Odd Rx behavior

Rod KQ6F

I recently acquired a 6400 and have been comparing its performance with other SDRs on hand.  One thing I've noticed is how it behaves with atmospheric noise.  My noise level is fairly typical but I hear almost no difference between when I'm connected to my antenna vs dummy load.   On 75M it's about S5 in both cases.  With other SDRs there is a big difference between antenna and dummy load.  I attribute this to Flex AGC and how it works which seems to bring up internal noise with dummy load.  But that doesn't explain why I hear no difference when going to the antenna.  I'm not employing the NR feature but it's almost as if there is one built into the DSP chain that runs all the time.

Don't misunderstand - I'm not complaining.  In fact I love it.  I just don't fully understand what's going on.

Rod

Bill -VA3WTB

Rod, did you buy your 6400 new? if not, how old is it? Is it possible a heat sink fell off the ADC chip? The very early 6400 radios had this problem that effected the noise floor a lot.

And are you really hear not difference or are you seeing no difference on the S meter?