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?
Best Answer
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.