Sansui AU-20000 Integrated Amplifier
Today were featuring a very special Sansui AU-20000 Integrated Amplifier! The AU-20000 produced in the mid 70’s until 1977, it was touted as the TOTL of Sansui’s AU/TU offerings designed to pair with the TU-9900 Tuner. Rated at 170/Wch at 0.05% factory via a X3 Push-Pull driver stage and a large low-magnetic-leakage toroidal transformer and two 22,000 Microfarad filter capacitors.
F2508 Power Supply
The AU-20000 power supply was designed around an extremely robust handling capability with a symmetric dual rail output. Uniquely the power supply as drivers were designed as slotted card PCB designs which is very convenient. As shown in the Before photos below and typical of units in this age/era the regulating transistors exhibit a large amount of radiant heat affecting the PCB and associated electrolytic. Also noted, and particular to Sansui’s as we know, was a large amount of glue used for mounting the large vertical capacitors. This glue had spread to the near resistors network oxidizing the exposed diode and resistor legs causing corrosion to the devices.
All the original glue was soaked and then carefully removed, all associated resistors were simply replaced with carbon film KOA resistors, many increased to a 1/2Watt rating to better dissipate evident heat issues. All populated 2473 and epoxy VD1212 diodes were replaced with 4148 general diode types. As expressed above all four regulating transistors were replaced with a modern On-Semi heavier duty TO-220 MJE device and new thermal compound to better handle the current/voltage requirements with less heat stress. All the electrolytic were replaced with high temp (105C) long life Nichicon PW and Panasonic FC capacitors.
The chassis mount 470MFD electrolytics were replaced with an increase in working voltage to low impedance long life Nichicon PW capacitors as well as the 10MFD/100V electrolytics
The original large 22,000MFD 80V Nichicon filter capacitors require a significant amount of realestate within the chassis and in such proximity to the large heatsinks for the driver stage devices. I wanted to decrease the 76mm diameter footprint of the original so I settle on high quality Cornell CDM (105C) computer grade capacitors with a 50mm diameter mounting surface with new clamp hardware.
The AU-20000 features a trifold protection stage and Sansui’s proprietary ASO (Area of Safe Operation) integration. The ASO is a self sensing monitoring circuit the limits and supervises current consuming/limiting operations. DC deviation, temperature sensing and short circuit protections exists as well plus a soft start inrush limiter relay.
The in-rush limiting relay and protection were both replaced with a high quality MY2/4 low noise Omron relay’s. The protections wire-wound resistors were replacing with MOX type 2/7Watt resistors as the cement in WW type tend to breakdown excessively. The F2511 protection also was populated with carbon composite resistors that tend to drift with age and high heat applications, all the CC type resistors on this assembly were replaced with 1/2-1/4Watt carbon film KOA resistors. All the populated 1S2473 diodes were replaced with modern 4148 type and all the 10D1 diodes upgraded to a Ultra-Fast (UF) 400# type diode as well as the original common cathode and common anode rectifiers to a modern reliable MUR1602 TO-220 tabbed device’s. The electrolytic were replaced with high temp (105C) low impedance Nichicon PW and Nichicon BiPolar VP capacitors. Bipolar 1mFD electrolytic were upgraded to Polypropylene (PP) type Panasonic ECW film capacitors and both .01 films upgraded to ECW PP film capacitors.
Similarly to the power supply PCB the drivers are both on slotted card style PCB’s. The AU-20000 driver designs consists of a three way complimentary push-pull stage. Significant transistor device upgrades were applied to this restoration which lowered the overall harmonic distortion/noise and response of the driver stage.
First all of the electrolytic capacitors were removed and the caustic glue as detailed earlier was soaked and removed. The electrolytic capacitors were replaced with a high temp (105C) low impedance Nichicon PW capacitors and BiPolar VP as well as audio grade KT capacitors all with an increase in working voltages. The original 1S2473 diodes were replaced with modern 4148 type and the epoxy MV-13 diodes upgraded to a modern series equivalent in Vf drop. Both potentiometers were replaced with precision Bourns pots as well and associated resistors from he glue clean up with KOA carbon film type.
The differential pair transistors were replaced with modern low noise Fairchild 1% matched TO-92 NPN type, the pre-drivers were also upgraded to modern low noise Fairchild TO-126 transistors for dissipation in compliments, the driver stage had the heatsinks modified to fit the new TO-126 package firm. The final driver stage TO-220 devices were upgraded to modern low noise/low distortion On-Semi TO-220 devices with new thermal compound applied.
F2501 RIAA EQ
The AU-20000 features an extremely unique and effective equalization stage. Using a two stage differential stage drive design a CR gain for high frequencies and NF type for low-subsonic ranges. Due to variations in production design this PCB features several Schematic/Installation discrepancies. The construction of this PCB causes for a pin connector that affixes in a squared 90 degree fashion with other AF stage assemblies. The AU-20000 also features a variable input sensitivity switch, which on this unit was actually damaged as the inside shaft and slid out of the entire phenolic package so careful disassembly and reassembly was required, taking attention to match the gang/wiper contacts and epoxy the shaft to the inner potentiometer core to secure it in place.
Both initial gain differential stages were replaced with 1% matched low noise TO-92 fairchild transistors as well as tied 2SA726 type transistors to low noise PNP fairchild transistors. Initial input capacitors (another scheme error .22/1MFD) were replaced with high grade polypropylene MKP film capacitors. Electrolytic were replaced with low impedance PW and audio grade Fine Gold Nichicon capacitors as well as Nichicon VP Bipolar’s.
F2495 Tone & F2505 Flat Amp
The AF tone assemblies on the AU-20000 is a dual rail feed proprietary design using Sansui’s Triple Tone Control circuit for detailed separate left/right attenuation and a ton defeat linear response option. As above due to production variations their are some Schematic/installation discrepancies. All the associated switching and phenolic potentiometers were cleaned and lubricated during deconstruction.
The initial differential gain stage in each assembly were upgraded to modern low noise fairchild TO-92 devices match to 1% as well as associated compliment 2SA726 type. The initial input mylar films were upgraded to high quality MKP polypropylene .22MFD film capacitors. Electrolytics were upgraded to audio grade KT , low impedance PW and VP Bipolar Nichicon capacitors.
F2504 Filter Assembly, F2498 Filter Amp PCB’s and F2503 Tape Copy AF Stages
The F2504 Filter assembly houses each channels F2498 filter amp assemblies perpendicular to the main assembly,
The electrolytic capacitors on the F2504 assembly were upgraded to audio grade Fine Gold and KT Nichicon capacitors. On each F2498 assembly the electrolytics were replaced with 4.7MFD audio grade KT Nichicon capacitors as well as the the 2SA726/2SC2208 compliments to modern low noise fairchild transistors.
The full monitoring, deck to deck dubbing feature and tape copy assembly had the known 2SA726 upgraded to low noise fairchild TO-92 transistors as well as the electrolytic capacitors to low impedance, long life PW capacitors.
F2496 Meter Assembly
The AU-20000 features two precision variable output meters versus the other AU series integrated amplifiers.
The F2496 assemblies noted epoxy type MV-13 diodes were replaced with modern 4148 diodes in series as well as 1S2473 diodes to 4148 type as well as the input 2SC1313 transistors to modern low noise 1845 TO-92 devices. The 1M and 1K adjustment potentiometers were replaced with modern precision Bourns trimmers. The tantalum and electrolytic capacitors were replaced with long life, low impedance Nichicon PW capacitors with a schematic/install discrepancy noted with the 1MFD capacitors. The Bipolar 1MFD electrolytic were replaced with high grade Panasonic ECW polypropylene film capacitors.
New LED’s installed to present a very consistent and uniform green hue to the beautiful black faceplate. LED’s are important to decrease the effect of excessive radiant heat of the original incandescent.
Original Tinted Fuse Lamps
Modern Multi-SMD LED Green Fuse Lamps
Modern Multi-SMD LED White Fuse Lamps
Sansui TU-9900 Tuner
The TU series Tuner released by Sansui was injection with the release of the TOTL AU series components. The TU-9900 features an incredibly sensitive tuner and ranked up there amongst the best. Touting a 5 gang capacitor with a tiered multi stage IF ceramic filter circuits.
One of the challenges considering the filters onboard was matching the Green LED lamps for the meters and dial display to the above AU-20000 Integrated Amplifier. I ended up modifying the Meters and lamp display to accept high grade custom made SMD LED 8V festoons which turned out perfect!
F2527 Power Supply
The regulated single rails -12/+13V supply is relatively straight forward no the TU-9900. The main point is replacing the two 4.7/10OHM fusible resistors to 1Watt MOX KAO resistors.The regulating TO-220 devices which often exhibit heat in these applications were both replaced with heavy duty, modern On-Semi TO-220 MUR devices with new thermal compound applied. Also replaced were both common cathode, common anode rectification devices to modern, reliable MUR1620 type TO-220 devices. All of the electrolytic were replaced with high temp (105C) long life Nichicon PW capacitors.
RF Stages (IM/MPX ++)
The remaining stage and several tantalums found were all replaced with my main stay for Tuners which is a VP Bipolar Nichicon and the low impedance, long life Nichicon PW capacitor types.