I am having some troubles with my circuit. I need to increase my gain to reach about 15 volts but I cannot get it above approximately 1.7 V (see in graph). I have tried adding another amplifier but did not succeed. Any help would be great.
If the source resistance really is as high as 1k, then R1 and R4 are far too small, try 100k and 18k
Rc and R10 and R20 are all loading the first stage output.
Try removing Rc and R20, change R10 to 1k, and feed the collector output of Q3 to the bottom
of D4. That way you reduce the load on Q3 and give it more gain.
The output transistors need good sustained gain at high current, so need to be selected carefully,
certainly a switching transistor like the 2N2222 is unlikely to have good performance in an analog
Given the 8 ohm load the output devices will need to be heatsinked as they will dissipate several watts
For good gain at less than about 500Hz the output capacitor needs to be more like 2200µF