From: tdawson@iphase.com (Tim Dawson)
Subject: Re: transformerless DI box powered by phantom power
Organization: Interphase Corporation, Dallas Texas
Date: Thu, 5 Nov 1992 17:29:10 GMT

jay@cmn19.Stanford.EDU (Jay Kadis) writes:

>In article <1992Oct28.162056.9893@viewlogic.com>  
>bobdavis@mighty.viewlogic.com (Bob Davis) writes:
>> 
>> I'm wondering if any of you folks have ideas on a circuit for using
>> phantom power to power a transformerless DI box, using either opamps,
>> or a PMI SSM-2142, or something else?  This means you've got the
>> phantom power on from the board, and that is powering the devices used
>> to drive the balanced signal from the unbalanced input.  It should be
>> simple to do this, but I haven't been able to solve this "puzzle".
>> A complete design for such an item would also be great, of course.
>> Seems to me this should be super cheap to produce once a good circuit
>> and layout are done.
>> -- 
>> Bob Davis -- bobdavis@viewlogic.com
>> "Gumby for President (Write-in candidate, No-Bogosity[tm] Party)"

>Try reverse engineering a phantom-powered mic schematic (i.e. AKG C460 or  
>414).  They are somehow able to extract the phantom power and still drive  
>the signal out to pins 2 and 3.  From a quick look, a transformer is  
>involved...probably the reverse of the center-tapped transformer  
>arrangement that "creates" the phantom power.  Also, the Crown PZM has an  
>active power supply that can use either batteries or phantom power, so it  
>can be done. The problem you will face is using a monopolar supply with  
>bipolar ICs like the 2142.

>Jay Kadis
>jay@ccrma.stanford.edu
 

Typically, the phantom power pickup can be done two ways, depending whether you
are using transformers or active components.  Please excuse by "basic" 
character drawings, but they should give  you the idea:



      Sig 1  Sig 2  Signal from wherever, balanced or unbalanced.
       |      |
       |      |
       \/\/\/\/
       ========  (Transformer, bad rendition!)
       /\/\/\/\
       |  |   |    10K
       |  |   |__/\/\/\___    +
       |  |   |           \____ Power out, with reference to the shield lead.
       |_ | _ | _/\/\/\___/
       |  |   |
       |  |   |               -
       |  |__ | _______________
       |  |   |
     Pin2    Pin3
        Shield (Or may be floating to the transformer at least, but MUST 
                connect to the mic's case, or preamp ground.)

Power comes in at (typically) 48V on Pins 2 and 3, and can be taken of with
(typically - my old console did it this way, as do most of the EV Condenser 
Mics I have) with a pair of 10K resistors, which sever to decouple the two 
leads from each other and prevent loading the line from pin 2 to 3 - remember 
this is 600 Ohm stuff, so the 20K across the cable looks like an open to the 
audio signal!


On an active balanced scheme, things are very similar, but there are a few 
more parts without the transformer:



      Sig 1  Sig 2  Signal from wherever
       |      |
      __________
     |          |  Differential driver (typically 2 op amps or whatever) to 
     |__________|  drive the balanced line.
       |      |
       =      =   (Capacitors, also a bad rendition, typically 2.2uF NPO or so)
       |      |
       |      |    10K
       | gnd  |__/\/\/\___    +
       |  |   |           \____ Power out, with reference to the shield lead.
       |_ | _ | _/\/\/\___/
       |  |   |
       |  |   |               -
       |  |__ | _______________
       |  |   |
     Pin2    Pin3
        Shield (Must be connected to case or the unblanced side ground.)


Since differential inputs have all DC removed or ignored by either a transformer
or by coupling caps, you should be able to drive this with single ended op-amps
and provide a differential output with both signals staying constantly above 
ground potential, but differential, and have the output coupling caps strip off
the DC offset to drive the line true differential, which is what I beleive is 
typically done when a transformer is not involved.

Disclaimer:  While every attempt has been made to be fully accurate here, please
remember that I am working from memory and subtle items like component values
may be in error.  Get yourself a schematic of something which works if you 
are serious about making this work and clone the hell out of it - there is 
typically very little variation in phantom power pickoff circuits between 
manufactururs - all the differences come in the preams, elements, etc.
-- 
================================================================================
The optionions expressed above do not neccesarily represent those of my 
employer - as a matter of fact, sometimes they wish I would shut up!