Amplifier Topologies for Current Driven Loudspeakers

At the last conference, Reproduced Sound 9 [l], we considered in detail the theoretical aspects of current driven loudspeaker systems, showing that this approach led to worthwhile improvements in performance over the conventional voltage driven case.Much of the discussion was restricted to subwool ers, where a working arrangement using both velocity and acceleration feedback techniques was described, although little was intentionally said about the power ampli er design requirements for such a system.Consequently, the aim of this paper is rstly to review the options open to the designer of current driven systems.while discussing the limitations of conventional approaches to achiean a high output impedance, such as current feedback.which although ne for subwoot er applications, does not represent the ideal strategy for wide bandwidth systems It is demonstrated that for high frequency operation, the use ul'a cascade grounded base output stage delivers optimum performance.Usingthese techniques enables us to build up a full range active system.one speci c implementation being a recording studio monitor using DSP crossover techniques. BACKGROUNDThe moving coil loudspeaker can readily be shown to bene t in terms of linearity when controlled by a current source (having theoretiwa an in nite output impedance), as opposed to the zero output impedance voltage source ampli er conventionally employed [l.2].In order to establish the bene ts of current drive.it is firsl necessary to examine behaviour for the voltage driven case, The basic electro-mechanical model is represented in Fig. 1, showing ampli er and interconnect source impedance Z .voice coil reo'stance and inductance Re and Le respectively.Analysis of this model gives the transfer function between amplifier output voltage and cone velocity: vi Bl ' u = ...(l) *5 Zm ( Z5 + (Bl) I Zm} where u = cone velocity (ms l) Vin= ampli er output voltage (V) B _= motor system ux density (T) l = coil length in field B (m) Z = lumped mechaniml impedance (kgs l) Zs = lumped electrical impedance (Zg, R: and ch) (ohm)