| 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:
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)
|
2k2
|
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======== 10u
+---------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)
Notes:
+-----o---------------------330R--------+
| | |
| 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
Notes:
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