How to Modifiy a Radio Shack PZM Microphone to phantom power.

This is a copy of the original article. It has ben posted here for your convience.

PZM Modifications, by Christopher Hicks

Last modified and copyright, 2nd October, 1995. Non-commercial distribution encouraged; all other rights reserved by the author.

Firstly, You mess with your precious(?) RS PZM's entirely at your own risk! Of course I disclaim all responsibility for your destruction of your equipment - microphones, mixers, tape-recorders, lawn-mowers and food-processors included.

I should point out that I have tried very few of these ideas; consider this more as a starting point for your own experimentation. Most of the ideas I present are equally applicable to other electret capsules, such as those from Panasonic.

On with the interesting stuff...

Many people do not realise that the PZM as supplied by RS is actually a balanced microphone. To convert it for connection to a balanced mixer input is as simple as removing the moulded 1/4" plug, and replacing it with a male XLR3. The shield goes to pin 1 of the XLR, and the other two wires go to pins 2 & 3 - which is which doesn't matter too much as long as you do them all the same if you are converting more than one microphone.

Another simple modification is detailed in the manual. It is suggested there that better dynamic range and higher maximum SPL can be had by replacing the 1.5V AA cell with two 6V batteries which will also fit in the battery compartment. The 6V battery is a bit expensive and you can use a 9V PP3 if you bodge the connections and tape it to the outside of the box.
Mechanical and acoustic modifications I have heard of include:

  • a) removing the bit of black fuzz from the front of the capsule.
  • b) enlarging the hole in the front of the capsule.
  • c) reinforcing/sealing the rear of the capsule with epoxy.
  • d) abandoning the metal plate in favour of a 6" square of perspex (plexiglass for those in the US!)

Since the capsules are so variable in their manufacture it is difficult to tell ahead of time whether any of these mods will lead to an improvement in the sound. Mods a and b aim to modifiy the HF response - as supplied most of the capsules have a pronounced presence peak, and a fairly nasty phase contortion around 4-6kHz due to the small size of the hole. This is a deliberate resonance introduced to extend the hf response as far as possible towards 20kHz. The felt pad is there to tame this resonance a bit. Mods a and b in combination are an attempt to smooth out the treble response by removing this resonance, at the expense of not reaching quite as high into the stratosphere. Mod c attempts to damp the phenolic backplate (which otherwise bends in the acoustic breeze (allegedly!)) and sealing the back more effectively should, in theory at least, extend the LF response a bit. Note that this will not help compensate for the lack of bass encountered when the baffle size for the PZM is insufficient. Mod d replaces a highly resonant piece of metal which rings like a bell with a piece of plastic which doesn't.

This seems like a good idea to me, but whether its effect is significant I can't tell without trying. A number of electrical modifications are also possible, and their effects are a bit more predictable than for the mechano-accoustic ones! First, the FET inside the capsule can be replaced with a quieter one, though I have never managed to do this without destroying the capsule. This is more due to my own heavy-handedness than anything else. Definitely not an easy mod, but some folks have reported success. The electronics of the stock microphone are really let down by the transformer and electrolytic capacitor lovingly installed by RS. Both are really ghastly examples of their type, but what else can you expect for the money. To drive an unbalanced input from a battery-powered mic, but removing those two components is quite simple. I make no claims regarding the benefits or otherwise of doing this!

        +---------------------------- battery +ve (3 to 12 Volts)
        o---------- 10u -------o----- output
        |+                     |
     CAPSULE                  10k
        |-                     |
        +----------------------o----- GND, and battery -ve

The 10u capacitor can either be a high-quality plastic-film type (expensive and large), or an electrolytic in parallel with a small (eg 100nF) plastic-film capacitor (cheap and small). In the latter case the positive of the capacitor should go to the capsule, and the negative to the 10k resistor. See note 5 below for the capsule polarity. I have also designed two schemes for completely replacing the innards of the microphone with better, phantom-powered electronics. The first method is the simpler; the second is more complex, but provides a lower output impedance, thereby allowing longer lines to be driven. Both remove the horrible transformer, and both remove the equally horrible electrolytic capacitor from the original RS circuit. Neither is intended as a complete cookbook method, but both can be made to work well with a little electronic skill. Method 1========

    +---------o-------------||------o-----------------  HOT    (2)
    |         |                     |
    |         |-                    |
    |      CAPSULE                 22k
    |         |+                    |
    |         |            10u      |
    |         o-------------||------|------o----------  COLD   (3)
   2k2        |                     |      |
    |        2k2                    |     22k
    |         |                     |      |
    |         o--330R---o----o------o------+
    |        +|        +|    |+     
    |        10u       12V  10u
    |        -|        -|    |-
  --o---------o---------o----o------------------------  GROUND (1)

  • 1) The component "12V" is a 12 Volt zener diode
  • 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                 +---10k----+   |
      |     |                  |          |   |
      |     |                  |       E--o---|--------------- COLD
      |     o---------||----o--o-------B      |
      |     |         1u0   |          C      |
      |     |+              +-100k-+   |      |
      |  CAPSULE                   |---o------o
      |     |-              +-100k-+   |      |
      |     |               |          C      |
      |+    o---------||----o--o-------B      |
     10u    |         1u0      |       E--o---|--------------- HOT
      |-    |                  |          |   |
      |     |                  +---10k----+   |
      |     |                                 o----+
      |    2k2                               +|    |+
      |     |                                12V  10u
      |     |                                -|    |-
    --o-----o---------------------------------o----o---------- GROUND

  • 1) Notes 1, 3, 4, 5, 6 from above apply here too.
  • 2) Component EBC is a PNP bipolar transistor, eg BC479
  • Ideally these should be hand-picked for low noise and
  • matched gain. Bear in mind that VCE can be up to about 36V.
  • 3) The 1u0 capacitors should be high quality plastic film types
  • 4) This circuit will fit in the existing RS box, but a metal one
  • is recommended for the additional screening it affords.
  • 5) The circuit may benefit from the addition of 22pF capacitors in
  • parallel with the two 100k resistors.
  • 6) For minimum hum pickup the two 2k2 capsule bias resistors should
  • be accurately matched.


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