Great Sounding Homebrew Microphone For About 10 Bucks

When I built my AM transmitter I looked around for a nice broadcast quality microphone to use with it. After gasping at the prices, I decided to see what I could come up with for less (much less) money. I grabbed one of the cheap 2 lead PC condenser elements at a local Radio Shack and found that they actually sounded pretty decent. The biggest drawback was that they seemed to have a very boomy low end that made them difficult to work with. The solution to that was to use a pre-emphasis RC network similar to what is used in many FM transmitters to produce a flatter response and more vocal presence. The end result is what I have been using with most of my transmitters ever since. I get lots of unsolicited comments about how smooth and natural my audio sounds, and lots of laughs when I tell people what I'm using. If you are looking for a cheap way to improve your audio, go spend 10 bucks and give it a try. You can tell people it's a Heil PR-40 with a Symetrix 528E if you want. They'll probably believe you.

W1AEX - Electret Circuit


I build most of the circuit right on the XLR connector at the base of the microphone.

1


The microphone above installed and connected to the Flex 5000

3


There's nothing really critical about how you construct the circuit. Just make sure that it's mechanically stable and will fit into the microphone case you have on hand. In this project, the scavenged micophone enclosure was an old Peavey unit that was fairly slim, so I had to be careful about how the components were arranged to assure that the completed circuit would have clearance when inserted. Before I slid this assembly into the microphone case, I wrapped the components in black tape to prevent inadvertent shorts against the enclosure. In this particular microphone I added a 22k series resistor in the audio line to address the fact that the output of these microphones is far greater than most dynamic microphones. The extra resistance results in an output level that closely matches a dynamic microphone that I sometimes use with the intended rigs. After building several of these projects, I found that with my voice, a resistance of 4.7k in parallel with a .047uf capacitor was ideal for the pre-emphasis circuit. Eliminating the variable resistor makes construction a little simpler, but I would encourage you to use the variable until you determine the best combination of values for your own voice. There is no need to use shielded audio cable between the electret element and the pre-emphasis circuit. In fact, I ended up removing the shielded audio cable shown in the picture and replaced it with two lengths of very flexible #18 stranded wire lightly twisted together. The shielded audio cable was problematic because it was too stiff and put too much stress on the solder connections at both ends when it was jammed into the microphone case. In my station, these microphones have always proven to be highly resistant to RF feedback as long as the assembly is mounted in a metal case such as the one above.

So... once you have all the parts on hand, it takes about 15 minutes to put one of these together. For less than 10 bucks, and about 15 minutes of effort, you end up with a great sounding microphone that should last for many years! At the links below are some audio samples of a few of these microphones being used with various rigs at my station. Decide for yourself if you think one of them might fit your needs.

Back in 1989 someone recorded my signal on 75 meters while I was using the 10 dollar microphone with my 4-400 AM rig. The sound clip ended up in the AM audio vault at the AMfone web site. I never found out who made the recording, but they had a terrific receive setup. Listen for yourself and see how the 10 dollar project sounds to your ears. (Note: At the time, my callsign was WB1AEX.)

75 Meter AM QSO (4-400 rig) from 1989 (0.38 meg mp3 file)

During the summer of 2009 I resurrected one of my AM rigs and once again relied upon the same electret element microphone design. I get excellent reports with this setup in spite of the small investment in the audio system. The microphone is run directly into my computer soundcard where the audio is processed with free software called Voice Shaper. The audio output of the soundcard is then run through a DBX 166A compressor limiter to a solid state amplifer that drives the grids of a pair of 811A tubes. This audio clip was recorded by Bill, KC2IFR.

75 Meter AM QSO (Viking III rig) from 2009 (0.59 meg mp3 file)

The clip below was recorded by Dave, W2VW and demonstrates the homebrew "cheap microphone" as it is being used with my Kenwood TS-440S in the AM mode. The TS-440S was modified by Dave for optimum performance in the AM mode. A Collins 8kc filter was installed in the wide position of the transmit - receive 455kc IF chain, the ALC problem was eliminated, and a balanced line audio input that runs through a 1:1 low impedance tranformer straight into the AN612 balanced modulator was added to the back panel. The simple audio chain for the Kenwood starts with the cheap electret microphone project that runs into my station PC soundcard, with the audio processed by the Voice Shaper program, which is then run from the soundcard output through an old JBL 7110 compressor/limiter and then into the balanced line input of the Kenwood. The TS-440S is driving my AL-80B (single 3-500) amplifier with about 15 watts of AM carrier to produce about 135 watts of AM carrier output.

The SSB audio clip below was made by Eric - WB2CAU on 75 meters in the summer of 2010. He has a very nice Flex 1000 SDR station which is capable of making high quality recordings. The microphone is the very same 10 dollar microphone that I put together back in the early 80's. The conditions were quite good for a summer morning session and signals were fairly consistent. The transmitter I am using here is a Flex 5000A and the microphone is patched directly into the front panel mic jack.

75 Meter SSB QSO (Flex 5000A) Summer 2010 (0.18 meg mp3 file)

The audio sample below was recorded by Jeff - W2NBC as I was using the Flex 5000A on 75 meter AM. The homebrew electret is plugged directly into the front panel jack of the Flex and all the audio processing is being handled by Power SDR. The microphone exhibits smooth response across the range needed for natural sounding voice reproduction.


Transmit audio in ham radio is entirely subjective, but spending 10 bucks for a great sounding microphone is priceless! There is a fair amount of tonal adjustment available with the 10k variable resistor, so it is very simple to make the microphone sound very different to suit your taste. Using a .01uf capacitor instead of the .05uf will also have a big effect on the frequency response as well. It's worth playing around with the component values to find what suits your own voice. Good luck!


Radio Shack Parts List
:

10K-Ohm 15-Turn Cermet Potentiometer/Trimmer   Model: 271-343  |  Catalog #: 271-343    $2.69

PC-Mount condenser Microphone Element    Model: 270-090  |  Catalog #: 270-090     $2.79

0.047µF 50V 10% PC-Mount Capacitor    Model: 272-1068  |  Catalog #: 272-1068    $1.49

1.0µF 35V 20% Dipped Tantalum Capacitor    Model:  272-1434  | Catalog #: 272-1434   $1.79

10µF 16V 20% Dipped Tantalum Capacitor   Model:  272-1436  | Catalog #: 272-1436   $1.79