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From: cmh@eng.cam.ac.uk (C.M. Hicks)
Newsgroups: rec.audio.pro,rec.audio.tech
Subject: How to phantom-power a Rat-Shack PZM
Date: 6 Jul 1994 15:49:35 GMT
Organization: University of Cambridge, England
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kerkhoff@mail.utexas.edu (Jim Kerkhoff) writes:

>Does the mod in the archive also
>tell how to phantom power the mike?  I remember hearing that it was 
>possible but could never find the info...

The Rastoczny mod does not describe phantom power, so I will...


HOW TO PHANTOM POWER A RADIO SHACK PZM
======================================

Introduction
============ 

This article describes two ways of powering electret capsules (with
built-in FET), such as that used in the Radio Shack PZM, with phantom
power. The first method is the simpler; the second is more complex,
but provides a lower output impedance, thereby allowing longer lines
to be driven (and is probably cheaper).

Both remove the horrible transformer, and both remove the equally
horrible electrolytic capacitor from the original RS circuit.

Method 1
========

                           10u
    +---------o-------------||------o-----------------  HOT    (2)
    |         |                     |
    |         |-                    |
    |      CAPSULE                 22k
    |         |+                    |
    |         |            10u      |
    |         o-------------||------|------o----------  COLD   (3)
   2k2        |                     |      |
    |        2k2                    |     22k
    |         |                     |      |
    |         +---------o----o------o------+
    |                  +|    |+     
    |                  12V  10u
    |                  -|    |-
    +-------------------o----o------------------------  GROUND (1)

Notes:
  1) The component "12V" is a 12 Volt zener diode

                  |                |
                 +|             |------
                 12V      =        ^  
                 -|              /   \
                  |              -----
                                   |

  2) The 10u capacitors in the HOT and COLD signal leads should be
     high-quality plastic film types. The values of these may be
     reduced to 2u2 if the preamp input impedance is 10k or greater.
  3) The 10u capacitor in parallel with the zener should be a tantalum
     type, and can have a 10n plastic film cap in parallel if you wish.
  4) The cable to the capsule should be twin+shield. The shield should
     be connected to ground near the zener diode, and left unconnected
     at the capsule.
  5) The polarity of the capsule is important. The + side is the one 
     connected to the casing. (Odd but true, at least in the case of
     the RS PZM.)
  6) The pinout given is the standard for XLR3 mic connectors.
  7) If you want to use the existing RS box you will find that the 10u
     capacitors do not fit. If you *must* then use electrolytics for
     these (>50V working) and bypass them with 100n plastic film caps.
 

Method 2
========

                          +---------------------------------+
                          |                                 |
                         2k2                 +---22k----+   |
                          |                  |          |   |
                          |                  |       E--o---|--------------- COLD
                          o---------||----o--o-------B      |
                          |         1u0   |          C      |
                          |+              +-100k-+   |      |
                       CAPSULE                   |---o------o
                          |-              +-100k-+   |      |
                          |               |          C      |
                          o---------||----o--o-------B      |
                          |         1u0      |       E--o---|--------------- HOT
                          |                  |          |   |
                          |                  +---22k----+   |
                          |                                 o----+
                         2k2                               +|    |+
                          |                                12V  10u
                          |                                -|    |-
                          +---------------------------------o----o---------- GROUND

Notes:
  1) Notes 1, 3, 4, 5, 6 from above apply here too.
  2) Component EBC is a PNP bipolar transistor, eg BC479
  3) The 1u0 capacitors should be high quality plastic film types
  4) Noise floor *may* be lowered by filtering the 12V supply for 
     the capsule with a series R parallel C arrangement.
  5) This circuit will fit in the existing RS box, but a metal one
     is recommended for the additional screening it affords.

THE END
=======

Hope this helps...

Christopher
--
=====================================================
  Christopher Hicks     http://www.eng.cam.ac.uk/~cmh
  cmh@eng.cam.ac.uk           Voice: (+44) 223 332767
=====================================================