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Old 4th December 2016, 02:38 AM   #121
lhquam is offline lhquam  United States
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Show me Id vs Vgs and gm vs Id with Id in the range of 0A to 3A. The datasheets are useless in that range, and extrapolating the curves down into that region will not be accurate. Actual device measurements are needed at the operating conditions of the device.
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Old 4th December 2016, 02:47 AM   #122
2 picoDumbs is offline 2 picoDumbs  Australia
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Quote:
Originally Posted by lhquam View Post
Show me Id vs Vgs and gm vs Id with Id in the range of 0A to 3A. The datasheets are useless in that range, and extrapolating the curves down into that region will not be accurate. Actual device measurements are needed at the operating conditions of the device.
Yeah fair enough. They follow the data sheets fairly closely, but if the data sheets are wrong we're back to square one.
Either way my parts are on the way and it will be fun building it.
It should be fairly straight forward.
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Old 4th December 2016, 06:09 AM   #123
2 picoDumbs is offline 2 picoDumbs  Australia
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Don't know if this has any value but assuming the data sheet is reasonably accurate in the area of interest, here is more useful data for 40P50P. I have specialised software that does this work.
For what it's worth here it is.

Click the image to open in full size.
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Old 4th December 2016, 02:24 PM   #124
generg is offline generg  Germany
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Pass XA25?
Second channel is running. The bias is very stable with the screwed NTCs on the heatsinks.
Too lazy now to measure residual. More fun to hear music.....

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Old 4th December 2016, 03:08 PM   #125
wdecho is offline wdecho  United States
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Pass XA25?
I hope you share your schematic.
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Old 4th December 2016, 03:45 PM   #126
nashbap is offline nashbap  United States
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Old 4th December 2016, 06:22 PM   #127
generg is offline generg  Germany
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I used Lynns draft, with less voltage and no cascoding and no cascode feedback.

Villars 2007 Power Amplifier

of course less voltage +/-23V and biased at 1.3A following Nelson BAF talk.

But if I were you I would wait building!

Look some time at the Villars.
Biasing is very similar to F4, frontend is very similar to Sony II.

I am sure Lynn and 2picodumps will do better drafts.
Better bias stabilization and possible pos feedback.

Maybe we should take away this build inspired by the XA25 in a different thread.

The decisive is the use of the IXys and not so much Nelsons for sure more clever implementations we do not know.
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Old 4th December 2016, 07:26 PM   #128
needtubes is offline needtubes  United States
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Originally Posted by generg View Post
I used Lynns draft, with less voltage and no cascoding and no cascode feedback.
My SPICEd version is similar, except I used global feedback from output to the JFET sources.

How did you implement the thermistor to control the bias drift? Are you using TL431? I have a simple idea of how use a thermistor with the TL431, but am unsure what value I would need to accomplish my goal.

Last edited by needtubes; 4th December 2016 at 07:38 PM.
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Old 4th December 2016, 07:41 PM   #129
2 picoDumbs is offline 2 picoDumbs  Australia
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Originally Posted by generg View Post
I am sure Lynn and 2picodumps will do better drafts.
Better bias stabilization and possible pos feedback.
.
Maybe a little different but I don't think it will be better. I generally keep to the KISS to do the job, For example mine will probably have no thermal compensation, hahahaha.
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Old 4th December 2016, 08:33 PM   #130
lhquam is offline lhquam  United States
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After understanding that capabilities of the IXYS hocky-pucks better, I conclude that there is absolutely no need for Positive Current Feedback (PCF). Wrt. the bias circuit: I did some serious equation analysis a simple thermistor based TL431 bias circuit. By measuring the bias voltage needed for the desired idle current at 3 temperatures, such as 25C, 50C, and somewhat higher, one can solve a system of equations for 3 resistors in the bias circuit so that the ible current will be nearly constant between 25C and 50C. Shown are the circuit, and some Vbias .vs Temperature curves showing the output from the bias circuit as the temperature to the thermistor (NTC) changes.

  • The first plot shows big red dots where the circuit Vbias was measured for 1.3A bias idle current, and the behavior of the bias circuit that was fit to the measured data.
  • In the second plot I intentionally lowered the Vgs value of the 70C measurement so that the bias generator would exhibit a fairly constant current behavior between 25C and 50C, but gradually reduce the bias current at temperatures above 50C.
  • The third plot shows a family of possible bias curves for different amounts of Vgs reduction at 70C.
Attached Images
File Type: jpg TL431-thermistor-bias-circuit.jpg (20.7 KB, 529 views)
File Type: jpg TL431-thermistor-design-3-point-fit-A.jpg (17.7 KB, 527 views)
File Type: jpg TL431-thermistor-design-3-point-fit-B.jpg (18.3 KB, 516 views)
File Type: jpg TL431-thermistor-design-3-point-fit-C.jpg (24.4 KB, 513 views)
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