The Serious Flaws of Voltage Drive
The following is a brief overview of the destructive effects that voltage drive has on the performance of electro-dynamic loudspeakers. A detailed analysis of the subject with measurements and illustrations can be found in chapter 4 of the book.
Today, all commercially available audio amplifier and loudspeaker equipment works on voltage drive principle without significant exceptions. This means that the amplifier acts as a voltage source and therefore exhibits low output impedance. Thus, the amplifier as though forces the voltage across the load terminals to follow the applied signal without any regard to what the current through the load will be.
However, both technical aspects and listening experiences plainly indicate that voltage drive is the wrong choice if sound quality is of any importance. The fundamental reason is that the vague electromotive forces (EMF), that are generated by both the motion of the voice coil and its inductance, seriously impair the critical voltage-to-current conversion, that in the voltage drive principle is left as the job of the loudspeaker.
The driving force (F), that sets the diaphragm in motion, is proportional to the current (i) flowing through the voice coil according to the formula
F = Bli
where the product Bl is called force factor (B = magnetic flux density; l = wire length in the magnetic field). B is the flux density that exists when the current is zero. (The current always causes its own magnetic field, that may react with adjacent iron, but the effect is not related to this equation.) This force, then, determines the acceleration (a) of the diaphragm, that in the main operation area is given by the Newtonian law F = ma and which determines the pressure radiated.
The most remarkable thing here, regarding loudspeakers, is that the voltage between the ends of the wire does not appear anywhere in these equations. That is, a speaker driver obeys only and solely current, not minding what the voltage across the terminals happens to be.
There is not any valid reason why voltage has been adopted as the control quantity. It is only due to the historical legacy originated almost a century ago most likely by cheapness and simplicity; the quality and technical reasonability of operation have not been issues in this choice. Engineers are also more accustomed identify electrical signals as voltages than currents.
At least the hifi/high-end community should be interested and able to see better through this, but alas, they too have taken the state of affairs as a given, being largely conditioned to the fallacious imagery that tight voltage somehow 'controls' it all, even up to middle and high frequencies, when such a belief does not have any real scientific background and it can be clearly shown by basic analysis and modelling that any damping effects that voltage drive can have on driver operation are strictly limited to the bass resonance region.
The Components of Impedance
The electrical equivalent circuit of a moving-coil drive unit can be depicted as the series connection of a resistor and two voltage sources, as shown below. Rc represents voice coil DC resistance; voltage source em represents the motional EMF (so-called back-EMF) of the driver and is calculated by em = Blv (v = voice coil velocity); and voltage source ei represents the inductance EMF that is generated by the lossy inductance of the voice coil.
Both em and ei are subject to a multitude of disturbances, that corrupt the flow of current when the circuit shown is fed by a voltage source.
In the impedance curve of a typical moving-coil driver, these two components, being of almost opposite phase, largely mask each other, whereby one can easily be mistaken that em is significant only near the fundamental resonant frequency or that ei is significant only at the highest operating frequencies. Instead, in a typical cone or dome driver, the sum of the magnitudes of em and ei is in fact in the whole operation band at least of the same order of magnitude than the voltage drop in Rc.
Microphonic Action of the Voice Coil
A moving-coil drive unit is a bidirectional device that functions as a microphone all the time, whether it is wanted or not. Thus, all the noise that is generated inside the speaker cabinet, and much of which typically penetrates out through the woofer cone, is picked up by the voice coil as a microphonic EMF. This cabinet noise EMF, that is also affected by panel vibrations, summates to the total motional EMF voltage em, modulating accordingly the flow of current and hence the very drive signal of a voltage-driven speaker. Thus, there is established a poor-quality feedback loop that circulates an even poorer quality reverberation signal.
At lower mid-frequencies, the magnitude of this cabinet noise EMF component is even in a low-sensitivity hifi driver typically several percents of the driver's terminal voltage and increases with increasing sensitivity, so that even at values below 95 dB/W, the proportion of this noise EMF is well beyond 10 percents of the terminal voltage. This is also one reason why PA speakers sound as they are known to sound, but even in domestic equipment, the magnitude of the interference is totally unacceptable.
Cabinet-noise passage through the cone at lower mid-frequencies is a serious yet widely neglected problem in all contemporary enclosed speaker designs. It can only be addressed properly by stuffing the cabinet tightly with relatively heavy damping material, such as cotton cloth, but for vented designs this is not possible. (Note: so-called electrical damping is not of any bearing here, as we are dealing with frequencies above the resonance region.) It can be quite easily demonstrated that in a typical middle-sized hifi loudspeaker with a 6.5-inch woofer and even with comparably good damping, the magnitude of the cabinet noise penetrating the cone is in the 300 Hz region more than 10% of the directly radiated sound's magnitude. This means that the ratio of the directly radiated sound to the leaked cabinet noise is worse than 20 dB! However, by heavy and careful stuffing, this figure is possible to be improved to around 40 dB.
Voltage-driven tweeters are also subject to similar microphone EMF feedback interference due to their back cavity reflections. Only the frequencies affected are about 20-fold compared to woofers.
Outward Microphonic Interference
Microphonic coupling also occurs outwardly between adjacent drivers, the induced disturbance EMF being roughly inversely proportional to frequency. The magnitude of this effect is lower than that of the inward coupling but extends higher in frequency.
In coaxial drivers, quite popular today, microphonic coupling between the low- and high-frequency units is also remarkable. The magnitude of the disturbance EMF depends, again, on the sensitivity properties of the interacting units but additionally is also strongly modulated by the cone position.
Indefinite EMF Generation due to Mechanical Non-Idealities
Extraneous and indefinite electromotive forces are also generated by the driver itself, without external pressure. All these EMFs also summate to the total motional EMF, em, and are reflected as such to the current of a voltage-driven speaker.
At least the following non-idealities are able to introduce in the voice coil parasitic mechanical vibrations, regardless of signal level:
- Reflections returning from the cone rim.
- In a dome diaphragm, the returning of the mechanical wave back to the joint of the coil former
- Loose mass and reflection effects of the cone's inner suspension
- Modification of the effective mass due to waving and disconnection of the diaphragm.
- Bell modes developing in the cone at certain frequencies, causing the diaphragm to deform and divide into sectors that vibrate in different phases.
- Air currents through a perforated coil former and through the air gap of the magnetic circuit
- Stirring of ferrofluid around the voice coil
- Flexing of the voice coil adhesives and coil former. At the high end of the frequency range, cone excursions are so tiny that even a slight compliance in the glue layers can introduce response alterations and even hysteresis.
- Air loading required for acoustic radiation. Especially on the part of backward radiation, this loading may include vague attributes.
Phase Modulation of Current
Stemming from the geometry of driver's impedance components, variation in the force factor Bl during large voice coil displacements introduces variation in the driver's impedance angle, notably in the lower midrange. On voltage drive, the outcome is phase modulation of current, giving rise to rapid frequency wavering, or jitter.
Position-Dependent Voice Coil Inductance
As the voice coil moves in and out in the air gap of the magnetic assembly, the magnetic reluctance and hence inductance of the voice coil varies accordingly. Even in quality drivers, impedance variations caused by this inductance modulation typically reach to +/-10% within the rated linear excursion range. On voltage drive, all this impedance fluctuation is directly reflected to the current, giving rise to mighty amounts of non-harmonic distortion, especially at upper mid-frequencies and above.
Current-Dependence of Impedance
Voice coil's inductance and hence the impedance of a typical speaker driver is strongly dependent on current level (regardless of any mechanical movement), especially at the upper end of the operation band. Even 10% variation in impedance due to instantaneous signal level is not uncommon, as is quite easy to demonstrate with basic equipment. As with all impedance fluctuations, also this one is directly transferred to the signal of the voltage-operated speaker.
Magnetic Coarseness of Iron
The magnetic nonlinearity of the pole pieces also gives rise to remarkable harmonic distortion in the voltage-to-current conversion performed by the voltage-driven speaker. On current-drive, this distortion appears only in the driver's voltage, without any effect on sound. Likewise, the magnetic hysteresis effect found in the steel as well as possible Barkhausen noise, generated by the stepwise movement of the elementary magnets, are done away with.
As the voice coil warms during high power levels, its resistance can increase more than 50%. In addition to the ensuing signal level compression, in voltage-driven systems this change is able to significantly alter the low-frequency tuning and balance between different loudspeakers and frequency bands.
On current-drive, the matching between drivers is also improved as the manufacturing variances in the load impedances become inconsequential.
The contact resistances accruing in speaker connectors, relays, and possible switches and fuses can exhibit due to aging and contamination nonlinear behavior. The ensuing current distortions on voltage drive are mostly 3rd-order products and quite easily reach the thresholds of audibility, adding to the corresponding products from the driver itself.
Filter Component Nonlinearities
The nonlinearities of crossover filter components, especially series inductors, also become injected to the driver currents under voltage operation. In a current-driven system, these distortion currents can be largely eliminated, enabling e.g. the use of ferrite-cored inductors in many applications where otherwise only air cores would qualify.
In amplifiers operating on one-sided voltage supply, current-drive makes it possible to prevent the non-idealities of the large DC blocking capacitor from being transferred to the load.
If the established convention is compared to car driving, an appropriate parallel would be that a car driver would not take hold of the steering wheel with his hands but by the agency of some kind of sticks, by which he attempts to control the wheel. The sticks represent the electromotive forces and other factors acting between the voltage and current of a speaker; and current-drive corresponds to the normal way of controlling the wheel directly with the hands. It should be obvious in which driving mode the vehicle keeps more accurately in the lane.