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Old 13th March 2018, 05:39 PM   #1381
lhquam is offline lhquam  United States
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Default FE bias sanity check

People (aka greedy boys) have spent a lot of effort to stabilize the front-end (FE) mosfet bias current. Here are actual measurements of an amplifier I have built with the topology shown below. As you can see, there is no servo (or thermistors) in the FE of that amplifier.

From a cold start I made three voltage measurements and a heatsink temperature measurement at 9 points in time, spanning just over one hour.

Across positive rail Rsen (0R27)
Across positive rail J313 source resistor (22R1)
Output offset voltage.

The OS and FE bias currents were computed from the voltages.

Conclusion: No need for a servo to stabilize the FE bias vs. temperature.
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File Type: jpg temps-20180313.txt.jpg (47.7 KB, 168 views)
File Type: jpg Ideal-ZD25-series-opto-OS-gate-1.jpg (76.5 KB, 172 views)
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Old 13th March 2018, 05:51 PM   #1382
Zen Mod is offline Zen Mod  Serbia
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agree with you 100%

I did what I did from several reasons ...... first we always are figuring how exactly Papa did it , further how to make it different

not necessary better (possible or not ) , but necessary not (much or at all ) worse than Papa's original

well , that I can say for my self for sure

long time past the point of must have new amp ......... never going to past the point of must have fun

(Italics written in Homer Simpson voice)

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Last edited by Zen Mod; 13th March 2018 at 05:54 PM.
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Old 13th March 2018, 08:18 PM   #1383
Ian Macmillan is offline Ian Macmillan
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Quote:
Originally Posted by lhquam View Post
People (aka greedy boys) have spent a lot of effort to stabilize the front-end (FE) mosfet bias current. Here are actual measurements of an amplifier I have built with the topology shown below.

Conclusion: No need for a servo to stabilize the FE bias vs. temperature.
Agreed, did anyone previously disagree? This really is no different to the regular PL front end albeit with capacitor bypass, i.e. it has DC local feedback due to the MOSFET source resistors. The need for a servo, if there is one, would arise only if you wish to remove the MOSFET source resistor and the associated capacitor.
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Old 13th March 2018, 09:09 PM   #1384
vitalstates is offline vitalstates  United Kingdom
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Quote:
Originally Posted by Zen Mod View Post
agree with you 100%

long time past the point of must have new amp ......... never going to past the point of must have fun

(Italics written in Homer Simpson voice)

so true

but never going past the point of understanding for me...it's still a learning process...but still fun...

so, sorry for the naive questions...I am aware that you guys have been at this thread for a long time, and also aware of the effort put in.....

ok creep mode over
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Old 13th March 2018, 09:51 PM   #1385
Zen Mod is offline Zen Mod  Serbia
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you had all legitimate questions



to be precise - all questions are legitimate ; only in case when asked by someone demanding spoonfed , they become questionable

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Old 13th March 2018, 11:22 PM   #1386
flancfuster is offline flancfuster  Romania
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Hi everyone,
I'm following this thread with great interest and I'm gathering parts to build the "beast". For now I have the hockey pucks and I'm looking for a transformer, but I don't know what voltage to choose for secondary. Any recommendation?
Lynn, in post #1290 you had positive results with stability of OS. The rails are 40V. Do you recommend to try that OS bias method? I'm thinking of mixing Sony VFET PII FE with OS from post #1290. Any thoughts?
Thanks.
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Old 15th March 2018, 02:42 AM   #1387
lhquam is offline lhquam  United States
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Ian:
Here is a topology without DC degeneration of the j313/K2013 MOSFETs. I found resistor parameters that work well.
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Old 15th March 2018, 03:21 AM   #1388
lhquam is offline lhquam  United States
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Amendment: I have not found parameters that pass the ZM pulse test.
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Old 15th March 2018, 06:50 AM   #1389
Zen Mod is offline Zen Mod  Serbia
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what is max. value [mA] you can take as passed?
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Old 15th March 2018, 08:41 AM   #1390
Ian Macmillan is offline Ian Macmillan
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Quote:
Originally Posted by lhquam View Post
Ian:
Here is a topology without DC degeneration of the j313/K2013 MOSFETs. I found resistor parameters that work well.
Interesting twist using an opto LED (U23) as a Ďdiodeí matching the voltage drop of the bias opto (U21) Iíve not tried this but I did do something similar using an LM385 voltage reference. In my case I used it just on the FE but I wasnít happy with the amount of servo gain, i.e. it didnít control the FE MOSFET current very tightly. Does your variant work better taking the sense from the output stage MOSFETs?

More importantly, does completely eliminating the degeneration resistors and associated capacitors in this way actually improve performance of the amplifier?
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