All of the preceding responses are correct. A theoretically ideal audio amplifier or preamplifier will have zero output impedance and infinite input impedance. That is obviously not practical for many reasons. The numbers cited above (several hundred ohms output impedance, or perhaps even one or two thousand ohms output impedance, and 50K or 100K input impedance) are typical for preamp inputs and outputs and power amp inputs, and are completely compatible.
Power amp output impedances have to be far lower (a fraction of an ohm) in order to drive speaker impedances, which are usually less than 8 ohms.
If an output impedance is not much lower than the input impedance of the device it is driving, a consequence is that a fraction of the voltage put out by the driving device will be wasted across its own output impedance, instead of appearing across the load device. The fraction lost would equal the output impedance of the driving device divided by the sum of that output impedance and the input impedance of the driven (load) device.
That assumes that stray capacitances and inductances in the circuit are negligible. That is not always the case, and if it is not the situation can be much worse. If interconnect cable capacitance or inductance, or the input capacitance of the load device, are significant, and the driving device's output impedance is too high, than the loss across that output impedance will be frequency dependent, which will cause the system's overall frequency response to be non-flat.
All of this applies only to audio frequency devices. At radio frequencies (such as antenna inputs to FM tuners, or high speed digital signals) completely different considerations apply, which usually require source, cable, and load impedances to be matched. Different considerations can also apply in certain specific situations involving audio frequencies, such as differential balanced outputs of professional microphones, which are often specified to work into load impedances that essentially match their output impedances.
Regards,
-- Al
Power amp output impedances have to be far lower (a fraction of an ohm) in order to drive speaker impedances, which are usually less than 8 ohms.
If an output impedance is not much lower than the input impedance of the device it is driving, a consequence is that a fraction of the voltage put out by the driving device will be wasted across its own output impedance, instead of appearing across the load device. The fraction lost would equal the output impedance of the driving device divided by the sum of that output impedance and the input impedance of the driven (load) device.
That assumes that stray capacitances and inductances in the circuit are negligible. That is not always the case, and if it is not the situation can be much worse. If interconnect cable capacitance or inductance, or the input capacitance of the load device, are significant, and the driving device's output impedance is too high, than the loss across that output impedance will be frequency dependent, which will cause the system's overall frequency response to be non-flat.
All of this applies only to audio frequency devices. At radio frequencies (such as antenna inputs to FM tuners, or high speed digital signals) completely different considerations apply, which usually require source, cable, and load impedances to be matched. Different considerations can also apply in certain specific situations involving audio frequencies, such as differential balanced outputs of professional microphones, which are often specified to work into load impedances that essentially match their output impedances.
Regards,
-- Al