Single Ended to balanced for F4

[Joshua_G]

I'm going to build 2 F4 mono-blocks balanced, to get 100 wpc@8 Ohm, which my loudspeakers would love. 25 wpc@8 Ohm isn't enough for them.

Alas, my preamp and phono stage have only balanced output. So, I need Single Ended to balanced conversion. I\d like it to be all FETs, preferred all FFETs, with no GNFB.

The circuit I came up with is attached.
All comments are welcomed.

[Joshua_G]

P.S,
I also wanted no capacitors in the signal Path.
A DC servo is intended, to keep the outputs at ground DC potential, or zero potential.

[Zen Mod]

Originally Posted by Joshua_G

.....

Alas, my preamp and phono stage have only balanced output......

typo ?

you meant unbalanced ?

anyway - you need more swing for full throttle F4

that means - you must go with cascodes

[Joshua_G]

Thanks.
Indeed typo, my phono stage and preamp have only single ended outputs.
As for the voltage swing, R18 sets the gain. I changed R18 to 3.3K and now 1 volt at the input gives over 22 volt between the outputs.

Another typo is that I prefer that stage to be all JFETs.

[Joshua_G]

P.S,
The stage in the schematic is folded cascode.

[Zen Mod]

folded yes

but not enough voltage and

not cascoded in right place

[Joshua_G]

Just say how much voltage is needed and R18 will take care of it.

[Zen Mod]

just look what's voltage swing of balanced F4

then take measure that jfets are still in safe voltage area

Papa sez that they're good to 40-43V ..... bit observing dissipation is must , too

[Joshua_G]

As far as I recall, Papa said in the F4 thread that it takes +/-20V. If +/-40V or +/-45V is needed, it can be done with the above schematic by changing the value of R18, which sets the voltage gain of the stage plus maybe raising the supply voltage.

[Zen Mod]

exactly ;

Maximum unclipped output +/-20 Volts (SE)
+/-40 Volts (Balanced Mono)

that means - you need slightly more than 80Vpp for balanced F4