reshp1 said:
More current, not voltage. Passive pickups can't source the current to generate rapid voltage changes (high frequency signal) across a low impedence input. An active pickup, even though it's designed for max efficiency with Hi-Z inputs, can source more current and therefore their performance doesn't degrade as much when plugged in a low-Z input.
You are right in that a passive source depends on impedance to generate voltage, and it's output voltage is limited by current draw, among other things. But current draw affects level, not response. The current draw in a magnetic pickup is soooo tiny, 1/10,000 amp for 1 volt into 10,000 ohms. That's one tenth of a milliwatt. Even into a 1500 ohm mic pre, it only would draw .0006 amps for one volt. I can't see a pickup not being able to draw that much.
Eventually, yes, I agree the signal would be affected by current limits. But not until very low impedances. And only affected because the voltage would be so limited, the output would be heavily disorted from clipping. But cable and pickup losses would have long since made the signal unusable.
Response is affected in a passive magnetic pickup with incorrect impedance mostly because the pickup coil inductance and input impedance form a low-pass filter. If the input impedance is high, the rolloff frequency is above the audible range. If it is low, the rolloff is audible. So even if a passive pickup could source enough current to put 50,000 volts into a lo-Z input, it still wouldn't have any high end.
As another example, mics don't like really high Z, even though current draw is minimal, because the transformer starts to saturate, due to eddy currents, and inductance also comes into play. The transformer can't work into the high impedance, and starts to act on itself. The signal collapses as current draw falls closer to zero, but impedance keeps rising.
Active sources have constant voltage, impedance-independent. They will put out the same voltage into 1 ohm as they will into a million, just with more or less current. They get in trouble when the impedance is so low they run out of current to support the output voltage. The outputs clip, as the device can't generate enough current to support clean output voltage. They also suffer from cable losses due to mismatch, just like passives.
Actives don't mind very high impedances, unlike passive sources. Their current draw is determined by the impedance and voltage just like a passive, but since voltage and current have separate sources, no harm is done. Current draw can fall to almost zero with no effect on voltage.
A lower-Z active instrument can plug into very high Z, no problem. A lo-Z active is designed to plug into a lo-Z input, but can also easily plug into a hi-Z input. It is more expensive to build this way, as a side note, due to increased current demands at lo-Z.
A high-Z active is designed for hi-Z inputs, and will most likely choke if plugged into a low-Z input, due to lack of sufficient current, and will suffer cable losses due to the mismatch anyway.
