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Old 27th May 2012, 04:07 PM   #201
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Quote:
Originally Posted by buzzforb View Post
Diy curve tracer psu. Need 36V+ and about 5A of clean power. I could do big clc, but didnt know if that was adequate as i think the needs for big Class A power amp are not as great in comparison to digital baseed stuff.
basic know how

LM317 PASS TRANSISTOR REGULATOR - Electronic Circuit and Kit - freecircuit.net
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Old 27th May 2012, 04:13 PM   #202
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Quote:
Originally Posted by StevenOH View Post
And if you have to stay with SJDP only, you just combine the two sketches
I have posted into one attached.
The new negative voltage reference will set both JFets to the selected current bias, pot in the voltage divider will set the output node to the ~half of the supply voltage. Just find the lowest value for R2, that will work for
your speakers and your ears (again, it's a balancing act). Here R4 will be tens KOhm.
So we have come full circle with some nice changes. We have traded self bias for fixed and added feedback to improve Zout. Nice. Is the top biasing mechanism, consisting of V divider and cap, referred to as a gyrator. Ratio of R6/R5 set gain, correct.
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Old 27th May 2012, 05:42 PM   #203
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<<Is the top biasing mechanism, consisting of V divider and cap, referred to as a gyrator?>>
No. C2, 300 ”F is just a coupling capacitor, and, as demonstrated, is not unavoidable. Gyrators, generally, convert impedances - one into another.
The name of this whole arrangement is SRPP.
<<Ratio of R6/R5 set gain, correct?>>
Not exactly. Since the OL gain here (2ŚRload / R2) is not high enough to sacrifice for the FB, I suggested to increase it at the expense of some other parameters by reducing R2. So, for the CL volt gain of let's say 15, that ratio may be ~30.
Besides, R5 will be acting together with C1 and Signal source impedance in series. This is why I stressed the requirement for C1 value. And I have to remind you, that to drive that high and nonlinear Cin of power FETs you will need pretty low source Rout with good output current capability.
SS 120R085 Depletion Mode Jfet
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Old 27th May 2012, 06:20 PM   #204
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The last paragraph in this post:
L'Amp: A simple SIT Amp
is a good illustration to what I have said here.
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Last edited by StevenOH; 27th May 2012 at 06:28 PM.
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Old 27th May 2012, 07:04 PM   #205
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Does this seem to be a reasonable and accurate description of what is going on?
http://valvewizard2.webs.com/SRPP_Blencowe.pdf
It suggest that the when attached to a load, the top fet is in parallel with the load.
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Old 27th May 2012, 07:19 PM   #206
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it always is

just imagine that top one as CCS

then imagine that that CCS can be modulated to our liking

and it is - by AC going through drain resistor of lower one
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Old 27th May 2012, 07:25 PM   #207
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So much pressure toget this right now. I love it. Feelslike i am back in school,but school is fun this time. Thank youboth,very much. I nned to go delete my sudscription to blowtorch thread. Instead of brilliant men advancing audio, universal ******* contest.
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Old 30th May 2012, 12:57 AM   #208
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Waht do yo think? Pay no attention to funny things with Rs. Not even going to be used. Working of old schematic. Just realized I dont have pull down R in front of cap. Will add.
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File Type: jpg R085 SRPP 001.jpg (354.9 KB, 380 views)
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Old 5th June 2012, 01:16 PM   #209
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Here is update based on Steven' Recommendations. I have jumpers that will allow me to try self bias DC coupled version as well. Maight as well try both ways.
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File Type: jpg RO85 SRPP.jpg (159.3 KB, 320 views)
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Old 3rd July 2012, 10:27 AM   #210
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I redid the boards and etched them last night. I have a question about the Rs for the lower fet. Sy is a big fan of LED bias in some of his tube stages. I believe he says that they were tested and found to be quieter than reistors in terms of noise and offer other advantages, such as lower AC impedance. While an LED is not possible at these bias levels, It might be an iteresting experiment. I caould do mulitple, but the number would be too great. This got me thinking about the diodes from the F5. In theory, thewre should be little difference in the two, with the main difference probably being Vf and AC impedance. The diodes from the the F5T, have a larger Vf than I was looking to use, but there are some failry inexpensice schottky diodes with lower numbers. What do you think of the idea and how do i test the AC impedance of one of them.
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