In this Youtube video, the experimenter connects the Joule Thief to the audio output of an amplifier, and lights a string of LEDs. About the only thing new about this is the source of power: the amplifier. I would say that it’s less than desirable to use the amplifier for power for the obvious reason that it takes away from the power to the speakers. But assuming there’s enough power, using the amplifier’s power is a novel idea. Apparently the audio output is not rectified or filtered; the AC signal is directly applied to the JT. This eliminates the 0.8 volt loss across the rectifier diode.
Since the Joule Thief is not very efficient, about half the power is wasted, so if the LEDs get a half watt of power, the JT is using about 1 watt total. This leads me to conclude that it would be more efficient to use a transformer instead of a JT to increase the voltage enough to drive the LEDs. The audio signal is AC, so it is exactly what is needed to drive a transformer, which is far more efficient than the JT. The transformer could be a regular power transformer with a 12 volt AC secondary, but with the audio connected to the secondary, and the LEDs connected to the power line side.
One problem with the use of the audio as power is that the volume control has to be turned up to high volume before there is enough to power the LEDs. This applies to the JT as well as the transformer. A potentiometer could be used between the amp and speaker to reduce the volume, but the pot must be high power to handle the power going to the speaker.
Unfortunately his all-nighter has caused a number of mistakes. The 2N4401 is a NPN transistor, not a PNP. But the schematic is drawn as if the transistor is a PNP. It is very difficult to determine many of the parts in the schematic because of the blurring caused by wild camera swerving and gyrations. I should mention that he’s pointing at something on the schematic and that point on the schematic isn’t even in the picture (I think he could have used a bit of sleep before he did this video). Also, it’s the first rule of Joule Thief experimentation that you should never remove the LED when the JT has power applied. Always remove the battery before removing the LED, or you risk damaging the transistor.