[Elecraft] K2 SSB Improvements?

[Jim Brown]

On Fri, 19 Dec 2003 17:53:50 -0800, Ron D'Eau Claire wrote:

>Henry, if you are getting ALC action at the compression setting you like,
>raising the value of R14 more won't help at all. 

I agree, but for an additional reason. 

The difference between 4.7K and 1K should be only about 1dB with a low
impedance mic, but could be 10 dB or more with a high impedance mic. 
R14 forms a simple voltage divider with the source impedance of the
mic. Low impedance mics are typically 150 ohms, so the loading of 1K
will drop the signal by about 1.2 dB for a mic of that impedance. 
Changing R14 to 4.7K simply reduces the loss to 0.27 dB. You would
never hear that.  Even if the output impedance of the mic was as high
as 500 ohms, the existing 1K gives a 3.5 dB loss and 4.7K gets you to 1
dB, so going further is still not going to buy you much. 

In general, it is considered bad engineering practice to load a mic
with a load impedance higher than about 10X its source impedance, and
you don't want the input impedance of an input that's exposed to RF any
higher than it needs to be.  I like the input impedance around which
the K2 is designed. If you look at most pro mic stages, you'll find
input impedances of 750 - 1,500 ohms. 

There certainly is virtue in rolling off the low end of the mic preamp,
as others have suggested. But simply reducing the value of C34 (the 2.2
uF input cap) is not the easy solution that it appears to be, at least
not by much. Why? Because the value of C34 is already a compromise
between the two settings of the input gain. Look at the input circuit,
and you'll see a switched 180 ohm resistor that says "Atten." The low
cut frequency is determined by the time constant of that cap and the
parallel combination of the 180 ohm and 1K resistors in series with the
mic impedance. When the attenuator is switched in, the rolloff of the
current design is about 250 Hz, and about 60 Hz with no attenuator.
Reducing it to 1 uF gives us 120 Hz and 500 Hz respectively. That's
about as far as I would like to go in the attenuated condition, but
much less than I would like in the full gain position. Adding a cap in
series with R15 would solve the problem of the variable rolloff, in
which case it does make sense to take C34 down to 0.47 uF or 0.33 uF. 
The cap added in series with the 180 ohm resistor probably wants to be
something on the order of 2 uF, and you end up with a low cut somewhere
between 300 Hz and 400 Hz with either setting of the attenuator. 

All of that said, count me among those who would like another 6-10 dB
of gain in the mic preamp so that i can use my pro mics with the K2
without the bandaid of a stepup transformer or external preamp.  As it
is now, I can't get enough level from my highest output dynamic mic to
reliably trigger the VOX or get any ALC!  It was a real PITA
handholding the Heil hand mic in the recent Sweepstakes while trying to
simultaneously maintain the log! 

Jim K9YC