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Why do amplifiers with a high output impedance have trouble delivering power/current to low impedance speakers/headphones?


Major Contributor
Feb 6, 2020
It's voltage divider effect that causes the high source impedance source to have trouble driving a low impedance load. That, and the current demand goes up when driving a low impedance load, and that can cause problems for a wimpy amplifier.
Last edited:


Feb 25, 2020
However, I've always noticed that amplifiers which have a high output impedance and corresponding low damping factor were never able to deliver much power/current to low impedance speakers/headphones. Is this at all related to damping factor, or is there other variables involved here? What causes this to happen? I feel like this is a really simple question but I haven't really found a simple answer to it yet.
I don't know electrical engineering, I will offer the analogy I made up in my mind to answer the same question. I am interested in more knowledgeable people validating it or - more likely I suppose - pointing out where it fails:

Current is the movement of free electrons across a wire (conductor). Voltage (potential) is how many more electrons are on one end of the conductor than on the other. This difference of electron accumulation is what "pushes" the free electrons inside the conductor to move across it.

Power (measured in Watts) is a concept. It expresses how quickly a force is applied (i.e., the rate at which "work" can be done). So in a car engine, you can have high power either because the pistons push down with high force but slowly, or they push with a low force but very rapidly (high engine RPM). You could then theoretically use a 50000 rpm relatively tiny engine to move a heavy tractor, by using gears to trade high rotational speed for force. So power in cars "is" the product of rotational speed times the force (RPM × Torque). Power in electricity "is" the product of current, times the voltage - i.e., the rate/"speed" at which electrons move through the wire, times the force that is pushing them, i.e. the voltage: (Amperes × Volts).

To "visualize" how it works then, I think of electricity as water flowing through, either narrow (high resistance) or large (low resistance), pipes.
If the speaker is a pipe of some diameter (some impedance) then how quickly the water can reverse direction inside it is a measure of how well it is controlled.
  • If the speaker is a large pipe (low impedance) connected to an amplifier (water pump) with narrow output nozzles (high amplifier output impedance):
    • I intuitively expect that trying to rapidly reverse the direction of water flow in the large pipe will be limited by the rate at which the water flows through the narrow nozzles. There will be too much water in the large pipe for the pump's narrow nozzles to "slap" around left and right as rapidly as I would like. Even if the pump can push the water with great force (very high voltage by the amp), the rate at which water could flow through the nozzles would still be too limited (think blow the nozzle or burn the wires) for our purpose of shifting the large volume of water, in the fat "speaker" pipe, back and forth.
  • If, however, we reverse the setup and the pump has very large output nozzles (low amp output impedance), connected to a narrow pipe (high speaker impedance):
    • I intuitively expect that the pump can get the water in the narrow pipe to change direction very rapidly, as the limiting factor in terms of speed is now the "speaker" pipe. And this is OK because that pipe is designed to do, within its own limitations, the final work we expect from it
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