May 27, 2010 Articles
A loudspeaker is a bilateral transducer, that is, the cone moves in response to an applied voltage, and a voltage is developed at the terminals in response to cone movement (as in a dynamic microphone).
The most common form of power loudspeaker is the moving coil type.
A plate or cone of rigid material is affixed to a tube upon which is wound a coil of wire.
The tube is constrained in a circular gap which forms the pole pieces of a large magnet assembly.
The cone is mechanically held in a central position, both radially and axially, by a “spider” assembly.
This spider returns the cone to constant rest position in the absence of either electrical influence, or external mechanical influence.
The moving coil loudspeaker has a relatively low impedance in electronic terms; for power systems, this is normally between 2 and 16 ohms.
According to Lenz’s law, a mechanical movement of the cone will result in an electric current in the voice-coil, which, if short circuited, will produce a magnetic field that will try to oppose the movement.
This effect is termed “damping”.
All amplifiers have some output impedance, and in semiconductor designs, it is normally very low (< 0.5 ohm>)
Valve amplifiers, by contrast, were arranged, in the interests of output transformer efficiency, to have an output impedance EQUAL to the loudspeaker load, ie, 4,8 or 16 ohms.
The relationship between the loudspeaker impedance and the amplifier impedance is described as the “damping factor”.
It can be seen that the transformerless semiconductor amplifier will impose a much greater damping factor than a properly matched valve amplifier.
The loudspeaker driver also experiences mechanical damping by the spider assembly, and by air pressure (or not) inside the cabinet.
Most importantly, in multiple drive unit cabinets, the way in which the drivers are wired can have a profound effect on the way they sound, particularly at the low frequency end.
It is generally thought that a high damping factor (source impedance lower than speaker impedance) provides a more accurate sound, due to a more accurate correlation between amplifier drive and cone movement.
However, there are some schools of thought that consider that allowing the cabinet and loudspeaker assembly to influence the sound more is not a bad thing.
The combined assembly of speaker and cabinet has a fundamental resonant frequency … a high damping factor will diminish this effect, but, in cases where one is trying to get a lot of bass out of a small speaker, this resonance may be tuned to enhance the bass end artificially, so, a lower damping factor could be considered beneficial.
Such tuning is often in the form of a ported enclosure.
The most common configuration of a 4 x 12 speaker cabinet is of two, parallel-connected, series pairs, resulting in a cabinet impedance close to that of an individual drive unit (all drive units should be identical).
A switch configured to join the two midpoints of the series pairs will demonstrate any difference in tone that results from the arrangement, which then becomes two series connected parallel pairs, and a doubling of the damping factor.
Generally speaking, for semiconductor amplifiers, 2 x 12 (or any other size) cabinets should be wired in parallel, and for valve amplifiers, wired in series (and the output impedance matching adjusted accordingly).
The nature of the cable connecting the cabinets to the amplifiers, and, especially, the connectors used, is of paramount importance in reducing unwanted losses, or even un-controlled, non-linear distortion.
The ubiquitous jack socket is probably the worst type of connector that could have been used as a high-current loudspeaker connector … it was originally designed for low-voltage, low-current telephony !!
The problem is even more serious with today’s high power, low impedance rigs … much less in the old days of higher voltage, lower current, 16 or even 32 ohm speakers.