The correct placement of a volume pedal in the signal chain depends on the expected function of the pedal. Both variants below are used by different guitarists, and both make sense in certain situations!

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Gain control (instead of volume knob on the guitar)

Any change in the volume of the signal before the input of the amp affects the clipping behavior of the preamp. If you want to use a volume pedal to replace the function of your guitar's volume knob (keyword: gain control), it must be placed in front of the amplifier input.

Control of the output volume (master volume)

If you want to control the overall volume of the amp without influencing the level of gain, the pedal must be looped in after the preamp in the (important: serial!) effects loop.