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Last weekend and the weekend before, I have been trying very hard to get a good small 8 ohm speaker amp with a preamp for a dynamic mic to work with no luck. It seems like every design I try has problems like this weird oscillation I got changing pitch if I touched different wires.

I even half-broke my left output from my iPod with one design It now will not play music quiet (scratchy sound), but oddly works above 80% audio level to the output jack. I will have to try to build an external POT volume control I guess :( .

So I have a dynamic mic , or a telephone pickup coil. How can I amplify this with the least distortion, and highest efficiency to drive an 8 ohm small speaker with a volume adequate enough to hear in a semi-quiet room (maybe a small fan running) up to 8 feet away from the human ear.

I also have one more question:

How can you learn from experience if you can't identify the problem and just hope the circuit works the next time you attemt to build it.

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    \$\begingroup\$ Can you post the circuits you tried? \$\endgroup\$ – Bitrex Sep 17 '13 at 1:54
  • \$\begingroup\$ Also, least distortion and highest efficiency are usually contrary goals when it comes to audio, unfortunately. \$\endgroup\$ – Bitrex Sep 17 '13 at 5:15
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    \$\begingroup\$ I'd find some lower-cost signal source for testing before hooking your shiny lifestyle gadget to it. Like the cheapest chinese copy shiny lifestyle gadget available on ebay or from the local crap store. \$\endgroup\$ – John U Sep 17 '13 at 8:21
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For learning about op-amps and their problems, I'm tempted to suggest App Note 47; it's very long and focuses more on higher speed applications, but it has a lot of practical information on debugging this kind of system.

"How can you learn from experience if you can't identify the problem and just hope the circuit works the next time you attemt to build it?"

You have to localise the problem, even if that's just to one half of the circuit or another. You have to either build up from something known to work, or dismantle down to a definitely faulty core.

You need a model of the system, either a mental one or a simulation. Then you need to find out where the reality differs from the model. For audio stuff, this will involve poking around with an osciliscope a lot.

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