toroidal SE output transformer

[tubetvr]

He said that his product gives 5hz to 50 khz. (20-20000HZ /-1db) While i discusting with him

This doesn't say very much, you need to know at what power level and at what DC current this was measured otherwise these are figures without value, you also need to know operating conditions to see that it is the same as in your case, Rp, RL, current.

Normally distortion at low frequency is the determining factor when designing transformers so please ask what the distortion is at full voltage swing and 20Hz. It is not so difficult to design a transformer with wide frequency range but to have both wide frequency range and low distortion is much more difficult. Unfortunately very few manufacturers give full data including operating conditions.

Regards Hans