Damping, Damping Factor
Context: That amp had a very high Damping Factor.
Description: One of the dictionary definitions of the word damp is to deaden a shock. In other words, a device or method is used to stop vibrations or oscillations caused by mechanical shock. In the case of your automobile, the shock absorbers deaden the oscillations of the suspension system in order to make the ride more comfortable and improve handling. Damping is also very important in loudspeakers. Quite simply, if the speaker system consisting of the voice coil, the cone, and the spider was allowed to vibrate freely without any kind of damping, it would be very difficult to achieve any kind of recognizable or useable acoustic signal from it. We want to be able to blast the speaker with a big signal, have it respond, then stop vibrating almost instantly when we remove our signal. Ok, so how do we do it? Strangely enough, since the voice coil is always immersed in a fixed magnetic field, it acts as a part time motor, part time generator. When you drive the loudspeaker with a signal from the amp, it causes the voice coil to move in the magnet field and then impart its movement on the cone to produce the acoustic signal. But, as soon as the signal from the amp goes away, the cone and voice coil start to fall back to the resting, or no-signal position. As the voice coil is moving back 'home', the windings of the coil cut across magnetic lines of force in that fixed magnetic field and the result is that a voltage is developed in the voice coil. It has now changed operation from a motor to a generator. The generated voltage is called back EMF, or flyback, and is fed back up the line into the amp and appears in the output circuit. This voltage would continue to be generated until the cone stopped vibrating and finally came to a rest at its home position. One way to stop the oscillations is to make it difficult for them to occur. The easiest way is to put a big load on the voice coil when it is in its generating mode of operation. Ever notice when you turn on the rear defogger in your car, the engine idle RPM goes down slightly? That's because the defogger requires quite a bit of current to operate so the load it puts on the alternator makes it more difficult for the engine to turn the alternator. You can take any simple permanent magnet motor, such as that used on a slot car or other toy car, and after temporarily shorting the terminals together, you'll notice it is more difficult to spin than with the terminals open circuit. This is electrical damping. With a typical guitar amp, although you always use the correct speaker impedance load as suggested by the manufacturer, as an AC signal power amplifier, or simply, an AC power supply, it has an internal source impedance that is lower than the impedance rating of the speaker connected to it. The ratio of this internal source impedance to the suggested speaker load impedance is called the Damping Factor, and is usually at least 5 or higher. The higher the number, the better, generally, and is a measure of how well the output circuit will quench or stall the oscillations in the speaker by providing a big electrical load to them during the time the voice coil is acting like a generator. Of course, the resulting electrical power doesn't just disappear, it ends up in the output circuit of the amp where it is dissipated as heat. It's odd to think of the output tubes acting as a dummy load for this flyback power, but that's how it works. Output tube bias, transformer turns ratio, and wire sizes all affect the damping to some degree. Sometimes those flyback voltages from the voice coil we spoke of earlier can get quite large. If you ever see a schematic of a guitar amp and notice the designer has installed solid state diodes between the plates of the output tubes and ground, it is to help quench those flyback voltage spikes.
