Sony vFET Amplifier Part 2

Jump to Latest
#4,082
re-adjusting bias

hi everybody!
today i was wondering if there is any drift in the bias setting with the time , so i measured the amp after almost 18 months. the result : yes there is some 🙂

at the left channel T18 was drifted from initially 7mv away up to 270mv but the rest of measurements were within the range.

at the right channel T18 was around 190mv (initially 6mv) , but the output DC offset was 350mv!(initially 19mv).

so to stay on the bright side of life, it is probably good to check the bias at least once per year and re-adjust. ;-)

any similar expediences with your vfet amp?

best wishes
 
#4,084
mr_whocares said:
hi everybody!

so to stay on the bright side of life, it is probably good to check the bias at least once per year and re-adjust. ;-)

any similar expediences with your vfet amp?

best wishes
Click to expand...

Do you use spring washers on the vfets to mount them?
It might also be worthwhile retightenting them to the heatsink.
I generally let the amp go through 3 thermal cycles of use, then once the amp has cooled down check every bolt and retighten mosfets down on heatsinks and every other bolted electrical connection.
Then recheck operating conditions, bias snd dc offset.

Kind of like what I do after a new engine rebuild on my motorbike. Hahahaha
 
#4,085
WBS said:
Was there an audible cue that made you think to remeasure?
Click to expand...

yes, indeed 🙂 ... i was facing the case that the sound stage drifts a bit to the left after 30-45 minutes of warm-up. i am keen to see if the output dc offset on the right channel caused this issue .

i have not had a new sound check yet, because i would like to re-check the bias again and maybe put another psu transformer in parallel to the existing one 🙂 ... but i will report if the issue got hopefully fixed 😉
 
#4,086
2 picoDumbs said:
Do you use spring washers on the vfets to mount them?
It might also be worthwhile retightenting them to the heatsink.
I generally let the amp go through 3 thermal cycles of use, then once the amp has cooled down check every bolt and retighten mosfets down on heatsinks and every other bolted electrical connection.
Then recheck operating conditions, bias snd dc offset.

Kind of like what I do after a new engine rebuild on my motorbike. Hahahaha
Click to expand...

thanks for the hint. the housing is still open for further tunings and i will check and retighten the screws !
 
#4,088
Eric said:
Thanks for the report on voltage drift. I suspect it has more to do with ambient temperature than anything else. Constructing in summer and re-measuring in winter will probably be the biggest difference. Lid on vs lid off is also likely to show some difference.
Click to expand...

yes, you are right; this is why the settings have to be done always with the lid on and i have been doing that as well. for that reason i use 5 DMMs for the ease of work 🙂

usually i measure the the bias every 10 minutes to get an idea how the changes are for about 90 minutes and fine adjust. the values over R32 , R5 an R6 do not change that much, but the rest (output DC, T18) continue to change and there no "saturation" even until 90 minutes. However, the most of the changes (highest slope) happen within the first 30 minutes, so i am good if i set up the amp after 60min and fine tune after 90 min. 😉
 
#4,089
Biasing

Below are my measurements after the new adjustment, measured for 90 minutes for the left and right channel.

best regards
 

Attachments

  • Messung_L_180220.jpg
    Messung_L_180220.jpg
    96.5 KB · Views: 472
  • Messung_R_180220.jpg
    Messung_R_180220.jpg
    93.3 KB · Views: 473
#4,095
Hi all, after a smooth build of one side of the Pass Sony Vfet I built the second side and ran into a problem. I'm at the stage of biasing the front mosfets and after doing the initial voltage checks which were all OK, R5 and R6 are totally unresponsive to P4,P3 as I try to bring them up to 1.5v. Can anyone give me an idea where to start to trouble shoot, any help would be greatly appreciated... Just to elaborate my read back from the test points are 000 on both sides
 
Last edited:
#4,096
Hi Bugeyes,

I'm no expert, and others can certainly help you a lot more than me, but to start:
Q5 and Q6: Toshiba or Fairchilds?
P3 and P4: 500R or 1K?
Also make sure it's actually P3 and P4 you are turning and not P1 and P2. On my first module I was too focused on the DMM readings and unwittingly turned P1 and P2 instead of P3 and P4.

Paul
 
#4,097
Thanks for the reply. Q5 and Q6 are fairchild's and per the supplemental BOM correct and the opposite functioning board correct. P3 and P4 have the correct code 501 for 500. Made sure to adjust the outside (relative to the board) pots. The inside pots, P1 and P2, worked perfectly to adjust the Vgs for the Vfets to be installed later. Made sure Q7 and Q8 were correct per the BOM...
 
#4,098
Assuming there are no damaged parts, I guess it's possible you have a
combination of low Idss jfets and high Vgs mosfets.

Can you test/verify a few things:

1) What voltages do you measure across R35 and R34, and what value resistors do you have there?

2) Can you measure the voltage between T7 and T10 test points (ie, one probe at T7
and the other at T10) as your adjust P3 through its range? What voltage range do
you see?

3) Can you repeat 2) for T9-T12 and with P4?

Thanks,
Dennis
 
You must log in or register to reply here.
Top Bottom