Keep in mind that most audio amps are rated to deliver X watts into an R ohm
load. Example: Amp rated for 2 watts into 4 ohms. Hang 8 ohms on the
output and the available power is only 1 watt. It is therefore limited by
the maximum voltage available from the power supply. At the same time put
2 ohms on the output and it might deliver 4 watts, depending on its current
capacity as it will attempt to deliver the same value of voltage. Drop much
lower than this for R and it will likely produce smoke. To prevent short
circuit or a low impedance load causing failure of the device the designer
usually adds a series R which raises the source Z and greatly raises the
damping factor the result being of little to no feedback to the stage. This
also then reduces the power to the speaker as the Z goes lower.
One must consider the value of THD acceptable in the audio output as well.
Most of these devices are quite good at low power values but get nasty
rather quickly when pushed toward their maximum limits. Usually about 2% to
3% is considered acceptable for these types of devices. Some companies will
rate their system as X watts into R ohms with a THD of 10%. Here you will
see higher wattage output values. In this case the distortion knee is being
rapidly approached and a few more tenths of a watt will easily drive it to
20% to 30% distortion or higher. In the FM mode this is where SINAD values
are very revealing.
As to speaker Z values, the impedance is also dependant on the acoustic
loading of the speaker by the enclosure. Most speakers rate the value of Z
based on their lowest value in free air. In systems where speakers are
provided in a specific enclosure then it is possible to determine the lowest
Z presented to the amp by the combination of the speaker Z and the acoustic
loading of the cabinet. It is not uncommon for professional speakers to
have a load Z vary between 3 ohms and 15 ohms, depending on frequency, and
this is for a 4 ohm rated speaker and cabinet. The DC value of R of the
voice coil also comes into play but by no means can a person determine the
impedance of a speaker by measuring the simple DC resistance alone.
Same conditions true for headphones as one writer has previously described.
73
Bob, K4TAX
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