F5 type amp but with higher power

[fab]

Hi,

I have already built the F5 as per original schematics. I like the sweet sound of the amp for moderate listening levels but I would like to get more power and more voltage gain while keeping it simple (like the F5).

I saw some posts on making the F5 balanced X amp but it is not my goal.

This is the schematics I came up with to get about 80-90W rms (8 ohms).
The heatsink size would allow about 10Wrms (8 ohms) class A and the rest of power in class AB.

It has a cascode input to handle higher voltage rails.
It has a mosfet driver stage to increase the drive current (35ma) for the 4 pairs of mosfet outputs. This vertical mosfet driver also allows to get more gain from input stage (higher drain resistance) at the same time because of the high threshold voltage compared to lateral mosfet or BJT...

To keep enough feedback factor while increasing the voltage gain to about 26db, the open loop gain is increased by paralleling output mosfet and modifying the input stage bias (3 ma).

Can you comment if this design has some potential...

Thanks

Fab

[bear]

seems like a possible way to go.

I'd not want to use any bipolar transistors myself...

Of course it is no longer an F5 at this point.

Where are you picking up the extra gain... maybe you can post the gains per stage (OL)? Is it all in the cascode?

_-_-bear

[fab]

Originally Posted by bear

seems like a possible way to go.

I'd not want to use any bipolar transistors myself...

Of course it is no longer an F5 at this point.

Where are you picking up the extra gain... maybe you can post the gains per stage (OL)? Is it all in the cascode?

_-_-bear

Hi Bear,

Most of the extra gain comes from the added parallel mosfets where the mosfet in common source configuration has a gain of about RL * gm where RL = 8 and gm = [5 -7] for IRFP240/9240. Thus each added pair adds about 5 of gain or so. Thus if 3 pairs gives about 15 of gain than it corresponds to the loss of feedback factor due to the closed loop gain increase from original F5 design...

The cascode should not add much gain but the Rload of the jfet input is increased by reducing the Id and having the VGSoff the IRF610(9610) requiring more voltage drop across Rload.

Yes it is no longer an F5 but can it provide good sound (at higher audio level)even if different?

I wonder if only 3 pairs with higher bias would provide better sound than with 4 pairs...

Also, regarding the cascode with bipolar, is my "floating" reference a good idea?

Thanks

Fab

[fab]

Originally Posted by bear

seems like a possible way to go.

I'd not want to use any bipolar transistors myself...
....
_-_-bear

Like this cascode:

[Nelson Pass]

The placement of the limiters will probably blow out the
drivers. Probably better to just take the limiters out.

[fab]

Originally Posted by Nelson Pass

The placement of the limiters will probably blow out the
drivers. Probably better to just take the limiters out.

I do not understand why. It takes quite a large amount of load current to activate them. My simulator does not show clipping until less than 1 ohm load. Unless you mean that I should limit more the current...

thanks

Fab

[a.wayne]

1 ohm Hmmmmm .... i can test for you .....

[fab]

Originally Posted by a.wayne

1 ohm Hmmmmm .... i can test for you .....

Of course, this is a simulator...

[Nelson Pass]

Originally Posted by fab

I do not understand why. It takes quite a large amount of load current to activate them. My simulator does not show clipping until less than 1 ohm load. Unless you mean that I should limit more the current...

M9 and M10 are Mosfet followers. They can deliver a lot
of current, and you are more or less shorting their Sources
to the rails.

Current will flow, sparks will fly.

[fab]

Originally Posted by Nelson Pass

M9 and M10 are Mosfet followers. They can deliver a lot
of current, and you are more or less shorting their Sources
to the rails.

Current will flow, sparks will fly.

I think I understand now...
Do you mean that if the current limiters are activated one day than I could blow off the M9 and M10

Thanks

Fab