#1
Hello, I have passive pickups, and have installed an EMG active preamp on a push/pull knob into my guitar (EMG AB After Burner). Will my guitar signal be low impedance regardless of whether or not the preamp is engaged?
#2
Depends on what you mean by 'low impedance.'
I mean, most normal passives are in the 7-10kO range.

Anyway, when you disengage the boost it -should- be bypassed in a manner which makes your guitar electrically identical to if the boost were not installed, or very near to it.
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