Sansui AU-666 integrated amplifier

General: 

This amplifier made & produced in the early 1970’s by Sansui Company inc. Japan.
 

Photos:

front ^

Front detail ^

 

Specifications:

Sansui AU-666	Stereo Integrated Amplifier
Output power: main amp.	Music power (IHF) : 100W (4ohm) 80W (8ohm)  RMS Output        : 45+45W (4ohm) 35+35W (8ohm)
THD	< 0.5%  (RMS power)
IM distortion	< 0.5%  (RMS power) (SMPTE, 60Hz:7kHz=4:1)
Power Bandwidth	10Hz - 40KHz  (8ohm) (IHF)
Frequency range	10Hz - 70KHz
Channel separation	> 60dB @  (1kHz)
Hum and noise	> 100dB (IHF)
Input sensitivity/impedance	1V  @ 50 kohms
Load impedance	4 Ohm-16 Ohm
Damping factor	> 40 @ 8-Ohm
Preamplifier :
Output voltage	Maximum output: 5V RMS Output: 1V  @ 50 kohms
THD	< 0.1%  @ output voltage
Frequency range	10Hz - 40000Hz, @ 1dB
Channel separation	Phono1, 2: >50dB  Tuner, AUX: >60dB
Hum and noise (IHF)	Phono1, 2: >70dB Tuner, AUX: >80dB
Input sensitivity (1kHz, Voltage, imp.)	Phono1, 2:2mV / 50kohm MIC: 3mV / 50kohm Tuner: 180mV / 100kohm AUX: 180mV / 100kohm Tape Play: 180mV / 100kohm Tape Recorder (Din): 180mV / 100kohm
Rec out	Tape Rec:180mV Tape Recorder (Din):30mV
Equalizer	Phono:RIAA NF type MIC:Flat NF type
Tone Control	Bass: 30Hz, @ 15dB Midrange: 1500Hz, @ 5dB Treble: 15000Hz, @ 15dB
Tone Switch	low filter: 20Hz-10dB high filter: 20kHz-10dB
Muting	-20dB
Overall:
Semiconductors:	Transistors: 31 Diodes: 5
Supply voltage	100V~250V, 50Hz/60Hz
Power consumption	240W max
Dimensions	Width 415x height 127x depth of 278mm
Weight	9.9kg
Year	1970-71
Price 1970	52,800 Yen

 

 

>>> Brochure of the Sansui AU-666 click here to view

 

detail back ^

Bottom original view ^

Original interior view with main pcb's ^

 

power on, front detail ^

 

RIAA phono pcb updated ^

 

The regulated power supply pcb updated ^

 

pcb of 1 of the power amps and the heatsinked driver transistors ^
 

Tone amp pcb ^
 

Special futures:

This amp is well known by audio enthousiasts, it has the mid-range tone control, very typical for this AU amplifier range of Sansui.
All AU-series high-quality amps of Sansui made in that time frame are equipped with mid-range tone control.

Old broken NEC power transistors ^
 

New Toshiba power transistors: PNP shown here ^

 

Schematic:
 

Direct-coupled & fully symmetric/complementary power amplifier, with differential-amp input circuit and moderate feedback,
incorporating a split +/- power supply.

Technical info:

Click here to see the main AU-666 schematic diagram in PDF format

 

Click here for the user & service manual of the Sansui AU-666 in PDF format

 

The original power amp pcb not modified ^  and the RIAA phono pcb not upgraded >

 

Obsolete circuits that should be taken out of the amp:

Low filter, there are 2 inside the amp ^
 

Obsolete protection-crowbar pcb (part) ^

 

Crowbar replacement circuit shown here:

The new speaker protection-circuit: sitting on the place of the old crowbar pcb, fully wired and operational, with green power led and wiring to the blue "OK" led. ^

 

Modifications:

Exchange the crowbar-circuit pcb for a better and more modern DC-offset protection-board as on the photo.
Sansui has a habit to wire amplifiers with too long wiring, in this AU-666 its advised to shorten many wires.
Most electrolytic capacitors can be replaced. The driver transistors can get quite noisy, they can be replaced by
modern drivers if required, in this amp they were ok.
The power-amp input transistors can be replaced by modern input transistors when the amp starts producing noise,
a 2SA1015 (pin compatible) or a BC560C (not pin compatible) (& complementary) will do the job ok.
The diode bridge of the power supply should be replaced by modern >100V 10A diodes or a 10A bridge.
Coupling capacitors in the signal path should be replaced as they will be out of specs.
The 3 tone knobs are L & R separated, but often they start sticking together after many years, separate them again:
spray a very little amount of contact spray or put some oil on the shaft of the aluminium/brass joint:
Turn the shafts separate 10 times and things start to work ok again.
There are some electronic circuit design flaws in this amplifier, you should be aware of this:
If you are familiar with electronics and amplifier design and amplifier building principles,
you will be able to make this to a very good HiFi amplifier with outstanding specs
and very good sound, even lift it to a higher level and outperforming many competitor amps.
(There are many tricks that the electronics expert knows to get the most out of a Sansui AU-666).
This amplifier is worth updating: The driver section of this amp should be changed to let the amp work correctly.
Heatsinking should be checked and set correctly when you upgrade this amp,
the power transistors can get hot if the electronic circuit design flaws are not corrected.
You can get the driver transistors overheated too due to the same electronic circuit design flaws,
this must be corrected to let the power amps work cool & correctly.
The Hitachi driver-transistors used by Sansui are ok, so is the driver heatsinking, but should be checked.
The Mitsubishi 2SC871 may suffer from silver-oxide creaping disease (black feet), you may replace these transistors when required.
There is a low filter between the tone- and power-amp section on both channels, that is obsolete with the install of
the new DC-offset protection-board, it can be removed together with the long wiring and the mounting strips.
If you keep the crowbar pcb in the amps circuit, leave the low filter in the amp.
You may change the low-cut filter switch into a "subsonic" switch, it will be very helpful when you play vinyl records, using the phono input.
Exchange C731 and C732 of 0.01µF into 0,1µF (100nF) 50V. Then if you switch the low cut to ON, the lows will graduately
roll off from 15 Hz to approx. -15 dB at 2 Hz.

 

Upgrade power amp shown below:

This amp-section should be updated for better current and temp. stability in the power-amp ^

^ The additional 2x 1W resistors install as indicated,
* when you use other type new driver transistors, the value of 390 Ω may differ.

 

Obsolete parts with obsolete wiring, only to be removed when the crowbar circuit is not used. ^
 

2SC871 with black leads ^

 

The blue LED shining through the hole of the Treble knob, it shows that the protection-circuit is operational and DC offset is OK and speakers are connected. ^

 

Sound:

The sound of this Sansui AU-666 is powerful and very fine, a real high-quality HiFi amp, better than many competitors from the same time frame.

When you do the upgrade right, this amp will be capable of up to 2x 40 Watts RMS output power.
Today we would call this upgraded amp a High-End product. This amp is a true competitor to the Luxman SQ505x.

This amp has more power and has cleaner sound at higher levels than the Sansui AU-555A.
Its obvious when listening to this AU-666 that the electronic technology inside the AU-666 is more advanced and at a
higher level than the AU-555A or even a AU-888, although the AU-888 can deliver more power per channel.

These Sansui series of audio components soon got legendary by their outstanding build quality and extra features and good sound.

 

 

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© Hans Hilberink - PE1MMK ® - 2003, Last update: 16-08-2020.