Is there anything special about hooking an amplifier to a loudspeaker? Here is a brief look at some parameters that can affect the sonic performance of this important interface.

The Ideal Case

Most analog interconnects in a sound reinforcement system are of the constant-voltage type, meaning that a low impedance output is used to drive a high impedance input. The result is the maximum transfer of the voltage component of the audio signal and minimal current transfer. Such interfaces are not optimized for power transfer, which requires that the circuit impedances be matched. Ironically, the amplifier/loudspeaker interface is of the constant-voltage type.

This may at first seem counter-intuitive. After all it is a power amplifier. Constant voltage interfaces deliver sufficient power to the loudspeaker load, while maintaining good stability and minimizing the effects of the loudspeaker cable.

The rule-of-thumb followed when interfacing line level audio equipment is to maintain a 1-to-10 ratio between output and input impedances. This is sufficient to prevent an input from loading an output (causing a voltage drop), and provides the additional benefit of making the output voltage of a component independent of the load impedance. When loaded in this manner, a further increase in input impedance does not yield better voltage transfer. Current transfer under these conditions is minimal (but not non-existent!) and there is little danger of the driving device not meeting the current requirements of the driven device.

This is a good place to inject an example. Consider a large dam and hydroelectric power station. When the gates are closed, the turbines do not move, and the dam is under great pressure from the weight of the water. When the gates are opened, the turbines are driven by the escaping water, yet the pressure on the dam is relatively unchanged. This can be considered analogous to the constant-voltage interface between amplifier and loudspeaker. Constant voltage (pressure) can be maintained while still developing significant power and maintaining a current (water flow) reserve. Figure 1 - The equivalent circuit of a cable as seen by the amplifier and loudspeaker. The affects of both capacitance and inductance vary with frequency, and must be considered within the audible passband of the system.

FIGURE 1

The Loudspeaker Cable

Much attention is given to the cable connecting the amplifier and loudspeaker. This is a vital system component, and its effects must be considered. Figure 1 shows an equivalent circuit of a loudspeaker cable. It contains a series resistance (both send and return), a parallel capacitance, and a series inductance in each leg of the circuit. The value of the resistance is independent of frequency. The values of the inductive and capacitive reactances are frequency-dependent, so we must perform some simple calculations to determine if and when they become significant. Some practical guidelines are required to continue. Let us assume that the reactive parameters become significant when the level change caused by their presence approaches 1 dB at any frequency within the bandwidth of the human hearing system. This will happen when their reactances rise to approximately one-tenth the value of the loudspeaker’s impedance. An impedance of 4 ohms will be considered, since this represents (hopefully) a worst-case scenario of the actual impedance of a loudspeaker with an 8 ohm rating.

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