Bargain AU-X1, Did I get a Good Deal
- Thread starter fuelie
- Start date
fuelie
New Member
Thanks Dr*Audio, I did find and have read those mods before, thank you -
I checked the Flat Amp and discovered that it has what appears to be a factory modification. They added a small cap and resistor RC network on the bottom of the board coupled to the Negative Feedback of the amp that must have been intended to reduce the flat amp bandwidth.
However, the Cap value was only 2pf, quite small even though it is used in an active circuit. I tested a couple values that were on hand to see if I could improve the bandwidth limitation, 33pf was too much and reduced the normal audio signal by 30%. I tested several values and finally settled on 10pf because it reduced the noise/oscillation significantly enough without any apparent loss of the high audio bandwidth.
The Power Amp also has some factory modifications from the original schematic, I have noted all these factory modifications to my copy of the original schematic and this small change too. (see attached photo of flat amp)
I will be placing this X1 amp in the listening room prime playing position tonight. It has such a HUGE dynamic sound stage and presence over any amp I have that I expect to be comparing everything to this big machine for quite some time.
In comparing the sound stage, presence and separation, my AU-919 is a factor of ten over my AU-717 and this AU-X1 it is at least a factor of five over the AU-919, and to me the 919 has a fantastic hard to beat sound.
Thanks to all for all you good comments and support.
I checked the Flat Amp and discovered that it has what appears to be a factory modification. They added a small cap and resistor RC network on the bottom of the board coupled to the Negative Feedback of the amp that must have been intended to reduce the flat amp bandwidth.
However, the Cap value was only 2pf, quite small even though it is used in an active circuit. I tested a couple values that were on hand to see if I could improve the bandwidth limitation, 33pf was too much and reduced the normal audio signal by 30%. I tested several values and finally settled on 10pf because it reduced the noise/oscillation significantly enough without any apparent loss of the high audio bandwidth.
The Power Amp also has some factory modifications from the original schematic, I have noted all these factory modifications to my copy of the original schematic and this small change too. (see attached photo of flat amp)
I will be placing this X1 amp in the listening room prime playing position tonight. It has such a HUGE dynamic sound stage and presence over any amp I have that I expect to be comparing everything to this big machine for quite some time.
In comparing the sound stage, presence and separation, my AU-919 is a factor of ten over my AU-717 and this AU-X1 it is at least a factor of five over the AU-919, and to me the 919 has a fantastic hard to beat sound.
Thanks to all for all you good comments and support.
Attachments
dr*audio
Fish fingers and custard!
Hmm, you say changing the cap reduces the oscillation "enough." If it doesn't remove it completely you are not done. Did you replace all the black flag caps? If so, I think it is likely that what you are seeing is ether a parasitic oscillation, probably in the final stage of the flat amp, or the oscillation could be somewhere else in the amp and it's being amplified by the flat amp. If you short the input to the flat amp and you still see the oscillation, it probably is in the flat amp. If shorting the input gets rid of it, it may be elsewhere.
vanbasten624
Member
I know exactly how you feel. I cost me more though, but the sound, THAT SOUND, my ears are in heaven man. Great for you! Nice catch!
fuelie
New Member
Not sure,- how far to take this.
Should I do a full re-cap when the unit is able to function after some minimal strategically placed component changes.
As originally mentioned, I am not sure if this signal is internally created or simply HF noise (now virtually non existant, down to about 4mv).
The flat amp active devices have a bandwidth up to 100MHZ, so does the Power amp and the original Power Amp Finals bandwidth to 50 & 70MHZ respectively.
When the early production units began suffering from oscillation problems, the designers had to go back and make corrections. They probably didn’t want to reduce the effective bandwidth too significantly because it could negate the original intention of the AU-X1 model. So from looking at the apparent factory mods and deviations from the original schematics, I believe they determined that a few small changes strategically placed could provide small incremental reductions of HF noise passing that overall could significantly reduce the possibility of oscillation, yet probably did not totally eliminate the HF noise signal. The noise/oscillations signal I have seen in this unit may simply be the remnants of this HF signal that the designers tried to reduce, and it may have always been there.
I was able to employ a slight additional HF band pass reduction to help assure that this unit will not suffer from the oscillation issue. I could keep going but when is enough, enough? It might never be able to be completely eliminated. Then again if I did completely eliminate it, how would that effect the sound? Some might say that the bandwidth is so high that what difference would it make, but I believe this is what makes the AU-X1 what it is and I hesitate to change it all. I just wanted to be sure that it will not self-destruct. And it is now working flawlessly after several weeks of testing every day.
I guess that only time will tell. I could be singing the blues later - then you can say "T o l d - y o u - s o" - but for now it is a pleasure to have it as near to the original design as possible.
Thanks
Should I do a full re-cap when the unit is able to function after some minimal strategically placed component changes.
As originally mentioned, I am not sure if this signal is internally created or simply HF noise (now virtually non existant, down to about 4mv).
The flat amp active devices have a bandwidth up to 100MHZ, so does the Power amp and the original Power Amp Finals bandwidth to 50 & 70MHZ respectively.
When the early production units began suffering from oscillation problems, the designers had to go back and make corrections. They probably didn’t want to reduce the effective bandwidth too significantly because it could negate the original intention of the AU-X1 model. So from looking at the apparent factory mods and deviations from the original schematics, I believe they determined that a few small changes strategically placed could provide small incremental reductions of HF noise passing that overall could significantly reduce the possibility of oscillation, yet probably did not totally eliminate the HF noise signal. The noise/oscillations signal I have seen in this unit may simply be the remnants of this HF signal that the designers tried to reduce, and it may have always been there.
I was able to employ a slight additional HF band pass reduction to help assure that this unit will not suffer from the oscillation issue. I could keep going but when is enough, enough? It might never be able to be completely eliminated. Then again if I did completely eliminate it, how would that effect the sound? Some might say that the bandwidth is so high that what difference would it make, but I believe this is what makes the AU-X1 what it is and I hesitate to change it all. I just wanted to be sure that it will not self-destruct. And it is now working flawlessly after several weeks of testing every day.
I guess that only time will tell. I could be singing the blues later - then you can say "T o l d - y o u - s o" - but for now it is a pleasure to have it as near to the original design as possible.
Thanks
Attachments
Last edited:
Kale
Super Member
Almost finished...
The left is old Binding Post, the right is a new one. As you can see, I dont like fancy gold binding post on vintage audio, I tried to find a new one as close as the old was...
Binding Post adopted to AUX1
Board for connection Binding Post
I have to adopt the nut of new binding post
... the result...
... finished....
The left is old Binding Post, the right is a new one. As you can see, I dont like fancy gold binding post on vintage audio, I tried to find a new one as close as the old was...
Binding Post adopted to AUX1
Board for connection Binding Post
I have to adopt the nut of new binding post
... the result...
... finished....
:thmbsp:
Power guess.......205WPC
Kale
Super Member
It is Sansui SR838.
Kale
Super Member
Kale, one question...
Where did you get your speaker binding posts ??? - they are so nice and original looking.
I have had a good look around and can't find anything that looks quite as nice
Go to the www.farnell.com and search BP31-10B and BP31-10R
