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Fig.1 Loudspeaker protection Thank you for producing the Automatic Level Control for PA systems in the latest (March 1996) issue of SILICON CHIP. If it works as well as claimed it would be ideal for bands and public address systems. I was hoping that the unit could be adapted to my particular application and that you could describe how to set it up. I am responsible for maintaining a public address system which is used by many people and some are not as caring of the equipment as they should be. Consequently, there is the risk of them blowing the loudspeakers by, for example, dropping a microphone. This can produce excessive signal levels and cause the amplifier to go into clipping. Can the Automatic Level Control be used as a limiter so that signals above a certain level will be restricted? This will prevent the main amplifier from clipping and possibly damaging the PA loudspeakers. (J. connected transistors or a 555 which is delivering current pulses which are too long. Are the time-constant components at pins 6 & 7 correct? Of course, if the 555 is defective it, too, could cause excessive current drain. Components for fluorescent light ballast Can you advise me where to pur- Fig.2 B., North Lambton, NSW). • It is certainly possible to use the ALC as a limiter since the relevant parameters of attack, decay and gain limit are adjustable. In fact, when the ALC is used normally for volume control or compression, any overload signal is quickly attenuated back to normal levels and so it acts as a limiter by default. If the ALC is to be used purely as a limiter, the gain limit would be set so that compression occurs at a low enough signal level before the power amplifier clips but high enough so that normal signals are not compressed. Typically, power amplifiers have an input sensitivity of around 1V RMS. The attack and decay rates should be at their fastest settings so that limiting will occur almost instantly and then quickly recover. The accompanying digital scope plot (Fig.1) shows ALC response when a signal above the gain limit is applied. The top trace is the input signal of 600mV RMS with bursts at 1.27V RMS (3.6V p-p). The lower trace is the output of the ALC with chase a kit for the Fluo­rescent Light Electronic Ballast published in the October 1994 issue of SILICON CHIP? (R. B., Pomona, Qld). • This design is not available as a kit. The MC34262P can be obtained from VSI, phone (07) 262 5200. Other parts can be ob­tained from Jaycar Electronics, phone (02) 743 5222 and Farnell Electronic Components, phone (02) 645 8888. The PC board can be ob- the gain limit set at a nominal 1V RMS. The output is limited above 2.72V p-p (960mV RMS). However, input headroom is not good since the input amplifier (IC1a) will clip at 1.35V RMS. This is undesirable so the input amplifier gain should be reduced from 5.5 to unity by removing the 22kΩ resistor from pin 6 to ground. This will allow the input signal to rise to about 8V RMS before it clips. You will need to set VR3 (the output preset trimmer) so that the ALC produces 1V output with a 1V input. The second scope shot (Fig.2) shows the performance with this gain modification. It shows a 1V RMS signal together with a 6V RMS (16.8V p-p) burst. Note the short 1ms overshoot in the output at 10.8V p-p (3.8V RMS). Recovery time after the 6V RMS input burst takes about one second at the fastest decay setting. The unweighted signal-to-noise ratio is -85dB with respect to 1V out (20Hz to 22kHz bandwidth). The A-weighted figure is also -85dB. tained from RCS Radio Pty Ltd, phone (02) 587 3491. Notes & Errata Insulation Tester, May 1996: the overlay and wiring diagram on page 34 is incorrect. It shows the battery connections re­versed. Also the 47kΩ resistor adjacent to the 36kΩ and 120kΩ resistors should be 43kΩ. SC June 1996  93