This amplifier is a further development of "Versatile Audio Amplifier with the Current Output Signal". In this version I gave the input transistor T1 more linearity by using a MOSFET transistor BSS110. This resulted in the possibility of realizing the voltage gain > 1. Furthermore, the input resistance can be made high-resistance so that an electric guitar could be connected.

The operating voltage Vcc can vary between +12V (Pout = 1.5W) and +19V (corresponds to the voltage of the notebook power supply, Pout = 4W). Using RV1, the T3/T4 collector voltage is set to the middle value Vcc/2. That's the whole adjustment. 

The "Line" position of the gain switch SW1 provides minimal distortion: approx. 1.5% of second harmonic at 1W. In the "Guitar" position this value increases to 4.5%. The clipping has no sharp edges. The cutoff frequency is 150kHz.

Care must be taken to ensure sufficient cooling of the output transistors T3 and T4 because the circuit has no thermal protection.