here

European link
Farnell 

OEP A262A3E

I would say most of these parts can be had from the usual suspects. Mouser, RS, Alpha, Eagle, ect...

Building

There is nothing complicated about Bo's DI box but you should follow a few of his tips. You can find these on Bo's own page about his DI box. 

(This info below is directly ripped from Bo's site. Please rather go there and read it all. But for quick reference look below)

metal box/case groundThe metal case/housing for the DI-box electronics, shall only be grounded on the XLR output connector pin 1, and also connected to a solder tag fixed in one of the XLR fasten screws.(this is the only ground connection to the metal case)
Then a additional cable from XLR pin 1 to the electronic ground system rail (the bottom line in the schematic) in series with the 0,1 uF, 100 ohm and the two diodes.

isolated plastic jacksThe two tele jacks must be isolated from the metal box, use plastic jacks, with plastic nut or crome nut with isolated washer, the jacks ground/sleve shall only be connected in the electronic ground system rail.
This is very important, to get the ground loop suppressor to function properly.(this is the only ground connection to the metal case)Then a additional cable from XLR pin 1 to the electronic ground system rail (the bottom line in the schematic) in series with the 0,1 uF, 100 ohm and the two diodes.
isolated plastic jacksThe two tele jacks must be isolated from the metal box, use plastic jacks, with plastic nut or crome nut with isolated washer, the jacks ground/sleve shall only be connected in the electronic ground system rail.This is very important, to get the ground loop suppressor to function properly.shortet input jackThe input jack should be shorted to ground when it is un-plugged.There is an extra connection/solder tag for the rear tip contacts that is the brake switch, this connection/solder tag shall be connected to the ground on the input jack.(the link can be seen in the picture of the input jack mounted in the lid)This removes noise, hum and interference with un-plugged input jack.transformer case groundThe metal casing of the transformer shall only be connected to the electronic ground system, and not in contact with metal box.it is not unususal that there is an extra cable or connector pin for the transformer core, this must also be connected to the electronic ground system.LED type and connection (pc-card version)Use a 3 or 5 m.m. red LED, because the low voltage drop. (other colours have higher voltage drop)If the LED is mounted visible from the outside from the box, it must of course be connected with loose cables from pc-card.The long leg shall be connected next to the two 6,8K resistors.If the LED is not used, use a link instead of LED.
some small electronic corrections 
not corrected in the schematic 

improved the bias stability 
Change the two 470 kohm to 100 kohm
Change the 1 uF to 10 uF/35 voltnot corrected in the schematic 

improved the bias stability 
Change the two 470 kohm to 100 kohm
Change the 1 uF to 10 uF/35 voltChange the two 470 kohm to 100 kohmChange the 1 uF to 10 uF/35 voltto reduce the component supplyChange 47 ohm to 100 ohmChange  0,047 uF to 0,1 uF

Source Bo Hansen 

Enclosure's 

Its a good idea to use aluminium enclosures. This way you can ground the chassis to shield the circuit from noise and interference. The Hammond 1590S seems to work well. The 1590B's are a little shallow if I'm not mistaking.  I used a rather oversized hammond enclosure for my prototype and it looks really cool. The silver handle on the top came from my local hardware shop and is actually used for a cupboard door. The finish I did in my usual way with water slide decals and plasticote spray.

To round up

These DI boxes are simply amazing. They cost around $50 to make and sound fantastic. I highly recommend building one of these babies for your rig. I started my "prototype" on a sunday morning and finished it by lunch time. SO its a quick, easy and fun project to sink some noob teeth into. So heat up your soldering irons and get cracking, Bo's Di is a winner.

Gunna see if there is some time to do some test recordings and post them here at some point (Jeez, I shouldn't promise that!)