Pressure Zone Microphones - History and other information

In 1982, Ron Wickersham and Edward 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 and a boundary. The invention requires that the diaphragm or major entry port of the transducer be oriented substantially parallel with and proximate to a boundary. A formula is disclosed by which the proximate relationships between the diaphragm and a boundary can be determined for any desired frequency range, which eliminates the discrimination between the frequency spectra of the direct and random incidence acoustical signals on the transducer while allowing the rejection, at the transducer, of unwanted higher frequency acoustical signals, by means of acoustical cancellation, which causes a filtering action without the undesirable effects of non-uniform phase or group delay caused by high order, sharp cut-off filters normally employed to reject these higher frequency signals. In the device, an acoustical-to-electrical transducer, having a uniform pressure versus frequency characteristic in the desired frequency range, is mounted with its diaphragm substantially parallel with and proximate to a surface forming part of a boundary and supported by a structure which is adjustable, or fixed during manufacture, to control the proximate distance between the diaphragm and its substantially parallel boundary. A surface connected to the adjustable or fixed structure, which forms part of a boundary, can also be used when it is only slightly raised from the boundary by shock mountings to mechanically isolate it from the boundary under which circumstances it functions as part of the larger substantially parallel boundary.

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 33-1080, 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, 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 (I think) 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.

Modifications Galore

Several people have published modifications for this microphone (models 33-1080, 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. But the puny output transformer remains, and it has no more magnetic shielding than your middle finger. It is common to hear people complaining of hum.

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.

I mention these other approaches to provide some history and background. Some time ago, around 1994, I got tired of all the hoopla about these mikes 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:

Get rid of the battery, run the thing from the P48 supply
Lower the distortion
Get rid of that nasty little transformer

I posted the circuit to one of the forums on Compuserve, and later put it on my own website. A kind soul on Compuserve (Ivan F.) did a PCB layout of my design and sent me a couple of panels of boards. At the time, RS was selling the -1080 and -1090 models, 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 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 original RS PZM, model 33-1090B.	The 33-3020 model Boundary Microphone.	The boundary microphone, the 33-3022 model.	The 33-3041 Business Microphone. (whatever that means.

The New RS PZM Board

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

With the 33-1080, 33-1090, 33-1090A, 33-1090B licensed products from Crown. These units have the tradename PZM marked on the boundary plate.
With the 33-3020 and 33-3022 boundary microphones.
With the 33-3041 business microphone.
With generic electret capsules, such as those made by Panasonic.
As a phantom-powered active direct box for sources requiring a very high input impedance.

The different circuit configurations are realized by altering what is stuffed on the PCB. It is not possible for one PCB configuration to accommodate all four applications. Thus, there are three different stuffing configurations 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.

I've 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. This is still not a job for the 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. (800k pdf) Sometime in 2006, I ran out of the original boards. I did a small amount of redesign, changed a few things, and added an optional LED that can serve as a power indicator. Of course, costs tend to keep going up, and so has my price. The semi-kit is $40USD postpaid to any US or Canadian address. Other destinations please inquire. I can accept payment via PayPal. It's 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. You get to do that part yourself. I will now begin offering assembly services and some of the ancilliary parts needed to complete the project.

In July 2006, member EU countries adopted a set of rules governing nearly anything electronic aimed at curtailing the presence of hazardous substances in landfills. These rules are known by their acronym RoHS (Rules of Hazardous Substances). For electronics, the primary banned substance is lead, which is a component in tin-lead solder. Not only must electronic equipment use lead-free solder, the components used must also be free of lead.

In the United States, California is on the edge of adopting similar legislation. It is just a matter of time for other states.

After July 2006, any kits that I ship to EU countries will be RoHS compliant. Originally I was not going to ship to the EU, but when I ran out of boards, I was able to have the new boards manufactured with a lead-free process. Getting the other components is fairly trivial, so I will be able to ship kits to EU countries again. I will not be including any solder with these kits. (FWIW, the RoHS components are compatible with tin-lead solder). 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. Regardless, I am making my attempt to comply with the directive.

NOTE: No license is granted with respect to this circuit. Permission is hereby granted to construct the circuit and use it for your own use. Permission to use this circuit in a commercial context (i.e. turn it into a product for sale) is not granted.

Even after all this work, remember the microphone's humble beginnings. It will never be a Neumann.

Yet More Stuff

Here are some pictures of the raw boards, completed boards, and a completed preamp. Remember that when you buy the kit, the cool metal case is not part of the deal. The metal case is made by Hammond (p/n 1590BBK) and it can be purchased pre-painted. In the picture of the completed unit, the output connector (larger) is on the left and the input connector (mini-xlr) is on the right.

One of the completed boards shows phoenix connector blocks mounted on the board. These are an alternate way to get the wiring off of the board. They are not included in the kit. We can supply them for $2 each if they are ordered with the kit.

Price List
The new kit manual (800k pdf)
The manual for the 2003 version (377k pdf).
Circuit description of my original mod|(which is close to but not exactly like the circuitry on the new circuit board)
Schematic of my original mod.
Christopher Hick's mod
Phil Rastocny's mod

Last modified 3/22/2008.