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Are you worried about burning out your loudspeakers? Build this audio power meter & worry no more With the wide dynamic range of today's compact discs, it is all too easy to over"".drive your speakers without being aware of it. By using this low-cost project, you can monitor power levels from 200 milliwatts up to 100W RMS using a LED bargraph display. By DARREN YATES Imagine that you've just whacked your favourite CD on the deck, turned up the wick and are now enjoying a golden blast of sound. But then you know that the next few bars will really make the speakers belt it out and maybe you think you should wind the wick back a bit, just to be on the safe side. If you listen in that way, the chances are that you are regularly driving your amplifier and your loudspeakers well into overload without really being 22 SILICON CHIP aware of it. And while the speakers may not protest too much if it only happens intermittently, if you do it too often you will end up with damaged tweeters at the least and possibly cook the woofers too. Paradoxically, the problem is more severe if you have a lower powered amplifier than if you have one rated at 100 watts per channel or more. You are more likely to overload a smaller amplifier and if you do drive it heavily into clipping, it can deliver a great deal more power than its nominal ratings suggest. For example, a nominal 30 watt per channel amplifier driven badly into clipping might deliver 100 watts for a brief burst and that can spell goodnight for your tweeters. By contrast, if you are using an amplifier rated at 100 watts or more, not only are you less likely to overdrive it but your spe,akers are more likely to be rated to take a lot more power anyway. So as you can see, if you are not careful in the way you use your amplifier's volume control, it can spell trouble for your speakers. Now you can end the guesswork with this simple little power monitor. It connects across one of your loudspeakers and uses a LED bargraph to indicate the . output power from your amplifier in 10 steps from 200 milliwatts to 100 watts. Circuit diagram Let's take a look at the circuit dia- gram - see Fig.1. It uses a 741 op amp (IC2) and an LM3915 logarithmic LED display driver (IC1). This latter IC does most of the work and so warrants a closer look. Inside the LM3915 is a string of 10 comparators which have their noninverting (+) inputs connected to a voltage divider string and their inverting inputs connected to an incoming DC signal. Each comparator drives a LED. If the input signal is higher than the non-inverting input for a particular comparator, then its LED will be lit. If the signal voltage is higher than the non-inverting input on all comparators, then the top-most LED will be lit. The voltage divider for the noninverting inputs is arranged so that the comparators progressively turn on their LEDs for an increase in signal level of +3dB. There are 10 LEDs, so the total signal range indicated by the display is 30dB. When we are talking about power, a 3dB increase corresponds to a doubling of power. Hence, as each additional LED is lit, it means that the power has increased by a factor of two. OK, let's now go back to the circuit diagram of Fig.1 and see how the LM3915 fits into our circuit. The signal from the power amplifier is taken directly from across the speaker terminals. This signal passes through a 100kQ trimpot and is then fed to a 741 op amp (IC2) which is connected as a precision half-wave rectifier. IC2 has diodes Dl and DZ inside the feedback loop and so allows the circuit to accurately rectify quite small AC voltages. ' The resulting DC is fed through a low pass filter consisting of a 3.3kQ resistor and a 0.15µF capacitor and thence to pin 5 of ICl. From here on, the circuit acts as described above. The DC signal at pin 5 fluctuates in proportion to the AC signal across the speaker and the LEDs are lit accordingly. The time constant of the RC filter at pin 5 is a compromise between fast attack and slow decay on the one hand and flickering with low frequency signals on the other hand. We have selected the filter components so that the display has a sufficiently fast action to catch signal transients. The lkQ and 4. 7kQ resistors on pins 6, 7 & 8 set the LED brightness. They +12V LE02-LED11 10-LEO BAR DISPLAY LE011 10 K 100W 11 K 330k + 50W 3.3k VR1 100k 12 + 0.15 FROM SPEAKER 25W 13K IC1 LM3915 13W 14 6W 15 K 10k 3W 1k 16 1.6W ~ -12V 17K 4.7k ~ S1 er-/ 12VAC INPUT 0.8W 18 0.4W 03 1N4004 0.2W OUT +12V ~ 04 1N4004 ~ LED1 4.7k OUT ~ -12V ffi ffi IGO A~K GI O AUDIO POWER METER Fig.1: the input signal is derived from one of the loudspeakers & is fed to op . amp IC2 which is connected as a half-wave precision rectifier. The resulting DC signal is then filtered & used to drive an LM3915 logarithmic LED display driver (ICt). IC1 then drives the LED display to show the power level. also set the internal reference voltage and hence the overall signal sensitivity of the circuit. Pin 9 of IC1 determines whether the LEDs are lit as a bargraph display or one at a time (ie, single LED dis- play). We've selected the single LED display mode, by leaving pin 9 unconnected, as it reduces the overall power consumption. The power supply is derived from a 12V 300mA AC plugpack. Two 3-ter- The two 3-terminal regulators must be bent against the PC board as shown in this photo so that they don't foul the lid of the case. Be sure to use the correct regulator type at each location & double-check to ensure that they are both correctly oriented. APRIL 1993 23 Before you begin construction, check the board carefully for any shorts or breaks in the copper tracks. These should be repaired before proConstruction ceeding further. Assembly can begin by inserting All of the components for the Audio Power Meter, except for the power the 'Wire link, the resistors and diodes. Follow the overlay diagram of switch and the AC power input socket, are installed on a PC board coded - Fig.2 and make sure that you install the diodes and electrolytic capacitors 01105931 and measuring 100 x 55mm. in their correct locations and the right way around. Next up, install trimpot VRl and PARTS LIST the two !Cs, followed by the 3-termi1 PC board, code 01105931, nal regulators. Don't install the 10100 x 55mm LED bar display or the indicator LED 1 plastic zippy case, 130 x 67 x for the moment. 42mm The next task is to drill the holes in 1 red 4mm banana socket the case. The mounting holes for the 1 black 4mm banana socket PC board should be fai rly straight1 SPST miniature toggle switch forward but the cutout for the LED bar 1 2.5mm DC socket display (in the case lid) will take a 4 15mm x 3mm tapped spacers little work. This is best done by using 1 12VAC 300mA plugpack the label as a template for drilling a series of small holes around the inSemiconductors side perimeter of the cutout area, then 1 LM3915 logarithmic LED knocking out the centre piece and display driver (IC1) filing the job to a smooth finish . 1 LM741 op amp (IC2) Once the holes have been drilled, 1 7812 +12V regu lator slide the LED display and the indica1 7912 -12V regulator tor LED into their holes on the board, 1 10 x red LED bargraph display then secure the board to the lid using 1 5mm green LED (LED1) four 15mm-long tapped spacers. The minal regulators produce ±12 VDC rails with LED 1 indicating that the unit is on. 2 1N914 signal diodes (D1, D2) 2 1N4004 rectifier diodes (D3, D4) 1 100kQ 5mm horizontal trimpot TABLE 1 Watts 8-ohms 4-ohms 0.2 1.26V 0.89V 0.4 1.79V 1.26V 0.8 2.53V 1.79V 3.58V 2.53V 1.6 ' 3.2 5.06V 3.58V 6.4 7.16V 5.06V 13 10.2V 7.2V 25 14.1V 10V 50 20V 14.1V 100 28 .3V 20V LED display and LED indicator can then be pushed into the lid and their leads soldered to the PC board. The on/off switch, power socket and input terminals can now mounted and the wiring completed. Now apply power and check that the supply rails from the 3-terminal regulators are correct (ie, +12V from the 7812 and -12V from the 7912). Don't expect to see the display light up yet because it won't. The only LED that should come on is the green power indicator LED. If you have an audio frequency gen- Capacitors 2 220µF 25VW electrolytics 2 100µF 16VW electrolytics 1 1µF 50VW electrolytic 1 0.15µF MKT polyester Resistors (0.25W, 1%) 2 330kO 1 3.3kQ 1 10kO 1 1kO 2 4.7kQ Miscellaneous Screws, washers, nuts, hook-up wire, solder etc. Fig.2: make sure that all polarised parts are correctly installed on the PC board. The 10-LED bar display & the indicator LED (LED 1) are not soldered to the board until after it has been attached to the lid of the case. RESISTOR COLOUR CODES 0 0 0 0 0 0 24 No. Value 4-Band Code (1%) 5-Band Code {1%) 2 1 2 330kQ 10kQ 4.?kQ 3.3kO 1kQ orange orange yellow brown brown black orange brown yellow violet red brown orange orange red brown brown black red brown orange orange black orange brown brown black black red brown yellow violet black brown brown orange orange black brown brown brown black black brown brown SILICON CHIP Fig.3 (right): this full-size front panel artwork can be used as a drilling template for the front panel (or you can attach the finished label to the lid & use that as a drilling template instead). + 0.2 0.4 0.8 1.6 3 6 12 25 50 100W SPEAKER INPUTS 12VAC IN ~~=== AUDIO POWER METER + Fig.4 (below): check your etched PC board for defects before mounting any of the parts by comparing it against this full-size artwork. erator handy, set it up to deliver about 6V on a sinewave of lkHz or thereabouts. Feed the signal into the input and adjust the sensitivity with trimpot VRl. As you rotate VRl over its full travel you should see each LED light up in sequence. If that checks out, you have a working power monitor. All that remains is to set the sensitivity so that the reading is accurate. Interestingly, no test instruments apart from a digital multimeter are necessary for this job. All you have to do is set the resistance oftrimpot VRl to 41.Zkn if you have 8Q loudspeakers and to 26.Zkn is you have 4Q loudspeakers. This done, your audio power meter should give a correct indication on all LEDs within about ±ldB. On the other hand, if you have an audio signal generator and digital multimeter and you wish to check the linearity of the power meter, the voltages that appear across a loudspeaker for different power levels are listed in Table 1. And that's about all there is to it but we'll conclude with a warning: don't wake the neighbours trying to light up the 100W LED! SC :."-'\ ~ [?~ ~ .· ; - i ~J4;,"{} t~i<,;~, ii~ - ·/4# The PC board is secured to the lid of the case on 15mm tapped spacers as shown here. Adjust the LED display so that it sits flush with the lid before soldering its leads. APRIL 1993 25