The Sansui AU-7900, released from 1975 to 1977 was the little brother to the release AU-900/11000 featuring the stylish black/bezel look such as the popular 999. The AU-7900 had some superior features such as load impedance selector and two phono inputs, Sansui’s exclusive TTC (triple tone controls) and a separations rear switch.
F-2596 Motherboard
The mains filter, protection and driver stages are all located on the F-2596 motherboard as outlined below;
Section I: Ripple Filter/Protection
The AU-7900 has a unique rectification stage utilizing a standard full-wave bridge for the Mains rectification and a unique 3-Phase rectification for the -/+50V regulated rails The protection circuit incorporating a DC devotion and thermal discrepancy protection.
All of the 3-phase diodes were upgraded to Ultra-Fast, Soft Recovery Vishay axial diodes. VD varistors were replaced with modern Vishay 4148 general axial diodes and a MUR type TO-220 diode. New low impedance Nichicon PW capacitors with an increase in operating voltages.
BEFORE
AFTER
Hardware
The original 15,000MFD capacitors were increased to 18,000MFD computer-grade Nippon Chem-Con capacitors for better filtering/reserve capacity
Section II: Driver
The driver stage employs a shared heatsink for the output driver stages devices in a DC OCL output array.
The common emitter dual PNP devices were replaced with 1% matched low noise TO-92 Fairchild devices as boded below. Associated TO-92 devices were replaced with low noise TO-92 Fairchild type and high-seed ZTX type. Fusible resistors were replaced with 1/2 watt KOA and new precision Bourns potentiometers.
New Dual PNP Boding
Electrolytic capacitors were replaced with audio grade KT/KA and low impedance Nichicon capacitors with an increase in operating voltages. High grade WIMA polypropylene film capacitors were also installed.
BEFORE
AFTER
Bias Confirmation
F-2599 Motherboard
This stage houses both secondary PS and RIAA Phono supply.
Section I: Reg. PS
The secondary PS establishes the regulated +22/-29Vdc rails for the AF front end.
Both regulators were replaced with a modern, robust Fairchild TO-220 device thermally coupled with new Wakefield fully synthetic compound, new low noise TO-92 Fairchild devices were also installed.
New low impedance Nichicon PW capacitors were installed with an increase in operating voltages.
BEFORE
AFTER
-29/+22Vdc Confirmation
Section II: RIAA PHONO
The RIAA stage employs a NFB and only -/+0.3dB Curve RIAA which is quite impressive.
The input film was upgraded to a high grade MKP polypropylene film capacitors, as well as new high grade WIMA polypropylenes. New low impedance Nichicon PW’s were installed with an increase in operating voltages and low noise modern TO-92 Fairchild devices.
BEFORE
AFTER
RIAA Diff Matching
F-2610 & F-2611 Tone / Filter CTRL
The AU-7900 employs a simple common emitter differential and npn/pnp output stage with the “exclusive” Sansui Triple Tone Control circuit.
F-2610
For proper cleaning the controls and volume potentiometer were removed and Caig Labs D5/F5 treatments applied.
High grade WIMA polypropylene films and audio grade MUSE Bipolars Nichicon capacitors were installed as well as KT/KA audio grade Nichicons with an increase in operating voltages. All new small signal low noise TO-92 Fairchild devices installed.
BEFORE
AFTER
F-2611
The Filter control electrolytic were replaced largely with high grade WIMA polypropylene films and audio grade KT/KA and VP Bipolar Nichicons with an increase in operating voltages. New small signal low noise TO-92 Fairchild devices installed.
BEFORE
AFTER
AUDIO NOTIZEN
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