Temperature changes and AC128's

[Mike Watterson]

I suspect the output transistor or amp is amplifying other noise. Four obviously noisy output transistors sounds mad. Note an output stage could be 20 times more noisy than the first audio amp and you might not notice. The first stage noise is amplified by entire amp gain. Which is why an RF pre amp needs to be much less noisy than IF amp, but recieved noise level is dramatically lower at VHF, lower still UHF and much lower at microwave, so while 1dB NF amp is useful at 11GHz, such is irrelevant on LW/MW/SW and pretty much on VHF.