Luxman 5M21

[rfitts46]

carlmart,


quote [have you checked the junction between the FET and the cascode transistors]


no I have not. are we talking about this junction

 

[carlmart]

Yes, that's it. At least you should know if the oscillation comes from the FET.

The oscillation I got on my simulation was cured in the driver's stage. Perhaps you might try it.

 

[rfitts46]

QUOTE [Yes, that's it. At least you should know if the oscillation comes from the FET. ]


carlmart, Wouldn't oscillation be seen there because of the oscillation being supplied back to the input card via feedback.


Couldn't the oscillation still be occurring downstream from the FET and the oscillation being supplied to the input of the FET from the feedback?


As mentioned the DML modules have been swapped and the oscillation did not follow the DML module.

 

[carlmart]

OK. This is the complete original 5M21 schematic, included parts inside the DML.

The other schematic shows, in red, what cured the first stage oscillation.

You may try it and see if it works for you.

Start with isolating the driver resistors from the NFB and add the 10uF cap. That should be easy to implement, with just one retrofittable track mod.

For lowering the resistors to 4.7, you can just add 10R resistors on the copper side.

 

[carlmart]

Forgot to say that it cured the oscillation on the LTSpice simulation, of course.

 

[rfitts46]

Finally straightened this amp out. Ended up removing the 2pf capacitor in the feedback circuit. Adjusted the bias, checked channel over entire frequency range with no anomalies noted and buttoned it back up. Just did my first sit down with the amp and am very impressed with it. Very refined sound, stage presence, crisp, lots of headroom. Think i will leave it in the system.

 

[carlmart]

I'm very glad you could cure your problems on the 5M21.

Have you checked square wave response?