Posted by
Ron D'Eau Claire-2 on
Aug 28, 2005; 7:18pm
URL: http://elecraft.85.s1.nabble.com/EV664-tp381491p381506.html
There seems to be two issues here. One is how the frequency response affects
the speech compressor, and the other is the frequency response that gets
transmitted.
The frequencies transmitted are controlled mainly by the SSB (OPT1) filter.
When properly aligned, the SSB filter (OPT1) bandpass is positioned to roll
off the low frequencies so they don't get transmitted. Properly positioned,
the filter also allows the high frequencies be transmitted for best
intelligibility. That is most important when using the stock, narrower
bandpass OPT1. If it's set for too much low frequency response, the audio
sounds "muddy" or "muffled". That's not so much because of the excess lows,
but because the filter is to narrow to pass the high audio frequencies that
are required to hear the sibilant sounds (such as the letter "s"). Those
highs get cut off by the OPT1 filter if it's position too close to the
carrier frequency.
In my experience, after roughing in the BFO setting with Spectrogram, final
adjustment requires listening to one's own signal and making minor tweaks to
the BFO frequency using CAL FIL as recommended in the SSB manual. My K2
sounds best with the OPT1 filter rolling off below 400 Hz and very steeply
below 300 Hz. That fits with "good Ham practice" as I've seen it applied for
many years. Even the old AM rigs rolled off frequencies below 200 or 300 Hz.
My voice has a lot of low-frequency drone in it and a little more aggressive
roll-off at the low-frequency end works best for me.
I tried my K2 using a "hi-fi" mic designed for music and found that it was
not as "crisp" or nice-sounding as my electret. I didn't pursue it further
because it wasn't an issue to me.
I've been following this thread because there seems to be an issue with how
the speech amp in the K2 is reacting to different mics. I did all of my
testing with the compressor off. I'm wondering if the difference is just a
difference in impedances, or whether something else is going on.
Ron AC7AC
-----Original Message-----
From:
[hidden email]
[mailto:
[hidden email]] On Behalf Of Francis Belliveau
Sent: Sunday, August 28, 2005 10:19 AM
To: Elecraft List
Subject: [Elecraft] K2 SSB Mic Input Frequency response?
I am now confused by the existence of two different mod's with the same
goal.
It makes perfect sense to me that I should not be wasting Tx energy on audio
frequencies as low as 20Hz. Beyond that I am somewhat in the dark.
Jim Brown and Stewart Baker have each suggested a modification to improve
this situation.
I am at a loss to know which, if either, I should implement. I must admit
that I an very tempted to incorporate the indicated improvements. However,
I would have no means of evaluating the results and therefore look to the
remainder of those on this list for further opinion.
Summary of the mod's are:
from Jim:
Change R14 from 1K to 2K.
Change C34 from 2.2hF to 0.22 uF.
Add 1uF in parallel with R15
from Stewart:
Change C34 from 2.2uF to 0.47uF
Change C31 from 2.2uF to 1uF
Change C20 from 0.33uF to 0.047uF
Is the situation severe enough to warrant an official mod for this?
Inquiring minds want to know.
Fran
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