December 2003 1

By Sam F. Kennedy Jr, KT4QW

With modern commercial amateurtransceivers becoming increas-ingly difficult to work on, most

ham builders have turned to other endeav-ors. Looking around for a fresh project,and one that had a reasonable chance ofsuccess, I decided to build a “scratch-built”condenser microphone. One of my friendshad been working on piezoelectrichomebrew microphones, so the condensermicrophone seemed like a good choice. Iknow that this is 1920s technology and thatyou can buy a very good microphone rea-sonably, but it is interesting, challengingand educational to retrace some of the de-velopment processes of those “times goneby.” I set the following objectives for mynew homebrew microphone:• Sound as good or better than the Heil

Goldline microphone• Work directly into a transceiver with-

out external equalization or process-ing

• Require no special tools to fabricate• Use no manufactured microphone parts• Use no special materials—only those

found in the “junk box” or local store• Be attractive and complementary to my

station• Be inexpensive

I’ll not bore you with all the things Itried that didn’t work—I will simply ex-plain the final model that met all my ob-jectives. The final product, pictured here,is the result—a condenser microphonewith a very large diaphragm (1.6 inchdiameter). The coarse screen mesh“cricket cage” serves as a combinationwindscreen and Faraday shield for theelement. Adequate shielding is very im-portant to ensure that no RF enters theaudio stream. Because of the high gainrequired to amplify a condenser element,RFI can be a problem, so I shielded allthe components thoroughly.

A HomebrewCondenser MicrophoneBuild a condenser microphone that will performlike the costly commercial units…plus, you won’tneed an expensive equalizer. Sounds good!

Some Basic TheoryCondenser microphones use a thin

lightweight conducting membrane as a dia-phragm and a fixed plate closely spacedbehind it. The two facing surfaces becomethe plates of an air dielectric condenser (orcapacitor if you prefer). Sound pressureagainst the thin membrane causes it tomove. This movement changes the spac-ing between the plates and therefore thecapacitance. When the condenser is polar-ized (charged or biased) with an externalvoltage, it causes a changing electricaloutput proportional to the sound pressure.The source impedance of the condenserelement is very high, several megohmsbeing typical. A built-in field effect tran-

sistor furnishes the impedance step-downnecessary to deal with input to typicalsolid-state amplifiers. The FET has thecapability of accepting a very high imped-ance input and producing a reasonably lowoutput impedance of 1-2 kΩ, while pro-ducing moderate gain.1

When considering commercial studiotype condenser microphones, “phantompower” is the term normally used for anexternally supplied bias or polarizing volt-age. A microphone of this type is said tobe a “pure condenser” type as contrastedwith an “electret condenser” microphone,which requires no external polarizing volt-age.2 The homebrew microphone under1Notes appear on page 00.

Figure 1—The microphone amplifier schematic together with the parts required forconstruction. Note that the polarizing or bias battery is fabricated from multiple cells,as described in the text. R1 and R2, the 10 MΩΩΩΩΩ resistors, can be made of two 4.7 MΩΩΩΩΩ,1/4 W resistors if 10 MΩΩΩΩΩ resistors prove difficult to locate. C1 is used to keep themicrophone polarizing voltage from appearing at the FET gate and C2 is used tobypass the FET source resistor. M1 is the microphone element and its construction isdescribed in the text.

B1—48 V battery (see text).C1, C2—0.1 µF, 50 V capacitor.M1—Fabricated element (see text).

Q1—MPF-102 JFET, RadioShack276-2062.

R1, R2—10 MΩΩΩΩΩ, 1/4 W resistor.R3—150 ΩΩΩΩΩ, 1/4 W resistor.

2 December 2003

Figure 2—The overallfrequency responseof the homebrewcondensermicrophone astransmitted by anICOM IC-756PRO andreceived by a Ten-TecPegasus transceiver.Note that the fre-quency responseranges from 100 Hzto 2750 Hz. The humpat the low frequencyend is a typical ICOMtransmitter charac-teristic. See editor’snote in text.

Figure 5—The assembled microphone element.Figure 4—The main parts of the microphone element.

Figure 3—The microphone element mechanical details and basic construction steps.More detail can be found in the text.

discussion uses a polarizing voltage de-rived from a very small internal 48 V bat-tery mounted within the microphonehousing. It does not use an external phan-tom power system. As is the case with anelectret microphone, it does require asmall voltage for the FET impedancetransforming circuit inside the micro-phone. And, as in many electret micro-phones, the FET’s drain load resistor andvoltage source are located downstream, inthe amplifier. No specific power supplyis therefore necessary when operating thismicrophone with an electret-compatiblecircuit, such as that used by ICOM. As anexample, this operating mode providesadequate microphone drive to satisfy theIC-756PRO transceiver. Figure 1 is theschematic of the microphone amplifierand the parts required.

In critical sound applications, the con-denser microphone is often preferred forits uniform frequency response and itsaccurate response to transient sounds.The natural design of a condenser micro-phone ensures an outstanding low-fre-quency response, and the low mass andhigh tension of the diaphragm allows asmooth high-frequency response. Thisresults in a clean, natural and clear sound.It is said to produce outstanding “trans-parency and detail.” These characteristicsmake the condenser microphone a natu-ral choice for professional use or otherdemanding applications.

Some manufacturers produce con-denser microphones with diaphragms thatare less than 1/10,000th of an inch thick.For amateur service use, not all of thesecharacteristics are relevant. Since mostamateur transmitters roll off most audiofrequencies above 3 kHz, the high fre-quency characteristic of the microphoneis not overly important. Condenser mi-crophone design does provide the oppor-tunity to shape the audio response withoutthe use of external equalizers and proces-sors. In the case of the homebrew micro-

1. The microphone base is made of around disk of 1/16 inch double-sided PCboard.

2. The spacer ring is made of 1/8 inchbrazing rod, bent into a 1.6 inchdiameter circle and soldered tosurface of the base.

3. The diaphragm is 0.001 inch aluminumfoil (lightweight household foil).

4. The condenser plate (inner) consists of1/16 inch double side PC board andpositioned 0.005 inch below thediaphragm.

5. The diaphragm is attached to spacerring with epoxy cement. Stretch foil astightly as possible without tearing.

6. Flexible wire is connected to front surface ofthe condenser plate. Connects to the inputcoupling capacitor and to the JFETtransistor and the polarizing voltage (bias)resistor.

7. Epoxy mix holds inner condenser platein position.

8. Dress hooks soldered in place forelastic band suspension of themicrophone element (4 hooks solderedwith 90° spacing around elementperimeter).

9. Foam rubber doughnut (see text).10. Pour hole, 1/4 inch, for pouring epoxy

mix into cavity to hold the condenserback plate in position.

December 2003 3

phone under discussion, a very large dia-phragm (1.6 inches) is made of 0.001 inchthickness aluminum foil. The large dia-phragm increases the low frequency re-sponse and the thickness of the materialdoes limit the high frequency response,but the microphone operates very wellwithin the frequency range we are con-cerned with.

Transmitted Spectral ContentUsing a HamAlyzer3 FFT audio spec-

trum analyzer on my notebook computer,I was able to test for frequency responseand to “tweak” the microphone to achievebest results. Figure 2 shows a HamAlyzertrace of the microphone’s over-the-airresponse when used with my ICOMIC-756PRO transceiver. This trace wascaptured by W5TOM, using his Ten-TecPegasus.

[Editor’s Note: A bit of caution here.The HamAlyzer records a system spectralresponse that includes both the transmit-ter modulation and the receiver demodu-lation characteristics. If that measurementis taken by a distant station, it will alsoinclude any RF path characteristics thataffect the transmit/receive audio frequencyresponse. It includes, too, the sound cardcharacteristics of the particular computerused, unless that is taken into account inthe calibration procedure. Additionally, theoutput impedance variations of the micro-phone over frequency can contribute toaudio “coloring” with a particular trans-ceiver. And, the sonic characteristics of theroom will further influence the measure-ment results. For accurate objective spec-tral response measurements of a mi-crophone, an audio spectrum analyzer withan accurate audio frequency transducerdriven by a calibrated audio sweep gen-erator is required. The microphone shouldbe tested in an anechoic chamber (a testchamber free of echoes and reverberation).Commercial microphone manufacturers doextensive measurements on their products,using sophisticated audio test equipmentin laboratory environments. While relativebasic measurements can be interesting, thefact that a microphone “sounds” good orbad with one transceiver may, in fact, bemisleading, unless accurate testing is doneunder carefully controlled and calibratedconditions.]

After extensive testing, I discoveredthat this microphone had the best “pres-ence” and overall sound quality when theelement was suspended in air with noenclosure other than the element itself.It produces a cardioid pattern over thewhole frequency range, but the pattern isless pronounced at low frequencies. Itdoes pick up room noise and it shows less“proximity” effect than other micro-

phones I have used. I did not concernmyself with frequencies above 4 kHz, inthat my transmitter cuts off everythingabove about 3 kHz anyway.

Building the ElementBy referring to Figures 3, 4 and 5 you

can see that the element is constructedby building four main subassemblies,then combining them. These are:

1. The base plate, a circular, 1.7 inchdiameter disk of double sided 1/16 inchprinted circuit board. I used a school-typecompass for layout and sawed the baseplate out using a small jigsaw. Note that a1/8 inch hole is drilled into this part. Thisserves two purposes. One is to furnish anexit path for the wire that connects to theinner condenser plate and the second is torelieve some of the acoustical back-pres-sure. It is important that the element isvented to atmospheric pressure to avoid aninternal pressure differential and thus dis-tort the diaphragm. The hole in the centerof the back plate is 1/4 inch in diameter.This hole is for pouring in epoxy behindthe condenser back plate and is filled inlater.

2. A 1.6 inch diameter brass ring ismade of 1/8 inch brazing rod. It is formedinto a circle by wrapping it around a 3/4

inch schedule 40 PVC pipe couplingwhich is clamped in a vise. The couplingis only 1.3 inch OD, but the rod springs

back to form a somewhat larger circle. Itis formed around the coupling for abouta turn and a half and then adjusted so thatit forms a 1.6 inch circle. The brazing rodbehaves much better if you anneal it byheating it with a propane torch to discol-oration and then letting it air cool. Afterthe bending operation, clamp the loop ina vise and cut the both ends at the sametime with a hacksaw. Solder it togetherbeing very careful to align the two endsto ensure a smooth circle.

3. The back plate of the condenser is acircular 1.25 inch diameter disk made ofdouble sided 1/16 inch printed circuit board.The layout for this was also done with asimple school-type compass and then theback plate was sawed out using a small jig-saw. Since most of the diaphragm motionis in the center, there is very little advan-tage realized by using all the outer spaceunder the diaphragm. The 1.25 inch diam-eter leaves plenty of room around theedges for assembly and alignment.

4. A foam rubber doughnut the sameOD as the back plate is required. Whilenot critical, the doughnut’s center holeshould leave about 1/4 inch of foam andthe height should be about 1/4 inch. Thiswill allow it to be compressed to approxi-mately 1/8 inch in the final assembly. Thisdoughnut, or “O-ring,” serves to containthe epoxy poured in from the back andproduces pressure on the back plate so

Figure 6—Applyingthe aluminum foildiaphragm to theelement using ahomebrew testfixture. Notice thebead of epoxy cementaround the edge ofthe spacer ring. Thediaphragm is held inthis position until theglue dries fast. It isthen trimmed, closeto the ring.

4 December 2003

Figure 7—The 48 V polarizing (bias)battery assembly and parts list.2 each, 12-V packs (RadioShack Keyless

Entry System, RS 23-279 [GP27A]. Eachpack contains two batteries for a total offour).

1 each, fiberglass board, 1.25×1.75×1/16

inches.1 each, Velcro patch 3/4×3/4 inch. Apply to

back of board for mounting.MiscShort length of very small gauge

stranded hookup wire for interconnectsand external leads.

Small quantity of 5 minute epoxy to affixthe batteries to the board and to clearcoat the finished unit for insulation.

Small section (0.5 inch) of shrink tubingto contain the two battery wires.

Figure 8—Anexample of atypical microphonehousing, with partsand fixtures.

Figure 9—The dimensions of the microphone housing example. Note that the elementis suspended by hooks and elastic.

that it will align with the brass ring whenclamped to a hard smooth surface.

Smooth the edges and the flat surfaceswith 400 grit wet-dry sandpaper. Place thesandpaper on a firm, flat surface and lapthe surfaces carefully to make certain theyare flat and have no rough spots or burrs.

The following steps are necessary toassemble the element:

1. Solder a small, flexible insulatedwire to the condenser back plate. This con-nection must be made to the front surfaceof the disk in such a manner so as to avoiddisturbing the flatness of the front surface.I drilled a 1/16 inch hole from the back butnot all the way through the front copper.Then press a dimple in the copper over thehole and drill a hole through the copperlarge enough to terminate the small con-necting wire. Carefully sand the surfaceto make certain the solder does not pro-trude above the back plate front surface.

2. Solder the brass ring to the base plate(large disk) in three places. Use a largesoldering iron and a minimum amount ofsolder and make sure that the solder flowsto both ring and plate. See Figure 6 for theproper position of the ring.

3. The most critical operation of theentire project is mounting the condenserback plate so that it is 0.005 inch belowthe plane of the lip formed by the brassring. This is done by temporarily pastinga piece of heavy paper (0.004 to 0.005 inchthick) onto the front surface of the con-denser back plate. This shims the surfaceof the plate a controlled distance behindthe plane of the lip. When both surfacesare pressed firmly onto a flat surface and

glued in place, the proper spacing will havebeen established. The objective of thisoperation is to establish the minimumspacing possible while ensuring enoughclearance, to avoid shorting of the con-denser plates when stimulated with highsound pressure levels. The closer the spac-ing, the higher the microphone’s outputlevel and the better the signal to noise ra-tio. [A higher polarizing (bias) voltagemight work to advantage here.—Ed.]

Application of the DiaphragmInitially, I had difficulty applying the

aluminum foil diaphragm to the base ringof the element. Figure 6 illustrates a fix-ture I built to make the task easy. Remem-ber that the foil must be tensioned to themaximum extent possible without burst-ing. The following procedures will makethis critical operation easy. Note that it isnot essential to use a special fixture. A drillpress that can be locked into position or asimilar device will work equally well.

The fixture allows a larger than neededpiece of foil (lightweight aluminum foilfrom the kitchen) to be held smoothly andtightly while the element is pressed into

December 2003 5

i t . A large-mouth plastic jar with athreaded top is used to hold the foil. Thecenter portion of the jar top is removedwith a sharp pocket knife. With a little careand experimentation, it becomes a verysimple operation to form a “drum head”with the jar. The foil need not be tensioned,just clamped into place smoothly with noripples. Lubricate the threads on the jar andthe lid with a very light application of non-staining oil or petrolatum. This makes itmuch easier to get the foil in place. Makecertain that the lubricant does not get onthe foil in the center area or it will inter-fere with the cementing operation.

Once the element is pressed into thecenter of the “drum head,” apply a lightcoat of epoxy cement around the edgewith a small brush. Just a few bristles ofan old brush held together with a pieceof masking tape works fine and can bediscarded when finished. Allow the ep-oxy to set (I used fast set epoxy but youmay need more working time to avoidrushing) and then remove the excess foilby simply rough-cutting the excess foilfrom around the element. When it’s re-moved, carefully trim the edges, beingcareful not to injure the tensioned dia-phragm. Verify that the foil has a stableelectrical contact with the ground portionof the element and that it does not makecontact with the back plate of the con-denser.

Polarizing (Bias) BatteryThe 48 V battery assembly is made up

of four 12 V batteries wired in series andmounted on a small fiberglass board.Since the condenser microphone uses thisvoltage to furnish only element bias, thereis no current drawn from the battery andyou should expect many years of full out-put potential—essentially the normalshelf life of the battery. The internal bat-tery was used rather than an externalpower supply for economy and simplic-ity. Figure 7 shows the wired battery as-sembly.

The Microphone HousingBecause everyone has different me-

chanical capabilities and facilities at theirdisposal, I have not attempted to furnishexact mechanical details for constructionof a housing. A number of satisfactorydesigns are possible. The housing shownin Figure 8 was constructed by W5TOM,using no more than simple hand tools andequipment. Figure 9 gives the overall di-mensions for the housing. While I didn’tintend to furnish exact construction de-tails for the microphone housing, thesephotos and notes should provide somebasic ideas for the construction tech-niques used to make a suitable housing.Since I use a boom microphone, my con-struction followed that path. A very nicedesk mount could probably be con-structed without much difficulty. So, sitback and enjoy your new condenser mi-crophone—you’ll be proud to say, “Themic here is totally homebrew!”

Notes1Editor’s Note: The condenser microphone was

invented by Dr Edward C. Wente, of Bell Labo-ratories, in 1916, with a patent granted onMarch 16, 1920. The condenser element is acurrent source, with the current dependentupon the rate of change of capacitance (bysound pressure) multiplied by the applied biasvoltage, which is a constant (Q = C × V, sod[Q]/dt = I = d[C]/dt × V). Sensitivity doesdepend on bias voltage and some commer-cial units run the bias voltage as high as 200V dc, depending on diaphragm size, materialand structure. The high impedance currentsource is converted to a voltage by a biasresistor and an impedance converter, whichcan be a FET or a vacuum tube.

2Editor’s Note: That polarizing voltage comesfrom a permanently charged electret “battery”which is part of the microphone element. Anelectret is a solid dielectric material that hasa near-permanent electrostatic charge. Elec-trets are similar to permanent magnets, butthey are electrostatic rather than magnetic.They are made by heating certain dielectricmaterials (special plastics, Teflon com-pounds and waxes) and then letting them coolwhile they are in a strong electric field.

3www.hamalyzer.com.All photos by the author.

Sam Kennedy, KT4QW, has been interestedin radio since the age of 7 (65 years ago) butwas actually first licensed in the 1950s asK4DEP. Relicensed in 1996,when he was as-signed his current call, he earned theAmateur Extra ticket shortly thereafter. Samhas attended both commercial and US Navyelectronics schools and has worked with mili-tary radio, radar and navigation equipment.He enjoys the technical aspects of ham radioand, as can be seen from this article, he hasa special interest in microphones. You cancontact him at 57 Huxley Pl, Newport News,VA 23606 or at kt4qw@arrl.net.

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