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Old 30th September 2012, 03:56 PM   #2131
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Originally Posted by flg View Post
lhquam, I was slower at the keyboard but, I beleive I agree
lhquam and flg. Clearly you know your stuff; but unfortunately the 'sweet spot' continues to be elusive; although within the near grasp of lhquam per his matching JFET study. ZM showed a picture of the SIT-1 amp with a meter [for Idss?] on the front panel. Is the sweet spot related to a certain idle Idss of the output stage? My thinking in my previous post was as follows by this example: If the upper JFET has a Gm = 8.0 S, then a Vgs = 1.0 Volt enables an idle Idss = 8.0 A to flow through it. By comparison, the lower JFET has a Gm = 7.0 S. So the same matching Vgs = 1.0 V as for the upper JFET, generates an idle Idss = 7.0 A to flow through it. The two JFETs are connected in series, and thus must pass either 7 or 8 A; otherwise create a voltage divider [DC offset]. So, the DIYer tweaks the bias pot of the upper JFET down and/or the bias pot up of the lower JFET to get either 7 or 8 A and zero output DC offset. Except, the range of Idss [of 7-8 A] is where the sweet spot resides; but at which tweakable steady state of idle Idss. Will it be at 7.1 A which is closer to the Gm of bottom JFET or by contrast at 7.9 A. Thus, one has 10 incremental steps = 0.1A Idss [or more or less] to tweak [with the same bias pots] so as to nail this sweet spot down. Except; an attendant output DC offset must develop with each tweak. Can't have your B-day cake and eat it too [your guests took care of it]. Flow with [aka tolerate] this offset to find the sweet spot.
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Old 30th September 2012, 04:00 PM   #2132
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DC offset must remain in safe boundaries , while varying THD content

already shown how to in this very thread

anyway - different constructions , different ways for varying Amount of Zen .... , but none of them is allowed to alter DC offset
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Old 30th September 2012, 04:27 PM   #2133
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Originally Posted by Zen Mod View Post
DC offset must remain in safe boundaries , while varying THD content

already shown how to in this very thread

anyway - different constructions , different ways for varying Amount of Zen .... , but none of them is allowed to alter DC offset
lhquam's search for an answer in the Zen Pot of the SIT amp is an interesting development. Clearly, the issue of device matching is irrlevant vis a vis SIT; a solo performer. Is the Zen Pot tweaking Idss and Vds of the SIT? If the answer is yes, then this tweak is moving or shifting or off-setting Vds [new status] from its parent value before the tweak as one hunted for the sweet Idss point. Uh Oh.. I am saying exactly what you said all along. SIT has a DC output blocking cap, and thus one will easily ignore a +/- 1 Volt shift in its Vds. It is irrelevant to its DC offset; it has none. But a similar DC output shift in offset for F6 clone is not tolerated, and is clearly managed with a blocking cap once its sweet Idss is found.
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Old 30th September 2012, 04:44 PM   #2134
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speaking strictly about F6 and MR's amp - with pot you're varying degree of either direct AC modulation , or AC degeneration (indirectly influencing AC modulation)

in both cases it's strictly area of AC , while biasing/DC offset is strictly related to static, DC area

so , how's that suddenly connected with DC offset ?
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Old 30th September 2012, 04:51 PM   #2135
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Originally Posted by Zen Mod View Post
speaking strictly about F6 and MR's amp - with pot you're varying degree of either direct AC modulation , or AC degeneration (indirectly influencing AC modulation)

in both cases it's strictly area of AC , while biasing/DC offset is strictly related to static, DC area

so , how's that suddenly connected with DC offset ?
Please ignore my earlier post because it is not relevant.
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Old 30th September 2012, 05:00 PM   #2136
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Originally Posted by Antoinel View Post
....do you?
obviously neither you nor I , have enough mileage or knowledge to understand how young or less young Pass was/is thinking ......

sometimes I see something twisty/funny in his schematics , but that's only making me happier , not really smarter ....... but that's not his fault , but mine

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Old 30th September 2012, 05:07 PM   #2137
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Originally Posted by Zen Mod View Post
obviously neither you nor I , have enough mileage or knowledge to understand how young or less young Pass was/is thinking ......

sometimes I see something twisty/funny in his schematics , but that's only making me happier , not really smarter ....... but that's not his fault , but mine

Poetic and wise words. Are the potentiometers in the schematic [L'Amp; A simple SIT Amp] independent or ganged/track?
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Old 30th September 2012, 05:12 PM   #2138
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I thought we had this figured out
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Old 30th September 2012, 05:19 PM   #2139
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Originally Posted by buzzforb View Post
I thought we had this figured out
The accidental and equal settings of 60% on both of the 10 K pots gave the impresion they were ganged. You are correct. The 10 K pots are independent. One 10 K controls bias [to -10V], the second 10 K which is hooked to the 47 uF cap is the Zen Pot.

Last edited by Antoinel; 30th September 2012 at 05:23 PM.
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Old 30th September 2012, 05:36 PM   #2140
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You neer to build it and test hypothesis activley.
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