AU-999 weak and distorted channels

Get more balanced speakers guys, its not the amp that lacks the bass, its your speakers.
If you find you are turning up the bass, its time to think about speakers which can deliver a more appropriate balance. Otherwise you are simply going to run out of headroom by making the amplifier work hard in the low end.
 
No sin in not liking an amp, or rather, not have it fit with your rig and personal taste. You've done everything possible to give it a fair shot. Kudos.

But....there might be one more thing. It border lines herecy, and it will mess with the core of the 999's soul. But because it is so easy - just one resistor -, and the results so noticeable.....it might be worth a try before you give up on an amp of such high build quality in which you have put in spuch effort.

A few month back Smurfer77 did an extensive 999 overhaul, he noticed that the value of the feedback resistor changed as compared to the near identical 888. So in typical Smurfer77 style he rigged up a full battery of test with lap equipment and came to the conclusion that the value of the feedback resistor had significant effect on the sound signature. He tested 3 values, one that made the 999 sound like it's cap coupled siblings, warm and bassy, the stock 999 value, and then finally the 888 value that gave the 999 a tighter bass.

I would normally provide a link, but on a smart phone now headed to work, so either PM Smurfer77 or look through some of his recent threads. The mod is documented to a very high standard. And since so easy might be worth a shot.

Link is in this thread, as well as summary and links to (most) other mods for the AU-999. I've been running my AU-999 with the AU-888 feedback setup since then and am still happy with it.

Get more balanced speakers guys, its not the amp that lacks the bass, its your speakers.
If you find you are turning up the bass, its time to think about speakers which can deliver a more appropriate balance. Otherwise you are simply going to run out of headroom by making the amplifier work hard in the low end.

FWIW I run my (modified, i.e. bass enhanced) AU-999 with flat tone controls but I do as a rule run neutral (bordering on bass strong) speakers (mostly Allison One or Allison Two speakers, some other brands but nothing bass-weak). I could see some folk might want to turn bass up a notch. For me I really love the AU-999 (modified) as well as the AU-888/666 which have a similar presentation, striking (IMO) a rather unique balance between the old boomy sound and the 'newer' hifi balanced sound with arguably more detail. With these amps, I sometimes have a feeling of lack of bass, but when the track/source really has bass, these amps provide it with fidelity, so I get what you are saying. It's the eternal struggle for me.... the desire for neutral clarity and fidelity and the slightly exaggerated woolly voicing of the older models that really gets the heart thumping (but does get in the way of ultimate fidelity and woofer control).

I ramble again, but for the reasons above I think I will have an 888/666/999 as well as a 777 or similar in my quiver as they sit either side of a clear line in history to me.... the singular biggest paradigm shift in hifi flavour
 
FWIW I run my (modified, i.e. bass enhanced) AU-999 with flat tone controls but I do as a rule run neutral (bordering on bass strong) speakers (mostly Allison One or Allison Two speakers, some other brands but nothing bass-weak). I could see some folk might want to turn bass up a notch. For me I really love the AU-999 (modified) as well as the AU-888/666 which have a similar presentation, striking (IMO) a rather unique balance between the old boomy sound and the 'newer' hifi balanced sound with arguably more detail. With these amps, I sometimes have a feeling of lack of bass, but when the track/source really has bass, these amps provide it with fidelity, so I get what you are saying. It's the eternal struggle for me.... the desire for neutral clarity and fidelity and the slightly exaggerated woolly voicing of the older models that really gets the heart thumping (but does get in the way of ultimate fidelity and woofer control).

I ramble again, but for the reasons above I think I will have an 888/666/999 as well as a 777 or similar in my quiver as they sit either side of a clear line in history to me.... the singular biggest paradigm shift in hifi flavour

I get that and it is one way of doing it, but this is my reasoning for not going down that route of influencing the frequency response/curve of an amplifier.
As soon as you are boosting or influencing say in this instance, the bass, you start to also shift the phase, now this is something I really don't like doing in an amplifier. There is enough phase shift in speakers thank you very much, so if you are causing a phase shift in the amplifier, and then also mix that with what happens in the speakers, it makes a rather messy situation.
I am not being conceited, but I can hear that and I don't like it, even though there is more bass, to me there is now a factor in the sound I don't like. Same happens if you introduce an EQ filter into the mix, it shifts the phase, and to me then sounds odd. OK for one track that lacks a bit if bass, but as a standard system setting, its not the way I like to do it.
The designers have specified the feedback in conjunction with the gain of the amplifier and I'd just leave it as intended, unless there is an actual design shortfall like there is with the pre-amp. Even changing the cap in the negative feedback can affect the phase as well.
When I did a frequency vs Phase plot of the 999 I worked on the other day, the plot looked very good and personally I would not mess with it.
People never talk about phase, but in my experience its one of the most important things in sound reproduction.

At the end of the day, we like to try things and some folk may like this approach you are experimenting with, and thats a good thing for the community....and Kudo's to you for trying things out.

This is why I have an Active Bi-amped system as my main, with the MC452 on the mid/high and the Crown driving the 15" Bass, they aren't subs so to speak, they are a "low extension" to the 8xTi coaxial speakers.
This way I am using two amplifiers with perfect phase, and to increase the "fat", "warmth", "punch" whatever you like to call it, I just balance out the two amplifiers.
It's "set" as a system and I do not ever change the level of the Crown, I find I never need to.

Of course using a Linkwitz Reily 24db/oct Filter in the active crossover, there is phase shift at the crossover frequency, but it has minimal negative effect on the sound once the sources are aligned.
 
I get that and it is one way of doing it, but this is my reasoning for not going down that route of influencing the frequency response/curve of an amplifier.
As soon as you are boosting or influencing say in this instance, the bass, you start to also shift the phase, now this is something I really don't like doing in an amplifier. There is enough phase shift in speakers thank you very much, so if you are causing a phase shift in the amplifier, and then also mix that with what happens in the speakers, it makes a rather messy situation.
I am not being conceited, but I can hear that and I don't like it, even though there is more bass, to me there is now a factor in the sound I don't like. Same happens if you introduce an EQ filter into the mix, it shifts the phase, and to me then sounds odd. OK for one track that lacks a bit if bass, but as a standard system setting, its not the way I like to do it.
The designers have specified the feedback in conjunction with the gain of the amplifier and I'd just leave it as intended, unless there is an actual design shortfall like there is with the pre-amp. Even changing the cap in the negative feedback can affect the phase as well.
When I did a frequency vs Phase plot of the 999 I worked on the other day, the plot looked very good and personally I would not mess with it.
People never talk about phase, but in my experience its one of the most important things in sound reproduction.

At the end of the day, we like to try things and some folk may like this approach you are experimenting with, and thats a good thing for the community....and Kudo's to you for trying things out.

This is why I have an Active Bi-amped system as my main, with the MC452 on the mid/high and the Crown driving the 15" Bass, they aren't subs so to speak, they are a "low extension" to the 8xTi coaxial speakers.
This way I am using two amplifiers with perfect phase, and to increase the "fat", "warmth", "punch" whatever you like to call it, I just balance out the two amplifiers.
It's "set" as a system and I do not ever change the level of the Crown, I find I never need to.

Of course using a Linkwitz Reily 24db/oct Filter in the active crossover, there is phase shift at the crossover frequency, but it has minimal negative effect on the sound once the sources are aligned.

I can respect that point of view. Two points you raise, phase response and also effect of feedback cap on phase, are discussed in my linked post above. Summary is that low end phase response is practically unchanged with the mod, and high-end response changes a bit (so I tweaked cap to get similar cut-off to original design). Also, note that the final value of feedback I settled on (i.e. AU-888 feedback settings) results in larger damping factor, taming the woofer resonance and associated phase response off real speaker loads, rather than makes them worse. So in your particular case where you are sensitive to phase shifts in bass, you might even prefer the mod feedback (which introduces neglible phase shift in bass due to mod, but damps woofer over stock.

Anyway, I definitely don't want to push the mod because amplifier stability is 'no joke' and should be taken seriously (it's the reason I haven't promoted this mod beyond the original post I made for interest sake), but perhaps one can argue the view that the feedback 'mod' is not so much a mod, but I think this might be another case (like preamp) where we might be implementing a correction made be Sansui themselves when moving from AU-999 to AU-888? Honestly, even after all mods, I still didn't like the AU-999 very much until I did the feedback mod... it's the reason I venture onto a more radical path. ymmv
 
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I can respect that point of view. Two points you raise, phase response and also effect of feedback cap on phase, are discussed in my linked post above. Summary is that low end phase response is practically unchanged with the mod, and high-end response changes a bit (so I tweaked cap to get similar cut-off to original design). Also, note that the final value of feedback I settled on (i.e. AU-888 feedback settings) results in larger damping factor, taming the woofer resonance and associated phase response off real speaker loads, rather than makes them worse. So in your particular case where you are sensitive to phase shifts in bass, you might even prefer the mod feedback (which introduces neglible phase shift in bass due to mod, but damps woofer over stock.

Anyway, I definitely don't want to push the mod because amplifier stability is 'no joke' and should be taken seriously (it's the reason I haven't promoted this mod beyond the original post I made for interest sake), but perhaps one can argue the view that the feedback 'mod' is not so much a mod, but I think this might be another case (like preamp) where we might be implementing a correction made be Sansui themselves when moving from AU-999 to AU-888?
Excellent work being thorough and monitoring the effects, perhaps there is something in this. Sansui perhaps dropped the ball on that part of the design?
I have a 999 here for some more work, I think I will try it out. You might be right, its perhaps not so much a bass boost as such, but the amplifier has better "control" over the bass, which in my book is a good thing.
I am a fan of amplifiers with high damping factor, not one to be narrow minded about these I am definitely going to give it a shot.
Thanks for the further clarification...
 
Get more balanced speakers guys, its not the amp that lacks the bass, its your speakers.
I think its actually the crappy room in our tiny flat which is causing the issue, its definitely a bit hard to troubleshoot a lack of bass in a bass-sucking shoebox.
 
I think its actually the crappy room in our tiny flat which is causing the issue, its definitely a bit hard to troubleshoot a lack of bass in a bass-sucking shoebox.
Where are the speakers? Put them in the corners if you can, you'll get some bass then...
 
Excellent work being thorough and monitoring the effects, perhaps there is something in this. Sansui perhaps dropped the ball on that part of the design?
I have a 999 here for some more work, I think I will try it out. You might be right, its perhaps not so much a bass boost as such, but the amplifier has better "control" over the bass, which in my book is a good thing.
I am a fan of amplifiers with high damping factor, not one to be narrow minded about these I am definitely going to give it a shot.
Thanks for the further clarification...

I think its actually the crappy room in our tiny flat which is causing the issue, its definitely a bit hard to troubleshoot a lack of bass in a bass-sucking shoebox.

Check out vintage Allison speakers... they are designed to be up against a wall for flat bass response. Very apartment friendly. PM if you want model advice... i have almost all of the older models.
 
What caught my attention with Smurfer77's post is that the near identical 888 would use a different value feedback resistor - I thought they were a no fuss straight calculated "gain" value, but as we have learned with Sansui, they admitted many revisions and service bulletin's including "a" version models. The 888 is arguable a 999a. It is an interesting idea if the 999's bass output - on my unit very satisfying after Kev's Eq mod - could be tightened up a bit aka "higher damping factor". At least it is the simplest mod to test. Very curious to hear some feedback (no pun) :music:
 
What caught my attention with Smurfer77's post is that the near identical 888 would use a different value feedback resistor - I thought they were a no fuss straight calculated "gain" value, but as we have learned with Sansui, they admitted many revisions and service bulletin's including "a" version models. The 888 is arguable a 999a. It is an interesting idea if the 999's bass output - on my unit very satisfying after Kev's Eq mod - could be tightened up a bit aka "higher damping factor". At least it is the simplest mod to test. Very curious to hear some feedback (no pun) :music:
Well as far as I can tell, tube amps have fairly low damping factors, so perhaps they were going for a performance closer to a tube amp on the 999....
Who knows.....but I can't think of any other reason they would do it unless they just screwed it up...
 
Check out vintage Allison speakers... they are designed to be up against a wall for flat bass response. Very apartment friendly. PM if you want model advice... i have almost all of the older models.
I think you are suggesting that I get rid of my KEFs.

Sorry bro that aint happening! lols
 
Excellent work being thorough and monitoring the effects, perhaps there is something in this. Sansui perhaps dropped the ball on that part of the design?
I have a 999 here for some more work, I think I will try it out. You might be right, its perhaps not so much a bass boost as such, but the amplifier has better "control" over the bass, which in my book is a good thing.
I am a fan of amplifiers with high damping factor, not one to be narrow minded about these I am definitely going to give it a shot.
Thanks for the further clarification...

Kev, the first SS amp of Sansui (777) uses even lower damping factor. I think the use of low damping factor on the early SS amps was not a goal in SS or tube; triode designs didn't require much feedback as they exhibit much (an order of magnitude) lower open-loop distortion than BJTs (and some other tube setups too). For those triode amps only a little negative feedback was needed and this was to achieve reasonable bandwidth, distortion and to handle the effect of load (which is problematic on triode designs with zero negative feedback).

Then the 777 and relative came along and they used low damping factor, partly because they had to make amps that sounded similar to the tube amps (?). The 777 sounds almost identical to the AU-70; I suspect it was on purpose to show that semiconductor isn't harsh and noisy like some think (Sansui started SS late, but some other brands did SS earlier before good transistors were available and it wasn't pretty). The 999, I think, just came at an awkward stage in between the old design philosophy - where a sweet sounding 20-20kHz with 1% distortion at full power was more than enough - and taking advantage of negative feedback.

I actually think the 777 sound and those early receivers has more to do with the damping factor (and associated speaker interaction) and voicing, than the often cited cap-coupled output. There are plenty of tube amps (necessarily cap-coupled) which do not sound boomy, vintage, or sweet at all.

I think the AU666/888 are in a sweet spot of vintage SS design in terms of complementary outputs as well as use of negative feedback. As we know, there spec wars followed and while I'm all for use of negative feedback, choices started to be based on achieving certain numbers rather than factors many of us care about more.

I may be able to confirm with a ex-Sansui tech I have contact with whether the early designs were just carry-over/evolution, or deliberate transition.

As stereofun says, the 888 is kind of like the 999A. I also think it's worth noting that the subsequent, but lower powered model, AU-666 (which shares much in common with the 999, but not quite as much as the 888) also has almost the same negative feedback value as the AU-888.
 
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Update. I have performed Smurfer77’s output board mod and currently have it at the 777 configuration. At first the change did not impress me all that much, but recent developments have changed my mind. I had originally paired the 999 with OLA’s, but I recently completed a side job which allowed me to purchase all of the parts I needed to restore my AR-3a’s. Wow is all I can say about the AR-3a and 999 pairing. Kev you were correct, the right speakers makes huge difference. I shouldn’t be surprised about this as I have a set of Cornwall’s that I hated until I paired them with my 1000a. Sometimes the amp makes the speakers and sometimes, in this case, the speakers make the amp. I think that the 999 might now be a keeper with the 3a’s. Speaker and amp pairing is really crucial. My next step will be to try the 888 ratio on the driver boards.

On a side note, now that there is more bass, I have been trying flat tone controls. Correct me if I’m wrong but in order to get good room filling sound the volume needs to be turned up considerably higher than when the tone controls are boosted. Does the amp work better (electrically) at higher volume levels? The wife doesn’t seem to care for those levels however! Ha!

Dave
 
Take the card modules out one by one and polish -by hand- the edgecard traces with non-abrasive "Blue Magic" metal cleaner to a bright shine...IT IMPROVES PREAMP DETAIL A LOT.
Also, the TO-66 drivers tend to go bad (at least in an AU-999 I'd found in 1998). TO-66 case transistors are hard to find replacements for now (without resorting to rewiring a suitable TO-220 type in their places), but I do know the drivers in there cross-ref'd to a NTE 38 and a NTE 175.
 
Take the card modules out one by one and polish -by hand- the edgecard traces with non-abrasive "Blue Magic" metal cleaner to a bright shine...IT IMPROVES PREAMP DETAIL A LOT.
Also, the TO-66 drivers tend to go bad (at least in an AU-999 I'd found in 1998). TO-66 case transistors are hard to find replacements for now (without resorting to rewiring a suitable TO-220 type in their places), but I do know the drivers in there cross-ref'd to a NTE 38 and a NTE 175.

I work and have worked on a lot of AU999's. I have one here right now in fact for some more development work.

I have never found, even on the rough ones, that the connections on the pre-amp cards oxidise at all. The worst 999 I had through here which was pretty rough, had a nice clean bright surface on the pre-amp cards. Of course they get cleaned during the restoration process, but unless they have been unplugged and plugged in many many times, they rarely present any issues.

The TO66 driver transistors are not a weak or failure point on the AU999, I have only had one through which has had blown drivers. The drivers are robust as they come for this amplifier and do not "tend to go bad". It is all the black and white CDC8002/9002 the XA495's and the 2SC458's which are the problem in these amplifiers.
I do not replace the drivers in the restorations unless they are blown or leaky. Have not found a leaky one as yet.
 
Update. I have performed Smurfer77’s output board mod and currently have it at the 777 configuration. At first the change did not impress me all that much, but recent developments have changed my mind. I had originally paired the 999 with OLA’s, but I recently completed a side job which allowed me to purchase all of the parts I needed to restore my AR-3a’s. Wow is all I can say about the AR-3a and 999 pairing. Kev you were correct, the right speakers makes huge difference. I shouldn’t be surprised about this as I have a set of Cornwall’s that I hated until I paired them with my 1000a. Sometimes the amp makes the speakers and sometimes, in this case, the speakers make the amp. I think that the 999 might now be a keeper with the 3a’s. Speaker and amp pairing is really crucial. My next step will be to try the 888 ratio on the driver boards.

On a side note, now that there is more bass, I have been trying flat tone controls. Correct me if I’m wrong but in order to get good room filling sound the volume needs to be turned up considerably higher than when the tone controls are boosted. Does the amp work better (electrically) at higher volume levels? The wife doesn’t seem to care for those levels however! Ha!

Dave
You will need to increase the level with the tone controls set flat to compensate. With the tone controls boosted you'll be sending more level to the power-amp.

Amplifier + Speaker combinations are very important...

Good that you are experimenting and trying things out..
 
I work and have worked on a lot of AU999's. I have one here right now in fact for some more development work.

I have never found, even on the rough ones, that the connections on the pre-amp cards oxidise at all. The worst 999 I had through here which was pretty rough, had a nice clean bright surface on the pre-amp cards. Of course they get cleaned during the restoration process, but unless they have been unplugged and plugged in many many times, they rarely present any issues.

The TO66 driver transistors are not a weak or failure point on the AU999, I have only had one through which has had blown drivers. The drivers are robust as they come for this amplifier and do not "tend to go bad". It is all the black and white CDC8002/9002 the XA495's and the 2SC458's which are the problem in these amplifiers.
I do not replace the drivers in the restorations unless they are blown or leaky. Have not found a leaky one as yet.

I agree with all of the above!

BTW... "more development work"... colour me interested Kev!
 
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