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I have attempted to build many amplifier circuits with little luck. The audio is either severely distorted, the gain is not high enough, or I pick up FM and AM broadcast stations.

I would like to build something with variable gain via a Pot, output to drive a small, handheld speaker (1/4 watt or so?) and power from a 9v battery. The dynamic range (from the Adjustment of the Pot) would be desirable to pick up signals from the audio level of a guitar pickup coil.

Is there a circuit which would accomplish this? If not, is there a more general circuit with formulas for my parameters? What is the next step to accomplish a simple design like this on my own?

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    \$\begingroup\$ If you just want a small amp, use a off the shelf small amp IC. If you want to learn about the electronics, show us a schematic of one of the amps that you built and what exactly the problematic symptoms are. We can hopefully explain what went wrong and how to adjust the circuit to make it work better. \$\endgroup\$ – Olin Lathrop Oct 30 '14 at 21:44
  • \$\begingroup\$ ti.com/product/lm4951a \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 30 '14 at 23:51
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For some reason a lot of the really simple discrete circuits that you find on the Internet have a lot of gain. But an electric guitar can put out 2-4 volts peak-to-peak (Vpp) if you really jam on it. So if you have a gain of only 2 and your supply is 9V, you're almost clipping the signal already. A lot of the "guitar preamp" circuits on the net are really just silly and pointless.

So you have to decide what you want to do. It might be reasonable to make a little jfet preamp with "feedback" to linearize the JFET just to make a high impedance input. That would make the guitar sound really clear and you would hear every detail.

Or you might make a circuit that progressively clips the signal (meaning it gradually rounds it off over a 6-12dB range) so that it sounds compressed like in Reggae or Jazz.

Or you could make a trivial little op-amp based circuit with a gain control.

But if you really want to actually know what you're doing, you need get a good book that describes transistors and how they really work. Silicon transistors are not great for processing small signals. They make great switches but when used as an amplifier of small singals, transistors are highly nonlinear (they have an exponential relation between input and current). So extra "feedback" circuitry has to be used to linearize things. But then they clip sharply and it sounds horrid because if you really clip hard in the feedback loop, the feedback is broken and the circuit does weird things. It will make horrible raking noises.

So keep the gain low.

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  • \$\begingroup\$ Ok, I did not know the physical coils would produce that much voltage. I only gave it as a reference, I want to amplify very small signals off of mics, induction pickup coils and more \$\endgroup\$ – skyler Oct 31 '14 at 16:20
  • \$\begingroup\$ Then you might need more gain. Microphones often need like 40dB of gain (20dB = 10x so 40dB is 100x). If you follow my username look at my post called "Electret Mic Circuit Using JFET Bootstrap". I have used that circuit to make cheap mics that perform very well (electret mics are actually very good devices). Just don't hard clip a linearized circuit or you will get horrible harmonics. \$\endgroup\$ – squarewav Oct 31 '14 at 17:19
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One candidate would be using LM386. On some circuits it can drive guitar pickup coil,

but an additional preamp with single transistor might help.

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