Loudspeaker operation: The superiority of current drive over voltage drive

-October 22, 2013

This is an overview of the destructive effects that voltage drive has on the performance of electrodynamic loudspeakers. A more comprehensive treatment of the subject can be found in the book Current-Driving of Loudspeakers: Eliminating Major Distortion and Interference Effects by the Physically Correct Operation Method by Esa Meriläinen.

Today, practically all available audio amplifier and loudspeaker equipment works on the voltage drive principle without significant exceptions. This means that the power amplifier acts as a voltage source exhibiting low output impedance and thus strives to force 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 equally indicate that voltage drive is a poor choice if sound quality is to be given any worth. 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, which 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 well known 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 induces its own magnetic field, which may react with adjacent iron, but the effect is not related to this equation.)

This force, then, determines the acceleration (A) of the diaphragm, which in the main operation area (the mass-controlled region) is got from the Newtonian law F = mA. The radiated pressure, in turn, follows the instantaneous acceleration and not the instantaneous displacement, as many mistakenly imagine.

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, the speaker driver in the end obeys only current, not caring what the voltage across the terminals happens to be.

There cannot be found any scientifically valid reasons that justify the adoption of voltage 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 physical soundness of operation have not been considerations in this choice. Engineers are also more accustomed to identifying electrical signals as voltages rather than currents.

At least the hi-fi community should be interested and able to better see through this discrepancy. But they too have taken the state of affairs as a given, being largely conditioned to the wishful thinking that tightly held voltage somehow "controls cone motion," even up to middle and high frequencies. Such a notion doesn't have real scientific grounds, and it can be clearly shown by basic analysis and modeling 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 in Figure 1. Rc represents the 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. This is the proper end essential representation of the electrical system for examining the amplifier-speaker interface. Any wiring resistances and possible output resistance of the voltage amplifier simply add to Rc and thus don't need any specific attention.

Figure 1: The electrical equivalent circuit of a moving-coil transducer with two kinds of feeding sources.

Both Em and Ei are subject to a multitude of disturbances that corrupt the flow of current when the circuit is fed by a voltage source (Uo). Thus the magnitudes of these two are of utmost interest. When the feed comes from the current source (Io), Em and Ei only appear as additional voltages in the amplifier output, having no influence on the current.

In the impedance modulus curve of a typical moving-coil driver (|Ztot| in Figure 2), Em manifests itself as the high peak at the resonant frequency, while Ei is responsible for the gradual rise typically starting in the whereabouts of 300 Hz. When looking at such a curve, one can easily be mistaken to assume that Em is significant only near the fundamental resonant frequency or that Ei is significant only at the highest operating frequencies. In reality, however, these two components are of almost opposite phase in the midrange and therefore largely mask each other near the impedance minimum.

Figure 2: Composition of the impedance magnitude curve of a typical cone driver. Both the motional impedance Zm and the inductance impedance Zi have considerable magnitude throughout the main operation area.

Figure 2 thus also shows the actual and typical magnitudes of the impedance components separately, which are also measurable by special techniques. It is seen that the sum of |Zm| and |Zi| is in fact in the whole operation band at least of the same order of magnitude than Rc. Increasing driver efficiency also increases both |Zm| and |Zi|.

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