NFB network compensation help pls.

[nickb]

Hello,
I trying to implement some (6db arround) nfb into my project. For some reason testing at 1 kcps with square wave is ringing a little bit too much. I tried to compensate with a capacitor over 1.5k series resistor , I usually did, but seems nothing work. Started with 500p then 1n then 2.2nf then 4,7n with no results. At 10nf it already get severe oscillations. Any thoughts how can I make it little stable,please ? Thanks

[ATTACH=CONFIG]56596[/ATTACH]

Late edit: I tried cut the high freq bypassing plate resistor into first stage with no results. The fault is from closed loop and didn't know how to compensate. The classical method bypassing series resistor didn't work for this circuit

Can be a issue of the 100uf elco and its incapacity to decouple effective in high freq, pls ?

If you remove the NFB by connecting the right hand end of the 1.5K to ground instead of the output, do you still get ringing? If so, then the cause is not the feedback. Note the frequency of the ring and then try to find which part of he circuit changes that ring frequency when you add capacitance to it. The output transformer is would a good candidate (as HH suggested).

[Helmholtz]

Because of the phase shift involved with a (parallel) resonance in the open loop frequency response, NFB tends to emphasize the resonance.
Best remedy is damping the resonance by a Zobel filter.

[catalin gramada]

Similar arrangements into a Grommes amp, not quite same paraphase aplication.

[Chuck H]

Because of the phase shift involved with a (parallel) resonance in the open loop frequency response, NFB tends to increase the resonant peak.
Best remedy is damping the resonance by a Zobel filter.

This is actually what I do. Most of my experience is with EL84 amps which seem especially prone to ringing on square waves. Or so I've read. It's certainly what I've seen on the scope. Sometimes you can get more effect from the circuit by getting away from the typical formula of impedance x 1.25 for the series resistor. I've actually found that increasing the resistance, with adjustments to capacitance can result in better performance from the circuit. And I've found that it's best to wait for the wife to leave the house for awhile and make any filter adjustments with an actual speaker load in operation.

JM2C

[Chuck H]

Similar arrangements into a Grommes amp, not quite same paraphase aplication.

[ATTACH=CONFIG]56606[/ATTACH]

I think you've fallen behind the process here We've moved on to trying to diagnose the ringing. Just because I don't get the merit of that feedback loop doesn't mean I think it's the cause of your problem. And no one else has raised an eyebrow at it. If you're getting the feedback ratio you want then it's probably fine. You need to test the circuit without the feedback loop by connecting the 1.5k resistor to 0V rather than the secondary tap as a diagnostic measure to see if eliminating the feedback eliminates the ringing you're experiencing. If it doesn't then your feedback loop is fine and you may want to affect the ringing as a separate problem.

[catalin gramada]

Hi,
Yes, is ringing also without nfb connected, same manifestation. Time to look for a Zobel alternative. Thanks for support.