Some effect pedals fire an annoying and sometimes loud pop when engaged. This is mostly due to large capacitors used in those pedals. These capacitors are not "silent" : they leak a tiny voltage of a few millivolts. When the pedal is engaged, a “pop” is heard.
To avoid that, you need to place Pulldown Resistors in the IN and OUT of the circuit, but NOT directly on the jacks. The Pulldown Resistors must be insensitive to the bypass switch.
Examples (taken from the excellent Jack Orman's www.muzique.com)
| Before | After |
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Note that the Pulldown Resistors (in blue) ars close to the circuit. Therefore, whether the Bypass switch is ON or OFF, they are still active.
Typical values : 2,2M or 1M. But not less, otherwise you will lower the overall volume of your pedal.
If the pop still occurs, there can be another cause called the LED popping that happens when the LED is powered. Jack Orman suggests this modification to avoid it.