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Old 4th October 2013, 07:34 PM   #1
juma is offline juma  Germany
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Default VFET PP amp

There's been some time since I've got 2 pairs of k60/j18 VFETs (thanks to sstrsat and bata bane) so I had no choice - bulding a VFET amp became unavoidable for me . First i did a nice little SE 5-Watter to get the hang of VFETs and the result was sonically very encouraging but I wanted something more powerfull.
Here is what I came at: complementary BJT input (Q1,Q2) stage with adjustable Ic (to adjust VFETs' Vgs, Id and DC offset at the output) followed by EF stage (Q3, Q4) that provides a low impedance drive for VFETs which are used in common source connection (drain follower, i.e. voltage and current gain). Feedback is done F5-style (so called "CF") for the CLG of about 20dB.
Before you ask: I tried the bootstrap loaded input stage (more OLG, more feedback) but it sounded a bit too "stiff" and cascoding the Q1, Q2 made the amp sound a bit "thin" to me. I also tried JFETs (cascoded k170/j74) as Q1, Q2 but BJTs sounded better (these are my impressions, you might feel different about it).
To bias the VFETs properly I used the higher voltage rails for the input stage and EF. I decided to bias VFETs at about 1A (Vgs of about 10V at 25V of Vds).
Values in PSU are carefully chosen, through experimenting, in order to provide correct RC constants for Vgs and Vds - VFETs can be destroyed if Vgs is slower than Vds.

Prototype worked fine and I plan to finish the amp as soon as big heatsinks (11"x8"x3.1/4") arrive. I tested the amp with small heatsinks so tests were short (about a dozen of 10 minutes sessions done in one day) but my conclusions are clear: VFETs are definitely "something else" and worthy of DIYer's attention.

In attachment: amp sch., PSU sch. and PSU module (amp prototype didn't get photographed because I needed the heatsinks quickly for another amp but it was a messy point-to-point job anyway).

EDIT: Due to small instabilities on difficult load, R18, R19, C11, C12 are added in the amp's schematic.
Also, R7 and R6 are changed to 4k7 in order to provide wider bias current adjustment range.

New schematic of the amp is in the post #69

Some changes in the PSU - post #76


VFET_PP1_psu.gif PSU-modul.jpg
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File Type: gif VFET_PP1_amp.gif (9.5 KB, 1664 views)

Last edited by juma; 8th January 2014 at 11:23 PM. Reason: small improvements
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Old 4th October 2013, 07:48 PM   #2
CFT is offline CFT  United States
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Thanks for sharing the design. Very nice, finally something refreshing
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Old 5th October 2013, 07:32 AM   #3
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Nice Juma!
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Old 5th October 2013, 08:14 AM   #4
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another nice amp as usual
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Old 5th October 2013, 02:13 PM   #5
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Most Excellent!

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Old 5th October 2013, 09:53 PM   #6
juma is offline juma  Germany
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Thank you guys!
I'm glad you like it Mr. Pass !
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Old 7th October 2013, 04:37 PM   #7
CeeVee is offline CeeVee  Portugal
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Hi Juma,

What about using the 2SJ28 and 2SK82 ?

I got 4 pairs....
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Old 7th October 2013, 07:47 PM   #8
juma is offline juma  Germany
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CeeVee,
just measure the Vgs they need in order to run the Id you wish (at chosen Vds), adjust the bootstrap resistors accordingly, and you are good to go!
And don't worry, you got 2 pairs to spare...
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Old 8th October 2013, 07:22 AM   #9
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Have you tried running the output outside of the feedback loop?
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Old 8th October 2013, 07:28 AM   #10
CeeVee is offline CeeVee  Portugal
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Quote:
Originally Posted by juma View Post
CeeVee,
just measure the Vgs they need in order to run the Id you wish (at chosen Vds), adjust the bootstrap resistors accordingly, and you are good to go!
And don't worry, you got 2 pairs to spare...
hehe guess i'll try one channel first...in case i fry a pair.
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