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This standard is voluntary, and many 8-ohm drivers have resistances of ≈5.5 ohms, and proportionally lower for lower rated impedances. Measured in henries (H), this is the inductance of the voice coil. The voice coil is a lossy inductor, in part due to losses in the pole piece, so the apparent inductance changes with frequency.
Sixteen ohm drivers (or loudspeakers systems) would be connected to the 16-ohm tap, 8 ohm to the 8 ohm tap, etc. This is significant since the ratio between the loudspeaker impedance and the amplifier's impedance at a particular frequency provides damping (i.e., energy absorption) for the back EMF generated by a driver.
The actual output impedance for most devices is not the same as the rated output impedance. A power amplifier may have a rated impedance of 8 ohms, but the actual output impedance will vary depending on circuit conditions. The rated output impedance is the impedance into which the amplifier can deliver its maximum amount of power without failing.
Speakers are usually designed to operate at frequencies above their resonance, and for this reason, it is the usual practice to define nominal impedance at this minimum and then round to the nearest standard value. [23] [24] The ratio of the peak resonant frequency to the nominal impedance can be as much as 4:1. [25]
Headphone outputs and line outputs are sometimes confused. Different make and model headphones have widely varying impedances, from as little as 20 Ω to a few hundred ohms; the lowest of these will have results similar to a speaker, while the highest may work acceptably if the line out impedance is low enough and the headphones are sensitive ...
An amplifier must be able to suppress resonances caused by mechanical motion (e.g., inertia) of a speaker cone, especially a low-frequency driver with greater mass. For conventional loudspeaker drivers, this essentially involves ensuring that the output impedance of the amplifier is close to zero and that the speaker wires are sufficiently ...
To match the impedances, both cables must be connected to a matching transformer with a turns ratio of 2:1. In this example, the 300-ohm line is connected to the transformer side with more turns; the 75-ohm cable is connected to the transformer side with fewer turns. The formula for calculating the transformer turns ratio for this example is:
The L pad attenuates the signal by having two separate rheostats connected in an "L" configuration (hence the name). One rheostat is connected in series with the loudspeaker and, as the resistance of this rheostat increases, less power is coupled into the loudspeaker and the loudness of sound produced by the loudspeaker decreases.