MIDI folks: As promised here is Phil Rastocny's PZM mod. The design has changed slightly I think since this was posted but I think the mod remains the same. Note that there is a followup from Greg Taylor which has some corrections to the original posting. Greg was the one who made me aware of this modification..... I think he called these mike "absolute killers" I dunno about that, but here it is. Sam Mullins -------------------cut here-------------------------- Article 8267 of net.audio: >From: pmr@drutx.UUCP (RastocnyP) Newsgroups: net.audio Subject: mods to the RS/Crown PZM microphones Message-ID: <957@drutx.UUCP> Date: Wed, 20-Aug-86 00:03:00 EDT Date-Received: Thu, 21-Aug-86 00:03:20 EDT Organization: AT&T Information Systems Laboratories, Denver Lines: 145 Many people have asked me to post something about the mods I made to the RS 3-channel mixer and the RS/Crown PZM microphones. I'll post the mods to the PZMs first since most folks have asked about them. 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/Crown PZM microphone is an omnidirectional electret microphone having an output impedance of about 600 ohms. It requires 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 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. There are compromises made when using this approach, but the benefits far outway the compromises. Modifying the Radio Shack/Crown 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. 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 a 1/4" phono mic input of most 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. 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 1.2K 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----- ----- "+" "-" --- 1.2K 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 note, if you are ABSOLUTELY POSITIVE that the input stage of your tape recorder has an input capacitor and then a load resistor, you can replace the 10 uF cap with a piece of wire. (See below.) REPLACE THE CAP WITH WIRE IF USE THE INPUT CAP ABOVE IF THE TAPE DECK INPUT LOOKS LIKE THE TAPE DECK INPUT LOOKS LIKE THIS: THIS: input input cap cap jack----||--------input stage jack----------||----input stage | | load load resistor resistor | | ground ground You should also replace the input caps of the tape deck in either of the above two cases with an equivalent value equal or higher voltage mylar or metalized polypropylene capacitor to obtain the best performance. Yours for higher fidelity, Phil Rastocny AT&T-ISL ..!drutx!pmr -- David "bd" Hsu hsu@eneevax.umd.edu seismo!mimsy!eneevax!hsu EE Computer Facility, Maryversity of Uniland, College Park, MD 20742 >From clyde!cbatt!ihnp4!ptsfa!lll-lcc!styx!ames!think!husc6!uwvax!astroatc!gtaylor Mon Feb 16 11:41:00 EST 1987 Article 691 of rec.audio: Path: clyde!cbatt!ihnp4!ptsfa!lll-lcc!styx!ames!think!husc6!uwvax!astroatc!gtaylor >From: gtaylor@astroatc.UUCP (Ou sont les neiges d'antan?) Newsgroups: rec.audio Subject: Re: Rastocny-fied PZM mikes .... Message-ID: <144@astroatc.UUCP> Date: 13 Feb 87 15:20:09 GMT References: <368@sbstaff2.UUCP> <217@srcsip.UUCP> <647@eneevax.UUCP> Reply-To: gtaylor@astroatc.UUCP (Mais, ou sont les neiges d'antan?) Distribution: na Organization: Haute Vulgarisation, Madison WI Lines: 36 Dave Hsu managed to get around to posting the Rastocnyfications to the PZMs before I did. However, Phil had included a mistake in his initial posting concerning the resistor values. It's supposed to be a 2.2 (NOT a 1.2) value. Here are the corrected secions of the posting. _____________ 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 polypropyle