Author Topic: Sansui-AU 111 revisited  (Read 9845 times)

Offline Ampdog

Sansui-AU 111 revisited
« on: May 07, 2015, 05:34:27 PM »
As some interest in the Sansui AU 111 has been rekindled by recent comments in the "For Sale" section on a restored model, perhaps some details. (This has in fact been posted before but I seem unable to access that thread. Perhaps PC memory also deteriorates with growing years ....)

It was hinted that I 'improved' the circuit, which is true, but not dramatically so in audible sense. This was about a more logical choice of component values, but also some improvement in the power amplifiers. The latter had again to do with better balance and improvement of NFB stability rather than distinctly better sound. If of any interest I could expound on some of these for would-be restorers.

May I in conclusion thank those members who there came forward with positive comments - I did follow the thread at the time. It is always gratifying to know that one's work is appreciated, even when hidden from normal view. Engineers also have their pride.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline King_Julian_S

Re: Sansui-AU 111 revisited
« Reply #1 on: May 07, 2015, 05:37:21 PM »
Much interest sire .. Please proceed...  :2thumbs:
Non audiophile music lover...

Offline Family_Dog

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Re: Sansui-AU 111 revisited
« Reply #2 on: May 07, 2015, 06:08:55 PM »
I'll make this thread a 'Sticky', so it can't disappear.

Am paying close attention, Ampdog!


-F_D



-Eric

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Offline Ampdog

Re: Sansui-AU 111 revisited
« Reply #3 on: May 11, 2015, 09:10:20 PM »
I have now located the previous thread - from an internet source! Not much interest shown here now, so I will just quote this for further present interest. Any fresh questions can then be dealt with here.
www.avforums.co.za/index.php?topic=6745.0
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline Family_Dog

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Re: Sansui-AU 111 revisited
« Reply #4 on: May 11, 2015, 09:37:39 PM »
Thanks Johan,
it is now a 'sticky' so it won't be lost again.

My first AU-111 requires some TLC, it has become softer.... and softer.... and softer. In volume, that is. Haven't looked at it yet but suspecting power supply problems as it affects both channels.


-F_D



-Eric

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Offline Kent Kassler

Re: Sansui-AU 111 revisited
« Reply #5 on: May 11, 2015, 09:41:49 PM »
I'll take it.
Audiophile Sound Sommelier Extraordinaire....aka Manic Depressive Temporary Void Filler Deluxe.

Offline Family_Dog

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Re: Sansui-AU 111 revisited
« Reply #6 on: May 12, 2015, 06:47:07 AM »
Aah kent, you know I will bring it back to life! Juat one of the many projects at hand. It cannot be anything major, I restored this one some 10 years back with new valves & caps and no, it looks nothing like Johan's fabulous restoration, I stuck to Sansui's original layout. I'm lazy, you see.

Actually, I am tempted to do it similar to the way that Johan carried out his rebuild, but if I do one, I will do three of them and that works out to many, many man-hours. Hence, I'm only considering it at this stage.

-F_D



-Eric

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Offline King_Julian_S

Re: Sansui-AU 111 revisited
« Reply #7 on: May 12, 2015, 08:45:12 PM »
Do it FD do it ...
Do it FD do it ........
Do it FD do it .....
Do it FD do it .....

Hopefully the cheerleading will persuade you sire 😄👍
Non audiophile music lover...

Offline Ampdog

Re: Sansui-AU 111 revisited
« Reply #8 on: May 12, 2015, 10:38:40 PM »
I'll take it.

Sorry, Chum - Pretoria is closer to Klerksdorp than Cape Town ....
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline Family_Dog

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Re: Sansui-AU 111 revisited
« Reply #9 on: May 13, 2015, 07:13:06 AM »
Aaahhhh.... it's nice to feel wanted :)


-F_D



-Eric

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Offline Ampdog

Re: Sansui-AU 111 revisited
« Reply #10 on: May 18, 2015, 02:13:57 AM »
Studying the history again in some spare time:

Rant Warning: One sore point (again referring to a schematic): That funny 'PRESENCE' switch at the upper left side of the front panel (S9a-b).

This gives a rise in response from below 120 Hz. I have always associated 'presence' with some accentuation in the 2 - 4 kHz frequency region. What this one does is create a rather unbalanced sound to me. Worse, it can cause instability in the l.f. region, visible as an alarming pulsating glowing of the 6L6 screen grids with any volume above average.

(It increases gain by NFB reduction by insertion of a 0,3F capacitor (C65-C66) in the main amplifier NFB circuit. This is considerably worse than deviating from the original 0,3F coupling caps as the designer warned in a previous reference!)

Out with it! Though I have not done that with my restored models. A far more realistic sound response is created by having the 'LOUDNESS' switch on (S8a-b). Safest is to short it (i.e. a short over C65.R81 and C66.R82), or replace those components by a permanent short. Or one can figure out a more suitable way of introducing a real 'presence' feature somewhere in the pre-amp stages.

One does not do 'tone' controlling in the main amp feedback loop! That is optomised for proper and stable NFB and then left alone.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline Ampdog

Re: Sansui-AU 111 revisited
« Reply #11 on: May 18, 2015, 03:06:39 AM »
6L6 bias adjustment (separating subjects):

It might be noticed in the recent photos that I have provided independant bias controls for every 6L6 by doubling [VR12 - R101 - C81] and [VR13 - R102 C-83].  What with a sometimes large spread in valve characteristics these days, it makes little sense to design a sophisticated output circuit without ability to separately set the bias current for each valve. I have also converted the bias circuit to a bridge rectifier one rather than half wave rectification (cheap enough these days).  As said before I have changed the resistive networks round this to give a less mechanically sensitive setting procedure. Previously one touched the trimpots and the reading dashed all over the show - not good to make a setting too sensitive to mechanical slider contact.

It may not be clear on the schematic of the 'TREBLE' control (VR3 - VR4) which is the slider and which the center tap. The sliders are on the left sides of the control sketches (going to R111 - R112). The tap goes to pins 7 of the valves V2, V3.

Then the PHONO STAGE:

Transistors T1 and T2 are erroneously shown as PNPs. They are NPNs. The numbers are correct.

I have replaced by low-noise BC550s. I have also reproportioned the RIAA networks and other caps to give a sharp cut-off below some 30 Hz. This is done without extra components, merely by correctly proportioning the relevant R.Cs. (For me a necessity in RIAA stages, to dump all rumble and 'bump' disturbances at no extra cost. As said before, why not done more often .....  :thinking:) My crippled PC cannot download diagrams at present; hopefully at a later stage for those interested.

Permanent Reminder:

Change those impractical vaues of C1-C2-C59-C60 to 1f/63V polyester types as mentioned before.

Do check the rather inferior loudspeaker slide switches S10a-b, S11 and S12a-b. Better models are available with slight changes of the mounting holes (also mentioned before).

Finally for now:

I found some AU111s with the large 12K 30W resistor R39 (mounted on top beside the power transformer and other hot resistor R98) open. This is difficult to source locally.  I rebuilt this with two cement 3,9K/10W and one 4,7K/10W in series, sandwiched between two alu plates, suitably mounted in the same place. This is a bit of a schlepp. (In these days of small semiconductors one can rather design a more suitable and cooler transistor regulator to replace these wasteful resistors.) 
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline henniekuyper

Re: Sansui-AU 111 revisited
« Reply #12 on: June 14, 2015, 04:54:28 PM »
Hi Ampdog


I have repaired quite a few of these amps.
You are correct in the fact that the transistor shown should be marked NPN. Just as a note the later or final edition of the Au-111 indicates 2SC650 as the also used from the factory.

This is the parameters of the 2SC650:
2SC650 Transistor Datasheet. Parameters and Characteristics.

Type Designator: 2SC650

Material of transistor: Si

Polarity: NPN

Maximum collector power dissipation (Pc), W: 0.2

Maximum collector-base voltage |Ucb|, V: 30

Maximum collector-emitter voltage |Uce|, V: 25

Maximum emitter-base voltage |Ueb|, V: 6

Maximum collector current |Ic max|, A: 0.03

Maksimalna temperatura (Tj), C: 175

Transition frequency (ft), MHz: 110

Collector capacitance (Cc), pF: 5

Forward current transfer ratio (hFE), min: 100

Noise Figure, dB: -

Package of 2SC650 transistor: TO1

I have used the BC182L with great success in the circuit.

I have experimented also with various brands of tubes in the amp. Tungsol , Electro Harmonics,  Tesla etc. Found both the Tungsol and EH to be a working great in the amp. Once the Bias is set corretcly it really sounds great!

All the amps that I did, I replaced all the electrolytic and oil capacitors.

Hope it helps a bit.

Kind regards
Hennie


Offline King_Julian_S

Re: Sansui-AU 111 revisited
« Reply #13 on: June 15, 2015, 06:22:54 AM »
Maybe not inline so pls delete if not...

But whoever owns one of these Ampdog restored amps, pls do yourself a favour and try a highend (/or any that you like ) preamp and use this amp as power only ... Absolutely sounds beautiful .... These are not just keeping pricing for nostalgic reasons.


Non audiophile music lover...

Offline henniekuyper

Re: Sansui-AU 111 revisited
« Reply #14 on: June 15, 2015, 09:18:51 AM »
Hi K J S

I full y agree with you. I have seen them going up and up in price. If I recall the one went for approx. R45k.

Yes you can easily use it with another pre-amp. I have found that when all the necessary electrolytic replacements are done the pre-amp sounds not to bad.