AF Power Limiter

Owing to their relatively low rating, tweeters (high-frequency loudspeakers) form the weak link in an audio system. If the volume is suddenly turned up too high, the risk of the tweeters being damaged irreparably is high. Such an impetuous and costly mistake, can, however, be avoided in two different ways. The first is to curb your desire to turn up the volume to levels that the loudspeakers cannot handle. The second is to install a power limiter presented in this article…it’s much safer than controlling yourself when you’re adjusting the volume of your beloved audio system.

There will be many readers who, after reading this article, will say that this does not concern them. They have a 100 W amplifier and the loudspeakers are also rated at 100 W. So, nothing can go wrong. Really?

Unfortunately, things can go wrong, since the rating given by the loudspeaker manufacturers is true only for average music signals. In arriving at this rating, account is taken of the fact that the energy contained in music signals is strongly dependent on frequency. Of the power delivered by the output amplifiers roughly 75 per cent is applied to the woofers (low-frequency loudspeakers), 25 per cent to the mid-frequency loudspeakers, and only 5 per cent to the tweeters. This means that of the power output of 100 W only about 5 W is applied to the tweeters.

Equally unfortunately, not all music signals are average. For in-stance, in the case of synthesizer music it can happen that a sudden burst of high-frequency music is produced, which at that instant contains more than half the total emitted energy. This means in this example that some 50– 60 W of music power is applied to the tweeters instead of the average 5 W. Many tweeters just cannot cope with this sort of power. There is yet another aspect concerning the specified rating of tweeters. Although in the case of woofers and mid-frequency speakers the ‘true’ rating is given by the manufacturers, this is not so in the case of tweeters. For these units, the specified rating applies only if they are used with a cross-over filter! On close examination, it appears that a rating of, say, 50 W applies only if the speaker is used with a 2nd-order high-pass filter with a cut-off frequency of 4000 Hz. If, however, the cut-off frequency is, say, 2000 Hz, the rating is lowered to 20 W. Without a filter, the rating appears to be only 5 W! All this is, of course, reasonable, since, at lower frequencies, a diaphragm has to move over a larger distance and tweeters just are not designed for this. Nevertheless, it goes to show that loudspeaker constructors should be well aware of how ratings are specified.

Fuse or Zener diode?

The question that arises in view of the foregoing is how the tweeters can be protected effectively.

The simplest way is merely to connect a fuse in series with the tweeters. However, this gives only a limited degree of protection, and also introduces a few drawbacks. If a fast fuse is used, chances are that it will blow at the first peak in the music signal. A slow fuse on the other hand does not guarantee that it will always be faster than the tweeters. In other words, the tweeters might still give up the ghost before the fuse blows. Add to this that any fuse introduces a certain resistance, which may vary from some tenths of an ohm to more than an ohm. This should undoubtedly be borne in mind, since, unless compensating measures are taken, it will inevitably lead to some attenuation of the high- frequency sound.

A variation of the standard fuse is a special device with positive temperature coefficient (PTC), which is available from many loudspeaker dealers. It is a semiconductor element that reacts just like a slow fuse when the current through it becomes too high. Unlike a fuse, however, it recovers when the danger is past: it need not be replaced, therefore. Unfortunately, its resistance is slightly higher than that of a fuse. It is clear that series current limiting by a fuse or PTC device has its drawbacks. What other means are there?

One is a voltage limiter across the tweeter. In its simplest form, this could consist of two anti-series connected zener (power) diodes, assuming that the necessary series resistor is already present in the cross-over filter (damping resistor).

It may be asked whether such a simple protection is sufficiently effective, to which the answer is yes and no. The difficulty is that this sort of protection is too effective. This is because the zener action normally commences at fairly small currents when the zener voltage is nowhere near its nominal value. This results in untimely limiting, which causes a compression effect even at fairly small signals. Another, practical, problem is that power zener diodes are not easy to come by.

What we need is a protection that is faster and more reliable than a series element and does not have the disadvantages of a pair of zener diodes in parallel. This requirement could be met by a sort of simulated power zener diode that has a sharply defined starting point.

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