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Old 25th August 2012, 12:22 AM   #1101
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Nelson Pass View Post
I took the time this morning to review my history of posts in this thread,
and I conclude that you would benefit from some additional information.

Notice the particular placement of capacitors, resistors, and transformer
secondaries which give greater output DC offset stability without
sacrificing the symmetry of the two halves at AC.

You might note that there was considerable discussion of "Bias Method B" vs Bias Method A" back in post #742. F6 Amplifier Zen Mod steered me away from method B which is the one you just presented.
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Old 25th August 2012, 02:00 AM   #1102
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Method B is acceptable on the negative half of the circuit. Bias voltage
referenced to ground is preferred on the positive half.

The issue is whether the difference in loading presented by the high
impedance bias resistor is different enough to affect the symmetry of the
drive.

On the positive half, the bias resistor sees the output swing, which loads
the secondary more than on the negative side, where it sees a constant
value.

The arrangement I use has the bias voltage bootstrapped by the Source
voltage through the capacitor, so that the difference in loading is
negligible.

Of course, as pointed out previously, this presumes perfect symmetry as
the goal.

And of course you are still free to otherwise load the secondary arbitrarily
for adjustment of the open loop response of the transformer. My
experience was that it did not need adjustment.

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Old 25th August 2012, 02:06 AM   #1103
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Nelson Pass View Post
I took the time this morning to review my history of posts in this thread,
and I conclude that you would benefit from some additional information.

Notice the particular placement of capacitors, resistors, and transformer
secondaries which give greater output DC offset stability without
sacrificing the symmetry of the two halves at AC.

There could be dangers in having the capacitor Cb and biasing resistor Rb too high.

The first graph is a Bode plot for Cb=220uf and Rb=100k. You should note that the phase margin is about 150 degrees, meaning it will oscillate at around .1 Hz. I have observed such low frequency oscillations.

The second graph is Bode plot for Cb=22uf and Rb=47k. The phase margin is about 110 degrees. This will not oscillate, and should not degrade the low frequency performance of the amplifier.
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File Type: jpg F6-R100-v5a-bode1.jpg (84.2 KB, 400 views)
File Type: jpg F6-R100-v5a-bode2.jpg (75.8 KB, 393 views)
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Old 25th August 2012, 02:15 AM   #1104
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Quote:
Originally Posted by Nelson Pass View Post
I took the time this morning to review my history of posts in this thread,
and I conclude that you would benefit from some additional information.

Notice the particular placement of capacitors, resistors, and transformer
secondaries which give greater output DC offset stability without
sacrificing the symmetry of the two halves at AC.

Thank you !!
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Old 25th August 2012, 02:21 AM   #1105
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Quote:
Originally Posted by lhquam View Post
There could be dangers in having the capacitor Cb and biasing resistor Rb too high.
As a practical matter, you won't have Rb values of 100K due to the
Gate leakage figures of the Jfets. The 10K range is a more likely value.

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Old 25th August 2012, 06:09 AM   #1106
Zen Mod is offline Zen Mod  Serbia
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well , Pa .......... you cheated ..........

didn't said that Jensen is that good

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Old 25th August 2012, 08:20 AM   #1107
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We can see no DC blocking caps are used (according to original schematic).
Dc through the coils are opposite directions and their effect on the core of the transformer are cancelling eachother.
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Old 25th August 2012, 11:40 AM   #1108
lhquam is offline lhquam  United States
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Quote:
Originally Posted by bobodioulasso View Post
We can see no DC blocking caps are used (according to original schematic).
Dc through the coils are opposite directions and their effect on the core of the transformer are cancelling eachother.
The only DC thru the coils is gate leakage currents of around 2uA. No big deal.
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Old 25th August 2012, 11:42 AM   #1109
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Quote:
Originally Posted by Zen Mod View Post
but that biasing scheme is flawed in few things more ;

you really need to have stable bias ref voltage (either voltage reference or resistor bypassed wit cap ) , then biasing net must be connected to modulating point through resistor of some higher value

as I wrote previously - last few Pa's articles (simple amps ) , same as BA1 FE detail are enough
Thanks zm, had a look at the ba1 - and yup the Zener+var r looks like it will do the trick nicely ! Cheers
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Old 25th August 2012, 01:12 PM   #1110
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Sticking with Ba 1 voltage source?
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