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Sony VFET Amplifier Part 2
Sony VFET Amplifier Part 2
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Old 11th August 2017, 04:24 PM   #3121
Zen Mod is offline Zen Mod  Serbia
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Sony VFET Amplifier Part 2
instead of VFets , solder 1K8 resistors

resistor between D and S of VFet

plain MF 600mW resistor will do

be sure that FB net is enclosed

power on, set both VFet bias voltages to max and same voltage (P1 and P2 , voltage say + and - 12V) at gates of VFets

now check all interesting nodes

power off ,desolder 1K8 resistors, mount VFets (check torque , check isolation to heatsink) , power on and set Iq and offset
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Last edited by Zen Mod; 11th August 2017 at 04:42 PM. Reason: edited +/-12V
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Old 11th August 2017, 04:30 PM   #3122
Gyuri is offline Gyuri  Hungary
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Zen said.
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Old 11th August 2017, 09:43 PM   #3123
dbis is offline dbis  United States
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Sony VFET Amplifier Part 2
Quote:
Originally Posted by Zen Mod View Post
instead of VFets , solder 1K8 resistors

resistor between D and S of VFet

plain MF 600mW resistor will do

be sure that FB net is enclosed

power on, set both VFet bias voltages to max and same voltage (P1 and P2 , voltage say + and - 12V) at gates of VFets

now check all interesting nodes

power off ,desolder 1K8 resistors, mount VFets (check torque , check isolation to heatsink) , power on and set Iq and offset
Thanks Zen Mod,

Sounds interesting. Bear with me, but if I'm understanding right it's a way to check that my circuit's ok without risking the Vfets, isn't it? I just have couple questions:

1. Right now I have P1 and P2 adjusted to the numbers written on the Vfets + 1V. When I remove the 1K8 resistors do I set P1 & P2 back to those voltages?

2. I know, silly question, but what's the "FB net"?

Thanks
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Old 11th August 2017, 10:00 PM   #3124
Zen Mod is offline Zen Mod  Serbia
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Sony VFET Amplifier Part 2
1.bias voltage higher than V+1 will just brake VFets even more (when you mount them in circuit) , minimizing any possible damage ; setting these two voltages to same value will result in , say, proper range of final output offset ....... remember that's just a test of entire circuit , but without risking precious ones
2. Feedback net
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Old 11th August 2017, 10:33 PM   #3125
dbis is offline dbis  United States
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Sony VFET Amplifier Part 2
Thanks Zen Mod.
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Old 12th August 2017, 02:12 AM   #3126
alvinlim is offline alvinlim  Malaysia
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Quote:
Originally Posted by dbis View Post
Hi Gyuri, Yeah seemed a little funky to me so I think I may have measured it wrong. All my resistor and voltage checks before applying power to the front end were ok. I always use a dim bulb to power on for the first time and that was fine too. Thanks.


I did not use the "dim bulb power on" with the VFET installed due to this statement in the documentation....

"If you have a Variac for this, you will want to use it, but keep in mind that it is only useful for providing a relatively low inrush current to the power supply capacitance – you cannot raise the AC line voltage too slowly or you risk having the VFETs draw too much current while the bias voltages are being established.
I suggest that you raise the AC line to voltage you have a Variac for this, you will want to use it, but keep in mind that it is only useful for providing a relatively low inrush current to the power supply capacitance – you cannot raise the AC line voltage too slowly or you risk having the VFETs draw too much current while the bias voltages are being established.
I suggest that you raise the AC line to voltage over a period of less than 5s"

I think the bulb will limit the AC line voltage - more technical members can comment on this
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Old 12th August 2017, 07:30 AM   #3127
Gyuri is offline Gyuri  Hungary
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First time, when I powered up amp with VFETs, I used a variac and a lightbulb series with it. I worried about it: "you cannot raise the AC line voltage too slowly".
What can be too slow here?
So I watched voltage drop on R32. At low voltages, it raised quickly because VFETs are normally on. So it was better to turned it off.
Then, I've got a big breath, and switched it on with light bulb at about 180VAC.
Worked flawlessly.
Later I have found with the variac's involvement, that below somewhere half of the PSU voltages, current raises again, as VFETs gets closed.
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Old 12th August 2017, 07:17 PM   #3128
Nelson Pass is offline Nelson Pass  United States
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Sony VFET Amplifier Part 2
The main point is - don't dawdle with the Variac, or if you do, then watch
the current draw carefully.
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Old 20th August 2017, 04:19 AM   #3129
Joca is offline Joca  Portugal
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Is a 9ma idss a no go? I can only find this in europe. What sources do you have in europe?
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Old 20th August 2017, 07:18 AM   #3130
Gyuri is offline Gyuri  Hungary
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Do you asks about 2SK170/2SJ74s?

I think 9mA Idss will be fine.

But be careful, is it original?
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