Oscillating Amplifier

[Telnet100]

Hello
Looking at the Attached Circuit, I would appreciate any comment as to why this Guitar Power Amplifier circuit does not have any sort of Zobel Network on the Output Terminals. This Particular Amp I am working on is suffering from an amount of high frequency instability, and I was thinking that it might be appropriate to add one?
Thanks for any insighthttps://www.diyaudio.com/forums/attachment.php?attachmentid=961398&stc=1&d=1624257194 -Telnet100

 

[JonSnell Electronic]

None of the other amplifiers of this design have a network and they are fine until the output transistors or MC1436 are replaced with Chinesium fleabay replacements. Then you will get issues much like yours. Check the PFLoop components, yes Possitive Feedback Loop R133 etc, it compensates for poor loudspeaker efficiencies at odd frequencies to try and smooth out the response.

I suspect, if original, the issue is elsewhere and the amplifier is only reproducing what is on its input.

 

[duncan2]

I would change C68 +C69 first , the IC is internally compensated .

 

[Telnet100]

Hello
Thanks for that -- I will do a suggested, and maybe try to disconnect the Input from the Power amp -- easy to do on this, as You only have to plug an unconnected 1/4" Male plug into the Stereo Return Socket, which then isolates the power amps.
I will report --
Regards, Telnet100

 

[Ylli]

Check the PFLoop components, yes Possitive Feedback Loop R133 etc,

You sure of that Jon? How do you see that loop, either the AC or DC portion, as positive feedback?

 

[jan.didden]

It is negative feedback for me, but it is current feedback, the feedback is current-sampled instead of the more common voltage sampled. Maybe that is what Jon means.

Jan

 

[Audio1Man]

Many times in a guitar amplifiers the OUTPUT Z is boosted (POSTIVE FEEDBACK) and lets the speaker RING. Maybe your amp has too much feedback?
Duke

 

[Ylli]

It is negative feedback for me, but it is current feedback, the feedback is current-sampled instead of the more common voltage sampled. Maybe that is what Jon means.

Current sampled for the AC FB, but there is also a more conventional DC path through R120 et al.

 

[jan.didden]

Yes you're right, I missed that!

Jan

 

[Rallyfinnen]

I would change C68 +C69 first , the IC is internally compensated .

Maybe add C67 to that list?

 

[Ylli]

This Particular Amp I am working on is suffering from an amount of high frequency instability

Perhaps if you would expand on what you mean by high frequency instabilities. What symptoms are you seeing?

 

[duncan2]

Maybe add C67 to that list?

Your right I missed that one --your thinking is the same as mine.