I am updating this instructable based on Tom Benedict creating a quite awesome little capsule holder for the JLI microphone capsule. You can get it here from Shapeways: https://www.shapeways.com/product/2CQ5JD22G/pzm-ar...

It is designed in two pieces with little clips on one side to hold the Mogami wire in place after you place the capsule in the hole. The photo's show the new assembly process, which is much simpler!

Solder the Mogami wire to the capsule per the original instructable
Press the capsule into the holder
Press the wire into the little clips to hold it in place.
Glue and glue the two pieces together using E6000 glue. I used a blob on top on the capsule to lock it into place. You want it flush with the bottom.
Hold the two pieces together with some rubber bands
Glue the assembled capsule holder to the acrylic base and clamp in place until dry.

Done!

I really want to thank Tom for doing this. It so simplifies the build. It also has the correct capsule gap designed in.

Supplies

Parts List: (All Builds)

Capsule JLI 61A Capsules
Wire: Mogami W2697
Lexan: Amazon

Capsule Holder:

Shapeways https://www.shapeways.com/product/2CQ5JD22G/pzm-ar...

XLR Build: “Simple P48 build”

3.5mm Build: “PIP Power build”

3.5mm Jack

Tools:

Razor knife
Soldering Iron
E6000 Glue

Step 1: Build a PZM or Pressure Zone Microphone

The first microphone I bought in my youth was a Radio Shack PZM microphone. It was very cool and I used it for years. I’m not actually sure when I lost, it but I no longer know where it is! So after doing a little research, I decided to build a few of them and this instructable will show you how you can too.

PZM or Pressure Zone Microphone is actually more of a technique than a product. They work by having a small microphone very close to a flat surface. Urban legend has it a microphone fell out of its stand ending up on the floor in front of drum kit and everyone in the control room immediately knew they had something great. Well, that may not be exactly what happened but it makes a good story. They were used a lot to capture drums by placing either one in the middle of the studio floor in front of the drums are a pair with the kick at the center of an eight to ten foot triangle. Before they were specifically built, audio engineers used to place microphones close to walls and floors to achieve this. There was a product made called the “Stage Mouse” that you could put a small diaphragm mic in and it would hold it close to the floor. In 1978 Ken Wahrenbrock built a prototype of a stand alone Pressure Zone Microphone. He used a screw mechanism to adjust the spacing between the capsule and the flat surface. It turns out spacing isn’t super critical but should be as small as practical. Specifically we want it less than 1/16th of an inch. That number comes out of a Patent from 1982 for this concept as a product. We are going to use some thin card stock to space the capsule.

US4361736.pdf
Download

Step 2: Acoustic Theory Part

When a sound is heard, or reaches a microphone in an enclosed space, there are multiple sources. There is direct sound and reflected sound. Based on the difference in time arrival of the sound there is a delay in the reflected signal. Depending on the frequency of the sound and the timing difference, the two signals will either be in phase and add together or out of phase and cancel each other out. In theory, for a single frequency, this could cause complete cancelation or a doubling of the sound level. Reality is a bit different. What happens is called comb filtering where some frequencies are boosted and some are reduced. This results in a frequency graph that looks like a “Comb”. A PZM microphone placed on a large flat surface does not have this effect. So how noticeable is this effect? Let’s build some and go find out!

Step 3: The Capsule

The first thing we need to do is find a small full frequency microphone capsule. Panasonic used to make one, the WM-61A, which is now discontinued. Rumor has it that this capsule was used in many test microphones as it has a ruler flat frequency response. I found a company that makes a direct replacement, JLI Electronics. They list it as the JLI- 61A and it is all of $2 each, less if you buy 10. This capsule is key to the project. One of the other properties of a PZM microphone is a 6dB boost in level due to the pressure zone area that the capsule is in. If the capsule doesn’t have a flat frequency response, this gets augmented. I tried the PUI5024 capsule from the Sound Sleuth Instructable, which has a couple dB rise in the 10-13K range. That rise sounds good by itself but not when used in a PZM.

Step 4: Interfacing the Capsule

To interface the capsule to a recording device we have two options, a 3.5mm jack for small recorders and cameras, or an XLR jack for professional recorders. I built several of both. This capsule contains in internal FET so the electronics are the same as in the previous Instructable with one crucial change: The value of the resistor in the P48 version. The value is selected to match the FET internal to the capsule, usually by experimentation. This circuit was originally published by David McGriffy. Specifically for the Panasonic capsule. I caught up with him on this and asked him how he came up with it. He said he started from the data sheet that shows a single resistor and capacitor but for 2-10 VDC. He extrapolated out for 48 volt phantom power and stumbled on a what is arguably the simplest way to do this. He now runs a company VVAudio that does ambisonic sound field processing. His joke to me is that he is good with ones and zero’s but not so good with things in-between. The original circuit used a 147K resistor and that is what we are using with the JLI-61A replacement. I kept the capacitor value the same at 3.3uF. Anything between 1 and 10uF should work fine. It needs to be electrolytic and rated for above 48V.

I had multiple questions on how this worked after the Sound Sleuth build. Specifically two questions: “Why are you connecting the “+” on the capsule to Pin 3 on the XLR, which is the negative input” and, “I wired the XLR up and there is 48 volts on the resistor, I’m not connecting that to my capsule!”.

So here is how this all works. Most modern preamps that supply 48V phantom power do so through two 6.81K 1% resistors. The signal is coupled into the actual gain stage internally via two capacitors. The resistor internal to Pin 3 on the XLR and the external resistor bias the FET for proper operation. When a sound wave makes the diaphragm move in (positive pressure) it gets closer to the back plate which has a permanent charge. As this happens, a small voltage change is sensed by the gate of the internal FET. This causes the FET to conduct more. As that happens, the voltage on the source lead on the FET, which is connected to the 147K resistor, goes up. That increasing voltage is coupled into Pin 2 on the XLR and thus the Mic Preamp. Simultaneously the voltage on the drain, which is connected to the 6.81K resistor, via Pin 3 of the XLR goes down. Voila, we have the correct phase polarity feeding our Mic Preamp in what is the “simplest” way to do this. To see how to do the wiring for the XLR connector, see this Instructable. The only difference is the resistor value.

For the 3.5mm version we are using a better jack than I used in the Sound Sleuthers. I had an issue with a second batch of the Amazon ones I was previously using. A couple of them were slightly bent which caused an intermittent connection with my Sony recorder. I found these at Redco Audio. They are a little more expensive – less than $2 each but much higher quality. We are connecting the Red wire to both the Tip and Ring connection. This allows the audio to go to both the Left and Right on any device that records stereo.

Step 5: Build the PZM

Now that we have a capsule, we need to do two things. First connect it to some wire. I’m using the same wire that I used in the Sound Sleuthers instructable. Second, mount it just above a flat surface. For the flat surface, I chose a small piece of 1/8” thick Lexan. I bought a 12”X12” piece and then cut it into four 6” squares. They are lighter than the old Radio Shack one I had, and perfect for taping to a piano lid or setting on a table. Now, how to mount the capsule… I tried a couple things and then came up with small flat piece of wood with a hole drilled in it that is the same diameter as the capsule. Then I inserted the capsule face down, flush with the bottom, and held it in place with some glue. All the was left was to glue a small piece of cardstock to the bottom of the wood to provide spacing between the capsule and Lexan. I routed a groove in the wood to contain the wire and then held it in place with some glue. The capsule is .236” in diameter and I used a drill bit one size smaller than ¼” 15/64” which works out to .234”. My first one was a bit hard to press in. On the second one I lightly sanded the hole to make it easier to press in. I cut matching pieces of the ¼” wood for the top of the capsule holders. These were glued together with wood glue to create a ½” by ½” by 3” piece with the capsule flush with one edge. After the glue dried, I sanded the two side and top edges smooth. And, since I was curious as to what that would sound like, I connected the microphones to my recorder while sanding. You can hear this in the build video.

Once those were sanded, it was time to glue them onto the Lexan pieces, using the small piece of card stock as a spacer. It only covers half of the bottom to provide a gap for the sound to enter and create the pressure zone. I used E6000 glue as it would work with both wood and plastic. Clamping this while keeping everything aligned was a bit of a challenge. You need flat parallel surfaces so that things don’t slide around. And expect a bit of the glue to get out around the edges. Once dry, the mics are complete.

Step 6: Testing and Summary

PZM’s are used differently than most mics. You place them on larger flat surfaces to capture sound. Due to COVID I can’t get to a Piano as of this publication. I am working on that. However, I was able to get with the Orchestra Director from a local high school who graciously played some double bass for me. It is one with an extension so we got some really low notes. I recorded him with a pair of my TSB2555B condenser mics spaced about three feet out of the f-holes and simultaneously with a pair of these PZM’s about 10-12 feet away on the floor of the orchestra room. How did they sound? Quite good! You can hear this at the end of the build video

Summary:

These easily replicates the old Realistic PZM if not improving on it, especially when you consider you can build it to work with either and XLR connector or 3.5mm. Total cost is about $8-12 a microphone depending on how you build it. I am planning on building a few more but without the Lexan part. You could then directly attach them with some double sided tape to a wall, piano lid etc. These aren’t ideal for every recording situation. However they are a fantastic addition to your Mic Locker. What, you don’t have a Mic Locker? Well build a pair of these and start one today. If you are interested in microphone building and other things audio related you should join the mic builders group Thanks for reading this instructable. If you like cool sounds and recording subscribe to my YouTube channel: Sound Sleuth. Everything I record uses microphones I built and have or will feature in an Instructable.