[Elecraft] Tx distortion

[Jim Brown]

On 2/2/2019 12:28 PM, mustang65 wrote:
> The symetrix output impedance is 200 ohm and it is recommended to not load it lower then 600 ohms.

It's been at least 15 years since I looked at a K2 schematic, but Don is 
very well equipped to help you there. The photo on the data sheet for 
the 528e shows that none of the TRS jacks appear to be properly bonded 
to the shielding enclosure, as specified by AES48. This is a classic Pin 
One Problem, and a path into the 528e for RF if a cable is connected to 
one of them. It isn't clear whether the XLR is properly bonded. If it 
isn't, that's another possible path for RF. That could easily be the 
cause of audio distortion -- what we commonly call "RF feedback."

This unit is designed to operate at an output level in the range of 2V 
RMS, which as W2XJ observed can easily overdrive the K2. The best way to 
drive the audio chain of a rig like the K2 is by the addition of simple 
voltage divider. I'd start with 26 dB, which is a 20:1 ratio. I'd go 
with 1K for the series resistor and 47 ohms for the load. You will, of 
course, need a cap in series with your drive to the K2 so that you don't 
upset the bias.

The K2 SSB board is low on gain. When I was using one, I worked out a 
simple mod to increase the gain by about 6 dB and also roll off the low 
end a bit. This allows the mic to hit the peak limiter a bit harder. At 
the time, I was using an EV RE11, also a pro mic, but without the 
proximity effect bass boost of an SM58. I gave notes on that mod to Don 
many years ago. No new parts are needed -- I simply moved two or three 
resistors and caps around.

I recall that there were other mods done in the audio chain to increase 
gain.

73, Jim K9YC