New idea. Instrument tracking d.i. box

[Morning_Star]

I was looking into what it would take to make a d.i. input for a mic pre because I was thinking of getting a two channel mic pre for tracking and it doesn't have d.i. inputs.  This way I could track line audio and instruments and keyboards and such.  So then I started looking around for how to do this, then I came across come d.i. box circuits that use phantom power for juice. 

So I thought that you could make a 1U 19" rack module with a selector switch for instrument d.i. input (on the front), and mic input (on the back) passive directly to the output.  Then just turn on the phantom power to turn it on and you have an instrument tracking unit for your mic pre.  So you would just plug your mic into the tracking d.i. unit and the phantom power would still work to power the mic as well.

Does this seem possible and easy?  If so what are some good phantom powered d.i. circuits to start with. 

 

[tv]

Look here - scroll down to FIG 8:

http://www.ethanwiner.com/gadgets.html

alternatively, you could modify the "schoeps" circuit for electret mikes - it uses a fet phase splitter running into two emiter followers balanced  (the above schematic is unbalanced):

http://www.twin-x.com/groupdiy/displayimage.php?album=256&pos=61

 

[BradAvenson]

Shameless plug:
http://avensonaudio.com/smallDI.php

I don't use the schoeps topology since it adds 6 db of gain and I found that unity was much easier to deal with. 

 

[JohnRoberts]

I don't think I have a web handy schematic, but i did a simple phantom powered DI back in the '80s that was small enough to build inside a 1/4" plug housing.

I made one with a pair of Nch JFETS configured as a simple differential. This accepted a balanced input and delivered a balanced output to the mic pre. Gain and operating point could be scaled to not overload the mic input.

For modest input levels one could use a pair of Pch JFETs as followers which is even simpler.

Note: depending on you tolerance for complexity, there is enough power available to design in a reasonable current dual opamp (like TL072). I did a commercial design that works from phantom power a couple decades ago and AFAIK it's still in production.

JR

PS: Not to be overly critical of Ethan's circuit (He's an old friend), he used a very low noise bipolar device, more appropriate for a mic input than instrument. His circuit is also unbalanced, which isn't optimal for inputs.

 

[conleycd]

Hi there,

My suggestion - never reinvent the wheel if necessary. 

Joe Malone's www.jlmaudio.com DIs are very nice.  Affordable too with the pleasure of DIY.

Coupled with his "Go Between" for Polarity reversal/Pad/Phantom you will be well suited.  They really seamlessly integrate with the Mic Pres so they switch your audio automatically from your mic input to your line/instrument when you plug into them.  They're affordable and sound great.  I would suggest the Discrete DI just because the jack is longer.  The IC DI that he has is very nice and good but you can't have a thick panel or the threads won't reach and/or will strip.

I suggest the Go Between kit too because you can make nice use of the PAD for line inputs into a Mic preamp but it is also a super easy way to add polarity and phantom power to a pre.

CC