F6 Amplifier

[lhquam]

Originally Posted by Nelson Pass

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.

[Nelson Pass]

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.

[lhquam]

Originally Posted by Nelson Pass

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.

[kasey197]

Originally Posted by Nelson Pass

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 !!

[Nelson Pass]

Originally Posted by lhquam

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.

[Zen Mod]

well , Pa .......... you cheated ..........

didn't said that Jensen is that good

[bobodioulasso]

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.

[lhquam]

Originally Posted by bobodioulasso

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.

[kasey197]

Originally Posted by Zen Mod

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

[bobodioulasso]

Sticking with Ba 1 voltage source?