The preamp is intended as a low noise differential preamp for a PC sound system when measuring power supply noise or mixer low frequency noise.
The circuit uses a low noise audio differential input preamp U101(THAT 1512) together with a differential audio driver U105 (THAT1646). Either a differential input or a single ended input audio system can be driven. Input servo U104 (LT1012) ensures that the 100K bias return resistors do not saturate the THAT1512 when operated with high gain. Output servo U101(LT1012) ensures that the dc offset at the THAT1512 output is low. U103(OPA134) has a gain of 20dB and drives U105 (THAT1646) For an overall gain of 80dB.
Input protection has been added so that (with care) the preamp can be used to measure power supply noise. Battery operation is preferred to minimise problems with ground loops.