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NYCOne 29th August 2010 11:50 PM

Balanced F5 question
1 Attachment(s)
I've asked a few times in the main F5 thread, but I didn't have a schematic. I put together a rough sketch of my question on the attachment.

Briefly, I have a standard F5 set of parts, and I'm interested in whether two boards can easily be converted to one balanced channel. I've seen Patrick's balanced F5 schematic, but he uses the Toshiba outputs, I have the IRFs.

Can the attached setup work without substitution of a lot of resistor values, or is the effort more complex?

I'd appreciate any input, as modeling circuits is well beyond my abilities.

AndrewT 30th August 2010 10:31 AM

the two halves of the single ended input are connected at the source resistors to form an LTP. But there is a tail missing to carry current to the other supply rail.

NYCOne 2nd September 2010 08:49 AM

Is this solved by connecting the +24v and -24v rails on both sides of the circuit, or is there more to it than that?

AndrewT 2nd September 2010 10:54 AM

Oh Sh..., Oh dear, is allowed.
I noticed that I had misread your partially inverted pdf. I intended to come back with an "oops" and ask you to explain what was happening to the two sides of the inputs.
But, I forgot. Sorry!

If the left side is restored to non-inverted, then the two new links get twisted to form the X, that is part of this balanced arrangement.
If you draw that part with the X and with both sides non-inverted, how does your proposal compare to the other examples of balanced topologies? I think there are about 4 ways to make this balanced work.

Itsmee 3rd September 2010 05:37 PM

Is it me or does a balanced F5 look like a variation of an UGS :spin:

NYCOne 4th September 2010 09:30 AM

I guess I can always try it out. I'm new to amps, I hope I don't blow something up.

EUVL 4th September 2010 10:38 AM

The balance topology is basically correct.
As shown, it delivers its full power for a 15 ohm load.
If you have a lower load impedance, then you are not using the rail voltage to the full, and you could have used with more current.

This is why the circuit I published has 16V rails and 2A bias.

But it will work.
And I suggest you ground the intersection of the "X" to start with, trim everything properly, before you float the X for final trimming.

Good luck,

PS This balanced circuit requires a balanced input signal.


AndrewT 4th September 2010 12:33 PM


Originally Posted by EUVL (
......And I suggest you ground the intersection of the "X" to start with, trim everything properly, before you float the X for final trimming.

is this your recommended setting up procedure?

Ground the X and set up the amp resistors/currents.
Then unground the X and trim to final currents/offsets?

If ungrounding shows up an error, can we read the errors to work out where they are coming from and make circuit adjustments to correct the circuit errors, eg. change the value of a source resistor rather than simply trim out an offset.

EUVL 4th September 2010 07:53 PM

Yes, grounding the X basically means that you separate the balanced circuit into 2 individual single ended circuits.

This is must easier to set up one by one.
Once you are satisfied that they are working on their own, you can then float the X and do the final trim.


NYCOne 4th September 2010 11:24 PM

Thanks Patrick. If I want to drive an 8 ohm load, are there simple resistor changes that will get me where I need to go?

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