F4 power amplifier

[Variac]

thoughts- that's what I'm looking for. Thank You.


I assume that Nelson will weigh in tomorrow with some too.
I hope......

[Nelson Pass]

The F4 gives about the same distortion numbers balanced
or not for any given wattage short of just below the SE
clipping point.

You can get lower distortion yet by:

1) decreasing the Source resistors (more careful matching
reuired for that)

2) More bias (more sinking)

3) more devices (more devices...)



1 and 3 will increase the DF.

[Babowana]

Originally posted by Variac


OK I have rewritten my previous post that you are commenting on. I hope it makes more sense! I meant to say "Do I adjust P1 to lower the bias?" I think I need to lower the bias due to fewer output devices.

I admit that I had some drinks last night when I was one-finger punching the computer keyboard. And, it was around 2 o'clock early in the morning . . .

With amps of Papa's projects, I always start with high bias, and reduce it little by little, monitoring the sound to the level of a good compromised satisfaction, without sensing any distortion numbers. I usually sit down on total about 1.5A class A bias ^^.
Just my habit ^^.

P1 is there to adjust the bias as it controls the dc differential voltage between the gate pins of Q3 (Q4/Q5) and Q6 (Q7/Q8) while the mid point between Q3 and Q6 always remains at zero voltage (or almost zero). I must be right as long as it is true that the bias level is depending on the Vgs value.

Wishing you a good success!

[Manu]

Originally posted by Nelson Pass
The F4 gives about the same distortion numbers balanced
or not for any given wattage short of just below the SE
clipping point.

You can get lower distortion yet by:

1) decreasing the Source resistors (more careful matching
reuired for that)

2) More bias (more sinking)

3) more devices (more devices...)



1 and 3 will increase the DF.

3) How much more?

Manuel

[GRollins]

How low do you want your damping factor?
For any given design the output device--in this case the IRF MOSFET--and its associated Source resistor will have some characteristic impedance. Just to make things easy, let's say that every complementary pair has an impedance of 1 Ohm. After that it's just a case of impedances in parallel:
1 pair would have an impedance of 1 Ohm
2 pairs would have an impedance of .5 Ohms
3 pairs would have an impedance of .33 Ohms
4 pairs would have an impedance of .25 Ohms
5 pairs would have an impedance of .20 Ohms...etc.
Obviously, it's a diminishing returns sort of thing. Given that the F4 has no feedback, the Zout is what it is. It's set by the topology and parts choices you made.

Grey

[GRollins]

Variac,
I confess that I got lost several posts back. Let me see if I've got this straight. You want to build a dedicated balanced stereo F4, right? I think I saw that your speakers were pretty efficient, so that works in your favor.
Two questions:
--What impedance are your speakers?
--What is your target power?
(These may have been posted earlier, but Wyatt's kicking and being a distraction and I'm having trouble feeding him and doing this at the same time.)
Yes, IRFP140s will be fine.

Grey

P.S.: Don't fret about heat. You have a decent heatsink, you have fans, the bias is adjustable. Some combination of those factors will get the job done.

[MiiB]

Originally posted by GRollins
How low do you want your damping factor?
For any given design the output device--in this case the IRF MOSFET--and its associated Source resistor will have some characteristic impedance. Just to make things easy, let's say that every complementary pair has an impedance of 1 Ohm. After that it's just a case of impedances in parallel:
1 pair would have an impedance of 1 Ohm
2 pairs would have an impedance of .5 Ohms
3 pairs would have an impedance of .33 Ohms
4 pairs would have an impedance of .25 Ohms
5 pairs would have an impedance of .20 Ohms...etc.
Obviously, it's a diminishing returns sort of thing. Given that the F4 has no feedback, the Zout is what it is. It's set by the topology and parts choices you made.

Grey

What if you reduced the resistors to 0,22 or 0,1 ohm, Then damping would increase, but would require closer matching??

[GRollins]

Yep, but Manu specified #3. There's no reason not to use lower Source resistor values and more devices...except heat and cost, of course.

Grey

[Manu]

Originally posted by GRollins
How low do you want your damping factor?
For any given design the output device--in this case the IRF MOSFET--and its associated Source resistor will have some characteristic impedance. Just to make things easy, let's say that every complementary pair has an impedance of 1 Ohm. After that it's just a case of impedances in parallel:
1 pair would have an impedance of 1 Ohm
2 pairs would have an impedance of .5 Ohms
3 pairs would have an impedance of .33 Ohms
4 pairs would have an impedance of .25 Ohms
5 pairs would have an impedance of .20 Ohms...etc.
Obviously, it's a diminishing returns sort of thing. Given that the F4 has no feedback, the Zout is what it is. It's set by the topology and parts choices you made.

Grey

Thx Grey,
But are the Jfet able to drive so much output devices (like says 4 pairs or 6)? We have sort of limit here?

[GRollins]

Yes, there's a limit, but where the limit is becomes a bit complicated. Different people will give you different answers as to how much current is required to drive a given amount of capacitance. The basic question is one of slew rate, which would seem to be one of those things that everyone could agree on...only it's not.
My personal feeling is that the F4 is pretty cool the way it is. If I were going to start strapping on more output devices, I'd want to look at beefing up the first stage. Nelson (and I'm putting words in his mouth, here) will very likely tell you that the front end is sufficient to drive at least two or three more pairs of outputs without alteration. Other people will have other opinions.
My suggestion is to ask yourself exactly what your goal is. Are you wanting to add output devices simply to increase the damping factor? Are you looking for more power into a low impedance? Are you trying to spread out the heat across the heatsink? Adding devices just to be adding devices is a waste of time and current. Do it intelligently.
Bear in mind that damping factor is one of those diminishing returns things. A damping factor of 1000 is not 1000 times better than a damping factor of 1. Even more damning, some tube amps (with pretty poor damping factors) have very tight bass indeed, much better than some solid state amps with far higher damping factors.
If you really want to do a number on the damping factor, think in terms of adding a front end and enclosing the output stage in a feedback loop. The more practical--though less impressive numerically--way is to choose Nelson's option for reducing the Source resistors.

Grey