Sansui AU-20000 Restoration and Upgrade Thread

[mattsd]

I recently got this amp from a very good friend to completely restore and upgrade where possible. This is one of my all time favorite amps, the looks and sound are amazing.

The front end is completely modular, all the boards except for the tape copy board (F-2503) can be taken out and worked on separately from the amp. Both amp boards, the power supply and the protection board can also be taken out as well. This amp is a joy to work on with this design.

Let us begin!

TONE AMP BOARD (F-2495)

This board is fairly straightforward. C01, C02, C11 and C12 are the input high pass filter caps, originals were 1uF 50v low noise caps and were upgraded with Wima 1uF film. C03, 04, 13 and 14 are for the feedback loop, originals are 47uF 10v polarized and were replaced with 47uF 16v bi-polar. C09, 10, 19 and 20 are the output high pass filter caps, originals are 4.7uF 25v bi-polar and were upgraded with Wima 4.7uF films. All the 2SA726 and 2SC1708 transistors were upgraded with KSA992 and KSC1845 transistors, with TR01 and 03, TR02 and 04, TR09 and 11, TR10 and TR12 all getting replaced with gain matched pairs.

EQ AMP BOARD (F-2501)

C01 and 02 are 1uF polyester film caps for the input high pass filter and were upgraded with polypropylene caps of the same value. C07, 08, 17 and 18 are for the feedback loop, originals were 47uF 6.3v bi-polar and were replaced with 47uF 16v bi-polar. C23 and C24 are the output high pass filter caps, originals were 3.3uF 25v bi-polar and were upgraded to Wima 3.3uF film caps. C601 and C602 are the local decoupling caps, originals were 10uF 50v and were upgraded to 47uF 50v FCs. All the 2SA726 and 2SC1313 transistors were upgraded with KSA992 and KSC1845 transistors, with TR01 and 03, TR02 and 04, TR09 and 11, TR10 and 12 all getting replaced with gain matched pairs. Also, R25 and R26 were 5.6K ohm carbon comps and were upgraded with Vishay RN60 resistors of the same value.

Edit: Need to add, I reversed the polarity of C601 and C602 by accident in the after photo, it was corrected after the photo was taken.

FLAT AMP BOARD (F-2505)

C01 and 02 are the input high pass filter caps, originals were .22uF polyester film and were upgraded to .47uF polypropylene film, the increase in capacitance was to minimize phase shift at the lower frequencies. C05 and C06 are 22uF 6.3v bi-polar caps for the feedback loop, replacements were 22uF 25v Nichicon ES. The output high pass caps are C11 and 12, originals are 10uF 25v bi-polar and were upgraded to Wima 10uF film caps. TR01-04 are 2SC1313 and TR05 and 06 are 2SA726 transistors and were upgraded with KSC1845 and KSA992 transistors, with TR01 and 03, TR02 and TR04 getting gain matched.

C13 and C14 were 1uF 50v electrolytics and were upgraded with Wima film of the same value. C17 and 18 are 4.7uF 25v electrolytics and were replaced with 4.7uF 50v Panasonic FC. TR09 and 10 were 2SA726 transistors and were upgraded with KSA992. R23 and 24 were 5.6K ohm carbon comps and were upgraded with Vishay RN60 resistors of the same value.

More to come shortly!

 

[mattsd]

FILTER & FILTER AMP BOARDS (F-2504 and F-2498)

On the filter amp boards (F-2498), C01 and C02 are 150pF ceramic and were upgraded with C0G caps of the same value. C03 and C04 are the output high pass filter caps, originals were 4.7uF 50v low noise electrolytic caps and were upgraded with Wima 4.7uF film. TR01 and 02 were 2SA726 and TR03 and 04 were 2SC1708 transistors, all were replaced with KSA992 and KSC1845 transistors. On the filter board (F-2504), C09 and C10 are the input caps for the second filter amp stage, originals were 4.7uF 50v low noise electrolytic caps and were upgraded with Wima 4.7uF film. C601 and C602 are the local decoupling caps, originals were 10uF 50v and were upgraded to 47uF 50v.

TAPE COPY BOARD (F-2503)

The tape copy board contains a buffer amp between the output of the flat amp (or tone amp if switched in) and the filter amp. C01 and C02 are the input high pass caps and are 3.3uF 50v low noise electrolytic caps, they were upgraded to Wima 3.3uF films. C03 and C04 are 150pF ceramic and were upgraded with C0G caps of the same value. The output high pass filter is C05 and C06, originals were 4.7uF 50v low noise electrolytic caps and were upgraded to Wima 4.7uF film. C601 and C602 are the local decoupling caps, originals were 10uF 50v and were upgraded to 47uF 50v. R15 and R16 were 5.6K ohm carbon comp and were upgraded with Vishay RN60 metal films of the same value.

METER AMP BOARD (F-2496)

The input high-pass filter capacitors (C01 and C02) were originally 1µF polarized electrolytic types and were upgraded to Wima 1µF films . The 47µF capacitors C05 and C06 in the feedback loop high-pass filter were polarized electrolytics and were replaced with Nichicon ES bipolar caps. The 10µF capacitors C07 and C08 on the TA7136P output were changed to 10uF Panasonic FC. The 1uF bi-polar capacitors C11 and C12 were upgraded to Wima 1µF film caps. The .68uF tantalum capacitors (C13 and C14) were also upgraded to Wima film caps. C15 and C16 were 1uF 50v polarized electrolytics and were also replaced with Wima films. C601 and C602 are the local decoupling caps, originals are rated at 47uF 25v and were upgraded to 100uF 25v FC. All the 2SC1313 transistors were replaced with KSC1845 and both 15v zeners (RD-15E) were replaced with Vishay 2% equivalents.

Up next, the power supply and driver amp boards!

 

[mattsd]

POWER SUPPLY BOARD (F-2508)

The power supply board on this unit is a breeze to work on. It does have more extensive glue usage compared to the other boards, so first thing was to get that off.

And after!

C01 and 02 are 10uF 100v polarized electrolytics and were replaced with the same values. C03 and 04 are 3.3uF 50v polarized electrolytics (SM parts list and schematic lists them as 47uF 50v) and were replaced with the same values. C09 and C10, which shunt ZD01 and ZD02 respectively, were 33uF 16v polarized electrolytics and were upgraded to 100uF 25v, the voltage being upgraded as they see around 15v. C11 is a 2.2uF 100v polarized electrolytic and was replaced with the same value. C12 is a 3.3uF 63v polarized electrolytic (listed as 2.2uF 80v on the schematic but 3.3uF 63v on the SM parts list) and was replaced with the same value. C13 and C14 are 10uF 50v polarized electrolytics and were replaced with the same values. C15 and C16 are the local decoupling caps for the +65V and -65V rails, originals were 220uF 80v and were upgraded to 680uF 100v. C17 and C18 are the local decoupling caps for the +35V and -35V rails, originals were 330uF 50v and were upgraded to 1000uF 50v. ZD01 and ZD02 were 15v 0.5w zeners and were replaced with Vishay 2% equivalents. D601 and D602 were VD1212 diodes and were both replaced with two 1N4148 diodes in series. TR01, 03, 09 and 15 were 2SA850 transistors and were upgraded to KSA1013. The 850 has a thick center collector lead, the design was so heat could be dissipated through the lead into the foil trace, but often it leads to cracked solder joints around that lead with extended use. TR02, 04, 06 and 12 were 2SC1735 and are the NPN compliment to the 850 and have the same thick collector lead design. They were all replaced with KSC2383 transistors. TR13 was a 2SC1313 and was upgraded to a KSC1815. TR14 was a 2SA726 and was upgraded to a KSA1015. Four resistors affected by the glue (R13, R24, R28 and R35) were replaced as well.

CONNECTOR BOARD (F-2512)

(Note: the manual has the wrong number listed for this board, in the manual it is listed as F-1512 instead of the proper F-2512)

Fairly simple here, C01 and C02 filter the incoming raw DC rails to the power supply board (+85 and -85v). Originals were 470uF 100v and were upgraded to 1000uF 100v Nichicon HW caps. C03 and C04 filter the main +68 and -68V rails supplying the driver and predriver transistors, these same rails also supply the outputs through different lines coming off the main filter caps. Originals were 10uF 100V (listed as 22uF 75v on the schematic) and were upgraded to 47uF 100v.

DRIVER AMP BOARDS (F-2507)

The left and right driver amp boards are identical so they have been marked L and R for easier identification. Note I forgot to get a before of the left driver amp board, oops!

C01 is the input coupling cap and was a 3.3uF 50v polarized low noise electrolytic and was upgraded with a 3.3uF Wima film cap. C03, which shunts ZD01, was a 33uF 25v polarized electrolytic (listed as 47uF 16v on the schematic) and was upgraded to 100uF 25v. C06 is the feedback cap and was a 100uF 6.3v bi-polar cap, replacement was a 100uF 25v Nichicon ES bi-polar. C07 and C08 are the local decoupling caps, originals were rated at 100uF 80v and were upgraded to 390uF 100v. C09 is a 3.3uF 50v polarized electrolytic and was replaced with the same type and value. VR01 is the DC-offset adjustment pot and was replaced with a 25 turn Bourns equivalent. VR02 adjusts the bias and was replaced with a single turn Bourns equivalent. R35 and R36 are 4.7ohm resistors for the base drive on the outputs, originals were 1/2w carbon comps and were upgraded to 1/2w metal films. D03 is a VD1212 and was replaced with a pair of 1N4148 diodes in series. ZD01 is a 22V 0.5w zener and was replaced with a Vishay 2% equivalent. TR01 and 02 were 2SC1708 transistors and were upgraded with a gain matched pair of KSC1845 transistors. TR03 was a 2SA847 and was upgraded to a KSA992.

Almost done! Just the protection board, repinning the dual layer main output boards and the filter caps left to do!

 

[mattsd]

PROTECTION BOARD (F-2511)

This board had some caps already replaced with less than ideal replacements, C07, C10 and C905. C02 is a 47uF 50v polarized electrolytic and was replaced with the same value. C05 and C06 were 47uF 6.3v bi-polar caps and were replaced 47uF 16v Nichicon ES caps. C07 was a 1uF 50v bi-polar, someone replaced it with a bi-polar axial cap. This was replaced with a Wima 1uF film cap. C08 is a 3.3uF 50v low leakage cap and was replaced with a 3.3uF 50v Nichicon KL. C09 was a 47uF 6.3v polarized electrolytic and was replaced with a 47uF 25v cap. C10 was a 10uF 50v, previously replaced with a 10uF 100v cap. This was replaced with the original value, 10uF 50v. C11 is a 33uF 50v and was replaced with the same value. C905 controls the protection relay timing and is put in the position of R22 on the board. Original is a 10uF 50v, it was replaced with the same value. All the carbon comp resistors were also replaced with equivalent metal films. Both relays were also replaced and the common cathode and common anode rectifiers were upgraded to MUR1620 ultra fast rectifiers.

OUTPUT BOARD (F-2497)

These boards have several passthroughs known for causing problems and this amp was no exception, a couple of days before I received it to start the restoration, the owner claimed it went into protection and would not come out of it. I was guessing it may be the passthroughs causing the issue and indeed it was, once I opened it up and pushed on the left output board, the protection instantly clicked in for as long as I held it. So, both boards came out and were repinned. I circled the pins that were redone on both boards, there are more on there but they had component leads or pins next to them that act as passthroughs of their own due to being soldered on both sides, so I did not need to redo those.

FILTER CAPS

Filter caps are easy to do with one exception, you have to enlarge the holes for the copper ground bar and ring terminals for the wires. Not hard work, but tedious. After that everything easily went together. The new caps are slightly taller than the old caps and just barely fit, they are at the max height this chassis can take.

Old vs new:

And all done!

Coming next, first listening impressions and a full bench test!

 

[mattsd]

Helps when I add the after photo of the protection board lol. Post edited.

 

[Ronito6]

Wonderful presentation!

 

[Hyperion]

+1 A beautifully presented restore.

 

[richardpp]

Thanks for sharing your work, the details of the upgrades and replacements are invaluable! I like your 'common sense' approach to pinning the output boards.

 

[mattsd]

POWER UP AND FIRST LISTENING IMPRESSIONS

Power up went smoothly, started it on a 100W DBT and after checking everything out moved to line power, setting the offset and bias to manual spec. Let it idle for 2 hours just to make sure the bias and offset were stable. Hooked up my speakers and source, speakers are ADS L1230s that have been recapped, source is a Hifiberry DAC+ Pro with modded RCAs and modded power input. Speaker and RCA cables are both limited run custom cables made by Ultra Fi (they are no longer in business). Found a song I know very well, set volume to a good listening volume (80-85dB) and let her go. Wow, this thing is transparent now, before it had a slight veiled sound on the highs, but not anymore. Not only is it very transparent but it has amazing resolution, the detail and micro detail of this amp is far beyond what I thought it would sound like after the restore. It projects a very wide soundstage that is instantly immersive with excellent imaging, the speakers disappear and you hear nothing but a wall of sound with all the voices and instruments having their own space. I've heard this before with modern high end audiophile setups, but to hear it out of a mid 70s integrated amp and early 80s speakers was impressive to say the least. The amp is very easy to listen to for hours, it draws you into the music like only a few other amps I've ever heard.

I delivered it to my friend a few days ago and we set it up with his system. He uses Dynaudio Excite X36 speakers, which are in a class of their own, very detailed and highly resolving speakers and will reveal any flaw. We hooked the amp up to them and WOW! Everything I heard with the ADS speakers was even better with the Dynaudios, with several songs he played I thought I was listening to a expensive modern audiophile amp. The transparency, detail and imaging was jaw dropping, nothing seemed out of place and it was so natural sounding with some songs you could pick out the way a instrument was being played or the way a singer was moving their mouth, even the little breaths and noises were easily heard and imaged. Needless to say I am very impressed with this amp, it has exceeded my expectations in every way. The bench test specs are quite impressive as well, which I will post soon, just have put together everything into a easily read format.

 

[mattsd]

BENCH TEST SPECS

The readings I got from this amp are nothing short of spectacular, especially when compared to the original specs. All were very close between channels as well.

Max power @ 1% THD+N into 8 ohms (1KHz):

Left channel: 220.60W (42.01v RMS)
Right channel: 220.71W (42.02v RMS)

THD+N @ rated power (170W) into 8 ohms (manual spec is 0.05%):

Left channel: .00633%
Right channel: .00679%

OUTPUT/THD+N (1KHz):

S/N ratio @ rated power (170W) (AUX, A-weighted) (manual spec is "better than 80dB"):

Left channel: 94.57dB
Right channel: 94.84dB

S/N ratio @ 1W (2.83v RMS) (AUX, A-weighted):

Left channel: 73.77dB
Right channel: 74.48dB

Channel separation (AUX, 1KHz, rated power, shorted input) (manual spec is "better than 60dB")

L>R: 68.73dB
R>L: 74.38dB

Channel separation (AUX, 1KHz, rated power, open input)

L>R: 56.62dB
R>L: 56.16dB

FREQUENCY RESPONSE (20Hz-20KHz)

And that finishes up this restoration! Overall a very pleasant piece to work on and the results are amazing, both sonically and performance wise. If I had the opportunity to pickup this amp I would do it without hesitation, it looks amazing and the sonic quality is even more so. This will go down as one of my favorite amps to work on and listen to.

 

[Hyperion]

Outstanding.

 

[Doegee]

Matt, My name is Bill and I have an AU20000 that needs repair. Im not sure the best way to contact you other than this forum would you contact me at doegee@att.net so I can talk to you about repairs

Thanks

Bill

 

[EMenendez]

Another great AU-20K restoration!
Congratulation to the owner of this great amp!
Thank you for sharing your big work, I inheritade my AU-20K from my father, he bought this amp in 1975 the same year when I was born.
For me is very special amp the sound maybe is not the same that 20 or 25 year, I'm planning send my AU 20K for a complate restoration but always is not easy find the best people to do that.

 

[dsmart705]

Really nice work and thread. Your work is outstanding and I can attest to the greatness of the AU20K. One of the best looking and sounding units I've ever owned and something I plan on passing down to my son (he is 3 which means I still have alot of time with it!) I also like your choice in main filter caps....I used the same one but I think I went for the 33,000uf 80v ones which look like they are the same dimension as the ones you put in. They just fit

 

[Tina Louise]

CONNECTOR BOARD (F-2512)
C01 and C02 filter the incoming raw DC rails to the power supply board (+85 and -85v). Originals were 470uF 100v and were upgraded to 1000uF 100v Nichicon HW caps. C03 and C04 filter the main +68 and -68V rails supplying the driver and predriver transistors, these same rails also supply the outputs through different lines coming off the main filter caps. Originals were 10uF 100V (listed as 22uF 75v on the schematic) and were upgraded to 47uF 100v.

DRIVER AMP BOARDS (F-2507)
C03, which shunts ZD01, was a 33uF 25v polarized electrolytic (listed as 47uF 16v on the schematic) and was upgraded to 100uF 25v.

C07 and C08 are the local decoupling caps, originals were rated at 100uF 80v and were upgraded to 390uF 100v.

Hi, what went into the decision to upgrade the capacitance of all of these?

Great work, BTW.

 

[Al Casey]

Stunning and inspiring!