Help me count the frequency respon of Marshall MG-30 power amp

[Agung Kurniawan]

Hi guys...
back a few day I was asking about FDD switch on marshall mg30 power amp and I get the function is to change the frequency respon of the power amp.
And now I have a lot of question since I get interested at marshall mg series power amp. I have a question about filter on this:
section.
there is some filter belong there, so the question is:
- what is the formula to count the filter frequency?
- can someone please do the math for me to take some example how to count them?
- is that any formula to count the power amp section gain of this amp?
- can I replace R37 and C39 with gyrators?
thanks

[antonis]

- what is the formula to count the filter frequency?

The all time classic f_{3db cut-off} = 1/(6.28xRxC)
(it also stands for every "sneaky" combination, like simple or twin T-notch filter..)

- can someone please do the math for me to take some example how to count them?

You must be kidding... 

- is that any formula to count the power amp section gain of this amp?

Exactly the same formula which stands for EVERY inverting op-amp gain calculation ( V_out/V_in = - R_F/R_G)
(replace every cap with it's impedance (X_C = 1/(6.28xfxC) at the frequency of interest (f) and calculate the series/parallel equivalent resistance(s)..)

- can I replace R37 and C39 with gyrators?

No, because you'll also come back and ask for filter formulas & math calculation chores..

thanks

You're wellcome.. 

[teemuk]

Do also note that practical load is reactive and will have an impedance. Current feedback will increase effective output impedance and voltage gain becomes load-variant. Which is the whole point of frequency dependant damping.