So the consensus is that replacing the two 0.47 OHM resistors with either two 1.5 OHM resistors in R1 & R2 (or a single 0.75 OHM in R1 and shorting R2) yields more usable power out of the 2sj28's Vfets but at the expense of more thermal generation because the current is now increased.
It's not a consensus but fact. 🙂
The 2SJ28/2SK82 have higher current capacity because of their double die versus the single die of the 2SJ18/2SK60. The Part 1/Part 2 amplifiers were single ended whereas OS2 was mu follower. In single ended all of the gain is from the VFET whereas the mu follower has the Mosfet providing some of the gain (some push-pull) to make up for the lower power of the 2SJ18/2SK60.
Here are the distortion/power plots of the Part 1 amplifier and the OS2 amplifier taken from NP's writeups:
Part 1 - 2SJ28
OS2
The 2SJ28/2SK82 have higher current capacity because of their double die versus the single die of the 2SJ18/2SK60. The Part 1/Part 2 amplifiers were single ended whereas OS2 was mu follower. In single ended all of the gain is from the VFET whereas the mu follower has the Mosfet providing some of the gain (some push-pull) to make up for the lower power of the 2SJ18/2SK60.
Here are the distortion/power plots of the Part 1 amplifier and the OS2 amplifier taken from NP's writeups:
Part 1 - 2SJ28
OS2
Since this is "diy" audio, the adventurous can try 2SJ28/2SK82 at 1.6A in mu follower configuration. Split the 0.75R into two and place into R1 and R2 positions.
So that in the bold above would be what you had described by using the parallel 2W 1.5 OHM resistor, Ben?
Hi Benji, tone is hard to convey but I meant it with the best of intentions, which is to say it should sound fine at 1 ohm total also. Just like it would in a number of other places. But Nelson's dialed it in to the sweet spot for what the 2SJ28 could do. So, if you want to hear that, make the switch.
Okay, so the resistor values for R1 and R2 are 0.75 OHMS, but because they are in a parallel circuit the equivalent resistance for those two resistors now becomes 0.38 OHMS
Is there a distortion vs power plot for the 0S2 with the 2SJ28 in a P channel build or is the distortion vs power plot the same since the N vs. P is just more of a polarity characteristic for the designed Vfet circuit.
I hope these aren't dumb questions, but I work 65 to 70 hours a week and with a family I don't have the available time to study and research all this stuff.
No, please look at the OS2 schematic (mu follower), and note that R1 and R2 are in series. The suggested R1=R2=0.38R was meant for OS2 only, not for Part 1 or Part 2. Do not put 0.38R in R1 and R2 of Part 1 or Part 2 amplifiers!
Do not attempt this if you do not understand the circuit.
If OS2 board is built with 2SJ28, R1=0.75R, R2=0R, then the distortion/power would be as Part 1 amp.
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Hi Rahul,
So the single 0.75 OHM resistor in R1 with the shorted R2 or two 1.5M OHM resistors in R1 & R2 is the ticket.
What wattage and tolerance should the replacement resistors be and what about P1 and R8?
Alazira posted this image below noting some other changes.
Single 0.75R resistor is 5W. Jumper in R2.
Alternate is 2 - 1.5R 3W resistors in parallel in the R1 position, not one each in R1 and R2. Jumper in R2.
P1 and R8 do not to be changed.
My mistake Ben as I do see that R1 & R2 are in series and not parallel for the 0S2 board.
Is resistor tolerance critical?
Ben, I have these in my Digi-key cart.
Would this fit the bill?
https://www.digikey.com/en/products...?s=N4IgTCBcDaIApwMIGYAiBGAdAVkQFQFoA5VEAXQF8g
Greets to all
This is a partial repost at Mark’s suggestion over in Ship of Theseus thread…as here’s where to find the VFET gurus…
I was lucky enough to find someone selling off a 2016 VFET kit in the swap meet not that long ago.
I put the kit aside when I started reading the documentation in the build process as I felt I needed some more amp building experience to take it on…I think I had only built a gainclone and ACA monblocks at the time.
Since then I completed my F2J monoblocks, a SONY VFET P channel amp and two Babelfish J2 stereo amps (still debugging one).
I’m always trying to listen to and compare my builds, and a recent listen to my VFET P channel SE got me thinking about all the other variations on the the VFET amps.
So I thought I’d like to try a few flavors.
My question is, if I want to use my pairs of 2sj28 and 2sk82 VFET’s to experiment with these SE designs more and then later on try building the 2016 push/pull amp with them…how do I preserve the precious VFETs?
Are there some sort of sockets that can be mounted to the DIYAudio store VFET OS boards so the VFETs can later be easily removed and used again?
I’ve ordered a pair of each of the N and P versions of the DIYAudio store OS stage kits as well as some Scourge and Marauder kits along with the SMPS.
I still need power supply filters.
I might have a lead or two. If anyone has some extra boards and any difficult to find components that might be needed to populate them please shoot me a PM?
Also…are there any other VFET chassis still around? I sent the store an email, no response yet.
This is a partial repost at Mark’s suggestion over in Ship of Theseus thread…as here’s where to find the VFET gurus…
I was lucky enough to find someone selling off a 2016 VFET kit in the swap meet not that long ago.
I put the kit aside when I started reading the documentation in the build process as I felt I needed some more amp building experience to take it on…I think I had only built a gainclone and ACA monblocks at the time.
Since then I completed my F2J monoblocks, a SONY VFET P channel amp and two Babelfish J2 stereo amps (still debugging one).
I’m always trying to listen to and compare my builds, and a recent listen to my VFET P channel SE got me thinking about all the other variations on the the VFET amps.
So I thought I’d like to try a few flavors.
My question is, if I want to use my pairs of 2sj28 and 2sk82 VFET’s to experiment with these SE designs more and then later on try building the 2016 push/pull amp with them…how do I preserve the precious VFETs?
Are there some sort of sockets that can be mounted to the DIYAudio store VFET OS boards so the VFETs can later be easily removed and used again?
I’ve ordered a pair of each of the N and P versions of the DIYAudio store OS stage kits as well as some Scourge and Marauder kits along with the SMPS.
I still need power supply filters.
I might have a lead or two. If anyone has some extra boards and any difficult to find components that might be needed to populate them please shoot me a PM?
Also…are there any other VFET chassis still around? I sent the store an email, no response yet.
I’ve never had issues swapping parts for different amps so long as I took my time with desoldering
