This circuit should be considered as more of an experimental circuit for AM demodulation, than as a practical application. In the associated ‘AM modulator’ we have raised the problems caused by mains hum, getting in the way of interference-free operation of the AM mains intercom. When the transmitter and receiver are not coupled through the mains then the quality is perfectly adequate. The ‘receiver’ used here is the same as found in the ‘mains remote switch’ and ‘mains remote control decoder’ (C1 to C5/TR1/L1).
The capacitor that connects the small toroidal transformer to the mains has been split into two, making the circuit a little bit safer. But please note: the complete circuit should be considered as being at mains potential. So don’t solder to the circuit or take any measurements while it is switched on. The input signal is first amplified to the right level by a fast opamp (AD827). P1 can be adjusted to give this stage a maximum gain of 20 dB. The actual demodulator is about as simple as you can get since it consists of nothing more than a diode, a capacitor and a resistor (D2/R5/C9). Due to the RC time constant and the diode, the voltage across the capacitor follows the envelope of the AM carrier wave. The circuit of IC1b, D1, and R4 make the characteristics of diode D2 somewhat more linear. This effect is fairly small, so if the simplicity of the circuit is important you could leave this part out.
The filter following this stage attempts to remove the worst hum from the signal. This passive double-T filter is tuned to 100 Hz because that was the strongest component of the interference. But in practice, all harmonics of 50 Hz are present, too many to suppress with a simple filter. Because the sensitivity of the power amplifier used here is fairly high, the double-T filter is followed by a potential divider and first-order high-pass and low-pass filters (C14/R9/P2/C15). This keeps as much of the speech signal as possible while removing more of the interference. The power amplifier is a TDA7052 (IC2) that is meant for 6 V operation but which can also be used at 12 V.
Take care that it doesn’t consume too much power (if you can get hold of one, a 16 Ω speaker is better). The amplification is approximately 40 dB. IC2 has been well decoupled from the supply by R10/C16/C17. Constructors have to make sure that both the ground and the +12 V for the power amplifier are taken directly from the regulator and are not used to supply any other part of the circuit. Preset P2 is used to set the volume level, although a ‘real’ (logarithmic) potentiometer could also be used. The power supply uses a standard circuit: a bridge rectifier (B1) with suppression capacitors (C21 to 24), a smoothing capacitor (C20), a 7812 regulator (IC3) and a final decoupling capacitor. The quality of the circuit could be improved by replacing the passive filter with higher order active filters (possibly switched-capacitor types), but this is clearly only something to try for dedicated experimenters.