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Old 18th August 2012, 01:43 PM   #851
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I think you will find something similar in bias setup of BA1.
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Old 18th August 2012, 03:14 PM   #852
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Also the links in Post #35.
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Old 18th August 2012, 05:04 PM   #853
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Quote:
Originally Posted by buzzforb View Post
I think you will find something similar in bias setup of BA1.
Hello buzzforb. I found the official BA #1 output stage in the article by Mr. Pass entitled Burning Amp Number Three. Please find attached a hand copy of what may be your underlined suggestion. Is it? I show the bias circuit for the upper and the different one for the lower MOSFETs. I inserted the secondary windings of the transformer where I saw possible. Can you or other answer the two questions regarding points A and B of the bias circuits.
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Old 18th August 2012, 05:08 PM   #854
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You can't develop a bias voltage across the coils, so it will only work for
those cases where you can bias at Vgs=0, in which case you don't need
all that circuitry anyway.



Correction: for depletion mode devices, the bias would be 0v between
the Gate and the other end of the Source resistor.
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Old 18th August 2012, 05:12 PM   #855
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Quote:
Originally Posted by Nelson Pass View Post
You can't develop a bias voltage across the coils, so it will only work for
those cases where you can bias at Vgs=0, in which case you don't need
all that circuitry anyway.



Correction: for depletion mode devices, the bias would be 0v between
the Gate and the other end of the Source resistor.
Thank you Mr. Pass. The dc resistance of the transformer's secondary is ~28 Ohms [previous post]. But suppose the same bias circuits are modified so as to supply ~45 mA each [past post] and generate the needed Vgs across the transformer's secondaries. This approach may be like that of lhquam who wanted to use CCSs.

Last edited by Antoinel; 18th August 2012 at 05:22 PM.
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Old 18th August 2012, 05:41 PM   #856
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Quote:
Originally Posted by Antoinel View Post
Thank you Mr. Pass. The dc resistance of the transformer's secondary is ~28 Ohms [previous post]. But suppose the same bias circuits are modified so as to supply ~45 mA each [past post] and generate the needed Vgs across the transformer's secondaries. This approach may be like that of lhquam who wanted to use CCSs.
Actually the idea was proposed by tadek in this post: http://www.diyaudio.com/forums/pass-labs/216616-f6-amplifier-80.html#post3128183. I was just exploring the implications of that idea.
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Old 18th August 2012, 05:47 PM   #857
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Quote:
Originally Posted by lhquam View Post
Actually the idea was proposed by tadek in this post: http://www.diyaudio.com/forums/pass-labs/216616-f6-amplifier-80.html#post3128183. I was just exploring the implications of that idea.
Thank you lhquam, and my apology tadek as I usually care for proper referencing.
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Old 18th August 2012, 06:32 PM   #858
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Nelson, Mayhave something up his sleeve, but i don't see getting away without a cap. THought about it all day. Only way i know is using CS, which ZM says is bad idea to load secondary( I tend to listen to him), or putting jfet after interstage and that's adding a stage. I wonder about a level shifter, but don't know much about them. ANyhoo, I dont know much,probably less than everybody else commenting, so I would follow their advice. As for BA1 bias scheme, I was just thinking in terms of simple Zener reference, but cap is still needed.
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Old 18th August 2012, 06:43 PM   #859
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here we go again

maybe I'm rotten , knowing (at least partially) direction in which Papa is heading ......... but everything is already written

not best , not only , not sole possible way of doing things ........ but certainly most elegant and practical way of doing it

I'm probably most stubborn guy around , always trying to do things different from Pa's way of doing it ........ even if that means that my solution is always more complicated and less elegant ...... but this time - there is no sane way of doing it without cap in series with secondary

at least not with small signal xformer
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Old 18th August 2012, 06:59 PM   #860
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Wouldn't bias currents in the secondaries cancel each other out? If there was a circuit that tweeked one of the current sources to trim offset, that would be an error that wouldn't be cancelled.
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