• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

AU-999 Mystery components and power switch quesions

rasenthiran said:
Kevzep,

Does that means we can only use speakers "C" when we want to use the Sansui AU 999 as power amplifier?
Click to expand...

Yes, if it is one which separates the pre-amp from power-amp with the speaker selector switch, some of them dont.....you may just have to check it....
You should see "separated" in brackets next to the C on the selector...
 
And don't forget to bypass that god-awful "protection circuit", which protects your speakers by shorting out the amp output if something goes wrong. I think I'd rather fix the speaker.
 
I am working on a different AU-999 now. It is weak on the right channel.

Turns out the subsonic filter, the 'Mystery Components' referenced in the title of this thread are the problem.

It was challenging to find this while troubleshooting because it wasn't the preamp, and it wasn't the power amp.

Rather than bother to fix it, I jumpered them out. This is easy to do by shorting together the tag board terminals that have the two inner conductors of the coax leads on them.

Earlier (or later, I haven't figured out which circuit is newer) production models of the AU-999 did not have this Subsonic filter block, so I don't feel bad about eliminating it.

This is now on my "do this to every AU-999 amp I see" list.
 
Is this the reason that some 999's have such poor bass response? I have to use a subwoofer with mine so this is of some interest to me. Would you mind posting some pics of the mods you made on the filter block?
 
To defeat the "subsonic filter block" bridge the two terminals shown in the pic with a piece of wire.
Remember to do it for both sides of the amp.
 

Attachments

  • subfilterdefeat3.JPG
    subfilterdefeat3.JPG
    69.3 KB · Views: 386
Last edited:
Landrew said:
Is this the reason that some 999's have such poor bass response? I have to use a subwoofer with mine so this is of some interest to me. Would you mind posting some pics of the mods you made on the filter block?
Click to expand...

I am going to look into this, a little while ago in a discussion on the Vintage Audio forum, I posted some screen shots of the frequency response of the AU999 pre-amp.
Here it is here.....You can see the low end dropping off at 70hz......and you can also see the phase shift involved as well....
au999preamp.png


Then here you see the G22000 Pre-amp, quite a significant difference....The phase starts shifting on the top end which is to be expected, its a fairly insignificant amount, in fact the G22000 pre-amp is fairly inpressive, compared to the fact the AU999 actually inverts the phase of the signal going in, and has significantly more shift across the bandwidth of the pre-amp, especially in the bass department....
g22000preamp.png
 
DCinDC said:
To defeat the "subsonic filter block" bridge the two terminals shown in the pic with a piece of wire.
Remember to do it for both sides of the amp.
Click to expand...

Im going to do that to mine, then do that test again and see how it comes up......
 
kevzep said:
Im going to do that to mine, then do that test again and see how it comes up......
Click to expand...

Im very interested to see the results :scratch2: it would be nice if you could get a little more bass out of it, because it can be pretty lacking in that department!

DC- did you hear any differences with yours bypassed?
 
It was broke when I started so I only used phones for testing.
When I bypassed the filter, I also added .1uF bypass caps on the main filter caps.
When I was checking it out after the repairs, I used small speakers at this phase of testing. But I know the source material intimately and these speakers have very good bass for their size. I heard good strong deep bass. We may be onto something here....

I was reminded of a comment someone made where the AU-999 doesn't really seem to need a loudness control. I questioned it when I first read it, but I was reminded of it when listening, and found myself agreeing with that thought.

I've developed a loudness circuit retrofit for my Franken 999 but haven't installed it yet. That will be interesting.
 
tobyj17 said:
Im very interested to see the results :scratch2: it would be nice if you could get a little more bass out of it, because it can be pretty lacking in that department!
Click to expand...

I've just bypassed them, in fact I just pulled them out altogether....

I have to go to work now, but I will do the test again when I get home.....

Results will be posted, I estimate it will change the sound of the amplifier quite drastically.
When I saw that response I initially thought the Low Cut filter was on.....and then I thought "what the hell is going on here?"

Well we will see........
 
DCinDC said:
We may be onto something here....
Click to expand...
Well it turns out we are onto something here....check out the screen shot below....

The green trace is the original AU999 with the subsonic filter in it.
Then the blue trace is with the Subsonic filter removed......There is significant improvement to the Phase as well as the response....
The response curve it the lower window, and the phase is the top window...he preamp still flips the phase 180˚, but you can see with that awful bass thief gone, everything looks a lot better down there now, still not as good as the G22000 but certainly a LOT more bass response than before....
I completely removed those tag boards, and took the incoming signal wire, cut it to the correct length to go onto the molex, looks very tidy in there now...
Feel free to ask questions...
au999subsonicfilterremo.png
 
kevzep said:
Well it turns out we are onto something here....check out the screen shot below....

I completely removed those tag boards, and took the incoming signal wire, cut it to the correct length to go onto the molex, looks very tidy in there now...
Feel free to ask questions...
Click to expand...

This is looking like yet another piece of superb investigative diy work gentlemen:smoke:

Any chance of a swift post hack n chop image Kev ?
 
Tsushima1 said:
This is looking like yet another piece of superb investigative diy work gentlemen:smoke:

Any chance of a swift post hack n chop image Kev ?
Click to expand...

Post hack n chop? I'm not sure what you mean? You can steal my image if you want....

I URGE EVERYONE, who has a AU999 to do this, I have just been listening to my amp for the last 30mins at quite a reasonable volume, and I can tell you I have NEVER heard this sort of DEEP BASS come out of this little amp before....The low end weakness has all come together
Quite the transformation.....
I guess when they designed and built this amp, no one was really that concerned about anything below 100hz.......
 
Tsushima1 said:
Merely my colorful terminology when referring to a modification Kev :D
Click to expand...

Ah yes I see, all good.......I've already put it back together.....We will get one of the other guys to snap a few shots, I know Toby will want to do his one....

I'm still listening to this amp, sounds FAT, and thats something these amps are generally not.......

Im very excited about the transformation here....:yes:
 
Last edited:
I'll definitely be doing this when i get home from work tonight! I probably won't eliminate, i'll just solder in a jumper. I'll also do a before and after comparison.. the kef's i have really like deep bass and i'll definitely be able to tell the difference...
 
tobyj17 said:
I'll definitely be doing this when i get home from work tonight! I probably won't eliminate, i'll just solder in a jumper. I'll also do a before and after comparison.. the kef's i have really like deep bass and i'll definitely be able to tell the difference...
Click to expand...

I will be very interested to see what you think.......:thmbsp:
 
You must log in or register to reply here.
Back
Top Bottom