Basic AM radio Recievers

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DGB

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For a hobby project I am experimenting with basic medium wave AM radio receivers and have had some success with an Op Amp radio from a circuit I found on the web. It is made using a MC 1458 dual op amp, with one element for the radio and the other as amplifier. It works fine with basic headphones but my problem is when I try additional amplifier circuits of various types they all suffer from severe distortion and “motor boating” noise.
I would like to know what may be going wrong and how to add extra amplifier stages to drive a small speaker.
I would prefer, for interest sake to make the amps. From op amps or individual transistors if possible.


Cheers
DGB
 

What is basic: single tuned circuit with diode, superheterodyne, zero IF, etc?
 

Your circuit is a Direct Amplification type receiver most probably similar with the one below:
http://www.ece.mtu.edu/labs/EElabs/EE3305/AM_receiver.pdf

This kind of receivers suffer because the get a lot of gain from different stages working on the same frequency, and parasitic loops could appear.
Layout, especially using a short and solid ground is mandatory.
Good quality decoupling capacitors on the power supply is also important. There should be used lower and relative high values in parallel.

Just for your information, the simplest superheterodine receiver will work better than most complex direct amplification receiver.
 
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    DGB

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Thanks for your helpful reply.
I have downloaded the PDF file and will try and build the circuit described.

Cheers,

DGB
 

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