A typical explanation for why passives sometimes don’t perform as well as actives is that the source doesn’t have enough voltage/current to push the signal through a passive to the amp. I think this was even mentioned by someone in this thread. While that can be true, I submit that this is all too often misunderstood and not the limiting issue.
Regardless of the line source type (CD, DAC, phono stage etc.), that source doesn’t know whether it’s connected to a passive or active preamp. All it "sees" is the impedance it’s driving into. If that impedance gets too low the source’s output stage won’t be strong enough (current capacity) to deliver the voltage signal properly because the current demand is too high (resulting in poor dynamics, flabby bass etc. ).
With active preamps, the source sees only the input impedance of the active preamp - typically 10k or higher. With passive preamps (resistive types like pots, stepped attenuators and LDRs), the source sees the combined impedance of the passive preamp in parallel with the amp.
If the passive has a stand alone input impedance of say 20k and is connected to an amp that also has a 20k input impedance, the combined effective impedance seen by the connected source is 10k. No different that the active in the above example.
If a given passive with a decent inherent input impedance disappoints relative to an active alternative, I would submit it’s not simply because the source can’t deliver the goods.
The related question of impedance matching (sufficient impedance bridging) is the number one topic with passive preamps. What we’ve found in actual practice with our LDR preamps in combination with countless different customer sources and amps is that impedance matching is rarely a limiting factor.
