IC555 can be used to generate PAM with one NPN transistor connected at output. The chip is configured in astable mode to generate pulse train to get samples of information (audio) signal. Its frequency should be at least twice that of audio signal. Usually it is 8 KHz (because audio signal is up to 3.4 KHz) but for better quality here I am keeping it 32 KHz. This pulse train output is fed to base of NPN transistor. The collector of transistor is coupled with low frequency audio (information) signal through positive clamper made up of capacitor C1 and diode D1.