Author Topic: Sansui AU111 Valve Amplifier  (Read 22592 times)

Offline Ampdog

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Sansui AU111 Valve Amplifier
« on: November 09, 2010, 03:52:33 AM »
This flagship has been under discussion before. I have just finished refurbishing one for a member, and I must say that it was the first amplifier where I found all the characteristics to measure as claimed and the valve design optimal. Only slight change necessary was in the main NFB circuit to cure some drastic h.f. damped oscillation. (I presume these things are checked in the final execution; then why oh why can't the designer get it right first time? I find that in many valve amplifiers, and it is mostly rectifiable fairly simply.)

There are a great many adjustments available, more than are really necessary. I guess it was designed when the folks over in the east thought all those were desireable. High and low pass filters cut off sharply, not the double RCs found in some designs. As stated before, it is only the third design I found to use the distributed load output stage with local feedback (a la Quad II) configuration. The output transformers are massive for the claimed output; as it is now an easy 40W/channel is obtainable.

The only other thing that needed attention were the (flimsy and corroded) output slide switches, giving an 8 - 16 ohm output impedance choice, and a phase reversal option. After consultation with the owner they were simply bypassed to 8 ohms and standard phasing.

I have no general experience of Sansui products, but if this one is representative, they could be purchased with confidence.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

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Re: Sansui AU111 Valve Amplifier
« Reply #1 on: November 09, 2010, 06:22:14 AM »
The old Sansuis rock!

Johan, did you perhaps keep notes on the changes you made to the amplifier?  I have three of them but only did the cap and rectifier diode changes, and also the electrolytic cathode bypass cap used in one of the pre-drivers.


-F_D



-Eric

That Guy in South Africa...
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Offline Ampdog

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Re: Sansui AU111 Valve Amplifier
« Reply #2 on: November 17, 2010, 01:19:17 AM »
Sorry, F_D,

...did not look here recently. Yes, I will PM you. As said, it concerned the NFB components in the main amplifier section. Several tens of cycles of damped oscillation on a square wave can be reduced to almost nothing. It is also stable with loudspeaker load up to 'no load', which reassures where loudspeaker impedance may go high at high frequencies. Lastly, I changed some of the rediculously high value coupling capacitors forcing use of electrolytics - I don't think one needs to consider going down to < 5Hz!
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline Moog

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Re: Sansui AU111 Valve Amplifier
« Reply #3 on: November 18, 2010, 08:04:11 PM »
Ampdog, I have been waiting paitently with great anticipation. I would like to use the amp with the Kef 105.2 speakers that are being refurbished  by another member from this forum. The KEFs are power hungry and i hope the SANSUI will have enough grunt to drive them.
I was impressed by Hendrix. His attitude was brilliant. Even the way he walked was amazing -

Richie Hugh Blackmore

Offline TheMooN

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Re: Sansui AU111 Valve Amplifier
« Reply #4 on: March 16, 2011, 04:32:28 PM »
Howdy Folks ~ Inaugural post here on this excellent forum  :)

I find myself intrigued upon reading Ampdog's comments regarding NF and HF oscillation on the AU-111 and would be most grateful for a little further information on this point .

All the best.

TM     

Offline Moog

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Re: Sansui AU111 Valve Amplifier
« Reply #5 on: March 16, 2011, 08:24:29 PM »
I would like to thank Ampdog for the the time he spent on working  on the SANSUI AU-111. He is one of those unsung heroes and and an   "audio wizard". When i bought this from another forumite, despite getting assurances from the seller, another forumite, that the amp was "working", on switching the amp "on", there was no sound at all. Thank you Ampdog. Let us meet again for for a cigar and some coffee.
I was impressed by Hendrix. His attitude was brilliant. Even the way he walked was amazing -

Richie Hugh Blackmore

Offline Ampdog

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Re: Sansui AU111 Valve Amplifier
« Reply #6 on: March 20, 2011, 01:46:12 AM »
Howdy Folks ~ Inaugural post here on this excellent forum  :)

I find myself intrigued upon reading Ampdog's comments regarding NF and HF oscillation on the AU-111 and would be most grateful for a little further information on this point .

All the best.

TM    

Sorry, TM - F_D alerted me to this; I did not read here recently. My alterations were as follows, also referring to the circuit diagram, which I hope you have. Apologies for the length. (I will refer to only one channel - changes obviously pertain to both channels.)

1.  In the power amp (V6 onward) I found an output square wave to have excessive ringing on the leading sides. Without a load the amplifier tended to oscillate, indicating conditional instability. The ringing resulted from a peak in response around 80kHz in the 6L6 stages. Smoothing first without global NFB and then 'shaping' matters after NFB, a more stable response resulted from adding C90 to both 6L6s and increasing the value to 68nF. This can be conveniently achieved by connecting one 33nF capacitor cathode-cathode for the 6L6s. Then there is the no-name RC network across R75 in the interest of keeping the loop gain controlled. For best results I found these to be 5K6 and 82pF respectively . Finally I added a phase lead compensating capacitor of 330pF over R73. A square wave then shows hardly visible under/over-shoot.

 I also changed R75 to 68K for somewhat improved linearity. Funally I increased R87, R89 to 100K. This gives a marginally larger total grid resistance than the maximum recommended for 6L6 fixed bias, but that specification is quite safe unless a 6L6 is quite gassy. The advantage is that the 12BH7 works into a slightly kinder load. [Loading R83, R85 heavily ac wise limits the output signal amplitude that can be achieved without serious distortion, particularly in this circuit where a large 6L6 grid input is required because of the local cathode feedback.]

In the loudspeaker circuit I got rid of S10 and S11, as they were quite corroded, replicas were not obtainable and the owner had no interest in phase changing or the alternative of a 15 ohm option. The fewer questionable devices in the loudspeaker circuit the better.]

2.  I found the 6L6 bias adjustment far wider than necessary, resulting in difficulty to set the bias with comfort, particularly with the carbon trim-pots used. I joined both outer sides of R12 - R15, getting rid of R101, R102, R115 and R116. Where the 'top' common junction went directly to the diode and C81, I inserted a 3K3 resistor to the 'top' pot junction. Instead of the discarded 15K resistors I inserted (now common to all four trim-pots) a 27K resistor. This should give sufficient bias adjustment tolerance for the normal spread of 6L6 characteristics.

Then there was the difficulty to exactly measure what current each 6L6 draws. For this purpose I permanently connected 1% 1 ohm 3W resistors from every 6L6 pin 8 in series with the rest of their circuits. One can then measure over these with a mV meter. Their presence has negligible influence on performance.

3.  I found a few (with respect) rediculously large value capacitors in the signal line, thus forcing the use of electrolytics. C1 could become 68nF - yes, as low as that! I modelled the RIAA input circuit and a good response down to 30Hz was obtained, providing a sharp roll-off below that, as it shoud be. Practice bore this out.

Another absurd value is that of C59. This output cannot provide a very low output impedance anyway; I made C59 100nF.

4.  For the rest, all capacitors/resistors  were replaced. C81 can become a 470F-100V; with modern small sizes it fits nicely. I further do not like very low voltage electrolytics; those cathode bypass capacitors could be 16V or more.

Electrolytic caps were replaced with 450V types (even C75-C76; C81 went to 470F/100V), and other capacitors were polyester, 630V, or ceramic for the compensating types.

This covers the component changes. Perhaps to add; I found 5881s in the output stages. The power stage currents and voltages exceed the ratings of the 5881; only 6L6GC should be used. (NOT EL34s - their heater ratings exceed the power transformer rating, and the power stage NFB stability will have to be reconfigured.)

Mains adjustment: Even with the selector set to 240v, there was still 6,7V across heaters. This can only be settled by the cumbersome method of a series mains resistor.

I re-iterate that these changes worked well in my model of the amplifier. The two channels were matched to within close tolerances, but I had experience with only the one unit. Others must comment on possible differences found with more models.

I trust that these notes will be of help.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline Ampdog

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Re: Sansui AU111 Valve Amplifier
« Reply #7 on: March 20, 2011, 03:32:17 AM »
No desire to belabour here ....

but I just read further comments from TM in a PM (oops...) to a common friend (jeepers - Engrish! I don't mean the friend is common ..)

Just to add for any interested parties: There appear to be several differences from model to model of the AU111 - perhaps well-intentioned improvements. Not all circuit diagrams look the same, and components found in the model I worked on did not show on the diagram. Yet it was fairly apparent that they were added in the factory, not wired in later by an owner.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline TheMooN

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Re: Sansui AU111 Valve Amplifier
« Reply #8 on: March 20, 2011, 01:59:01 PM »
Many Thanks for taking the time and trouble to list your experiences whilst servicing the Sansui Johan , Most Kind :0}

My thanks also to Eric for greasing the wheels of communication .

I shall enjoy digesting your recommendations and may well follow in your footsteps when my AU-111 service is a little further forward.

I have been looking at Fairchild Ultra Fast , Soft recovery types however these might be considered a tad overkill ?
http://uk.farnell.com/fairchild-semiconductor/isl9r860p2/diode-ultrafast-8a-600v/dp/1095128

Also wondering what Transistors folks have used in place of the 2SC-402/650's.

All the best.

TM.

Offline TheMooN

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Re: Sansui AU111 Valve Amplifier
« Reply #9 on: May 15, 2011, 04:16:31 PM »
Hi Johan ~ I'm not sure that I have interpreted your changes in point 1 correctly , or have I ? 

http://i177.photobucket.com/albums/w220/Tsushima1/AmpdogAU-111ModsV2.jpg[/img]]

Offline Ampdog

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Re: Sansui AU111 Valve Amplifier
« Reply #10 on: May 16, 2011, 02:52:03 AM »
Moon,

Not quite:

The 330pF is over (in parallel with) R73 (4.7K) in the NFB line, not R77 (1,5K), cathode of V6a.

The 33nF is connected between the cathodes (pins 8) of the 6L6s, not between cathodes tied together and point C1 as you show (the cathodes are not tied together!).  C90 is then omitted.

Other changes showed are correct.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline TheMooN

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Re: Sansui AU111 Valve Amplifier
« Reply #11 on: May 16, 2011, 10:51:54 AM »
Thank you for the qualification AD , re the 33nf , I had this correct in Version 1 then had doubts as the replacement for C90 is no longer tied to C1 hence V2 .

Offline Ampdog

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Re: Sansui AU111 Valve Amplifier
« Reply #12 on: May 19, 2011, 02:29:20 AM »
If you have access to a scope and square wave generator it is easy to tweak the cap over R73 (R74) so as to give no overshoot/undershoot into the load. (The generator, naturally, inputted at the 'main input', not through the pre-amp!). In more sophisticated amplifiers I include a trimmer capacitor with this cap so as to individually fine-adjust each amplifier.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline TheMooN

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Re: Sansui AU111 Valve Amplifier
« Reply #13 on: June 27, 2011, 11:32:20 AM »
Staggered Over The Finishing Line In The End  !!! Stock Circuit ATM , No Mods ~ For Now  :)






Offline ghostinthemachine

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Re: Sansui AU111 Valve Amplifier
« Reply #14 on: June 27, 2011, 02:14:01 PM »
Those sure are stout resistors in the driver stage!
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