Pour ceux qui n'ont aucun Manuel de USSA , voici les differençes principales au niveau sonore entre les différentes versions:
(excusez pour l'anglais ici)"1.1 Main Differences between USSA versions 1.1.1 USSA version 1: This is the 1st version. The sound is transparent and sound stage is deep. Dynamics are good but not extreme. Bass is expressive where
you can hear the detailed sound of bass strings. The sound is very natural as opposed to be exaggerated and cause
no listening fatigue.
1.1.2 USSA version 2: This version has the damping factor increased to about 20 compared to 15 for the USSA ver. 1. This is first achieved by reducing the degeneration on the output mosfets. The mosfet source resistances (R15, R16) value change from 0.25 ohms to 0.1 ohms. Because of that, one resistor value used for temperature compensation is lso changed accordingly. Indeed, R9 and
R10 values changes from 2k2 to 1k5 ohms. And secondly, the CCS current is reduced to 4.6ma. The bias current will need to be re-adjusted with P1 and P2 (therefore
DC offset too at the same time) otherwise it will be too high compared to original bias current value.
You will also notice that the initial bias current at power up of amp will be less high compared to USSA version 1 and may take slightly longer time to stabilise to the target bias value set point. There is slight improvement on the bass response depending of speakers where it can be more “tight”.
1.1.3 USSA version 3:
This version has jfet inputs, mosfet as voltage shifter/buffer stage and Lateral mosfet double die as output transistors. It has no CCS since it uses jfet property to set current in input stage.
The double die mosfet and buffer stage allow for enhanced output current thus more power and control in difficult load.
This version is more detailed for sound. The bass is a bit more tight and controlled and bass guitar is very natural. Vocals and cello are sweet sounding. Damping factor is more than doubled compared to version 1.
1.1.4 USSA version 4:
This version is an hybrid version between 1 – 2 and version 3. This version 4 has the input stage with CCS similar to version 1 and 2 and output stage is similar to version 3. The main advantage over version 3 is the use of bjt compared to jfet in input stage since jfet are difficult to match between N and P channels if you do not have many of them.
The double die mosfet and buffer stage allow for enhanced output current thus more power and control in difficult load. This version has a similar sound to version 3 but even a little better in all aspects.
1.1.5 USSA version 5:
This version is the same general topology as version 4 but has a CFP input stage and an improved CCS using 2 jfet instead of one. This is the latest crafted version of USSA and sound evaluation has not been performed at the moment of the writing of the manual.
1.1.6 USSA version B:
This version was designed just after version 1 and is similar to version 2 but uses Sanken darlington transistors as output. These transistors are excellent for audio and are not expensive.
This version is the same topology as the F5 thus uses the same number of parts and no modification to the F5 pcb required. However, input jfet transistors IDSS value have to be selected more precisely and output transistors are mosfet designed for audio (2SK1530Y/2SJ201Y) instead of IRFP240/IRFP9240 designed for switching. The gain is also 10 (as opposed to 5 for the F5) to allow to be the same as the other USSA versions and not require a preamp to reach the full available power. The damping factor is reduced from about 80 (F5) to about 30. The input current is also slightly reduced. The overall sound result is an improvement (more expressive bass and natural) over the standard F5 amplifier. "
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