amplifier oscilates in LTspice

[andyr]

Thanks for the LTspice pic:

does anyone know why this amplifier oscillates and what is the solution to fixing it?

View attachment 1375780

So if my (old, analogue) CRO screen shows a fuzzy sine wave like the one above ... the circuit must be oscillating, right?

But what about if the CRO screen shows a sine wave which is fuzzy only at the highest and lowest points of the sine wave?

Thanks,

 

[No ideas]

What if i replaced it with the cfp

 

[WhiteDragon]

what about this circuit? ill probably use mje3055/2955 or some better transistors?

Is it stable? Check phase margin

What if i replaced it with the cfp

Not much benefit, less diodes needed of course.
likely need compensation near opamp to get 45 degree phase margin
Having played with many of these circuits assume your near only 30 degree

 

[No ideas]

So darlingtons are better?

 

[No ideas]

Is it stable? Check phase margin

Not much benefit, less diodes needed of course.
likely need compensation near opamp to get 45 degree phase margin
Having played with many of these circuits assume your near only 30 degree

never bothered about it, im also intrested in the jlh and a few other circuits

 

[No ideas]

Is it stable? Check phase margin

Not much benefit, less diodes needed of course.
likely need compensation near opamp to get 45 degree phase margin
Having played with many of these circuits assume your near only 30 degree

it works ok although id need an extra resistor/trimpot between the diodes

 

[mr_zener]

Why not try a simple 'Diamond' output stage?

https://tubecad.com/2022/06/blog0558.htm

 

[No ideas]

done that should do, do i need to use two identical pairs or does the driver/output thing apply here too?

 

[No ideas]

Why not try a simple 'Diamond' output stage?
View attachment 1414005
https://tubecad.com/2022/06/blog0558.htm

wiring got a bit confusing when i was trying to make up an pcb but it think im just tired

 

[No ideas]

Why not try a simple 'Diamond' output stage?
View attachment 1414005
https://tubecad.com/2022/06/blog0558.htm

i made up an quick single sided pcb since i have some of these copper etching boards that are lying around, also Kicad dosent know what a jumper is so im left with empty test points

 

[egellings]

Should Q8 be a PNP rather than an NPN device? I don't see how the circuit can work with an NPN in there.

 

[No ideas]

Which photo are you talking about?

 

[No ideas]

Why not try a simple 'Diamond' output stage?
View attachment 1414005
https://tubecad.com/2022/06/blog0558.htm

here is an simulation i made, im still not sure about the transistor choice

 

[mjona]

View attachment 1376111
done used an random model of an relatively slow op amp and everything works fine. now im intrested in what op amp to use, the lt1056 is to fast the tl072 will be also too fast, maybe an ne5532 or an rc4558?

does anyone know why this amplifier oscilates and what is the solution to fixing it?View attachment 1375781View attachment 1375780

How about 10p?
Sim with square wave.

How about 10p?
Sim with square wave.

Put a comp capacitor from inverting input to opamp output

Put a comp capacitor from inverting input to opamp output

 

[mjona]

Agreed take off point from IC input is cleaner than taking NFB from the output stage where phase shifts due to driving reactive speaker loads exist