DAT-Heads Digest #195
Contents:
Which Mod is Really the Best for the Radio Shack PZM Mic & Why (Robertlars@aol.com)
Gerulf is dead, new e-mail address. ("Alexander W. Chin")
From: Robertlars@aol.com
Subject: Which Mod is Really the Best for the Radio Shack PZM Mic & Why
Date: Wed, 29 Jan 1997 08:22:41 -0500 (EST)
Hello Fellow DAT Heads:
I've included several of the Radio Shack PZM Mic mods that I've pulled
off the Internet. Questions: Which is the best mod and why, (please try to
explain your answer - technically)? Please e-mail responses back to
robertlars@aol.com
Mod # 1: From: cmh@eng.cam.ac.uk (C.M. Hicks)
Newsgroups: rec.audio.pro
Subject: Re: PZM mods (LONG - 500 lines)
Date: 28 Sep 1995 09:50:48 GMT
Organization: University of Cambridge, England
A large number of people have emailed me wanting details of
modifications etc to the Radio Shack PZM, so I have decided to post
them. I should point out that I have tried very few of these ideas;
consider this more as a starting point for your own experimentation.
Of course I disclaim all responsibility for your destruction of your
equipment - microphones, mixers, tape-recorders, lawn-mowers and
food-processors included. You mess with your precious(?) RS PZM's
entirely at your own risk.
Firstly, there are a few things you can do to improve the electronics
of the thing. Most of these electrical mods and circuits are equally
applicable to other, similar electret capsules such as those made by
Panasonic, though values of bias resistors etc may need to be altered.
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.
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!
I'll attach a couple of articles detailing more extensive electrical
modifications to the end of this post.
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 (which rings horribly) in favour
of a 6" square of plexiglass (perspex for those in the UK!)
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.
Mod # 2: INTRODUCTION:
The RS PZM microphone is an omnidirectional electret microphone
patterned
after a principle invented by Crown International called the pressure
zone
microphone (hence, PZM). The output impedance of the stock microphone
is about 600 ohms (unbalanced) and it requires a phantom supply voltage
from -1.5V to -12V DC for operation. The stock microphone has a supply
module and built-in line-matching transformer to convert 600 ohms
unbalanced
to about 10K ohms unbalanced. The problem with this stock PZM is
twofold:
1) you cannot use long cable runs on the mic since the line is
unbalanced
2) the matching transformer used in the module is terrible
So the mods outlined below address these two problems by describing a
method of using a standard balanced microphone cable in conjunction with
an unbalanced (single-ended) microphone input configuration common to
most consumer tape recorders. There are compromises made when using
this
approach, but the benefits in the case of this PZM far outway the
compromises.
MODIFYING THE RADIO SHACK PZM MICROPHONE
The stock assembly consists of a mic, a coax cable, a supply module, a
twinax (2-wire shielded) cable and a 1/4" phono plug as shown next.
===== ==============
|mic|---coax cable-----|power supply|----twinax cable---1/4" phono plug
===== ==============
1. Cut off the 2-wire shielded cable between the 1/4" plug and the
power
supply. Toss the phono plug.
2. Take the mic apart (screws on the bottom). Unsolder the coax cable
from the mic element and replace with the 2-wire cable from step #1
above. This is a somewhat static sensative device so work with a
grounded soldering station and appropriate clothing. Connect the
low side to the dark color wire and high side to the light color
wire. DO NOT CONNECT THE SHIELD TO THE LOW SIDE!
3. Connect the other end of the 2-wire cable to an in-line male XLR
connector. You should now have something that looks like this:
male XLR
mic n/c --------------------------------------- shield (pin 1)
electret high -------light wire---------------------- pin 2
element low -------dark wire----------------------- pin 3
4. Make some long mic cables from some twinax or 2-wire microphone
cable.
I made three 75' and three 25' cables for my setup. Shields are
connected on each end to pin 1 and the case on one side (I think
it's
the female side) as shown next.
female XLR male XLR
case------shield --------------------------------------- shield (pin 1)
high --------------------------------------- pin 2
low --------------------------------------- pin 3
The next step is to build an in-line supply that also adapts the XLR
connectors to the 1/4" phono mic input of most consumer tape recorders
as shown next. There should be one of these supply boxes built for each
mic used.
-----------------------
female XLR-------|supply/adapter module|-------------1/4" phono plug
-----------------------
5. Cut a 24" piece of 2-wire mic cable and connect an in-line female
XLR to
it as you did in step 3 above.
6. Cut a 24" piece of coax and connect an in-line 1/4" male phono plug
to
it.
7. Cut holes large enough in a small steel project box to run the
cables
through. Add chaffing and strain relief to these two cables.
8. Connect the shields from the two cables AND the low side of the
2-wire
mic cable to the same point (single point) on the project box.
(If you prefer to use chassis mounted XLR and phono connectors,
instulate
these connectors from chassis ground and wire the cases internally
to this
same single-point ground.)
9. Connect the "+" side of a 9V transistor radio battery jack to this
single point ground.
10. Connect the "-" side of this battery jack to a 2.2K ohm 1/4 watt
resistor.
11. Connect the other end of the resistor above to the high side of the
2-wire cable.
12. Connect a 10 uF mylar or metalized polypropylene capacitor from the
high side of the 2-wire mic cable to the center conductor of the
coax
cable.
You should now have something that looks like this:
female
XLR 1/4" phono
plug
1 ---shield-----+---+---- single-point ground
===============shield=====
3 ---low--------| |
======hot========
2 ---high----- ----- "+" "-" --- 2.2K ohm ----- |
| 9 volt | |
| battery | |
+--------------------------------------- |
| |
--------------||---------------------------
10 uF
input
cap.
When the mics are not connected, there is no drain on the battery so
there
is no need for a switch.
Close up the project box and plug in the microphones and the tape
recorder.
I think you'll be surprised by the improvement in these otherwise
inexpensive
and ho-hum mics.
ONE LAST THOUGHT
If you are *ABSOLUTELY POSITIVE* that the input stage of your tape
recorder
or mixer has an input capacitor (of adequate voltage) and then a load
resistor, you can replace the 10 uF cap with a piece of wire. (See
below.)
REPLACE THE INPUT STAGE CAP DO NOT REPLACE THE INPUT STAGE CAP
WITH WIRE IF THE TAPE DECK WITH WIRE IF THE TAPE DECK INPUT
INPUT LOOKS LIKE THIS: LOOKS LIKE LIKE THIS:
input input
mic stage cap mic stage cap
jack----||--------input stage jack----------||--------input
stage
| |
load load
resistor resistor
| |
ground ground
If you decide not to or cannot replace the input stage cap with wire,
you should replace the input stage caps of the tape deck or mixer with
an equivalent value of equal or higher voltage mylar or metalized
polypropylene capacitor to obtain the best performance.
FOR BALANCED OPERATION:
It's possible to feed a balanced preamp with a similar configuration.
However,
you'll want it to wire it up in the following way:
female Male
XLR XLR
1 ---shield-------------- grounded to
case-------------------shield-----
3
---low--------+--------------------------------------------low--------
2 ---high----- |-------- "+" "-" --- 2.2K ohm -----
======high=======
| 9 volt | |
| battery | |
+--------------------------------------- |
| |
--------------||---------------------------
10 uF
input
cap.
VARIATIONS:
You can eliminate any or all of the XLR connectors if you wish to make
a custom length, dedicated mic setup. The reason that I suggest the
XLRs is that as soon as you get serious about recording, you instantly
find out that you need about 10' more of cable than what the custom
lengths are to do what you want. With the XLRs, you can add or remove
cable for each situation.
For permanent installations in a mixer or tape deck, you could build
a phantom supply similar to what is shown next.
========== =========== ==========
|12V c.t.| |full wave| |-12 volt| 2.2K 2.2K 2.2K
|xformer |--| Bridge |-----| reg. IC|-----\/\/-----\/\/-----\/\/----->
-12V out
========== =========== | ========== | | |
--- | --- --- ---
--- 220uF| --- 220uF--- 220uF--- 220uF
| | | | |
--------------------------------------------->
gnd
You can gang the passive RC components together to run several channels
from
the same bridge. You could also put all of this inside of a "Bud" box.
I recommend using all similar value components since parts are cheaper
by
the dozen.
This concept provides more than adequate ripple rejection and if you
want a bit
improved high frequency clarity, shunt all 220uF caps with 0.1uF
polypro.
I've also done this for budget portable systems. I use one per channel:
2.2K 2.2K
9V battery--\/\/----------\/\/-----> -9V out
| | |
| --- ---
| 0.1 uF--- --- 220uF
| | |
-------------------------------> gnd
I drag a pair of these supplies with hard-wired 20' cables, a Sony
Walkman Pro, and a light weight pair of earphones out with me
backpacking
and get some wonderful wildlife and wilderness recordings on batteries!
You can also replace the massive square metal plate with a piece of
plexiglass with tapered edges. The edges do influence the response of
the microphone, but in some situations, what you place the mics on or
near will equally degrade the response, so what the heck. My portable
rig uses the plexiglass plates; I usually pack in about 45 pounds worth
of stuff and shaving off every ounce that you can helps.
QUESTIONS FROM PREVIOUS POSTINGS
One person asked ``Why such a big capacitor?'' Well, it has to do
with the uncertainty of the input impedance of your tape recorder or
mixer. If you have a low input impedance (say 1,000 ohms or less)
you need this big of a capacitor to get the low frequency response
available with this microphone. If you have a high input impedance
(say 10,000 ohms or more), you can get away with a smaller capacitor.
If you use a lot of different tape recorders and mixers or if you
don't know what the input impedance will be, it's better to use the
big cap (and that's why I recommend it).
Some folks have asked why I don't shunt the mylar with a small exotic
cap. The answer is simple: the PZM has a rising top octave response.
The mylar tames a little of the peak; a shunt cap would only exagerate
it.
Mod # 3: HOW TO PHANTOM POWER A RADIO SHACK PZM, by Christopher Hicks.
======================================
(last modified 5th October, 1994)
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.
Both remove the horrible transformer, and both remove the equally
horrible electrolytic capacitor from the original RS circuit. Neither
is really 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:
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
========
+-----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:
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.
==================== END HICKS ARTICLE =====================
Mod # 4: > DAT-Heads: Does anyone know of the month/year of this Recording
Magazine Mod??
Some time ago, Recording magazine had a piece on modifying the Radio Shack
>PZM microphones for better performance. The only part of it I've been able
>to learn so far was to run the mikes at 18 volts instead of 1.5V; this was
>to be accomplished by connecting two 9-volt radio batteries in series,
>then wiring the resulting 18 volts in place of the single 1.5V battery
>normally used. The idea was that this would raise the signal level well
>above the noise floor of the output signal, giving better signal-to-noise
>ratio ( hence more dynamic range ). Less hiss, in other words. Does anyone
>remember the rest of the story? Have a back issue index that goes back a
>few years?
According to the little booklet that came with the Radio Shack mikes,
you can run them at 3 volts. However, they say to remove these
higher voltage batteries after the session. The 1.5 volt battery can
be kept in the microphone between sessions. I would like to know
more about using the 18 volt supply.
Mod # 5: Replacing the 1.5 volt AA Battery with a -12 volt power supply will
dramatically improve their sound, SPL handling, etc., IMO. Make sure
that you build a NEGATIVE 12 volt supply. The mic uses a PNP based
circuit (Don't ask why) You can use (2) 6 volt batteries but they
cost! I've used these mics for years and gotten good results. Use the
large (2x2) plates for the bass and roll off the mids some if you can.
Good luck!
Greg
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)
Mod # 6: Live Sound! International Magazine supposedly had an article on
modifying a Radio Shack PZM Mic into a balance mic configuration with higher
voltage in the Jan/Feb issue (unknown what year). Any DAT-Heads out there
try this mod?
Thanks for the help guys. Please e-mail your responses back to:
Robert
robertlars@aol.com
From: "Alexander W. Chin" <alexc@newt.phys.unsw.edu.au>
Subject: Gerulf is dead, new e-mail address.
Date: Wed, 29 Jan 1997 21:41:49 +0800
Hi everybody,
I have some very bad news for all of you. Gerulf has decided to die
recently. Apparantly, the SCSI controller decided to chew up chunks of the
hard disk and thus corrupted the filesystem really bad. Either way you look
at it, the gerulf will be down for a very very long time.
So, this means my homepages will be down till I get back to Sydney around
mid February.
For everybody else, my new e-mail address will be
alexc@newt.phys.unsw.edu.au
If you have sent me e-mail in the last 2-3 weeks, it is most likely lost,
so please resend any important mail, especially concerning the 3 tape trees
I am in the middle of...
Also, please pass this message on to anybody that you think I may have left
out.
Finally, I wish to apologise for the problems and hardship that this may
have caused all, especially the managers of listservs.
Thanks.
Alex
** FOR YOUR REFERENCE **
To unsubscribe from this digest, please send email to
dat-heads-request@fedney.near.net with the word unsubscribe on
a line all by itself. If your email address has changed, you may
(optionally) specify 'unsubscribe oldaddress@olddomain' on a line
by itself -- an example is "unsubscribe liam@mit.edu." Problems or
questions about a subscription should be addressed first to dat
-heads-request@fedney.near.net, and then if needed postmaster@fedney.near.net,
never the list itself.
You can submit a message for inclusion in the next digest via this address:
Internet: dat-heads@fedney.near.net
Archives of DAT-Heads digests and related files are available on
the DAT-Heads home page:
http://www.solorb.com/dat-heads
End of DAT-Heads Digest
******************************