Vintage Hi-Fi Audio Restorations: R.Martin's Sansui AU-20000 Integrated Amp Restoration

Sansui AU-20000 Integrated Amplifier

The AU-20000 was one of the top tier integrated amplifiers offered by Sansui from 1975 to 1977. With a unique red/maroon metal shroud and black faceplate its a gorgeous looking unit. Boasting 170Watts at .05%THD factory the AU20K has some positive power for most application and a versatile RIAA phono stage.

F2508 Power Supply & Associated Filters C01~C04

The regulated power supply employs two full wave bridge rectifiers located on the F2511 assembly. The large current bridge array uses two 22,000MFD filter capacitors as well as C03/C04 chassis mount capacitors for the F2507 Driver assemblies. The smaller array feeds the F2508 regulated power supply with C01/C02 chassis mount as the initial DC decoupling capacitors. The F2508 establishes a -/+35 & -/+65 Supply.

The heatsink mounted TO-220 regulators were all replaced with a modern, robust TIP TO-220 devices and coupled with synthetic Wakefield thermal compound. All small signal devices were replaced with low noise, TO92L Fairchild devices for better dissipation handling.

Zener’s and varistor diodes were all replaced with modern Vishay axial diodes. All of the electrolytic capacitors were replaced with a low impedance, high temp (105C) Nichicon PW and Panasonic FC with an increase in operating voltage, output decoupling capacitors were increased in value for better filtering.

BEFORE

AFTER

Regulated -/+Vdc Af Supply Confirmation

C01~C04 Chassis Mount

The initial C01/C02 decoupling chassis mount capacitors were replaced with a low impedance type Nichicon PW with an increase to 1000MFD for better DC filtering. Also the C03/C04 chassis mount for the F2507 were replaced with a low impedance type Nichicon PW double in capacity for better DC filtering like C1/2.

BEFORE

AFTER

F2511 Protection

The protection assembly employs a Soft Start relay as well as DC protection input from both AF Front end and Driver assemblies featuring thermal tracking, excessive current consumption and DC detection. The large Bridge and 1A bridges are located on this assembly.

The initial 1A bridge rectifier was replaced with state-of-the-art MUR series TO-220 type common cathode, common cathode rectifiers, the 1A full wave for the relays were replaced with ultra-fast, soft recovery Vishay diodes. All small signal devices were replaced with low noise, TO92L Fairchild devices for better dissipation handling. All of the electrolytic capacitors were replaced with a low impedance, high temp (105C) Nichicon PW with an increase in operating voltage. New high grade WIMA polypropylene film capacitors and Nichicon PP type films installed. A new Omron MY/LY type relay installed.

BEFORE

AFTER

F2507 Drivers (PA)

The A/B classes drivers employ a differential pair, current mirror/pre/Driver output with a complimentary NPN/PNP output stage designed on a card style PCB arrangement with molex connectors. One of the drivers on this unit suffered a catastrophic failure resulting in several resistors burning up on the PCB surface.

New matched differential pair devices were installed (see mapping below) as well as all small signal devices with modern, low noise TO92L Fairchild devices for better dissipation. New TO-126 & TO-220 pre-drivers and drivers were installed and thermally coupled with synthetic Wakefield compound. All new diodes/varistors replaced with modern axial Vishay type. new precision Bourns potentiometers installed.

Differential Pair Mapping

All new electrolytic capacitors installed with low impedance Nichicon PW type and Muse BP type with an increase in operating voltages. New bypass high grade WIMA polypropylene film capacitors installed.

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AFTER

BIAS Confirmation

Due to the cascading damage a new bank of TO-3 Onsemi outputs thermally coupled with synthetic Wakefield compound installed including an array of new wire-wound emitter resistors. All solder connections on the output stage PCB were inspected and reflowed and with the PCBs cleaned.

F2498 Meter Control

The meter assembly routes many of the Af front end signals via the Muting as well as calibration and control of the variable-precision output/input meters.

All of the electrolytic capacitors were replaced with a low impedance Nichicon PW type with an increase in operating voltages as well as new high grade WIMA polypropylene film capacitors at 1MFD values. All varistors were replaced with modern axial Vishay diodes and new sealed, precision Bourns potentiometer adjustments installed. All new small signal devices installed with TO92L Fairchild devices for better dissipation.

BEFORE

AFTER

F2503 Tape Monitor / Coupling PCB

The F2503 assembly act as a large coupling stage for the Filter amp front end gain stage as well as input pass thru for the remaining AF front end stages. All pin connections were inspected, cleaned and reflowed if necessary.

The input stage small signal devices were replaced with modern, low noise TO92 Fairchild devices. All of the electrolytic capacitors were replaced with a audio grade KT/KA and low impedance type PW Nichicon capacitors with an increase in operating voltages.

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AFTER

F2501 RIAA EQ (Phono)

This assembly mounts perpendicular to the F2503 “coupling” stage and provides an extremely accurate and touted RIAA Curve response for the phono input. The AU20,000 also features two Phono inputs and variable load and input sensitivity settings.

All of the small signal devices here were replaced with a modern, low noise TO92 type Fairchild devices. Input and output signal capacitors were replaced with audio grade MUSE BP type and remaining electrolytic to a low impedance PW type with an increase in operating voltages. DC decoupling capacitors values increased for better filtering. Coupled films were replaced with high grade WIMA polyrpropylene film capacitors.

BEFORE

AFTER

RIAA Curve Response

F2505 Flat Amp

As above the front end stage mounts perpendicular to the F2503 “coupling” stage providing the nominal gain increase for the filter amp stages.

All of the small signal devices here were replaced with a modern, low noise TO92 type Fairchild devices. Input and output signal capacitors were replaced with audio grade MUSE BP type and remaining electrolytic to a low impedance PW type with an increase in operating voltages. DC decoupling capacitors values increased for better filtering.Coupled films were replaced with high grade WIMA polyrpropylene film capacitors.

BEFORE

AFTER

F2504 Filter Coupling & F2498 Filter Amp

This assembly acts as another “coupling” stage design with the filter/tone assemblies mounted via. The F2504 begins the Lo (40Hz/20Hz) & Hi (7Khz/12Khz) pass filter stages as well as the gain derived F2498 assemblies which mount direct and perpendicular the F2504.

On both assemblies all of the electrolytic capacitors were replaced with KT/KA audio grade and low impedance PW Nichicon capacitors with an increase in operating voltages. All of the small signal devices were replaced with modern, low noise TO92 Fairchild type. All molex connections were inspected, cleaned and reflowed where necessary.

BEFORE

AFTER

F2495 Tone Amp

The F295 mounts perpendicular to the “coupling” F2488 PCB that houses the control potentiometers. The tone amp provides the nominal gain stage for the Bass/Mid/Treble or defeat attenuation.