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Sony VFET Amplifier Part 2
Sony VFET Amplifier Part 2
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Old 30th November 2017, 11:00 AM   #3361
wdecho is offline wdecho  United States
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Sony VFET Amplifier Part 2
Quote:
Originally Posted by apaya View Post
I have both amplifiers and:
- I "absolutely love" the SIT-2
- I "love" the VFET amp

I also own 4 the original Sony VFET amps and have collected them since 1978, always loved them but the SIT-2 is entirely different. For lack of better expression, the VFET amp has an ultrasonic characteristic that is unlike any other amp I own or have listened to, the SIT-2 may be slightly less detailed but it is simply magical with my Altec Valencias!

Armin
Push pull vs SE, personal preference. Much the same with tubes, push pull vs SET. I enjoy both sounds depending on music. I have a friend that owns the SIT-2 and has built and owns a V-fet. He has said much the same as you. I heard some Valencias years ago. Fantastic sounding horns. I own modified Klipsch Lascalas.

Last edited by wdecho; 30th November 2017 at 11:04 AM.
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Old 2nd December 2017, 06:32 PM   #3362
PJN is offline PJN  United States
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Hi Everyone,


i'm hoping for some help with my problem. This spring I built the DIY Sony VFET, one channel worked perfectly the other not. The bad channel appeared to bias up correctly and all voltages were the same as the good channel, both channels played at very low volumes, but when the volume was increased the bad channel would just fade out. When the bad channel fades the voltage on T18-gnd goes from ~ 200 mV to ~ 26 V. I triple checked all of the resistors, everything was correct, all transistors were in the correct spots. Thinking is was a problem with the input section I replaced the input jfets with a net set from the DIYAUDIO store and Q3/Q4, and reflowed the board. didn't find any bridges, or cold joints. Still had the same problem. After that I put the project aside for a while. Now I have a little free time I'd like to try to get it going. Today I dusted it off and did a continuity check and compared the results to the good board everything was identical except that Q2 showed continuity between the drain and source (outer legs) on the bad board, and not on the good one. I have enough spares to replace all of the regulators and transistors with the exception of the 2SK82/2SJ28, so if their bad I'm out of luck. My plan moving forward was just to replace the regs and to see where I land, and if that doesn't do the trick replace all of the other transistors. But the Q2 continuity has me wondering. Before I rip everything apart I was hoping some of the more experienced members might have some ideas that could help point me in the right direction.

Thanks,

Paul
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Old 2nd December 2017, 07:53 PM   #3363
jan.didden is offline jan.didden  Europe
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Old 2nd December 2017, 07:58 PM   #3364
Rodeodave is offline Rodeodave  Austria
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Sony VFET Amplifier Part 2
Maybe, just maybe, the gate of Q2 is still charged and the mosfet thus turned on. You could ground the gate and measure continuity between D and S again, see if the mosfet turns off. Otherwise it would look like Q2 is fried... At least according to my understanding.
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Old 2nd December 2017, 08:28 PM   #3365
Nelson Pass is offline Nelson Pass  United States
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Sony VFET Amplifier Part 2
If all else fails, you can remove the channel and send it in for testing.

This procedure is a good one, but requires patience, which you seem
to have.
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Old 3rd December 2017, 02:21 PM   #3366
PJN is offline PJN  United States
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Thanks Nelson, something isn't working and I don't have fancy equipment or enough knowledge to zero in on it easily, so I'll methodically replace components until I get it working. If I am ultimately unsuccessful I may have to take you up on your kind offer.
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Old 3rd December 2017, 03:15 PM   #3367
wdecho is offline wdecho  United States
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Sony VFET Amplifier Part 2
Do some resistance readings at all points between good board and bad board before swapping parts. Some discrepancy may show itself. I think you already said you did V readings between good and bad. I would do that again as well.
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Old 3rd December 2017, 10:57 PM   #3368
PJN is offline PJN  United States
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Will do, thanks
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Old 3rd December 2017, 11:05 PM   #3369
Botte is offline Botte  United States
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PJN,

I had this problem on one of my builds and it’s was just that the bias on the second stage drifted with temperature. Let the amp warm up for an hour then recheck all the bias test points they will drift with temp. Re bias and then let the amp cool down and then re test once it’s back up to temp.

My two cents....
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Old 4th December 2017, 04:41 PM   #3370
juanitox is offline juanitox  France
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Quote:
Originally Posted by Nelson Pass View Post
You will be happy to know that this is a reliable seller, and he also has the
Tokin THF-51S, which is a similar part.
i have ordered a pair of this THF51-S what is the usual operating point for these is it the same as the 2SK82 20V 2A ? or is it better with high voltage or high current?

thanks
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