Circuit Diagram Knowledge

Active loudspeakers offer the only way f obtaining good bass reproduction from inexpensive or small enclosures.
The design described does not make use, therefore, of large, heavy enclosures to obtaion a good result, but of acoustic feedback. A microphone placed in close proximity of the bass drive unit unfailingly registers every movement of the loud-speaker. It is, of course, important that proper attention is paid to the maximum movement of the speaker.

The microphone output is coupled . into the negative feedback loop ofthe output amplifier. In this way, the input sig- nal to the amplifier is compared with the acoustic signal produced by the speaker. In practice, this arrangement appears to work well only with low-frequency signals. Experiments have shown that if  the microphone is placed about 10 mm from the cone of the woofer, signals at e frequencies of up to 500 Hz are fed back faithfully.

To make absolutely certain of  correct operation, in the present circuit the upper frequency has been set to 300 Hz; above it, the correcting action gradually ceases. Note, however, that the phase behaviour of the loudspeaker is corrected also for signals above 300 Hz. lf the change-over frequency ofthe cross- over filter ofthe loudspeaker lies at 300 Hz, it is advisable to make the cut-off frequency ofthe present circuit, determined by R6-C8, lower than 300 Hz. The gain of lC2 over the operating range of the circuit is 20 dB, which reduces to 0 dB for frequencies above 300 Hz. This amplifier, which provides the correction up to the cut~off point, also serves  as buffer for the microphone signal.

Preset Pl serves to set the signal level on the basis of the power rating of the n outpu amplifier and the efficiency of the microphone. lf this control is set too high, corection is also applied to frequencies above the cut-off point; if it is set too low, little correction will be applied and signals between 20 Hz and 300 Hz will increase along a standard lst order characteristic. The choice of microphone is a matter of some experimentation, particularly with high-power amplifiers.

That used in the proposed subwoofer bass enhancer prototype proved to work well with low-power systems with a relatively low efficiency. If another type is used, make sure that the potential across the microphone is about half the supply voltage. This is arranged by R8 and R9. Also make sure that the cut-off point set by P1-C9 remains well below 20 Hz (no signal at P1 results in an increase of the final amplification). The frequency up to which the microphone signal is compensated is determined by R8-P1-Cm.

 This time-constant must be equal to R6-C8. The present circuit can magnify frequencies down to 20 Hz by roughly 20 dB. Since most loudspeakers cannot cope with that frequency, the circuit includes a 3rd order Butterworth section with a cut-off point of 37 Hz.

This frequency may be altered by changing the values of C1, C2, and C3. This filter prevents the loud-speaker being loaded with signals which it cannot reproduce. The bass correction circuit is of particular use with active loud speaker systems.M2 makes sure that the loudspeaker phase is sh ifted by l80° to prevent positive feedback. This may be done by adding an inverter-buffer before K2. The circuit draws about t5 mA, of which only 0.25 mA is drawn by the microphone.