i just tried that test using an 8.3W bulb for a car brakelight. i hooked up the lineout from my computer to the audio in of the modulator and played some music. the bulb glows but it is pretty dull (especially compared to if i sort a 12V/3A power supply with the bulb) and it doesnt change brightness much at all. i guess i should try in the other room using my stereo receiver and see what happens.
same situation with my home stereo. on my scope i can see the audio modulating the carrier signal and it is 6VRMS but it is only 600mV Pk-Pk.
(in the attached image 0V is at the bottom of the window.)
Sol,
It looks like there's a problem with the circuit. It shouldn'T have any problems "swinging" the voltage to a double.
I hooked up my scope and made a little movie this morning:
http://youtu.be/uttBWvyDK1w(You can see that my DC at carrier level is not nice and flat under load of the RF deck! However this isn't noticeable
on the audio from the receiver)
I expect that something is not working right with your circuit. I'll attach the latest schematic of the circuit and that is basically
the one I'm using in the video. (I just noticed that the 10uF cap symbol after the 5K6 should be an electrolytic)
Maybe check pin 2 & 3 to the op-amp?