NAD USB RIAA Head and Line Buffer Amp PP-3 (PP3)-Schematic here-but several Questions

[tiefbassuebertr]

in the attached PDF you will find my own drawing of NAD's PP3 (PP-3, Phono Preamp 3), except the AD-converter section with PCM2905; the genuine diagrams are not available about NAD.
Unfortunately I don't understand the complete circuit - especially concerning certainly devices I don't know the exactly aim:
1) R181-R184, D101-D102 and R169-R170 (Voltage Regulator)
2) R150-R152, C138-C140 (RIAA-network, right channel and according devices left CH)
3) R156-R158, C154-C156 (RIAA-output, right channel and according devices left CH)

Additional I don't find a datasheet on the web for the device near by USB output. The SMD marking code is "RT93" (outline you will find in the attached pdf by the device numbers).
Who knows more about this device?

And a last question: The German PP3 folder mentioned a special circuit topology to realize a soft clipping function. Which devices are therefore in use? Thank you for your hints.

Some URLs for your information:
NAD PP3 folder
NAD Electronics :: PP-3 Digital Phono Preamplifier
Instruction Manual
http://nadelectronics.com/content/10...TAL_100628.pdf
Schematic NAD PP2 (PP 2 Phono Preamp 2) for comparing to the PP-3: by post #6 about
FFT Noise Spectrum of the LM7812 three terminal fixed voltage regulator

[tiefbassuebertr]

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