- Mar 1, 2018
- Gold Coast, Queensland, Australia
Clearly there is some form of that effect in many modern low-distortion amplifiers (including the Topping LA90), as you can see that as the output decreases from the maximum there is kind of a "sawtooth" of the noise/distortion floor until it finally reaches some minimum and then it just slopes up as the output level goes down.
The 'sawtooth' as you describe it, is the Audio Precision switching gain/attentuation ranges to keep the signal for the onboard data aquisition A/Ds in their relative sweet spots for the greatest accuracy/lowest noise.
At extreme low input levels, there is a heap of gain applied to the signals, using very low noise amplifiers, in order to keep the signals applied to the A/Ds in their linear ranges. The rise before the vertical drop is the AP, not the DUT. As the applied input levels increase, the gain/attentuation is switched out, and at high levels, way above the A/D's input ranges, the signal is attentuated in ever increasing multiples to keep the A/Ds right in their full scale ranges. Less gain equals less intrinsic noise.
The AP's automatic range selection matrix would be quite impressive to look at all the combinations of nested gain stages and attenuation stages.