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.
I build most of the circuit right on the XLR connector at
the base of the microphone.
The microphone above installed and connected to the Flex
5000
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