First Watt F7 review

[Sangram]

Nag,

Study all posts that have either conceptual or actually drawn circuits.

You have to have two feedback loops, one negative (voltage) loop and one positive (current) loop.

The circuit you are drawing here has no current loop. You have to measure the current through the load. This means that if you disconnect the load, there will be no positive feedback at all. This bit is critical and missing from your last schematic.

Sorry if that sounds cryptic but I don't think I want to give away more clues. As Nelson said, there is a post here that contains a decently working circuit. And there are at least three of them.

 

[Nelson Pass]

Whenever I read my brain goes somewhere. in F7 manual there is no degeneration resistor. But in image green resistor there. What is that green resistor value?. So much highly technical details are there in this thread. actually, I gained some knowledge in a complementary push-pull circuit from Mr. Nelson Pass articles. That credit goes to Mr.Papa. somehow I know voltage feedbacks. Still, I don't know current negative feedback and current positive feedback. Just I am simulating what I have learned.

The green resistor is 0.1 ohms in series with supply allowing reading of the
bias current.

 

[woofertester]

I got a 4K input impedance. 0.87 output impedance. 14.5dB gain. 2dB positive feedback.

The First Watt F7 output impedance is 0.08 ohms as per FIRST WATT

 

[Nag]

Thank You all. My Brain stopped working. I will come back once I understood current vs voltage feedback.

 

[Nelson Pass]

Perhaps you are over-thinking it. Here is a simple MicroCap example
of a working circuit to play with.

You can scale and vary the resistors to suit.

 

[Nag]

Thank you, Mr. Papa.

 

[Nag]

Hi All,
I got below from lt spice.
input impedance 11K.
output impedance -0.150ohm

8ohm load gain 14.665dB
4ohm load gain 14.760dB
2ohm load gain 14.918dB

If above is correct and I will post schematic if permission granted Mr.Papa or Zenmod.

Thanks,
Nag.

 

[Zen Mod]

OK, I am lifelong Adoptee candidate (whom Papa is ignoring all the time ) , but I really have nothing with grants of that sort



however , as one of Greedy Boyz brigade I can tell -- feel free to post fruits of your work, just don't expect confirmation how close is to real F7

all rest of us can and will say is - will it work .....

 

[Rewind]

And the world held its breath.

 

[Nag]

input impedance 11K.
output impedance -0.150ohm

8ohm load gain 14.665dB
4ohm load gain 14.760dB
2ohm load gain 14.918dB

I have a very basic knowledge of audio amplifiers. Please ignore if it is wrong.

 

[totaltech]

f7

G'Day All,
I have been following this thread having built a F5. See attached schematic, disregard values and output Mosfets. the only problem is is i appear to have one more component to Papas design.... Note r16 and r17 would be a pot.

 

[Zen Mod]

input impedance 11K.
output impedance -0.150ohm

8ohm load gain 14.665dB
4ohm load gain 14.760dB
2ohm load gain 14.918dB

I have a very basic knowledge of audio amplifiers. Please ignore if it is wrong.

looks nice

go back and read what Papa sez about 0R1 resistor ...... practically you don't need it

 

[woofertester]

input impedance 11K.
output impedance -0.150ohm

8ohm load gain 14.665dB
4ohm load gain 14.760dB
2ohm load gain 14.918dB

I have a very basic knowledge of audio amplifiers. Please ignore if it is wrong.

I have never built an F7. I have no secret knowledge of a real F7.

I can tell you that a negative output impedance is probably a bad thing. Tweak your feedback resistors until you have some small positive output impedance that suits you.

A negative output impedance may indicate too much positive feedback which could indicate that you have built an oscillator.

 

[Nag]

Thank you Zen Mod.
Below are the new results.

 

[generg]

You have probably the right amount of pos feedback, when the signal at the output is with 8 Ohm speaker load 2db higher than without.

:--))

 

[Nag]

Now irrespective of load it is at 14.0db constantly. just like F7.

 

[Antoinel]

At which higher value of R6 [ >0.3 Ohms] does the simulation predict oscillation due to a resultant increase in the extent of positive feedback?

 

[generg]

With no load you should have around 2dB less gain, because the pos feedback is not effective.

 

[oreo382]

Now irrespective of load it is at 14.0db constantly. just like F7.

Check the FW bandwidth specs. 3db down at 100khz.Do you have that?That's all i'm going to say.

 

[woofertester]

At which higher value of R6 [ >0.3 Ohms] does the simulation predict oscillation due to a resultant increase in the extent of positive feedback?

Instead of thinking of R6 as a variable to manipulate, R5 and/or R7 should be the variable(s). Just a thought.