Al:
I think he is measuring mainly white thermal noise as opposed to 60/120/240 Hz hum (via the tweeter).
I found something similar with the PS Audio GCPH. That pre-amp has a low noise input stage and a gain cell buffer/output stage. The gain on the input stage is determined by a set of fixed gain settings (48/54/60/66 dB) and the gain cells have a volume control. So the input stage is analogous to your the pre-amp and the gain cell similar to the line amp.
I found that the lowest noise (as measured by my Tektronix 7L5 spectrum analyzer) was found at the higher input gain settings, minimizing the output stage gain. In other words, the output buffer gain stages were noisier at higher gains than the input stage was. In fact, with gain increases of 6 dB of the output gain cells, I gained 8-12 dB in noise. But this trend was very non-linear, and this effect occured only over the last 6 dB of gain in the gain cells. Below that, the noise was dominated by input stage gain.
I think he is measuring mainly white thermal noise as opposed to 60/120/240 Hz hum (via the tweeter).
I found something similar with the PS Audio GCPH. That pre-amp has a low noise input stage and a gain cell buffer/output stage. The gain on the input stage is determined by a set of fixed gain settings (48/54/60/66 dB) and the gain cells have a volume control. So the input stage is analogous to your the pre-amp and the gain cell similar to the line amp.
I found that the lowest noise (as measured by my Tektronix 7L5 spectrum analyzer) was found at the higher input gain settings, minimizing the output stage gain. In other words, the output buffer gain stages were noisier at higher gains than the input stage was. In fact, with gain increases of 6 dB of the output gain cells, I gained 8-12 dB in noise. But this trend was very non-linear, and this effect occured only over the last 6 dB of gain in the gain cells. Below that, the noise was dominated by input stage gain.