AU999 Pre-amp mod.

kevzep

Its all about the Music
Disclaimer!!
This is experimental development work, I take no responsibility, enter at your own risk...:yes:
Originally I was not going to post this, but after listening to the AU999 for a few weeks, I cant believe how this amp has been transformed by this simple mod.
The AU999 is an amplifier with a lot of potential, it should sound a lot better than it does in standard trim....
The Subsonic filter was the first thing which made a large improvement, but I wanted more!!

Recent threads, questions and comments have come up here relating to the the disappointing sound of these amplifiers I started working my way through the amplifier to try an find out why, they do in my view sound a bit "small", so working through the input stage and got to the F1209 board which is where I found something...

I thought there was a fault, but there wasnt, the voltages looked all as they should be according to the schematic....but wow, it really runs out of headroom on this board (F-1209)
I called a friend of mine who is an audio design boffin....
He made some suggestions to bias TR705 differently. In standard trim this stage of the pre-amp is running out of headroom and this is before the volume control.
A healthy dose of input from a CD player or outboard DAC can certainly run this stage into a "semi" clip situation, it tends to clip the positive side first....it just makes it sound "compressed" and "not quite right".
Secondly TR707/708 seem to be lacking in the ability to swing enough voltage to drive the power-amp to full power before it (TR707/708) runs out of headroom.....

2 x 3.3KΩ 1/4watt metal film resistors
2 x 1megΩ 1/4watt metal film
2 x 1kΩ 1/4watt metal film
2 x 2.2kΩ 1/4watt metal film
2 x 100Ω 1/4 watt metal film

On the F1209 Boards
Replace R759 with the 3.3KΩ
Replace R761 with 1.0KΩ
Replace R769 with 2.2KΩ
Now, remove R755. You will put the 1megΩ resistor on the trace side of the board though the hole which is connected to the BASE of TR705, you will then connect the other end of this resistor on the trace which connects to the COLLECTOR of TR705....
Replace R763 with the 100Ω.....

That is all you have to do........
Its very easy to do, takes minutes...

This mod improves the performance of the pre-amp beyond anything it was designed to do....
The preamp will now do 7.2VRMS (+19.3dBu) before it clips......
Most importantly, it does not clip in the phase inverting class A stage before it gets to the volume control, which is what it was doing before....You can easily pump 3.5V into the AUX/Tape inputs with absolutely no danger of overloading the front end....but more importantly it now has headroom in the pre-amp......

I CANNOT BELIEVE how great this amp sounds now....the top end has really come alive, its punchy clear and just nice!!!!
I feel this has really brought this amplifier alive!! Brilliant
Now the amplifier can now produce 72watts easily before it clips.........before it would barely make its rated output, and when it did, its waveform was not nice, it was not producing a nice even sine wave, and all this originating from the pre-amp....

I am very interested in people trying this and reporting back on the sound...
 
Yes...And some before and after Pics...

Seriously? its 4 resistors on a circuit board, the rocket science was working out the values not posting pictures of where they go....
You do not need pictures to find these resistors, the AU999 is one of the simplest amplifiers to work on....
I almost posted pictures, but seriously, it looks like 4 resistors on a circuit board, its just a waste of AK's server space to be honest, it does not show anything revolutionary, as I said, its in the resistor values...

I am considering posting part 2 which is the gain and slope mod to the Bass EQ in the AU999.....That will need pictures, because the resistors which need changing are not on a board, they are not easy to find without directions and pictures.....
We will see how this thread goes first.....
 
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An Excellent Piece of Work Sir :thmbsp: I have found it to be the case ,more often than not ,that a little further tuning of even very well built and performing equipment can result in some discernible refinement .

As Kevin points out tho , this type of fine tuning often requires a good deal of time and effort on the behalf of the intrepid engineer in working through finite combinations of components and values of the same .
 
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An Excellent Piece of Work Sir :thmbsp: I have found it to be the case ,more often than not ,that a little further tuning of even very well built and performing equipment can result in some discernible refinement .

As Kevin points out tho , this type of fine tuning often requires a good deal of time and effort on the behalf of the intrepid engineer in working through finite combinations of components and values of the same .

Thank you, yes it has taken the old AU999 to an entirely new level of performance, I am very happy with the sound of it, and combined with the modification the the Bass EQ, it can be extremely full and juicy at low levels, but retain quite amazing detail too.....nice....:thmbsp:
 
Great thread Kev, and thanks for sharing this information.

I will be doing the sub sonic filter delete, and these mods shortly. My 999 has stock transistors on the F1209 board, and I will take before and after measurements in stages to compare with yours.

This is very timely, as I have just put the AU888 I acquired earlier this year on the bench for some work to get it up and running, and it is almost identical circuit wise - has F1209A boards - except for the loudness circuit on the volume pot. I will do some comparison tests on the 888 to see if it suffers from the same headroom issue as the 999.

And I might be in contact re the Bass Eq as well.

Cheers

John
 
Great thread Kev, and thanks for sharing this information.

I will be doing the sub sonic filter delete, and these mods shortly. My 999 has stock transistors on the F1209 board, and I will take before and after measurements in stages to compare with yours.

This is very timely, as I have just put the AU888 I acquired earlier this year on the bench for some work to get it up and running, and it is almost identical circuit wise - has F1209A boards - except for the loudness circuit on the volume pot. I will do some comparison tests on the 888 to see if it suffers from the same headroom issue as the 999.

And I might be in contact re the Bass Eq as well.

Cheers

John

Im very keen for people to try it out, I will be keen to see what you think John....

Yeah I managed to get some very effective performance out of the Bass, I will get something up very soon with some info, I will have to pop the bottom off my 999 and double check the values I ended up with, and do some kind of diagram, its difficult to follow where the resistors are for the bass control.....some of them are on the Mid selector switch, its very confusing in there at first......
I used Smaart Live 7 and the transfer function to monitor the effects of the changes I was making to the EQ.....Same as what I was using to look at the effect of taking out the Subsonic filter....

Great stuff.....
 
I'll be doing manual frequency/gain checks, and I might break out the distortion meter and take some measurements with this as well. It will be a couple of weeks before I can get the time to have a go at this, I'll post up here when I get something worthwhile to add to the thread.

and your right, this is great stuff!!
 
Nice work Kevin!!:thmbsp::thmbsp:..i have a couple of questions, will this affect the Phono 1/2 performance of the the amp or will it improve as well? I noticed it is geared towards a CD/DAC output..

Skippy: taking out the subsonic filter a MUST before this MOD? did i miss the subsonic filter thread???

AGAIN HATS OFF TO KEVIIN!!..nice work!!..AU999 CLUB workin its magic!!!
 
Well, well, well, - you could very well 'light up' all AU-999 owners with this Kev.

I can remember reading a thread about the virtues or otherwise of various Sansui's of the AU-999 era. I particularly remember our own EW grumbling that the AU-999 could scarcely meet its power output spec, but was a fair sounding amp even so (something like that) and some comments about clipping.

You have been soooo quiet recently :D but what a result !

This is a marvellous piece of work - many congratulations Kev :thmbsp:
 
Nice work Kevin!!:thmbsp::thmbsp:..i have a couple of questions, will this affect the Phono 1/2 performance of the the amp or will it improve as well? I noticed it is geared towards a CD/DAC output..

Skippy: taking out the subsonic filter a MUST before this MOD? did i miss the subsonic filter thread???

AGAIN HATS OFF TO KEVIIN!!..nice work!!..AU999 CLUB workin its magic!!!

The mod is not really "geared" towards CD/DAC type output, that wasnt my motivation, it was more about having the pre-amp performing to meet the demands of driving the poweramp properly. As a result the pre-amp now has more headroom, and it will only have the phono section performing better as result.....
The modification happens after the two emitter follower stages which come after the phono pre.....this would mean it would retain the Sansui signature we like.....
Where the problem happens is after these stages which is post EQ and also the last and most important part of the pre-amp stage....
The mod just means the pre-amp is more capable of handling higher input levels into the AUX and TAPE inputs that it did before because of the improved performance.....

Nice Job Kev! I'm looking forward to doing this!

You will appreciate it......I am interested to see what you think....

Well, well, well, - you could very well 'light up' all AU-999 owners with this Kev.

I can remember reading a thread about the virtues or otherwise of various Sansui's of the AU-999 era. I particularly remember our own EW grumbling that the AU-999 could scarcely meet its power output spec, but was a fair sounding amp even so (something like that) and some comments about clipping.

You have been soooo quiet recently :D but what a result !

This is a marvellous piece of work - many congratulations Kev :thmbsp:

Oh that is interesting John, standard they really do scarcely meet their output, but then apart from that, the waveform doesnt look good, its always puzzled me, the wave looks kind of "rounded" and "wrong"......

For some reason beyond me and my friend, they drop the voltage to the last stage of the pre-amp (R763), this seems to actually really limit the post volume performance......As you increase the volume, it just hits the wall really early clipping the positive side first, not to mention the pitiful voltage it swings......
 
Sansui did odd preamp things in the AU-20000 too. They up the gain about 20 db and then throw away 10 of it. Or something like that. My Croatian friend told me about this. Maybe he will chime in.

I'll do the AU-999 I have here soon, Kev, and give my impressions. But if it does as you say, I'm sure it will be an awesome improvement!
 
Here is the schematic with the new values for the resistors....
I will endeavor to lift the hood and measure some voltages today if I get time......
au999mod.jpg
 
Wow Kevin,

I've been so involved in my restoration I missed this thread till now. As soon as I get the kinks worked out on mine, I'll get my parts ordered.

Thanks for the modification.

George.
 
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