RS PZM History Other Mods Photo Gallery Uneeda Audio's PZM Preamp Kit

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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, 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.

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.

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:

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 -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.

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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

Kit manual (1.3MB pdf) Prices and Ordering info Photos Troubleshooting Tips

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

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.


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.

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 (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.


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. Uneeda Audio — PZM Modification Price List

Price List and Ordering Info

Be sure to refresh/reload this page to ensure that you're getting the latest version.

These prices apply to the US, Canada, and export. Within the US and Canada, most prices include first-class postage, except as noted. Export prices (aside from Canada) are exclusive of postage. I'll advise your options at the time of order inquiry. Please be sure to tell me where I'll be shipping to. For small parcels, international first class is affordable and my usual choice.

If you must have package tracking, it will cost a great deal more. All international destinations (including Canada) ship via USPS only. The rest are far too complicated.

USPS now has tracking by default for domestic mail.

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 10% 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 can supply completely assembled preamps, made to your order. This usually takes about 4-weeks. See the price list for more details.

To order:
image linked to My PayPal address is PayPal.Me/rickchinn PayPal.Me requires that you have a paypal account (like any other paypal transaction). You can use your credit card this way, OR I can send you a PayPal invoice that you can pay with a credit or debit card without having an account. My paypal address is NOT my address.

Prices effective May 2017
Postage costs are steadily increasing, especially for international mail, and I must pass this cost on to my customers.

The kit components are largely RoHS compliant, but since the kit is considered a repair part, it is exempt from RoHS rules.

Description Part Number Price (USD) US, postpaid (USD), Canada, UK, W. Europe, postpaid (USD) non-RoHS, elsewhere (plus postage)
Semi kit, configs 1 & 2. Includes the bare PCB, pcb mounting hardware, all resistors, capacitors, ferrite beads, transistors needed. pzm-sk12 $50 $60 $50
Semi kit, config 3. Includes the bare PCB, pcb mounting hardware, all resistors, capacitors, ferrite beads, transistors needed. pzm-sk3 $52 $62 $52
Assembled and tested circuit board, (any config) pzm-assy $115 $125 $115
Assembled and tested preamp, complete, includes TA4F for microphone. Delivery typically 4 weeks. I will attach the TA4F to your microphone at no extra charge. if you send the microphone to me. pzm-complete $195 $210 $195
3-pin Phoenix connector blocks (see photo), Phoenix p/n MPT 0,5/3-2,54 pzm-phoenix $7 per pair, when ordered with the kit.
mini-xlr mated pair (TA4F<cable> + TB4M<chassis>) xlr-minipair $15 when ordered with the kit.
All XLR connectors, TA4F, TB4M, D3M, with mounting hardware. xlr-all $20 when ordered with the kit.
Project box only, undrilled box-u $20 $24 $20
Project box only, with 3/4" hole for XLR connector punched, no other holes. box-d1 $22 $26 $22
Project box only, drilled box-d $60 $65 $60
Printed manual manual $3 when ordered with the kit. You can download and print your own for nothing via this link.
(1). The kit components are largely RoHS compliant, but since the kit is considered a repair part, it is exempt from RoHS rules.

Customers in countries outside of the USA are responsible for any and all applicable taxes, duties and RoHS issues.
Washington State customers must pay sales tax.

Harmonized TC: 8518.90.80, 851890

Last modified 05/13/2017.

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

Click the pictures to enlarge them.

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The 2012 (latest) revision for the Revision C PCB. 
Troubleshooting Tips 
Christopher Hick's mod 
Phil Rastocny's mod 
Back to Uneeda Audio 

The construction manuals for Revisions A and B are available upon request.

Copyright 2003-present by Rick Chinn. All rights reserved.

Last modified 05/13/2017.