The audio amplifier that I am referring to here consists of three stages. Among others, the amplifier also includes negative feedback (NFB) that is made of two passive resistors.
From the base of TR3 to ground, there is a NFB's resistor connected in series with capacitor C2 (referring to red square). What is the function of that capacitor in such circuit?
I know that this RC circuit in series represents a filter and limits the amplifier's gain bandwidth at lower frequencies. It obviously represents some kind of barrier to an audio amplifier. So, why wouldn't I just short it to ground? This would be most probably seen as improved amplifier's gain bandwidth.
Why did the first creators of such circuit topology put it there? With what purpose?
I don't see any, unless shorting that capacitor to ground and leaving only the resistor RF2 represents another biasing source for TR3's base, whereas RF1 is already biasing source for TR3's base. So, that would probably have some other effect.