2N5458 is a quite common JFET transistor for audio applications, pedals, pre-amps etc.
(datasheet by on-semi) On the image you can find two simple applications:
i) a buffer (e.g. before a tone-stack)
ii) a pre-amp/boost
The component values are real and both circuits can be used as is.
The boost circuit can be stacked with another similar stage to increase the gain, just make sure you use the correct type of output capacitors for both stages.
Adding a 100μF capacitor in parallel with the source resistor of the 1st stage will also give you increased gain and might overdrive the 2nd stage producing some smooth clipping, but it definitely depends on the level of the input signal.
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