Active bass correction: Active loudspeakers offer the only way of obtaining good bass reproduction from inexpensive or small enclosures. The design described does not make use, therefore, of large, heavy enclosures to obtain a good result, but of acoustic feedback. A microphone placed close to the bass drive unit unfailingly registers every movement of the loudspeaker. 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 of the output amplifier. In this way, the input signal 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 Microplane is placed about 10 mm from the cone of the woofer, signals at frequencies of up to 500 Hz are fed back faith0.111y. 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 behavior of the loudspeaker is corrected also for signals above 300 Hz. If the change-over frequency of the cross-over filter of the loudspeaker lies at 300 Hz, it is advisable to make the cut-off frequency of the present circuit, determined by R6-C8, lower than 300 Hz.
The gain of IC2 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 a buffer for the microphone signal.
Preset P1 serves to set the signal level on the basis of the power rating of the output amplifier and the efficiency of the microphone. If this control is set too high, correction is also applied to frequencies above the cut-off point: if it is set too low, the little correction will be applied and signals between 20 Hz and 300 Hz will increase along a standard 1st order characteristic.
The choice of microphone is a matter of some experimentation, particularly with high-power amplifiers. That used in the 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 PI-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-C10. 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 Butter-worth 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 loudspeaker being loaded with signals which it cannot reproduce.
The correction circuit is of particular use with active loudspeaker systems. Make sure that the loudspeaker phase is shifted by 180° to prevent positive feedback. This may be done by adding an inverter-buffer in front of K2.
The circuit draws about ±6 mA, of which only 0.25 mA is drawn by the microphone.