PWM test circuit one, a little test circuit put together by Boomer. It shows the circuit on the bench putting out a signal on 800 khz, the AM mediumwave band.
The modulator chip is a TL494, set for single ended output, 50 to 60 khz, emitter follower, through a low pass filter to the transmitter's final stage. keep in mind that the 494 is driving the modulated stage directly, and the power output is in the area of 500 milliwatts (.5 watt).
The modulation can reach down to DC for bottomless bass, and indeed the bass sounds excellent! The distortion you hear is the camera choking on all of the bass. The low frequency response here is only limited by the laptop's output section and a 2.2 uF and 100k input resistor.
On the high end, it's mostly flat with a few db peak above 5 khz, then a roll off above 8 khz, based on the sound, though I haven't tested the HF response yet.
This circuit is more complex than the simplest AM transmitter you could build, but it has the potential for higher audio quality and better frequency response. The efficiency is higher than most other circuits you could think of.