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The reason it sounds so bad is the BC547 is rectifying the line in, and only putting out the top half of the AC cycle to the speaker. The transistor needs to have forward bias to the base. However, if there was forward bias, the BC547 is too low power and would overheat and be damaged. Instead, use a circuit like this one. http://www.bowdenshobbycircuits.info/page8.htm#amp.gif The unmarked transistor can be a BC547, and the other two can be BD135 and BD136. April 17, 2012 5:23 AM
The transistor is marked. An this is the "simplest" but not the "best" mini amp.