Posted by
Jim Brown-10 on
Aug 27, 2005; 4:26pm
URL: http://elecraft.85.s1.nabble.com/EV664-tp381491p381493.html
On Wed, 24 Aug 2005 12:40:47 -0400, W3FPR - Don Wilhelm wrote:
>I was able to find some info on the EV 664 mic and found it is capable of
>being configured for 'high impedance' or 'low impedance', but I could find
>no reference for how low is low nor how high is high, and I found no mention
>of the output level. You could just try the low impedance connection and
>see how it performs. If it produces peaks on the K2 LED display at (or
>slightly below) the point where the RF output is indicated on CW (at the
>same power level), it is producing sufficient output.
I'm looking at an original data sheet for the 664! It does not tell me how high is
high, but it does say that low is 150 ohms. This is pretty much standard for
"pro"mics. I would guess that the "high impedance" output is probably on the order
of 10K-20K, but I'm only guessing.
Because I'm in the pro audio biz, I have a lot of good pro mics laying around, and
I use them in my ham station. As a result, I've taken the time to figure out how
to interface them well with my rigs.
I would do two things. First, I have modified the mic input circuit of my KSB2's
to raise the input Z and also raise the low frequency rolloff. This has the effect
of increasing the input senstivity of the KSB2 mic input, which is much lower than
is ideal. It also makes the audio a bit "punchier" by reducing the low frequency
content that would otherwise hit the peak limiter and reduce the audio level. The
low frequency rolloff will now be 250 Hz. When you're running QRP, every little
bit helps, and I would expect this to help audio punch by 3-4 dB (more than double
the power).
The mods are quite simple. First, change R14 from 1K to 2K. Second, change C34 to
0.22 uF. Third, add 2.2uF in series with R15.
If you can find some good audio input transformers (like the old UTC O-series or
A-series), you can pick up an additional 6dB of gain by using one that provides a
1:2 or 1:3 turns ratio. This will be most effective if you have done the above
mods to the input stage.
I've used pro mics with my K2, and these mods help a lot. Without the mods, the
K2's audio punch is a bit lacking. There are also some mods on the Elecraft
website that show how to increase the drive level quite a bit further through the
audio chain (after the peak limiter). I haven't done them, but they are VERY well
thought out, and should help even more. I probably will do them when I have time.
BTW -- all of these mods are good no matter whether you're using pro mics or
communications mics.
Getting back to the original question. I suggest that you first do my mods, then
try the mic both high-Z and low-Z, and use whichever configuration provides the
best sounding (and hottest) audio.
When a mic is wired for a high-Z output, it has more output voltage, but through a
higher resistance source. If the mic is loaded by a low value resistor, the source
resistor and the load resistor form a voltage divider, and much of the higher
output voltage is lost.
And if anyone wants a copy of the data sheet, give me a FAX number and leave your
machine on. It's four pages.
73,
Jim Brown K9YC
http://audiosystemsgroup.com_______________________________________________
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