Hi Anthony,
I’m sure it will be another super nice amplifier from Fab but I have not yet heard it because of social distancing. We’re also not yet allowed to enter someone else’s house.
When I can, I will definitely take the USSA3b for a spin.
All the best!
Do
Btw, tried to email you but got a bounce back saying your inbox was full.
I’m sure it will be another super nice amplifier from Fab but I have not yet heard it because of social distancing. We’re also not yet allowed to enter someone else’s house.
When I can, I will definitely take the USSA3b for a spin.
All the best!
Do
Btw, tried to email you but got a bounce back saying your inbox was full.
Hi AnthonyA
If you cannot locate GR grade jfet then you can use lowish IDSS grade jfet (<= 8ma). Using 2 pairs of bipolar as outputs will not help that much since each one will have half the bias. I had not checked this avenue yet to increase the open loop gain….
If you want to use bjt for input then you must switch to USSA-5.x build as the bjt require current sources as opposed to jfet self biasing. In fact, the USSA-5 is the bjt input version of the USSA-3 and all USSA-3 and USSA-5 versions use the same pcb you have.
Let me know if you need the USSA-5.x manual.
Fab
If you cannot locate GR grade jfet then you can use lowish IDSS grade jfet (<= 8ma). Using 2 pairs of bipolar as outputs will not help that much since each one will have half the bias. I had not checked this avenue yet to increase the open loop gain….
If you want to use bjt for input then you must switch to USSA-5.x build as the bjt require current sources as opposed to jfet self biasing. In fact, the USSA-5 is the bjt input version of the USSA-3 and all USSA-3 and USSA-5 versions use the same pcb you have.
Let me know if you need the USSA-5.x manual.
Fab
Last edited:
I had not in mind that you were building USSA3B where jfet IDSS needs to be lower than USSA3 with mosfet output.
Still it should work fine with IDsS of 6.x ma but damping factor will be less…
See below expected DF on USSA3B based on input jfet IDSS:
IDSS= 3ma —> DF= 60
IDSS= 7ms —> DF= 30
Some ussa3.0 amplifiers were built with DF of about 25 to 30 with good reported sound.
You can interpolate DF value based on that.
Fab
Still it should work fine with IDsS of 6.x ma but damping factor will be less…
See below expected DF on USSA3B based on input jfet IDSS:
IDSS= 3ma —> DF= 60
IDSS= 7ms —> DF= 30
Some ussa3.0 amplifiers were built with DF of about 25 to 30 with good reported sound.
You can interpolate DF value based on that.
Fab
Last edited:
Hi AnthonyA
For 8 ohms speakers a DF of 30 would be quite fine (but maybe not optimal for hard rock music depending on speakers
). We can push DF a bit higher by increasing the driver current too.
There would also be another way to increase DF which consists in replacing the darlington bjt drivers complementary pair with a high VGSoff mosfet complementary pair readily available.
This is what I am testing on the USSA-5.2 (new version). Bench tests are completed and listening test is on-going since last Saturday. I can adjust DF easily with the trim pots. Quite promising so far….
As for a new USSA-3.2B (mosfet drivers stated above) then I am not there yet but maybe on my to-do list…
it is just that USSA-5.x versions seem much more popular than USSA-3.x versions…
Thanks for your appreciation of my projects, this alone justifies my hours taken to share and support these projects
Fab
For 8 ohms speakers a DF of 30 would be quite fine (but maybe not optimal for hard rock music depending on speakers
). We can push DF a bit higher by increasing the driver current too.There would also be another way to increase DF which consists in replacing the darlington bjt drivers complementary pair with a high VGSoff mosfet complementary pair readily available.
This is what I am testing on the USSA-5.2 (new version). Bench tests are completed and listening test is on-going since last Saturday. I can adjust DF easily with the trim pots. Quite promising so far….
As for a new USSA-3.2B (mosfet drivers stated above) then I am not there yet but maybe on my to-do list…
it is just that USSA-5.x versions seem much more popular than USSA-3.x versions…
Thanks for your appreciation of my projects, this alone justifies my hours taken to share and support these projects
Fab
Last edited: