Sony vFET Illustrated build guide

Jump to Latest
D

dantwomey

Member
Joined 2002
Paid Member
Rodeodave said:
dantwomey, V-Fets are normally open devices, which means they will conduct current from drain to source even without a voltage present at the gate. This is why the bias supply is so important, the V-Fets have to be turned off or "throttled" before the supply at T3/T5 is present, otherwise they'll burn up.

For the first startup we want the V-Fets turned off (or throttled) more than necessary, just to be safe. The 2sk82 needs a negative bias voltage, so we initially make it more negative than necessary. The 2sj28 needs a positive bias voltage, so we initially make it more positive.
Click to expand...

Many thanks for the explanation. :)
 
D

dantwomey

Member
Joined 2002
Paid Member
dantwomey said:
Strange feeling when you follow the instructions and everything works out perfectly! ��
Click to expand...

Oooops! May have spoke too soon!

Slightly running away DC offset on one channel even though R32 is perfect and T18 to ground is under 100mv.

Questions.

Is R5(T6 and T7) and R6 (t8 and T9) supposed to be the same 1.5V after the VFETs are installed?

Regards,
Dan
 
D

dantwomey

Member
Joined 2002
Paid Member
Is R5(T6 and T7) and R6 (t8 and T9) supposed to be the same 1.5V after the VFETs are installed?
Click to expand...

R5 and R6 were around 1.1-1.2V so with meters installed I adjusted them back up (using P3 and P4) to 1.5V while keeping an eye on T18 to ground. So far this has not helped. My DC offset keeps drifting higher. About 2-3mv per minute.

Everything else seems normal. P1 and P2 keep T18 to ground inline much the same way the first channel did.

More to come.....

Regards,
Dan
 
B

Ben Mah

Member
Joined 2012
Paid Member
I have found that the DC offset varied from day to day and even from hour to hour. Temperature changes had a big effect on the offset. After a while, I just stopped worrying and stopped trying to chase the perfect setup.

Do you have the case top in place in between adjustments? The jfet temperature needs to be stable for stable operating offsets. Adjustments need to be done quickly and the top put back in place for the temperature to restabilize, and then further adjustments if needed only after a warmup period.

It can be very frustrating, especially since the adjustments are very sensitive to the single turn pots.
 
D

dantwomey

Member
Joined 2002
Paid Member
Ben Mah said:
I have found that the DC offset varied from day to day and even from hour to hour. Temperature changes had a big effect on the offset. After a while, I just stopped worrying and stopped trying to chase the perfect setup.

Do you have the case top in place in between adjustments? The jfet temperature needs to be stable for stable operating offsets. Adjustments need to be done quickly and the top put back in place for the temperature to restabilize, and then further adjustments if needed only after a warmup period.

It can be very frustrating, especially since the adjustments are very sensitive to the single turn pots.
Click to expand...

I was spoiled when the first channel turned out perfect. :(

Regards,
Dan
 
D

dantwomey

Member
Joined 2002
Paid Member
Two channels with DC offset shown. One is perfect. The other runs away thermally. More details and measurements when I have time.

IMG_0336_zpsiqevgm8m.jpg


Regards,
Dan
 
rumina

rumina

Member
Joined 2013
i build two of these amp and both are not as stable and easy to adjust as my f6 or f5t. you need to adjust the amp fully heated up (1-2h) and with lid closed.

but in contrast to the other amps i build i realised that setting (readjust) the offset of the two amps was a lot easier after the amps run for over 24 hours (not in one part but total overall runtime). maybee the "sleepy" vfet needs time to awake befor getting stable, first time for me that this effect of "burn/break-in" was so dominat.

i would suggest that you let run the amp for some hours and then try to adjust it. do both channel in parallel i needed 2-3 hours after heating up to find the right spot. now the amps are stable after 1 1/2 h with a offset under 15mv, but it needed a lot of time.

don't put them in a closed rack, bad ventiltated rack - changes the offset asap, it's a sensitive kid ;-).
 
Last edited:
D

dantwomey

Member
Joined 2002
Paid Member
rumina said:
i build two of these amp and both are not as stable and easy to adjust as my f6 or f5t. you need to adjust the amp fully heated up (1-2h) and with lid closed.

but in contrast to the other amps i build i realised that setting (readjust) the offset of the two amps was a lot easier after the amps run for over 24 hours (not in one part but total overall runtime). maybee the "sleepy" vfet needs time to awake befor getting stable, first time for me that this effect of "burn/break-in" was so dominat.

i would suggest that you let run the amp for some hours and then try to adjust it. do both channel in parallel i needed 2-3 hours after heating up to find the right spot. now the amps are stable after 1 1/2 h with a offset under 15mv, but it needed a lot of time.

don't put them in a closed rack, bad ventiltated rack - changes the offset asap, it's a sensitive kid ;-).
Click to expand...

Thanks for your input! Right now I'm monitoring and measuring like crazy!

Regards,
Dan
 
D

dantwomey

Member
Joined 2002
Paid Member
This from post #539 caught my interest.

I threw a bit of a tanty trying to set this amp up a few months ago and mothballed it. Yesterday I pulled it out again to finish off one side. After attacking this amp fresh - I found these steps helped immeasurably:

1. Ensure that you do the proper DC offset setup properly and thoroughly before installing the VFETs,

2. Once the VFETs are in - be prepared for the setup to take a while (all day if need be!)

3. Use three multimeters - I had my one good one, plus 2 imported cheapies. Connect the good one to your speaker terminals , one other one to your ground-T18 point, the last one across R32.

4. Aim to set your R32 100mV level first by using P1 & P2 adjustments.

5. Check that your 1.5V across R5 and R6 are still good - these wont change markedly by the following setup procedures.

6. Tweak the adjustments using P1-P4 - but with a focus on P3 and P4. The P1 and P2 adjustments will quickly re-adjust your R32 voltage (should that drop by a few volts).

7. I dont know if this is correct - but after I dialled in the R32 voltage, I then focused primarily on the speaker terminal DC offset. Once I had this close to 0v, I then fine tuned by adjusting all pots with micro-tweaks (and taps to loosen the spring tensions) to try to keep both voltages close to 0.

8. The voltage between T18 and ground was the hardest one to nail down. At the end however I had 99mV on R32, 18mV on the speaker terminals and 34mV on T18 - however the latter was the only one that was fluctuating. In the end I was only adjusting this reading by tweaking the pots.

9. The T18 measurement is extremely susceptible to heat changes. So at the last stages of adjustment (or preferably, sooner) do these adjustments with the lid ON - otherwise you will try to set (with the lid off) and the readings start taking off about 10 minutes after you replace the lid. ensure you have a small screwdriver that you can poke thru the lid.

I got it in the end but I will double-check and re-calibrate both sides once I have the time to get back to this amp. I had the one channel running for a while and I can tell this will be so worth the effort.
Click to expand...
 
You must log in or register to reply here.
Top Bottom