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Loudspeaker protector for stereo amplifiers This simple circuit will protect your expensive loudspeakers from overdrive or from amplifier failure. It can be fitted to just abol}t any amplifier & will also eliminate switching thumps. By BOB FLYNN Although som e audio amplifiers includ e loudspeaker protection circuitry as standard, there are many amplifiers that do not. In particular, this important feature is often left out of high-power amplifiers , either to reduce cost or to eliminate switching in the audio path. And that can be bad news for your loudspeakers. Almost all audio amplifi ers use direct-coupled output stages - ie, there is no output coupling capacitor. This means that if an output transistor goes short circuit, virtually the full supply rail to that part of the circuit will be applied to the loudspeaker. The result is usually a blown loud- speaker voice coil or damaged suspension, probably before the unfortunate user even realises that anything is amiss. For example , consider a 100W amplifier with ±50V supply rails driving a loudspeaker with a voice coil DC resistance of 6.5il If one of the supply rails is shorted to the speaker, the resulting power dissipation in the voice coil will be 50 x 50/ 6.5 = 385W (or at least it will be until the power supply fuses blow. Obviously, there are not too many voice coils that will withstand that sort of treatm ent for long. In fact, the voice coil of a typical 50W or 100W loudspeaker would burn out almost instantly. Depending on the make and model of the loudsp eaker, a typical driver can easily set you back $150 or more , so replacing them can be a very expensive exercise - more so than getting the amplifier repaired, in fact. And don 't think that the above scenario in quite improbable. Audio amplifier output stages can go short circuit for a variety of reasons ranging from power supply faults to straight out user abuse. Another way of damaging your loudspeakers is to overdrive them, either by running the amplifier into clipping or simply by advancing the volume control too far. This type of abuse can quickly burn out a tweeter voice coil due to excessive power dissipation In severe cases, it's also possible to damage other drivers in the enclosure, particularly if the amplifier output power greatly exceeds the rated power of the loudspeakers. This project is designed to protect your expensive loudspeakers if any of the above situations occur. In use, the Loudspeaker Protector monitors the DC conditions at the output of the amplifier and trips a relay to disconnect the loudspeakers if a problem is detected. Switch-on thump All the parts for the Loudspeaker Protector are mounted on a small PC board which is installed inside the amplifier chassis. This is the DC-powered version. 62 SILICON CHIP Another problem with many amplifiers is that they cause a large thump in the loudspeakers when they are switched on or when they are switched off. There are a couple of reasons why this can happen. First, the positive and negative supply rails may not rise (or fall) at the same rate and so the output swings towards one rail. Second, it takes a finite time for the input circuit to stabilise at switch on and gain control over the output stage. During this time, there is no negative feedback which again means that there is nothing to stop the amplifier swinging towards one of the supply rails. - ~ - ----- - --- - - - -, I '------+--<> TO LEFT SPEAKER FROM AMPLIFIER LEFT CHANNEL \ \ .----.- -...-----'Wt'lr-:,~--~--.---v. 100 01 02 270k \ + 16VW+ 470 + 25VWJ \ 56k 56k 2x1N914 I 47k RLY1 12V 160ll .,. TO RIGHT SPEAKER FROM AMPLIFIER RIGHT CHANNEL 22k 0.5W ,.. .., B EOc VIEWED FROM BELOW 22k o.sw 22k 47 BP! Fig.1: most of the transistors in the circuit function as switches. Normally, Q4, Q5 & the relay are on & the loudspeakers are connected to the amplifier. However, if a DC voltage appears at an amplifier output, then either Q3 or Q1 & Q2 turn on & Q4, Q5 & the relay turn off. D1, D2 & Q6 form the overdrive protection circuit. 47 BP! ~ 22k .,. BCE 04 1N4002 FROM AMPLIFIER .,. *R1 REQUIRED FOR~+45V LOUDSPEAKER PROTECTOR This project neatly eliminates the switch-on thump problem. It does this by using a simple time delay circuit to switch on a relay to connect the loudspeakers two seconds after power is initially applied. During this period, the amplifier has time to stabilise so that, when the loudspeakers are connected, no switch-on thump occurs. The Loudspeaker Protector can also eliminate any thump that occurs in the loudspeakers shortly after switch off. It can also reduce (but not totally eliminate) the sharp click that some amplifiers produce at the instant of switch off. This can usually be eliminated by correct selection of the suppression capacitor associated with the on/off switch. How it works Take a look now at Fig.1 which shows all the circuit details. The main part of the circuit uses six transistors, most of which function as simple switches. It is based on a circuit which was first used in a commercial Japanese amplifier some 20 years ago. We have added a number of features to it to arrive at the circuit presented here. Transistor Q5 controls the relay and is in turn controlled by Q4. When Q4 is on, it turns on Q5 and the relay to connect the loudspeakers to the amplifier. Base hias for Q4 is derived from a network consisting of a 2 70kQ resistor, two 56kQ resistors and a 100µF capacitor. When power is applied, the 100µF capacitor charges via the 270kQ resistor and, after about two seconds, provides sufficient forward hias to turn on Q4. Q5 and the relay then turn on to connect the loudspeakers at the end of this 2-second delay, thus eliminating any switch-on thump. Q1, QZ & Q3 are used to monitor the amplifier outputs for DC fault conditions. Both channels are monitored via a low pass filter consisting offour 22kQ resistors and two 47µF bipolar capacitors . This filter is there to ensure that legitimate AC signals at the ampl ifier outputs have no affect on the protector circuit. However, if the DC output of th e amplifier rises above +2.5V, Q3 turns on and pulls the base of Q4 low. Q4 thus turns off and so Q5 and th e relay also turn off to disconnect the loudspeakers. On the other hand, if the amplifier output exceeds -2 .5V, Ql 's emitter is pulled negative with respect to its base. Ql thus conducts and turns on QZ which th en turns off Q4, Q5 and the relay as before. Note that when the relay is de-energised, the moving contacts are shorted to the loudspeaker ground lines via the " unused" contacts . This has been done because if a large DC voltage appears at the amplifier output, an arc can be maintained across the contacts as they open; ie, the loudspeakers will still effectively be connected during this time. Shorting the moving contact to speaker ground removes any DC voltage across the loudspeaker terminals and blows th e amplifier fuses if the arc persists. The fact that this Loudspeaker Protector is intended for use with high power amplifiers which can produce considerable output currnnts, plus the JUL Y 1991 63 .IJ~TO LEFT ~--:-1:~r------~--~--~-~- FROM AMPLIFIER 2-n___ .04 ! 0B63VW ~ ~ d:;® ~ ~ \I -Dl[}-- ~a®' efM 10uF8+ 01 ~.---~ ~~:: + - TO RIGHT SPEAKER + FROM AMPLIFIER RIGHT CHANNEL Fig.2: here are the assembly details for the DC-powered version. Be sure to observe correct component polarity & use heavy-duty cable to connect the loudspeaker terminals & amplifier outputs. Resistor Rl (220Q 5W) can be replaced with a wire link for supply rails of less than +45V. need to protect against heavy DC fau lt currents, means that a heavy duty relay is call ed for. The one specified for this circuit has DPDT (changeover) contacts rated at 10 amps. Lesser rated relays can be expected to weld their contacts together under DC fault condition s. DC input voltages of less than Z.5V have no affect on the circuit. This is desirable since all direct-coupled amplifiers have a "normal" DC offset at their outputs and this can be as high as Z00mV or so. In any case, DC voltages of less than Z.5V are not going to cause damage to your loudspeakers. Overdrive cutout D1 , DZ and Q6 form the overdrive monitoring ci rcuit. In operation, D1 & DZ rectify th e AC signals at th e ampli- fier outputs and charge a 10µF capacitor. If the AC signals exceed a preset level , sufficient vo ltage will be developed across this 10µF capacitor to forward bias Q6. Q6, Q4 and th eir common 100Q emitter resistor form a Schmitt trigger. In order for Q6 to turn on, the voltage on its base must exceed the voltage on the base of Q4 (approx. 1V). When this happens due to excessive signal drive, Q6 turns on and Q4 , Q5 and the relay all turn off. As soon as Q6 turns on , the voltage across the 100Q emitter resistor drops since all current is now supplied via the 56kQ resistor in the co ll ector circuit. This means that the voltage on Q6's base must drop to well below the turn-on value for Q6 to turn off again. This translates to an inp ut h ysteresis level of about 3V AC and prevents relay chatter at the transition point. Trimpot VR1 allows the trip point to be adjusted to the desired signal level. Power supply Power for the circu it can be derived in one of two ways . Assuming that you intend mounting the circuit inside your amplifier, power can be derived from any convenient positive DC rail ranging from +30V to +65V. This voltage is fed to a series regulator circuit based on D4 , ZD1 and Q7 . D4 protects the circuit against reverse polarity voltages, while ZD1 sets the voltage on the base of Q7 to 18V. Q7 functions as an emitter follower and produces a regulated +17.4V which is then used to power the rest of the circuit. Also shown on the circuit is current limiting resistor R1 (ZZ0Q) . This resistor is only required if the supply rail is greater than 45V and is linked out for voltages below this figure. The alternative power supply uses a centre-tapped mains transformer to drive a full-wave rectifier made up of diodes D5 & D6. The resulting 17V DC supply is then filtered using a 470µF Z5VW capacitor (n ear Q5 , at the top righthand corner of the circuit). Construction All the parts for the Loudspeaker Protector are mounted on a PC board coded SC0l 105911 and measuring 134 x 71mm. Before installing any of the parts, carefully inspect the copper side of the board for possible defects. In particular, look for open circuit tracks or tracks which are shorted together due to incomplete etching. Two versions of the Loudspeaker Protector can be built, one incorporating the voltage regulator components (based on D4, ZD1 & Q7) and RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 64 No Value 4-Band Code 5-Band Code 1 2 2 4 2 270kQ 56kQ 47kQ 22kQ 2.2kQ 220Q 100Q 68Q red violet yellow gold green blue orange gold yellow violet orange gold red red orange gold red red red gold not applicable brown black brown gold blue grey black gold red violet black orange brown green brue black red bro"".'n yellow violet black red brown red red black red brown red red black brown brown not applicable brown black black black brown blue grey black gold brown SILICON CHIP PARTS LIST 1 PC board , code SC01105911, 134 x 71mm 1 24V centre-tapped mains transforme r**, Altronics M2854 or similar 1 12V DPDT relay with 10-amp contacts, Jaycar SY-4065 or DSE P-8014 1 TO-220 mini heatsink~ 1 5kQ horizontal mount trimpot 1 1-metre length 32 x 0.2mm hook-up wire Semiconductors In most cases, you should be able to mount the PC board on the rear panel of the amplifier, close to the loudspeaker terminals. This view shows the prototype installed on the rear panel of the SILICON CHIP Studio 200 power amplifier. the other using the mains sup ply. For this reason, we have shown two separate wiring diagrams (Fig.2 & Fig.3 ). Fig.2 shows the assembly details for the version with th e on-b oard vo ltage regulator. Th is is th e version to bui ld if you intend powering the unit from the amplifier's pos itive supp ly rai l. Begin the assembly by install ing PC pins at all the external wiring points , then install the resistors , trimpot, diodes and capacitors. Make sure that all the diodes and polarised electrolytics are oriented as shown on Fig.2. The two 47µF bipolar electrolytic capacitors can be installed ei ther way around. Note that the wirewmm d res istor Rl (220Q, 5W) is required only if the supp ly rail is greater than +45V. Mount the resistor so th at it sits slightly above th e board surface to all ow air circulation for cooling. The six small- signal tran sistors can be in stalled next. Push them down onto the board as far as they w ill comfortably go before so ldering the ir leads and check th e ori entation of each one carefully against the wiring diagram. Make sure also that you don't get the transistor type numbers mixed up. Fig.1 shows the transistor pin out detai ls. The BD649 transistor (Q7) is mounted flat against the PC board and fitted w ith a small U-shaped heatsink to keep it cool. To mount th e transistor, first bend its leads at right angles so that the metal tab lines u p with its mounting ho le. Thi s done, smear the tab with heats i nk compound, then bo lt the assemb ly to th e board and solder the leads. Finally, th e relay can be mounted on the PC board. The best way to go about this is to first solder a short length of stout tinned copper wire to each relay pin. Th ese wire leads are then pushed through the relay mounting ho les and soldered. Be sure to sit the relay down as far as it wi ll go, so that its pins contact the board surface. Don 't try to enlarge the board h oles to directly accept the relay p ins. You will damage the copper pattern if you do. Fig.3 shows the assemb ly details for the alternative AC-powered version . It is similar to the prev ious version but omits the vo ltage regul ator components and substitutes the transformer and diodes D5 and D6 instead. Alternative ly, if th e amp lifi er's power transformer has a 24 V centretapped winding, you can use this instead. The completed board assembly can be installed in any conven ient location in the amplifier chassis , although it's best to mount it close to the loud speaker termina ls. In most cases, you shou ld be ab le to mou nt the board on the rear panel using standoffs. If you are installing the AC-powered vers ion into an integrated amplifier, be sure 4 BC547 NPN transistors (01 ,03,04,06) 1 BC557 PNP transistor (02) 1 BC327 PNP transistor (05) 1 80649 NPN transistor (07)* 2 1 N4148 s ignal diodes (01 ,02) 4 1 N4002 diodes (03 ,04* & 05**, 06**) 1 18V 500mW zener diode (ZD1 )* Capacitors 1 470µF 25VW PC electrolytic 1 100µF 63VW PC electrolytic* 1 100µF 16VW PC electrolytic 2 47µF 50V bipolar PC electrolytic 1 10~1F 16VW PC electrolytic Resistors (0 .25W, 5%) 1 270k.O 2 56kQ 2 47kQ 2 22kQ 2 22k.O 0.5W 1 2.2kQ 1 2.2kQ 1W 1 220Q 5W* 1 100Q 1 68Q 1W Miscellaneous Heats ink compound , standoffs, machine screws & nuts , mainsrated cable. * DC-powered version only. ** AC-powered version only. to keep the mains transfo rmer away from th e sens itive preamp lifier stages . On ce the un it has been moun ted in position, connect up the power sup p ly w irirrg but don' t co nn ect the amplifier or loudspeaker leads unti l the unit has been tested . You w ill have to check your amp li fier 's circuit d iagram fo r a su itable DC supply rail and th is shou ld be verified using a m ult imeter before it is connected to the Lou dspeaker Protector. If the AC-powered vers ion is used, JULY 1991 65 FROM AMPLIFIER T"-J - AMPLIFIER TO LEFT .----------J ~ SOLLJtR _ / / ~ ~::: ' S ' DV 06 ~ ij~ 8~~ ~ ~ ~\I -L.=!!......J- ~ 0,.,.o e,:Fes• .... ~~$~ ~ ~~"'1\ ~ ~e--(ill]--e 1 !70u: POWER TRANSFORMER LEFT CHANNEL o~ ~ ~ ~ • VR1 ~ • • • RELAY ~ + - TO RIGHT SPEAKER + - FROM AMPLIFIER RIGHT CHANNEL Fig.3: the alternative AC-powered version omits the on-board voltage regulator components and substitutes a mains transformer & diodes D5 & D6. tery. Connect the battery across each input in turn, first with one polarity and then the other, and check that the relay immediately opens in all four cases. In each case, the relay should close again as soon as the battery is removed. If you strike trouble , switch off immediately and check the circuit for w iring errors. In particular, check the power supply voltage and ch eck that all parts are correctly oriented and in their correct locations. Assuming everything is OK, VR1 can now be adjusted to set the signal overdrive trip point. To calculate this trip point, you need to know the power rating (P) and impedance (R) of your be sure to in stall the mains wiring in et professional manner. Use mains-rated cable and solder the leads directly to the PC board. Do not use PC stakes for mains terminations. Noto also that a lead must be conn ected -from the ground pin on the board to th e main earth of the amplifi er. On no account shou ld you simply connect the ground pin to circuit earth. You could get a hum loop if you do. Testing To test the unit, switch on and check that the relay closes after about two seconds. Fault conditions at the input can now be simulated using a 9V bat- Fig.4: here is an actualsize artwork for the PC board. Note that the same pattern is used for both the DC-powered & ACpowered versions. Check your board against this pattern before mounting any of the parts. rI O 0 0 sco I 10591~ I~' 66 SILICON CHIP Liz· loudsp eakers . These values are then plugged into the formula P = v ~/R to derive th e trip point voltage. For example, let's say that yo ur loudsp eakers have an impedance of 8 ohms and are rated at 50W. If those values are substituted into th e above formula, we get a trip point voltage of V = 20V. Similarly, if your loudsp eakers are rated at 100W, the trip point voltage will be 28V. Th e adjustment-procedure i_s as follows: (1). Switch offal! equipment & connect the Lauds poaker Protector between the amplifier outputs and loudspeaker terminals using h eavy duty (32 x 0.2mm) hookup wire. Disconn ect th e loudspeake~ . (2). Connect your multimeter to the speaker terminals of the amplifier and set it to the 50V AC range. Connect an audio signal gen erato r to an input of the amplifier and sot VRl on the protector board to minimum (fully anticlockwise). (3). Switch on, set the signal generator to 50Hz and increase th e signal level (or volume contro l) until the multimeter shows the required trip voltage. Adjust VRl slowly until th e relay trips (opens). If you wish , the can check the hysteresis of the system by now reducing the signal level and noting the voltage at which the relay closes again. The difference will probably be about 3V although individual units can vary from this figure somewhat. That's it! Your expensive loudspeakers are now protected against signal overdrive and amp lifi er failure. SC