From: JOSEPHSON@AppleLink.Apple.COM (Josephson Engineering,VCA)
Newsgroups: rec.audio.pro
Subject: Phantom/T-powering FAQ
Date: 17 Nov 92 21:51:00 GMT

Going through a pile of unread posting, noticed several queries/comments/ etc
that I might have answers for, so here's a pre-digestified reply to the net:
 
>>>Gabe Wiener re FAQ... excellent idea, not only are there a lot of questions
frequently asked, there is also a real dearth of good reference material on the
subject. The FAQ might also include a bibliography of what _is_ available and
isn't a lot of pablum written by a magazine hack who doesn't know much about
the equipment. For recording techniques, start with Alton Everest & Ron
Streicher's new book.
 
>>>Phantom and T-power... Gotham Audio, former importer/distributor of Neumann
in the US, once claimed trademark protection on "phantom power". Neumann
invented phantom powering as a marketing ploy to sell condenser microphones to
radio stations in Germany that didn't want to fool with dedicated power
supplies. But they have since abandoned their claim to the name or the
technique as an early and wise example of open architecture. Although phantom
powering could certainly have been patented and licensed to all, instead they
gave it to the German government who made it into a specification (DIN 45596).
There is quite a bit of confusion as to what constitutes correct phantom
powering. The previous post on the topic is correct as far as it goes. The
basic concept is that phantom powering voltage appears as common mode DC riding
on the two balanced signal lines versus ground.  The circuit is:
                                                         R1
                           R3                  I-------/\/\/----I---- +V
               I---------/\/\/---I             I         R2     I   C1,C2
    ___________I______           I             I  I----/\/\/----I /
    [ mic electronics]-----):(---I-------------I--I---------I(----
    [                ]     ):(---I                I                 audio out
    [                ]-----):(--------------------I---------I(----
    __________________      \
                             \transformer or moral equivalent thereof
 
Domain:....microphone.............../..cable../..power supply.../external equip
 
"Standard" phantom is +V=48V, R1=R2=6.8K +/- 20% _but_ difference between R1
and R2 must be <27 ohms (0.4%). This is called "P48 Phantomspeisung". 24V and
12V are also allowed in the standard, for P24 use R1=R2=1.5K, for P12 use
R1=R2=680 ohm. R3 is internal to the mic and determined by the maker. C1, C2
may be replaced by a transformer. If capacitors are used, a lowpass filter is
formed with whatever load impedance is presented by the external equipment, so
the value and type of these capacitors must be chosen carefully.
 
The original DIN spec called for a maximum current of 2 mA. The original idea
was to power just an FET (Neumann FET-80 series, KM-84 typ.) which only
required 0.9 mA. The FET drove a step-down transformer which allowed good line
drive characteristics and got rid of a lot of input stage noise. Drawing 1mA
through R1 in parallel with R2 (effectively) still leaves the common-mode
voltage at 44V which was plenty to polarize the diaphragm. Schoeps wanted to
make a transformerless version because good transformers for this application
are very expensive (mfr cost in production qtys from Haufe, the German maker of
same, about $30!)and big, and require extensive tweaking to get the common mode
rejection down where it needs to be. The Schoeps topology (since used by nearly
all transformerless mic output stages) is a PNP emitter follower driving each
mic lead, fed by an FET phase splitter. The collectors of the output stage are
tied together and held a few volts above ground by a zener; this voltage powers
the input FET. This needs 4-5 mA to work properly, so the DIN spec was revised
to allow 10 mA maximum current. Pulling 5 mA brings the P48 voltage down to 31
volts, so most transformerless mics include a DC-DC converter to make the
capsule polarizing voltage.
 
 
                 I---------------------I
                 > R1                  I
                 < 2K        /---------)------------------- (mic conn pin 3)
                 I C1 .1  eI<          I
 signal          I---I(--b-I       R3  I
 from         I--I        cI\      5K  I  /
    ----)(--->I             I----/\/\/-I--I<------I-- (mic conn pin 1/case/gnd)
 capsule      I--I C2 .1  cI/           /(zener)  I
                 I---I(--b-I              6V      I
                 I        eI<                     I
                 > R2        \--------------------)-------- (mic conn pin 2)
                 < 2K                             I
                 I--------------------------------I
 
(BTW, this circuit makes a fine phantom powered DI box, for the posted query on
that topic; 1M from gate to gnd, 100 K's from bases to collectors, parallel
them with 470 pF disk caps to prevent oscillation)
 
>>>T-power, or "Tonaderspeisung" (ton=sound, ader=leads or conductors,
speisung=powering), as specified in DIN 45595, actually preceded phantom by a
few years. Sennheiser and others use this to power their RF condenser mics and
several companies make transformerless types using the above circuit but with
one of the transistors an NPN!  It's still quite common in the film sound biz.
T-power uses 12 volts _across_ the signal leads; +12 through 180 ohms to pin 2,
-12 (not 24 volts different from +12, just the other half of this _floating_ 12
volt power supply) through 180 ohms to pin 3 (*all pinouts in this post are for
the XLR version*). Of course a dynamic mic plugged into a T-power line will go
click, probably only once, as the power supply wil try to push 33 mA through
the mic's output transformer. Absent any shorts in the cable or grounded
transformer center-taps, a dynamic or ribbon mic can be plugged in to a phantom
supply without ill effect.
 
Both phantom and T-power are inherently balanced systems, although you can
cobble an unbalanced version that will work with most mics.
 
Xref: sparky rec.audio.pro:2364 rec.music.synth:18587