a.f. power limiter - protects tweeter
This limiter contents owing to their rela-tively low rating, tweeters (high-fre-quency loud-speakers)form theweak linkin an audio sys-tem. If the volume is suddenly turned up too high, the risk of the tweeters being damaged irreparably is high. Such an im-petuous and costly mistake, can, how-ever, 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 build the power limiter pre-sented in this arti-cle…it’s much safer than controlling your-self when
you’re ad-justing the volume of your beloved audio system. Design by T. Giesberts 38 protects tweeterThere will be many readers who, after reading the introduction to this article, will say that this does not con-cern them. They have a 100 W am-plifier 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 ar-riving 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 loudspeak-ers, and only 5 per cent to the tweet-ers. This means that of the power out-put of 100 W only about 5 W is ap-plied 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 in-stant contains more than half the total emitted energy. This means in this ex-Elektor Electronics 9/96 ample 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 con-cerning the specified rating of tweet-ers. 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 examina-tion, 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 di-aphragm has to move over a larger dis-tance and tweeters just are not de-signed for this. Nevertheless, it goes to show that loudspeaker constructors should be well aware of how ratings are specified.
FUSE OR ZENERDIODE? The question that arises in view of the foregoing is how the tweeters can be protected effectively. The simplest way is merely to con-nect a fuse in series with the tweeters. However, this gives only a limited de-gree 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 un-doubtedly be borne in mind, since, un-less compensating measures are taken, it will inevitably lead to some attenu-ation of the high-frequency sound. A variation of the standard fuse is a special device with positive temper-ature coefficient (PTC), which is avail-able from many loudspeaker dealers. It is a semiconductor element that re-acts just like a slow fuse when the cur-rent through it becomes too high. Un-like a fuse, however, it recovers when the danger is past: it need not be re-placed, therefore. Unfortunately, its re-sistance is slightly higher than that of a fuse. It is clear that series current limiting by a fuse or PTC device has its draw-backs. 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 (damp-5.6 V, the power applied to the tweeter is restricted to about 5 W. SIMULATED ZENER DIODE What we need is a pro-tection that is faster and more reliable than a series element and does not have the dis-advantages of a pair of zener diodes in paral-lel. This requirement could be met by a sort of simulated power zener diode that has a sharply defined start-ing point. The simple circuit in Figure 2 is such a zener diode, consisting of two discrete darlington transistors. Con-nector K1 is simply connected in par-allel with the tweeter terminals. There is no need of a supply voltage, because this is drawn from the loudspeaker signal. The alternating signal across the loudspeaker is rectified by B1, so that a pulsating direct voltage exists across network R1-R2-P1, which is averaged (to a degree) by capacitor C1. When C1 R1 (text) ContentsIt may be asked whether such a simple protection is sufficiently effec-tive, to which the answer is yes and no. The difficulty is that this sort of protection is too effective. This is be-cause the zener action normally com-mences 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. An-other, practical, problem is that power zener diodes are not easy to come by. T2, which consequently short-circuits part of the alternating signal.