Path: frisbee.net.au!chuckie.apana.org.au!curlew.mtx.net.au!news.adelaide.edu.au!news.cs.su.oz.au!inferno.mpx.com.au!news.mel.aone.net.au!imci4!newsfeed.internetmci.com!usenet.eel.ufl.edu!warwick!lyra.csx.cam.ac.uk!cmh From: cmh@eng.cam.ac.uk (C.M. Hicks) Newsgroups: rec.audio.pro Subject: (LONG) Re: Realistic PZM Mod? Date: 17 Apr 1996 19:25:39 GMT Organization: University of Cambridge, England Lines: 527 Message-ID: <4l3gjj$6d9@lyra.csx.cam.ac.uk> References: <4kmel9$nkl@newsbf02.news.aol.com> <4ks5th$taf@news5.erols.com> NNTP-Posting-Host: twg01.eng.cam.ac.uk Rob Howe writes: >I can't quite e-mail you the info., but I was able to find this exact >info. just by searching "radio shack" and pzm using netscape on the >web---I found someone from Cambridge University who had a modification That'll be me. Here's what I have on the subject: ======================================================================== ~Newsgroups: rec.audio.tech ~Subject: Radio Shack PZM modifications. LONG (500 lines) Hi All, 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. Now here are the articles I promissed. The first is by Phil Rastoczny, and was first posted about ten years ago. It is a very detailed description of some relatively simple modifications to improve the electronics, while retaining the stand-alone battery- powered portability of the microphones. The second is by me, and shows how to use phantom power instead of a battery to power electret capsules. Chris Christensen (regular contributor to rec.audio.*) also has a very simple method of phantom- powering the RS PZM with a minimal amount of work - unfortunately I don't have a copy of this. ================ START RASTOCZNY ARTICLE (325 lines) ===================== RECORDING TIPS WITH THE PZM About recording pianos, Crown recommends that you tape two of them inside of the lid. I place the mikes in various positions depending upon the room. When recording in a large hall, I place them on the floor about five feet apart and 12' from the bend in the sound board (it's an unconventional approach; I've never seen anyone else use it). When recording in a small room, I tape them to the lid in various positions, depending upon the type of piano. Crown has several published tips on using the PZMs. If you can find a dealer near you, they may have these articles in stock. PZMs are wishful-sinful mics: they sound pretty good but they need to be placed against a large surface to work properly. Sometimes this is just not possible and you have to try other mics or go to extremes to find a surface. And unfortunately, PZMs have a rising top octave response :-( But they are seldom seen by the audience! CAVEATS: All appropriate caveats about voiding warrantys and possible self-destruction of equipment apply here. If you're careful, you'll get a nice piece of gear; if you're not, you'll have a smouldering piece of junk. I assume no responsibility for any results, good or bad. These mods assume that you have some technical ability in that you know what you're doing when modifying a circuit. 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. SOURCES OF CAPACITORS Some sources for 10uF esoteric capacitors are: Manufacturer Type Part Number L x W (mm) DCV ChateauRoux m-pprop ? 64 x 22 ? 250 El. Concepts m-pprop 5MP12D106K 38 x 20 100 El. Concepts m-pprop 5MP12F106K 57 x 23 200 El. Concepts m-pprop 5MP12J106K 57 x 39 400 IAR "Wonder" m-pprop X series 10uF 57 x 29 310 Illinois m-pest 106MWR063K 32 x 14 63 Illinois m-pest 106MWR100K 32 x 19 100 Illinois m-pest 106MWR250K 44 x 20 250 Illinois m-pprop 106MPW160K ? 160 Illinois m-pprop 106MPW250K ? 250 Illinois m-pprop 106MPW630K ? 630 ?(Meniscus) mylar ? ? 100 Panasonic m-pest E1106 31 x 16 100 Paxton mylar 8uF 38 x 19 ? 100 Seacor m-pprop PMWAF100KG ? 100 Seacor m-pprop PMWFF100KG ? 100 Sidereal m-pprop ? 49 x 19 100 Sidereal m-pprop ? 57 x 27 200 Sprague m-pprop 735P106X9100USL 38 x 23 100 Sprague m-pprop 735P106X9200WVL 57 x 26 200 Sprague m-pprop 735P106X9400ZVL 57 x 42 400 I haven't had time to research all of the sources. I'd appreciate it if you could contact me if you have other sources to contribute or corrections/updates to this list. Addresses and telephone numbers for the above capacitors are: * Digi-Key, 701 Brooks Ave S, PO Box 677, Thief River Falls, MN 56701 Panasonic (800) 344-4539 * Electronic Concepts, PO Box 627, Eatontown, NJ 07724 (201) 542-7880 * Gateway Electronics, 5115 N. Federal Blvd., Denver, CO 80221 Paxton (303) 458-5444 * Illinois Capacitor, 3757 W Touhy Ave., Lincolnwood, IL 60645 (312) 675-1760 * Meniscus Systems, 3275 Gladiola SW, Wyoming, MI 49509-3224 Mylar; best prices on ChateauxRoux (606) 534-9121 * Seacor Inc., 123 Woodland Ave, PO Box 541, Westwood, NJ 07675 (201) 666-5600 * Sidereal Akustic, 1969 Outrigger Way, Oceanside, CA 92054 SiderealKap, ChateauxRoux (619) 722-7707 * Sprague Electric Co. There's probably a sales office in or near your town. Ask for Engineering Bulletins #2092 and #2752, and catalogs #ASP-420K and #C-567A. * TRT, Box 4271, Berkeley, CA 94704 IAR "Wonder Caps" no telephone number published ===================== END RASTOCZNY ARTICLE ===================== ================ START HICKS ARTICLE (100 lines) ==================== 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 ===================== -- --------------------------------------------------------------------------- Christopher Hicks cmh@eng.cam.ac.uk There's more to a good recording http://www2.eng.cam.ac.uk/~cmh than sample rate and word length - me.