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Pass Labs XA25 review
Pass Labs XA25 review
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Old 9th February 2018, 11:01 AM   #31
woofertester is offline woofertester  United States
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The pics are beautiful. Why the 6 x 1 ohm resistors between the drain and the power supply? My guess is that this the only way to measure the bias current through the transistor since there are no degeneration resistors at the source.

It could also be part of the 10A protection. Or both used to set bias and a place to monitor current during operation.
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Old 9th February 2018, 03:44 PM   #32
WalterW is offline WalterW  Netherlands
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Ha, new version with outputboards, nice!
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Old 9th February 2018, 05:38 PM   #33
wayne is offline wayne  United States
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Quote:
Originally Posted by massimo View Post
Very impressive!
I might be wrong, but at first glance it looks like the FE isn't really dissimilar from the DIY Sony VFet amp... and component values are silkscreened
A small revision on the boards and I put the values on. Nelson has them all in his head I don't.
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Old 9th February 2018, 05:44 PM   #34
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Old 11th February 2018, 01:33 PM   #35
kasey197 is offline kasey197
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Ooo these tease pics are always fun!
So we've got output zoble network now and those little silver caps make a reappearance.

I know that all the real magic is in the output stage but the front end voltage gain is also intriguing. based on the description in the latest owners manual, the front end sounds like mini F5 with cascoded input Jfets ?
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Old 11th February 2018, 03:42 PM   #36
kasey197 is offline kasey197
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I just saw Massimo's post and yes the front end does look to me like the diy vfet front end

but all the mystery here is the op stage and its biasing arrangements ...
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Old 11th February 2018, 04:04 PM   #37
PKI is offline PKI  United States
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I wonder if there are any advantages of the boards revisions in terms of performance, or mostly due to manufacturing/cost/and other mass production things?
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Old 11th February 2018, 04:14 PM   #38
woofertester is offline woofertester  United States
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Quote:
Originally Posted by kasey197 View Post
I just saw Massimo's post and yes the front end does look to me like the diy vfet front end

but all the mystery here is the op stage and its biasing arrangements ...
Per the picture, the OPS looks to me to be biased by 2 x TL431, no degeneration, and 6 // 1 ohm resistors between each drain and the power supply to monitor current in the OPS.

At 10A peak, the parallel resistors will consume 1.67v of the power supply. Any voltage greater than 1.67v across this resistor bank will trip the protection circuit.
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Old 11th February 2018, 04:58 PM   #39
Nelson Pass is offline Nelson Pass  United States
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We did put a delay on that, so you can have larger transients.
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Old 11th February 2018, 05:01 PM   #40
generg is offline generg  Germany
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Reviewers testing intermodulation might have a chance now.

:-)
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