Pressure Zone Microphones - History and other information

In 1982, Ron Wickersham and Ed Long were granted US Patent 4,361,736, "Pressure recording process and device."

A process for transducing acoustical signals, including music, speech, noise, etc. which embodies a method which eliminates the usual discrimination between the frequency spectra of the direct and random incidence acoustical components of the sound being transduced and allows the frequency range of interest to be controlled by adjusting the spacing between the diaphragm or major entry port of an acoustical to electrical transducer

In short, the big benefit was the elimination of comb filtering caused by sound arriving via reflections from nearby surfaces and sound arriving directly. Ken Wahrenbrock licensed this idea and produced the first commercially available microphones using this idea. Eventually Crown International (you know, the guys who brought you the DC300 amplifier) purchased the trademark and license and they produce microphones using the Pressure Recording Principle to this day.

You'll note that the patent expired in 1999, however Crown still owns the trademarked terms, "PZM and Pressure Zone Microphone."

The Radio Shack™ PZM™ Microphone

Many years ago, Radio Shack (RS) sold a Pressure Zone Microphone, models 33-1090, 33-1090A, 33-1090B under license from Crown International. These were an inexpensive alternative to the Crown product and worked OK. Not great, but OK. Oddly, RS told people in the manual that by substituting a higher voltage battery they could improve the performance of this modest microphone. Many tapers have gone this route, and while it does improve performance, there are still a few limiting factors: the puny output transformer, the non-standard output impedance, and the battery.

Some time later, RS decided (or perhaps Crown decided for them) to no longer sell the PZM microphone and came up with a replacement that they called a Boundary Microphone (getting past the trademark), model 33-3020. Unlike the PZM which uses an omni capsule mounted a fraction of an inch away from a boundary surface, the RS Boundary microphone uses a microphone capsule mounted 90 degrees to and adjacent to a boundary surface, much like the EV mike mouse. This model suffered the same limitations as the original unit AND it was wired differently. Later yet, RS replaced that model with yet another, model 330-3022. This too is called a Boundary Microphone and looks totally different than the previous unit. Unlike the #1 and #2 units, this had no output transformer. Instead it had an output choke, whose purpose is to short out any electret microphone bias present at the input, as it would be if the microphone were plugged into a consumer camcorder.

Sometime around 2008, RS dropped the 33-3022 unit and introduced their 33-3041 Business Microphone. This is not a boundary microphone nor is it a PZM. It is an omni electret capsule mounted so it is sticking up in the air. If you snip off the battery box, it conforms to the Configuration 2 wiring. RS also sells a boundary microphone made by Audio Technica. The information on their website is insufficient to even infer any details.

The 33-1080 Microphone

I originally thought this was a PZM, and the RS website would lead you to believe that. I came to find out that the 33-1080 is NOT a PZM, it's a conventional probe-style cardioid microphone. Here's the datasheet for the 33-1080. A special thanks to MichaelD in the UK for bringing this to my attention.

Modifications Galore

Several people have published modifications for this microphone (models 33-1090, 330-3020). All are aimed at making the things compatible with standard professional low-impedance balanced inputs, with or without phantom power. The easy modification is cutting the 1/4" phone plug off and rewiring the leads to a male XLR connector. The output of the first two models was balanced by virtue of the output transformer. Do this and add the 12V battery and you've already improved things. It still doesn't know a thing about phantom powering. The puny output transformer remains, and it has no more magnetic shielding than your middle finger (and about as much iron). It is common to hear people complaining of hum.

In the 1970s Phil Rastocny published a modification that has gotten more than a bit of attention. It is probably the origins of the "put a bigger battery in" modifications. Phil's mod changes the battery to 9V and raises the value of the load resistor to 2k2. He also gives the nasty little transformer the heave-ho. When you're done, you have a fairly vanilla electret microphone with an unbalanced output, unsuitable for connection to a balanced microphone input.

Christopher Hicks published a clever circuit that lets the microphone drive a balanced input. Chris' circuit puts the microphone element across the phantom powered input and uses a pair of PNP transistors running as emitter followers to drive the line. My objection to this circuit is its total dependence on the CMRR of the balanced input as neither side of the microphone element is grounded; you must replace the cable with 2C shielded twisted pair. That means that you get to solder the new wires to the microphone element, which is risky business at best, unless you have mastered the Art of Soldering.

I mention all of these other approaches to provide some history and background. Some time ago, around 1994, I got tired of all the hoopla about these mikes, these less-than-complete modifications, and tired about endless questions about operating them in a P48 environment. I designed my own interface for these microphones that was to end the problem and make a definitive step forward. The goals were straightforward:

I posted the circuit to one of the forums on Compuserve, and later put it on my own website. A kind soul on Compuserve (IvanF) did a PCB layout of my design and sent me a couple of panels of boards. At the time, RS was selling the -1090 model, which ran the capsule "upside down" both physically and electrically.

When RS introduced the newer -3020 model, I bought one and found that it was electrically incompatible with my design because this one ran the FET in the microphone as a common source amplifier, whereas the older -1080 and -1090 models ran the FET as a source follower. I intended to find a way to use my circuit boards with the newer model, but as these things go, I never got around to it, and when I finally did, RS had replaced the -3020 with the -3022 model. A look at the -3022 model revealed that this model was very different, and the modifications of days past were no longer applicable. About this time, I started getting a few orders for the board (I made a kit of parts available via the web) and I ran out of the original boards. This give me a reason to redesign the board and to make it compatible with all models.

Someone once said that a picture is worth a thousand words, so here are some pictures to help you identify the different models.

The New RS PZM Board

The new circuit differs from that previously shown on the website. The new circuit can be used:

The different circuit configurations are realized by 2 jumper wires. There is one stuffing arrangement to accommodate the four applications (the -3020/3022 microphones and the generic electret capsule take the came configuration).

The transformer inside the -1090 battery box loses about 3 dB of signal (by design), so when this microphone is modified, the output signal is about 9 dB hotter than it was before. The small size of the transformer also limits the low-frequency response; eliminating it removes the rolloff associated with the transformer and it's contribution to the harmonic distortion performance. If that isn't enough, the transformer is magnetically unshielded making it a very effective hum magnet.

I put together a semi-kit of parts with the new board and all of the electronic components needed. There's instructions now, and I even include some small-diameter solder (USA only). This is still not a job for a rank amateur, and if you don't know the business end of a soldering iron from a turkey baster maybe this isn't for you. The instructions and pictures can be found here. (2800k pdf) end The semi-kit is $75USD postpaid to any USA address. Other destinations are the same price, but postage is extra; please inquire first. I can accept payment via PayPal.

It's called a semi-kit because it includes everything that you need to assemble the printed circuit board but does not include a case, or the connectors. See the price list for other options. It does include:

The case needs to be made of metal, for shielding reasons. All of the holes can be drilled, although the 3/4" hole for the large XLR connector can be difficult. I use a Greenlee chassis punch. You can also use a Unibit stepped drill bit. Yes, you can get 3/4" drill bits, but even with a drill press, they are unwieldy and even a bit dangerous. NOT recommended.

Even after all this work, remember the microphone's humble beginnings. It will never ever be a Neumann, but it will be substantially better than it ever was. Refer to the audio samples referenced in the menu at the beginning of this webpage.

RoHS

The kit is considered a repair component because it extends the life of a non-compliant article that was placed on the market prior to 1 July 2006. By Article 2 Section 3 of the Directive, repair parts are exempt. If you are hassled by customs because of RoHS, then point out that this is a repair part, which is exempt. Regardless, I am making my attempt to comply because nearly all of the components are RoHS compliant. I do not include solder when shipping outside of North America.

Pressure Zone Microphones - History and other information

The Radio Shack™ PZM™ Microphone

The 33-1080 Microphone

Modifications Galore

The New RS PZM Board

RoHS

License

Here's What You Need

Price List and Ordering Info

Yet More Stuff