Power amp under development

[mrlord]

Photos of my NMOS350 Mk2

That's the my photos:

Square wave prfect but with no load...


... The parasitic oscillation on the negative half-wave, with load connected.


Testing....

 

[AndrewT]

post3880 still not showing pics.

edit, post3881 pics are showing, I think you are editing as I post.

But these are not attached, they are still downloading from a remote server.

 

[mrlord]

Correct... sorry for confusion!

 

[mrlord]

What think about?

 

[AndrewT]

negative oscillation.
Is that due to clipping?

What load and what voltage?

 

[mrlord]

Schematic

That's the schematic thats I have build.

 

[mrlord]

The load it's 8 ohm, both case dummy load or loudspeaker.
The voltage is: +/-69 V and 65V at full power 1,4 A on the posive rail whithout any measure resistor.
Nothe that the oscillation it's present only with load connected.

 

[AndrewT]

What is the voltage at the output when the 8r0 dummy load is connected?

 

[mrlord]

Ah.... I try to put a 39nf capacitor between the gate of T13 and the ground and the oscillation it's fixed, but I don't know if that cause other problem (limitate bandpass, ecc...). From my measure at the oscilloscope (fully sweep frequency) does not seem.

 

[mrlord]

Offset voltage? About 5mV....

 

[mrlord]

You have some idea, Andrews?

 

[AndrewT]

Gate of T13 to Ground seems wrong.
Does a cap from base to collector of the driver help?

I hope some others will come in.
I am not any good at AC problems.

 

[mrlord]

There are all in holidays....!!!!

 

[Vostro]

I'm noob, but I would try a 22pf from base of T2 to base of T5, this is easy to try, don't have to change anything on board.
If prob on output, did you use resistor values on driver bases shown on drawing, maybe increase to 150R, also same for gate resistors on outputs.

Anyway that's what I would play with at first, remembering I'm only experimenter, not expert

 

[mrlord]

Thank for reply. @ Vostro: what mean "prob" ? Im not perfect in english, sorry.....
Another question: 22 pF instead C3 (330 pF) or another cap in parallel to R6 - C3?

 

[Insomnia]

use Quasi ACRTK schematic this circuit have perfect square wave.

Nmos sound is not so good like many Class AB Designs, I made Nmos before and have changed to Class D, never want go back to Class AB.

Class D is big improvement in Sound with low heat compare to class AB

 

[mrlord]

I have tryed to put 39nF cap from base to collector of the driver (T11), as how said Andrew ... catastrofic result: sine wave orrible!!!!
Also I tryed 22pF from base of T2 to base of T5, as how said Vostro: nothing change and the oscillation remain the same.

 

[mrlord]

use Quasi ACRTK schematic this circuit have perfect square wave.

I ave already made 2 pcb for NMOS....

Nmos sound is not so good like many Class AB Designs, I made Nmos before and have changed to Class D, never want go back to Class AB.

I tink so...

Class D is big improvement in Sound with low heat compare to class AB

Maybe in the future.... in this time my amp MUST work well!

 

[Insomnia]

Maybe in the future.... in this time my amp MUST work well!

The amp can not work well because you need about + 10 V / bootstrap to drive negative rail for full rail swing / perefct sine wave without oscillation. Thats why quasi make Acrtk Nmos Amp. This amp have + 10 V to fix this problem, check the schematic you will see

Class D work same, using 2 N Channel Mosfets and have +12 V Bias / Bootstrap for drive negative Mosfets

 

[Vrystaat]

Plenty of the Nmos 350 original mk1 amplifiers worked 100 %. I don't know if the mk 2 worked.