@Bordodynov what is the function of Q3 and Q4?

These transistors to increase the gain. They have a positive feedback. This results in the two elements having a negative dynamic resistance. Two resistors in parallel transistors reduce the gain to an appropriate level. This amplifier is promising for the soft limit the audio signal (the same tube circuits). To reduce the output impedance amplifiers can be used for common collector scheme.

 

Thanks for the help guys

I need a gain of 800, 80dB Common mode rejection but that is flexible 7.5 +- swing across a 1k load

7.5 volts when the supply voltage of 15 volts to get from differential stage transistors do not get.
Why do not you want to use an operational amplifier? You want to make a discrete transistor amplifier? I for fun can offer you a the scheme.

 

See example