ozapaydin wrote:
It's not enough to understand what causes ringing in OPT. More details please.
Feedback amplifiers greatly extend the bandwidth of the signal chain. At higher frequencies, circuit parameters not normally considered at normal audio frequencies become important. These include both stray impedances from layout and the overall impedance of the loop gain circuit. The output transformer contains significant elements that come into play. See figure below.
Attachment:
Output Transformer Equivelant.png
In short, the feedback function is frequency dependent. At low frequency the feedback can be negative and degenerative, just as desired. However, due to the complex impedance at higher frequencies, the feedback can become positive and hence regenerative. All output transformers have a natural resonance above the audio band. If the overall loop gain is regenerative at this frequency then the circuit becomes unstable and will oscillate in the vicinity of this resonance. Normally the feedback circuit must take this into account. Changing any one part of the loop gain circuit without understanding the circuit parameters across frequency risks introducing instability.
The theory can be better understood reading the following paper:
H. S. Black,
“Stabilized feed-back amplifiers”, Elect. Eng., pp. 114–120, Jan. 1934.
The paper clearly explains how the loop gain function affects overall stability across frequency.