Sansui AU111 Valve Amplifier

[Viagara]

Those sure are stout resistors in the driver stage!

Indeed! They need to be given a hiding for the mess they made with the wiring

[Ampdog]

Moon,

Some differences in layout from the one I worked on. Seems like a later factory original ... or is it?

One thing - urgent! - get those 'stout' resistors (12K, 33k, 47k) AWAY from snuggling in between the capacitors! They get quite hot and heat is an enemy of capacitors. Those were originally mounted away, all in a row between 2 tag strips, in the space where the white paper (probably indicating output transformer connections) is stuck on now. Quite the best place for them (never mind the longer connecting wires, doesn't count in audio).

[TheMooN]

Yes I have to agree A-D , seem's odd practice ~ However the component layout appeared to be as factory untouched when I started the re-build of this first revision Black/Alu knobs serial No  96010372 .

I have seen other builds along this configuration ~



I shall see what I can do to give the components a little more personal space .

TBH , I was half expecting a slap for using 250V small value caps in the Pre-Amp stage instead of the circuit indicated 400v'ers 

[Ampdog]

TBH , I was half expecting a slap for using 250V small value caps in the Pre-Amp stage instead of the circuit indicated 400v'ers 

Ya-e-e-e-e-h ... One must be careful there. With a s.diode rectifier power supply most of the coupling caps will have the full unloaded h.t. over them for about 15 sec, until the power valves have warmed up, and that will be >400V. I am not sure what the safety factor is with poly-caps these days, but that is not safe. For a few Rand difference in cost I regularly use 630V caps only; small enough in size etc.

[Viv]

Hi Ampdog,

Valuable info here. It is very informative.

Sansui has produce some very good products before 1990. I once had an amplifier from my uncle which he purchased in the 60's and it was great for the time, however it blew a few speakers in its time.

Viv.

[Moog]

This amp first came to the market in 1965. When i listen to it ,  i realise that  by 2015, that is four years from now, the design wll be fifty years and it still sounds fabulous. For the same reason, I do not mind paying good money for a vintage McIntosh or Marantz tube gear, but unfortunately, in our shores, these are as rare as hen's teeth. It would appear that after all these years, not much has changed/improved in terms of amplifier design and sound quality. Tubes certainly have some magic.

[TheMooN]

Ya-e-e-e-e-h ... One must be careful there. With a s.diode rectifier power supply most of the coupling caps will have the full unloaded h.t. over them for about 15 sec, until the power valves have warmed up, and that will be >400V. I am not sure what the safety factor is with poly-caps these days, but that is not safe. For a few Rand difference in cost I regularly use 630V caps only; small enough in size etc.

Sound advice A-D ~ The power supply poly's are 1600V , 400V'ers on the HT of the Pre- and 630V/850V in the Output stage .

I used the 250V Wima's only on secondary-sub circuit duties such as the various Function and Tone controls , nothing running to ground or handling more than a few Volts ~ As  noted earlier in the thread , Sansui it would seem over-spec'd several of the capacitor ratings on the AU-111 . Perhaps a case of using componants that were in stock , or available at a good bulk price 

I plan to run the amp in stock configuration for a while , before looking at the mods resulting from your excellent research 

           

[Ampdog]

Thanks for inferred compliments.

Ah!

No I feared that you used the 250V types in general.

Regarding voltage specs in capacitors: It might be uneconomical to purchase too many different types in mass production; in small quantities it is often a question of what popular ratings a shop stocks. As said, for myself, I stay with 630V with valve jobs. But small poly-caps of various sizes for low voltages can make a difference in compact pc layouts. There again, locally, one sometimes has a job to find size fit, what with the various size configurations even for the same thing. In that case I use what the dealer stocks mostly, and hold thumbs for the future! 

[TheMooN]

@AD ~ I am wondering , taking a look at the circuit and the way Sansui have the amplification stage set up do you feel that keeping the value of the coupling caps at 0.33uf is sacrosanct ?

I have a few nice 0.47uf's 500v > 1200v at hand ie; Mundorf , Ampohm , VCap that might prove an interesting exercise !

TM.

           

[Family_Dog]

This is what Isao Asakura had to say re the coupling caps:

"Higher values of coupling caps are supposed to improve the low frequency characteristics.  Well, you can do that on non NFB amps with little considerations.  However, the AU-111 applies the global NFB within the power amp section.  If this is a case, the combination of coupling caps (in each amplification stage) under the global NFB need to be maintained with the certain values - otherwise, the phase in low frequency will be shifted at the end.  This is the reason why you need to use the 0.3uF coupling cap."

Isao Asakura was part of the original Japanese design team who designed the AU-III.


Reference: http://sansui1944.websitetoolbox.com/post?id=1412218


-F_D




 

[TheMooN]

Hey Eric ,

Yes Indeed , It was Isao's comments regarding global feedback that have kept me honest to the factory circuit .

I was just wondering whether AD might have taken a look at that part of the circuit whilst evaluating some of the other component values on Moog's 111 and prompted my enquiry.

Regards.

TM.

       

[Family_Dog]

I'm sure AmpDog will reply in his usual inimitable way at around 03:30 or so...



-F_D

[Moog]

AMPDOG, AMPDOG, Where art though?

[Family_Dog]

I think those little men doing hand stands & flip flops in your avatar are scaring him away....


-F_D

[Ampdog]

I'm sure AmpDog will reply in his usual inimitable way at around 03:30 or so...

Sorry, only able to reply now (05;30); was a mite busy this am.  (Seriously, tonight I am/was on radio control for the local CPA - Community Patrol Action - until 06:00.)

As said about NFB stability - in this case (l.f.) I will have to consult notes on a previously refurbished AU-111 and report in detail later; end-of-shift for me is approaching and I would like to get ready for bed! Basics perhaps for newbies: For NFB stability (in this the l.f. end), the normal approach is to have one RC coupling time constant start rolling off higher up until loop gain is about unity, before others start and cause excessive phase shift; that includes the output transformer which of course also has a l.f. limit (over-simplified, but to get the idea across).

I do not think the 'early' ones in this case were the 3300nF couplers; one usually do that earlier in the loop circuit. But until later, please. Just a remark previously made: One may use high value cathode bypass caps in the pre-amp circuits; those days they were relatively large. I never go as low as 6V nowadays - 16V types are still physically small and quality is usually excellent - modern materials science and such .....