[Elecraft] Microphones for K2/3

[Jim Brown]

On Wed, 05 Sep 2007 06:53:40 -0400, John Huggins wrote:

>What is it that makes any microphone 
>"optimized" for any radio.  Pin outs are obvious of course.

Many years ago, an international standards organization issued a 
poorly thought out standard for 2-way radio communications that, 
if followed, resulted in the bandwidth of transmitted audio being 
rolled off by 3-6 dB at 3 kHz. Because the higher audio 
frequencies (1-4 kHz) contribute the most to intelligibility, this 
resulted in reduced "talk power" and muddy audio if a mic with 
good (flat) frequency response is used. 

To compensate for this lousy standard, mic companies began 
building mics with a strong peak in their response around 3 kHz. 
This response peak is clearly visible in the response of mics like 
the Shure 450 -- there's a 10 dB peak at 3 kHz! Plug this mic into 
a pro sound system and it will sound really nasty, but connected 
to you ham rig it sounds just about right. 

Some ham transceivers provide a switchable peak in the audio 
response so that a pro mic with flat response can be used. The K2 
does not. 

Another element of good communications audio (that is, good "talk 
power") is to limit the low frequency response so that transmitter 
power is not wasted on the bassy parts of speech. That's because 
these low frequencies contribute very little to intelligibility. A 
good communications circuit will roll off sharply on the low end 
somewhere between 250 and 400 Hz. It's easy to design this into 
ham gear, and it's also designed into a few mics. The EV 635A, for 
nearly 50 years a mainstay of broadcasting, rolls off at about 150 
Hz. The K2 runs flat down to about 40 Hz, but can be modified to 
move the rolloff up to about 200 Hz by changing some capacitors 
and resistors. 

A mic also needs enough output voltage to drive the radio. This is 
not an issue with most ham rigs, but the K2 is a bit low on gain 
through the audio chain, so it takes a pretty "hot" mic to drive 
it well. Pro dynamic mics don't have enough output to drive it 
very well. 

Finally, the output impedance of the mic needs to be low enough 
that the input impedance of the radio doesn't load it down. Mics 
are not designed to be "loaded" (that is, terminated). They are 
designed to work into an impedance at least 5-10 times their own 
source impedance. The input impedance of the K2 is on the order of 
600 ohms, which is on the low side for a pro mic (typically 150 
ohms output impedance). This is easy to change with a few 
resistors. 

Summarizing -- for use with the K2, a mic needs relatively high 
output, should have an output impedance lower than about 100 ohms, 
should have a pronounced response peak around 3 kHz, and should be 
rolled off around 250-400 Hz on the low end. 

73,

Jim Brown K9YC