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...
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...