Making a DI box

Over the past decades making a DI box has been one of my recurring activities.

From repurposing scavenged  transformers into make shift 'passive' boxes to  experiments with unbalanced to balanced chips.

The problem is phantom supply. 

All though rated at 48volts it's purpose as a PSU for any circuitry is really limited, because the way it is implemented in almost every mic pre-amplifier:

The 48 volts is fed through 2 resistors of each 6k8 in each leg of the symmetrical input, thus giving an impedance of 3k4. Which means that if your circuit draws say 10mA of current only 14 volt of that supply is left to begin with.

Your own circuit will have some resistors in the supply lines too, to unload the output from your circuit. So the voltage to work with and thus the headroom of your DI will even drop more.

Hence the really limited headroom of a lot of modern, commercial DI boxes

Old skool '70's electronics to the rescue: 

 

This circuit is no rocket science and well known in every pro-audio circle, mostly credited to Bo Hansen..

Quite a few mods though, just as a reminder for myself a picture of the early limited (as in only 20 pcs of that specific transformer in my very old stock) prototype:

 

 

 

 

By popular demand we decided to try to do a reissue fairly recently.

The problem was finding a nice affordable transformer: these Swedish ones are really nice, but man, do they charge..Pure by coincidence i stumbled across OEP. Now those i do remember from the classic BSS AR116 DI. (Which has nothing to do with the modern AR133 !!) And with some googling I found them to be used by modern Telefunken as well (speaking of brand name marketing!!)

 

So here we go a batch of class A, current driven, transformer balanced DIs in a stainless steel housing. 

Yes, stainless steel. 

Every DI gets abused once and awhile as a stage weight, box tilt, or even as a hammer so to prevent them from changing into ugly lumps of corroded, dented metal, we folded and water jet cut some stainless steel sheets:

(of course I didn't do it by myself: credits to Koos Roadservice)

 

 

 

 

 

 

Still not totally satisfied by the end result I learned myself how to powder coat (not difficult at all) 

 

So here it is..open for orders now..hahahaha...

KBLsystems class A transformer balanced DI