Impedance Matching

In the early days of high fidelity music systems, it was crucial to pay attention to the impedance matching of devices since loudspeakers were driven by output transformers and the input power of microphones to preamps was something that had to be optimized. The integrated solid state circuits of modern amplifiers have largely removed that problem, so this section just seeks to establish some perspective about when impedance matching is a valid concern.

As a general rule, the maximum power transfer from an active device like an amplifier or antenna driver to an external device occurs when the impedance of the external device matches that of the source. That optimum power is 50% of the total power when the impedance of the amplifier is matched to that of the speaker. Improper impedance matching can lead to excessive power use, distortion, and noise problems. The most serious problems occur when the impedance of the load is too low, requiring too much power from the active device to drive the load at acceptable levels. On the other hand, the prime consideration for an audio reproduction circuit is high fidelity reproduction of the signal, and that does not require optimum power transfer.

In modern electronics, the integrated circuits of an amplifier have at their disposal hundreds to thousands of active transistor elements which can with appropriate creative use of feedback make the performance of the amplifier almost independent of the impedances of the input and output devices within a reasonable range.

On the input side, the amplifier can be made to have almost arbitrarily high input impedance, so in practice a microphone sees an impedance considerably higher than its own impedance. Although that does not optimize power transfer from the microphone, that is no longer a big issue since the amplifier can take the input voltage and convert it to a larger voltage - the term currently used is "bridging" to a larger image of the input voltage pattern.

On the output side, a loudspeaker may still have a nominal impedance of something like 8 ohms, which formerly would have required having an amplifier output stage carefully matched to 8 ohms. But now with the active output circuitry of audio amplifiers, the effective output impedance may be very low. The active circuitry controls the output voltage to the speaker so that the appropriate power is delivered.

Matching Amplifier to Loudspeaker

The maximum power transfer from an active device like an amplifier to an external device like a speaker occurs when the impedance of the external device matches that of the source. That optimum power is 50% of the total power when the impedance of the amplifier is matched to that of the speaker.

But modern audio amplifiers are active control devices, and the impedance matching of the amplifier to the loudspeaker is no longer considered best practice. Modern solid state amplifiers are sometimes referred to as "bridging" devices which take an input voltage from an audio source and form an amplified image of that voltage at the output. The output impedance is low, and the output voltage and power are controlled dynamically.

The implications of the simplified model for resistive amplifier outputs and speakers may nevertheless be instructive as a reference. For example, assume that the maximum distortion-free voltage from the amplifier is 40 volts:

To emphasize the oversimplification involved in the above model, it should be noted that the loudspeaker is not a simple resistor - it contains a coil or coils with significant inductance, and is typically composed of two or three speakers with a crossover network that has capacitance and inductance. So the impedance of the loudspeaker will inevitably vary with frequency. The only present day amplifiers that would have a characteristic output impedance like that shown would be those designed to operate with "valve" or "vacuum tube" amplifiers.

Note that it is safer in terms of total power to go to higher impedance speakers (series speakers), but more typical practice is to put speakers in parallel, lowering the impedance. Note in the table above that lowering the impedance below the output impedance of the amplifier not only reduces the output power but increases the internally dissipated power in the amplifier.

This diagram shows the relationships used to obtain the power values in the table above. Note that it assumes a resistive nature of both the loudspeaker impedance and the internal impedance, neither of which is strictly true.

Matching Microphone to Input

While impedance matching of a microphone to an audio amplifier is not the problem it was in the early days of high fidelity sound reproduction, there are some considerations that still apply.

In practical terms, the modern microphone needs to deliver optimal voltage to the preamplifier, and not necessarily the optimum power that would require impedance matching. Considering the microphone as a voltage source, the voltage delivered to the input of the preamplifier is given by

where V_source is the signal generated by the microphone mechanism, R_i the impedance of the microphone and R_L the input impedance of the preamplifier. The actual signal power delivered to the preamp can be expressed in decibels of loss compared to the microphone's generated signal . Assuming a resistive circuit so that the power if proportional to the square of the voltage:

As long as the microphone has enough signal strength to provide the minimum signal input to the mixer, it can be an advantage to connect a low impedance microphone to a moderately higher impedance input. From this point of view, current practice for "low impedance" inputs to audio mixers typically have impedances from 1000 to 2000 ohms according to the Shure Pro Audio website. They comment that as a rule of thumb, a signal loss of 6dB is acceptable.