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

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

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.