Pressure Zone Microphones - History and other information

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

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

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

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 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:

• Getrid of the battery, run the thing from the P48 supply
• Lower the distortion
• Get rid of that nasty little transformer to improve the bass response

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. 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.

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. (1200k pdf)   The semi-kit is $40USD postpaid to any US or Canadian address. $45USD to the UK and western Europe. Other destinations please inquire. 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.

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 unless you have a drill press, they are unwieldy and even a bit dangerous.

NOTE: No license is granted with respect to this circuit. Permission is hereby granted for you 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. Purchasing a kit from me grants you a per-instance license to construct/use/sell the item using that instance of this circuit.

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

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..

For more information, refer to this link (courtesy of Newark Electronics) and this website's RoHS Page.

July 2011.
The LM394 dual transistor used in the original design has been obsoleted by National Semiconductor. It was a unique enough part that it was never second sourced. I stockpiled a small quantity of RoHS compliant LM394s and they're now gone. If you can find them, they're priced to stun. Accordingly, kits shipped to the EU will be RoHS compliant with the exception of the LM394. Analog Devices says they have something comparable in the works (MAT-12), but it has not even sampled yet. Once again, since the kit is used to maintain the usefulness of a non-RoHS product (by keeping it out of the landfill), the kit is considered a repair part, and as such, is exempt from RoHS rules.

The same discussion applies to pre-assembled kits that are shipped to the EU.

License

Price List and Ordering Info

These prices apply to the US, Canada, and Western Europe. Other destinations please inquire. Within the US and Canada, prices include first-class postage. European prices include postage for international first class mail. If you need package tracking, it will cost a great deal more. International destinations (including Canada) ship via USPS only. The rest are far too complicated.

Washington State residents must pay retail sales tax, which is based on your place of residence. In the absence of this knowledge, I will charge 9.5% sales tax (yeah, it's high, but we don't have a state income tax), which is the rate in effect in Sammamish, WA.

For a price, I will supply completely assembled preamps, made to your order. This usually takes about 4-weeks. See the price list for more details.

To order:
The PayPal address will be sent upon receipt of your inquiry.

Last modified 06/11/2013.

Yet More Stuff

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. They are $2 each if they are ordered with the kit.

Click the pictures to enlarge them.

The 2012 (latest) revision for the Revision C PCB. 
The 2006 kit manual (Rev B PCB, 800kB pdf) 
The manual for the 2003 version (Rev A PCB, 377k pdf). 
Troubleshooting Tips 
Christopher Hick's mod 
Phil Rastocny's mod 
Back to Uneeda Audio 

Last modified 06/11/2013.