Designing an active loudspeaker system is a complex (but fascinating) matter because each drive unit needs its own output amplifier and cross-over filter. But, of course, an active system has several advantages over a passive one. The present circuit, comprising three filter sections. active bass corrective net­work. and three output stages, fits on just one printed-circuit board.

Two of the advantages of an active loudspeaker system over a passive one are that neither loudspeaker cables nor a passive cross-over filter are required. The lack of loudspeaker cables saves money, too, since good quality ones are expensive. A passive filter dissipates en­ergy in the inductors and capacitors. and this causes some deterioration in the quality of sound reproduction. An ac­tive filter uses no inductors and the ca­pacitors have a much lower value, so that their quality can be of better qual­ity. Moreover, in direct coupling there are no capacitors and inductors with loss resistances, and this means that the loudspeakers are under more direct control of the output stages.

Two of the drawbacks of an active sys­tem are its cost and higher complexity. These two go together, because it is the complexity of the electronic circuits (each loudspeaker needs it own output stage. for instance) that costs the money. In the present design. the output stages have been kept small through the use of modules for the medium and high fre­quency sections.

Some design considerations

The design (see block diagram) allows for various configurations of the cross­over networks. The audio signal from the preamplifier is applied to the three filters (bass, medium and treble) via a buffer stage. Each of the filters may be given a rolloff of 6 dB. 12 dB or 18 dBper octave depending on the value of certain components.The low-pass filter is followed by a bass correction network. The original design of this network is due to Linkwitz. It is particularly useful to lower the response of the woofer in a closed box.

The formulas for the calculations of the correct component values and the rolloffs will be given later.

The outputs of the filters are applied to amplifiers. Note that the LF stage has more than twice the power output of the middle and high frequency one. Since the LF stage has been designed with dis­crete components. its output is applied to the drive unit via a power-on delay. This delay is an integral part of the medium and high frequency modules.

3-way-spk-2Fig. 1. Block diagram of the active 3-way loudspeaker system.

Practical design

Opamp IC2b (see Fig. 3) buffers the applied audio signal to prevent this being loaded unnecessarily. The input impedance is determined by R1. The output of IC2b is split three way to IC1a, IC1b and IC1c. Each of these circuits forms a third order fil­ter, which may be converted into a first or second order type by omitting certain components. The middle-frequency sec­tion has two filters, IC1c, and IC1d, be­cause it needs a rolloff at the low frequency end and one at the high frequency end. With values as shown, the cut-off frequencies are at 500 Hz and 500 Hz re­spectively. The response is a Butterworth type.


Fig-3: Circuit Diagram of the Active 3-Way Loud Speaker System.

It is possible to convert the present sys­tem into an active two-way one by omit­ting the entire middle-frequency section and the associated output stage (1C4). The low-pass filter is followed by the Linkwitz correcting network that matches the frequency response to the low cut­off point of the box. In this way, an oc­tave is added to the lower portion of the response of the enclosure. It is. how­ever. only possible to use the arrangement with boxes whose Qtc and fc are known. The calculations of the values of the net­work components are given later.

The supply voltages for the opamps are stabilized by IC6 and IC7. These voltages are derived from the ±25 V supply for the LF output amplifier (and for the other output stages if this is desired—more about this later).

Output modules IC4 and IC5 need only a few external passive components and a feedback loop. If the supply to them is ±25 V, they deliver an output of up to 30W into 8Ω. Noteworthy in the diagram of their internal circuitry, shown in Fig. 2, are the many protection circuits. Their input has a mute stage to prevent on and off switching becoming audible.

Concentrating on IC4, capacitor C26 pre­vents any d.c. components reaching the opamp. Low-pass filter R39-C27 limits the bandwidth of the input signal to a de­gree that is suitable for the output am­plifier. The input impedance of that am­plifier is determined by R43. The a.c. am­plification of the module is set by feed­back network R42-R41. The d.c. amplifi­cation is limited to unity by C28. Network R43-R44-C29 forms a bootstrap that in­creases the full power available from the output stage slightly. Boucherot net­work R45-C30 in parallel with the loud­speaker serves as load at high frequen­cies when the loudspeaker becomes in­ductive. Power-on delay is provided by R46C31.

The presets between the filters and the loudspeakers serve to match the speakers as far as efficiency is concerned .

The LF output stage is a semi-dis­crete design to obtain a higher power since human hearing is less sensitive to low frequencies. The board has been designed to enable this amplifier operating from a power supply different from that for the middle and high frequency stages.

The supply for the differential input opamp. IC3, is derived from the main supply via networks R31-D3 and R32-D4. The input impedance of the opamp is de­termined mainly by R12. The bandwidth of the input to the opamp is limited by R11-C9. R15~C10 is a compensating network.

The output of IC3 is applied to a com­pound output stage, which provides not only current amplification, but also voltage amplification. This allows the opamp, although its output is limited by the ±20 V supply, to fully drive the power stage, whose supply is ±25 V.

The output stage consists of T4-T5 for the positive half of the signal and T6-T7 for the negative half. The power stage is driven by IC3 via two current sources, T1-D1 and T2-D2. The quiescent current through the output stage is determined by T3. which functions as a variable zener diode. The overall feedback is pro­vided by R13 and R14.

The relay contact at the output of the low-frequency power stage prevents on/off clicks becoming audible. At power on. the relay is energized after a delay by T8 and T9. Before T9 can conduct, C15 needs to be charged via R29 and D7 to a voltage of 2.7 V (D6) plus 0.6 V (base- emitter junction of T9). Diode D7 en­sures that the relay is de-energized im­mediately the supply voltage is switched off.