A simple AM modulator

Posted on Feb 5, 2014

An AM modulator for the MW band that will allow hearing the music of a CD or MP3 player during daytime on such radios. This is not a transmitter: the output power is very low (only 10mW) and is intended to be connected to the antenna input of the radio, or to be closely coupled with it. Amplifying the signal would be a bad idea, since transmitting in this band is strictly forbidden

A simple AM modulator
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in many countries. The audio signal is connected to CN1 and CN2. The input stage is designed for stereo signals of 0. 7Vrms (2Vpp) and has an impedance of 47k © because this is usually what is needed to connect the line output of a CD player. If one wants to use the earphone output of an MP3 player, shorting JP1 and JP2 will drop the input impedance to 33 © to match this kind of output. AM only supports mono signals, so both channels are just added together in U1:A. In case of connection of a mono signal source, one can use either CN1 or CN2, but should always short the jumper of the other channel to make sure it doesn`t pick up noise. The RC networks C3-R2 and C6-R6 limit the lower side of the bandwidth to about 50Hz preventing DC from shifting the bias of the modulator. U1 is an ordinary NE5532 low noise audio operational amplifier. U1:A is just an inverting additioner with a gain of -2 (6dB). The network R7-C7 limits the upper part of the bandwidth to about 4. 8kHz. U1:B is an inverting amplifier whose gain can be adjusted via R8 between -0. 45 (-7dB) and -4. 5 (+13dB). This is used to adjust the modulation factor as close as possible to 100% according to the exact amplitude of the input signal. To avoid distortion, 80% will be good enough, since power efficiency is not an issue for this application. If this range is not enough, one could change R9 and R8 to increase or decrease the gain. The network R5-C5 limits again...

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