Revisiting the Sansui AU-999 - repair, rebuild and mods

[smurfer77]

I recently purchased a totally restored ( to original form and specs) AU 999. Curious to find out what the fuss is all about. I was seriously underwhelmed. Lack of bass end and lacking any depth. I currently own a 9090, an au-717 and have previously owned an au-6500. All of these are satisfying to my ear.
My question is this:
Are the modifications suggested to help this integrated amp primarily in the amp section, preamp section or both? I am contemplating replacing the amp section with my Dynaco 150 and gauging the effect. Predictions?

Sorry for the slow reply. Look back to the first posts carefully in this thread and you can see descriptions of what each mod does IMO.

To answer more directly; the subsonic filter removal has biggest effect on bass (these filters are not present on all AU-999). The negative feedback mod too, but it's far more subtle. Don't expect the lovely 'kevzep bass mod' to give you a very different sounding amp; it doesn't change the level of bass when tone controls are flat (although is very useful for getting usable bass when you turn the bass knob up).

My AU-999 now sounds arguably as sweet as my 777A!

 

[playittwice]

^ Just curious, “as sweet as the 777a”, but still different. Is that a fair assessment?

I’m still searching for a 777a, but if they sound about the same I will wait, since I have the 999 completely modded.

 

[brando1]

Yes, there is a drastic improvement in performance, the essential ones are....
1, Removal of the Subsonic filter (if it has one)
2, Pre-amp mod. (there is an inherent design error in the biasing of one of the gain stages, this makes an enormous difference, it was corrected by Sansui for the AU888 which I believe came out after the AU999)
3, Differential Constant Current Source mod.

#1 +2 are in the pre-amp
#3 is in the power amp.

I feel that is quite narrow minded of the tech, great that he wants to keep it factory, but in this case the factory made errors, so it would be in his interests to at least look into these modifications.

The pre-amp offers quite a bottleneck in performance so replacing the amp with the Dynaco 150 is not going to bring any appreciable improvement.

Get the mods done and you'll have a very nice sounding integrated amplifier.

Kevzep what is the differential constant current source mod?

 

[brando1]

Yes, there is a drastic improvement in performance, the essential ones are....
1, Removal of the Subsonic filter (if it has one)
2, Pre-amp mod. (there is an inherent design error in the biasing of one of the gain stages, this makes an enormous difference, it was corrected by Sansui for the AU888 which I believe came out after the AU999)
3, Differential Constant Current Source mod.

#1 +2 are in the pre-amp
#3 is in the power amp.

I feel that is quite narrow minded of the tech, great that he wants to keep it factory, but in this case the factory made errors, so it would be in his interests to at least look into these modifications.

The pre-amp offers quite a bottleneck in performance so replacing the amp with the Dynaco 150 is not going to bring any appreciable improvement.

Get the mods done and you'll have a very nice sounding integrated amplifier.

I recently replaced the xa495's on the 1209 boards with german ztx795a's and it got real flat but brought back the bass. I was disappointed until I did your preamp mod and wow the mids and highs are back and sound great! There was definitely a big miss by sansui there.

 

[CBarboni]

SUBSONIC FILTER REMVOAL
I bypassed the subsonic filters on the underside of the amp (just jumpered over the 47k resistors so I could go back and forther between having filter in or out for testing).

Hi, I need help finding the subsonic filter. I think it's on the bottom side, could it be the F-1277? And how should I make the bridge?
Thanks in advance

 

[kevzep]

Hi, I need help finding the subsonic filter. I think it's on the bottom side, could it be the F-1277? And how should I make the bridge?
Thanks in advance

there are two filters, one for each channel.
you can find the signal cables which connect them to the pre-amp and just bypass, its very simple.

take a picture of the bottom of the amp and we can help identify, some 999's don't have the subsonic filters...

 

[CBarboni]

there are two filters, one for each channel.
you can find the signal cables which connect them to the pre-amp and just bypass, its very simple.

take a picture of the bottom of the amp and we can help identify, some 999's don't have the subsonic filters...

Thanks you very much!!! here a picture .....

 

[kevzep]

Thanks you very much!!! here a picture .....

View attachment 2639489

Too easy, your amp does not have the subsonic filter.....lucky!!

 

[CBarboni]

Too easy, your amp does not have the subsonic filter.....lucky!!

Well, thanks. But... why does the subsonic filter works? Or am I going crazy lol

 

[kevzep]

Well, thanks. But... why does the subsonic filter works? Or am I going crazy lol

It doesn't work to produce full range audio, this is why we remove them.

 

[bolero]

Hi all,

I have seen pics of bypass capacitors across the 2 main filter caps on some 999 rebuilds. But can't seem to find any details on what values they are, and what difference it makes?

Has anyone tried that out, and have more information?

Thanks!

 

[Hipocrates]

. But can't seem to find any details on what values they are

As rule of thumb I've seen some designers use 100:1 ratio.

and what difference it makes?

The goal seems to be to filter high frequencies on the power supply, it can add resonances I've read, a good PSRR amp should not care a bit is my guess, in my experiments do not add anything, but some say it improves the amp, this is totally subjective.

Has anyone tried that out

Go for it, you will not hurt anything.

 

[bolero]

Good to know, thank you!

 

[CBarboni]

Hi!! The changes to the pre-amp on the tone cards are spectacular.
For the amplifier what would be a good substitute for the TR. 2SC281????
I got some on eBay but the equipment started to heat up. I made more changes and the audio went mute. Everything is focused on the amplification stage.
I'm still looking for causes of the problem. Thank you

 

[Basspig]

SUBSONIC FILTER REMVOAL
I bypassed the subsonic filters on the underside of the amp (just jumpered over the 47k resistors so I could go back and forther between having filter in or out for testing).

I'm looking for this subsonic filter on the AU-999 that is on my bench now, and I'm not seeing it. Perhaps some models have it and others not? Does it only affect the power amp or both that and the pre-outs? I am measuring the preamp outs and they are down 3dB at 20Hz.
Where, physically is this subsonic filter located? I'm not seeing anything under the chassis that fits the description.

 

[brando1]

It is those little guys on the right, the other is the crowbar protection.

 

[brando1]

It is those little guys on the right, the other is the crowbar protection.

It sounds much better without them. Some did not have them.

 

[Basspig]

Bypassed them. Big improvement in the response at 20Hz, but over 2dB.

Thanks for pointing them out. Would have been a Pita to locate them through all the switches and wiring harnesses!

My customer won't be using the power amp, as he's going to be driving a Hafler 500 with this as the preamp. Presently, it will put out +31dBm before clipping and can take +18dBm input before the pre volume circuits clip. I'm inclined to leave the F1209 board as is, since the Hafler goes to full power with only +7dBm.

 

[quadmatrix82]

I believe some models don’t have them

 

[bolero]

Yes, my AU-999 did not have the subsonic filters, stock.

It still didn't sound that great IMO, but rebuilding it with all the mods here took it into new sonic territory. Well nearly all: I still need to do the constant current mod.