First Watt F7 review

[Nelson Pass]

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

Criteria is that the gain is exactly the same with or without load.

 

[gl]

Nelson,

What effect does the positive feedback have on the input impedance?

Apologies if this has already been discussed.

Graeme

 

[Antoinel]

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

Great thought; to determine the right balance between negative and positive feedback.

 

[generg]

Criteria is that the gain is exactly the same with or without load.

Thank you Nelson for correcting this, I mixed up in my brain the change of the input voltage with and without load, that should be around 2dB difference, with the output voltage.
Sorry for this guys.
The correct way was discussed here with the help of Nelson.

https://www.diyaudio.com/forums/pass-labs/275921-watt-f7-review-100.html#post5116140

And here Nelson himself!

https://www.diyaudio.com/forums/pass-labs/275921-watt-f7-review-99.html#post5115281

 

[Zen Mod]

my sims (of Real McCay) are showing difference of 40mdb of CLG , when load is changed from 8R to arbitrary high value (100K)

 

[Nelson Pass]

What effect does the positive feedback have on the input impedance?

As presented, very little.

 

[gl]

As presented, very little.

Thank you.

G

 

[Nag]

I measured input impedance and output impedance. I am getting almost zero but negative value.
Zin = Vin / Iin
VO8 = output voltage @ 8 ohm load
VO4 = output voltage @ 4 ohm load
Zout = (8 * (VO8 - VO4)) / ((2 * VO4) - VO8)

Input 10k
output -0.0689

 

[woofertester]

I measured input impedance and output impedance. I am getting almost zero but negative value.
Zin = Vin / Iin
VO8 = output voltage @ 8 ohm load
VO4 = output voltage @ 4 ohm load
Zout = (8 * (VO8 - VO4)) / ((2 * VO4) - VO8)

Input 10k
output -0.0689

Increase R5 until you obtain 0.08 ohms Zout.

 

[Nelson Pass]

The difference between .08 and 0.0 is pretty slight.

 

[oreo382]

Increase R5 until you obtain 0.08 ohms Zout.

That is not the only issue with that sim.

 

[woofertester]

That is not the only issue with that sim.

Yes I get that. I was just addressing the negative output impedance.

 

[Nag]

Thanks for the valuable info. So I tried 3 rd variation in post 442.

8R load 14.37dB
2R load 14.19dB

The difference is 0.18dB is that fine ?.

 

[woofertester]

Thanks for the valuable info. So I tried 3 rd variation in post 442.

8R load 14.37dB
2R load 14.19dB

The difference is 0.18dB is that fine ?.

What is your goal?

 

[Nag]

My goal is the lowest components in my amp 🙂.

 

[Nelson Pass]

In any case, that's a quite high damping factor for 2 ohms.

 

[Nag]

Nelson Pass: Four Decades of Innovation

 

[Nelson Pass]

We are at year 47....

 

[nicoch58]

Nelson

 

[bkdog]

Just another 3 years for a GOLD anniversary!

Nelson has very generously probably contributed more to quality audio reproduction, to a greater audience (through DIY etc), than anyone I can think of. There are other greats (Blumlein, Baxandal, Walker, Hafler, Neumann etc) but none have contributed so much to the mass audience like Nelson has - My life and listening experience is far richer for Nelson's generosity - thank you!