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This bare board version of the Universal Loudspeaker Protector can be built into a stereo amplifier to protect the loudspeakers and prevent switch-on and switch-off thumps. A universal loudspeaker protector for stereo amplifiers By LEO SIMPSON & BOB FLYNN This simple circuit is designed to mate with any stereo amplifier, music system or car sound system and will protect the loudspeakers from damage in the case of an amplifier failure. It could also prevent a fire. It has a turn-on delay and will elim­inate switch-on thumps. Do you leave your stereo amplifier or home music system permanently switched on in standby mode? Do you realise they could be a fire hazard? If you haven’t thought about this problem in the past, then this article is for you. Most power amplifiers these days are direct-coupled to the loudspeakers. This means that there is no output 54  Silicon Chip coupling capaci­tor in series with each loudspeaker terminal. This is true wheth­er you have a large stereo amplifier which delivers several hundred watts per channel or a typical home music system which can be turned on and controlled by a remote control handpiece. This means that if an output transistor goes short circuit or in the case of smaller home music systems, a hybrid power amplifier fails, virtually the full supply rail to that part of the circuit will be applied to the loudspeaker. The result is usually a burnt out loudspeaker voice coil or damaged suspension system. That’s expensive to fix but it may not be the end of the matter. In a worse case, the large DC current in the voice coil does not burn it out immediately but allows it to get red hot so that it sets the speaker cone on fire. From there, the acetate filling material in the enclosure and the grille fabric also catch fire, generating huge quantities of choking black smoke. Ultimately, your house may catch fire too. This is not an imaginary scenario. Stereo systems do fail and they Fig.1: this is the self-contained version of the Univer­sal Loudspeaker Protector, intended to be powered from a 9V or 12V DC plugpack. Q1, Q2 and Q3 monitor the output of channel 1 of the amplifier while Q4, Q5 & Q6 monitor the second channel. If a high DC offset is detected, the base current to Q7 will be shunt­ed to deck and this will cause Q8 and the relay to turn off. can cause house fires. That is why they should not be left on for long periods of time, especially if no-one is present to turn them off in the case of a fault. Why does this sort of amplifier fault cause so much heat in the voice coil of a loudspeaker? Well, consider a 100W per chan­nel amplifier with ±50V supply rails and driving loudspeakers with a voice coil resistance of 6Ω, a typical value for a speaker with a nominal impedance of 8Ω. If one of the amplifier’s output transistors fails, it will apply almost the full DC supply rail of 50V to the loudspeaker. The resulting heat dissipated by the voice coil will be 50V2/6 = 416W! No wonder the voice coil gets hot and burns out! Actually, the power dissipation is generally not as high as that because the power supply voltage will drop under such a serious load. If you’re lucky, the amplifier’s fuses will also blow before a fire starts, limiting the damage to just the ampli­fier and the victim loudspeaker. Fire insurance Now the only safe way to prevent a major fault occurring while you’re not listening to your music system is to turn it off at the wall socket. But faults can still occur while you are listening to the system and if you’re not actually in the room at the time to turn it off when a major fault occurs, the results will be costly. So to prevent damage to your expensive speakers you need to build the Universal Loudspeaker Protector presented in this article. The Universal Loudspeaker Protec- Advantages Of This New Protector This is not the first loudspeaker protector circuit we have published. The last one was featured in the July 1991 issue of SILICON CHIP. This new circuit was produced as a result of devel­opment work we have been doing on a high-power bridge amplifier. The new circuit is built onto a substantially smaller PC board and copes with an amplifier fault condition that would be ignored by the previous circuit. By using separate monitoring circuits for each channel of the amplifier, the ULP can respond to a DC fault condition in one or both channels of a stereo amplifier. The previous Loudspeaker Protector (published July 1991) had only one monitoring circuit which summed the active lines from the loudspeakers. If the amplifier in question failed simultaneously in both channels, it is possible that one channel would produce a posi­tive DC fault and the other channel a negative DC fault. If a common sensing circuit was used, these two fault conditions would effectively cancel each other out and the Loudspeaker Protector would fail to operate. Is it possible for both channels of an amplifier to fail at once? And with oppo- site faults in both channels? Definitely! It is certainly possible although we admit that it is un­likely with conventional stereo amplifiers. However, where a stereo amplifier is driving a single loudspeaker in bridge mode, it is highly likely. In most bridged amplifiers, one channel gets its signal from the output of the second channel. So if the second channel fails and its output goes high, the first channel will have its output forced low. So the fault condition will exist in both channels and both channels must be sensed separately, as in the ULP. April 1997  55 Fig.2: this version of the Universal Loudspeaker Protector is identical with that shown in Fig.1 except that it derives its power from the amplifier’s DC supply via regulator transistor Q9. Fig.3: this version of the Universal Loudspeaker Protector is mainly intended for protecting speakers connected to bridged output amplifiers in cars. tor (ULP) will continually monitor the DC conditions at the outputs of your stereo amplifi­er. If a fault occurs, the ULP will operate a relay to disconnect the loudspeakers. As a bonus, the ULP has a delay at switch-on and if it is built into a stereo amplifier, it will prevent switch-on thumps from the loudspeakers. Three versions We are describing three versions of the ULP. One is self-contained and 56  Silicon Chip powered with a 9V or 12V DC plug– pack. The second is intended to be built into a stereo amplifier and has its own on-board regulator. The third version is intended for bridged ampli­ fiers in cars. We’ll talk about these two latter versions later in this article. Fig.1 shows the complete circuit diagram of the self-contained version. Let’s talk about how Q1, Q2 & Q3 monitor the active output terminal of an amplifier. The active signal is fed via a two-stage low pass filter network consisting of three 22kΩ resistors and two 47µF NP (non-polarised) electrolytic capaci­ tors. This filter network effectively removes any audio frequen­ cies and ensures that only DC signals are fed to the following transistors. This is necessary because we don’t want normal audio sign­als to trip the ULP in any way. Now let’s see how the three transistors operate together. The line from the low pass filter is connected to the emitter of transistor Q1 and the base of The self-contained version of the Universal Loudspeaker Protector is housed in a plastic case and powered from a 9V or 12V DC plugpack. Note the resistor in series with the DC power socket. This is only required if a 12V DC plugpack is used (see text). transistor Q3. In effect, Q1 moni­tors for negative DC signals while Q3 monitors for positive DC signals. If a positive DC signal of more than 0.6V is present, Q3 will turn on. Similarly, if a negative DC signal of more than 0.6V is present, the emitter of Q1 will be pulled below its base and so Q1 will turn on and turn on Q2. Both Q2 and Q3 have a common 56kΩ load resistor (R1) and this normally feeds base current to Q7. Q7 feeds base current to Q8 and so both of these transistors and the relay are on. However, when either Q1 or Q3 turn on, the base current for Q7 is shunted to deck and so Q7, Q8 and the relay are turned off, disconnecting the speakers. The same working principle applies to the monitoring of the second amplifier channel, with Q4 sensing negative DC signals and Q6 sensing positive DC signals. Q5 & Q6 share the same common 56kΩ load resistor as Q2 & Q3. So if either of these transistors are turned on by fault voltages, they will also rob Q7 of base current and cause Q8 and the relay to turn off. Arc protection When the relay operates to discon- nect the loudspeakers, the moving contacts are shorted to the loudspeaker ground lines via the “unused” contacts. This has been done because if a large DC voltage (say more than 30V) appears at the amplifier outputs, the resulting high current can cause an arc across the relay con­ tacts. Until that arc is extinguished, the loud- speaker is still being subjected to the high current and the possibility of dam­age. By shorting the moving contacts of the relay to the speaker ground lines, the arc current is diverted and the amplifier fuses will blow if the arc still persists. The fact that this Universal Loudspeaker Protector can be used with high power amplifiers which can produce very large output currents means that a heavy duty relay must be used. The one specified has DPDT (double The amplifier and loudspeaker connections are run to the selfcontained unit via a terminal block at one end of the case. April 1997  57 Fig.4: use this diagram when wiring the self-contained version of the ULP. The missing components at the lefthand side of the PC board are for other versions. Fig.5: this is the wiring diagram for the built-in version of the circuit, as shown in Fig.2. Note that the external resistor RY is only required if the amplifier’s DC voltage supply is above 40V. discharged and no base current can flow via 56kΩ resistor R1. C1 then charges via 220kΩ resistor R3 and eventually sufficient voltage is present to allow resistor R1 to turn on transistor Q7. This turns on Q8 and the relay and so the loudspeakers are con­nected to the amplifier. This delay is several seconds and it allows the voltages within the amplifier to stabilise, so when the speakers are connected, no thumps are heard. When power is removed from the ULP circuit, the relay dis­connects the speakers almost immediately, preventing turn-off thumps. Note that this “thump” protection is only available if the ULP is powered from the supply rails of the amplifier, as in Figs.2 & 5. If it is built as a self-contained unit and powered from a DC plugpack, the thump protection will not be provided. Construction pole, double throw; changeover) contacts rated at 10 amps. Power supply As noted above, we are presenting three versions of this circuit. The first version, intended as a self-contained unit to be used with any amplifier or music system, can be powered with a 9V or 12VDC plugpack. The second version, presented as a PC board to be built into a stereo amplifier, can derive its supply from the positive amplifier DC supply rail and this can range from +30 to +75V DC. Its circuit diagram is shown in Fig.2. In this case, the amplifier’s supply rail is fed to tran­sistor Q9 and associ58  Silicon Chip ated components and these operate to provide a regulated +12V supply for the relay and other transistors. The third version, intended for bridged amplifiers in cars, takes its supply directly from the 12V battery line. Its circuit is shown in Fig.3. Turn-on delay So far we have described the main function of the ULP which is to prevent loudspeaker burnouts. The minor function, mentioned above, is to prevent thumps from the loudspeakers when the ampli­fier is turned on. This is achieved with resistors R1 & R3 and capacitor C1. When power is first applied, C1 is Let’s now describe the construction of the self-contained version. All the parts are mounted on a PC board coded 01104971 and the wiring diagram is shown in Fig.4. As you can see, some parts are missing from one end of the board. These are for the on-board regulator (Q9, etc) which are used only in the in-amplifier version. Fit the PC pins first and then the resistors. The four 47µF electrolytic capacitors can go in either way around since they are the non-polarised (NP) type. The 100µF capacitor is polarised and must be inserted the correct way around. The eight transistors and the diode can be inserted next. Check that you insert the correct type in each position and make sure that each is oriented exactly as shown in the wiring dia­gram. Don’t forget to install the wire link, LK1. This has been provided to enable a thermal cutout to operate the circuit but this feature is not used here. Finally, the relay can be installed. We mounted ours by soldering short lengths of stout tinned copper wire to each relay pin. These wire leads are then pushed through the relay mounting holes on the board and then soldered. We understand that some kitset suppliers may provide a PC board with slotted holes so that the tinned copper wire may not be necessary. With the board complete, it’s time to install it in the plas­tic case. You may elect to use a different case from our proto­type; as long as everything fits, the case size and shape are unimportant. You will need to drill a hole at one end of the case to take the DC socket for the plugpack. At the other end you will need to mount a six-way insulated terminal block and drill holes for wires to run inside the case. Install the PARTS LIST Self-contained version 1 plastic case, 150 x 80 x 60mm 1 PC board, code 01104971, 107mm x 55mm 1 9V or 12VDC 150mA plugpack with 2.1mm DC plug 1 2.1mm DC socket 10 PC board pins 1 Relay DPDT 10A 240VAC, 12V coil <at> 75mA, Jaycar SY-4065 or similar 6 3mm x 20mm screws 6 3mm nuts 4 6mm spacers 4 adhesive rubber feet Semiconductors 5 BC547 NPN (Q1,Q3,Q4,Q6) 2 BC557 PNP transistors (Q2,Q5) 1 BC327 PNP transistor (Q8) 1 1N4004 silicon diode (D1) largest terminal block you can obtain which will fit. The larger ones have larger wiring holes which makes it easier to connect the speaker wires, Capacitors 1 470µF 16VW electrolytic 1 100µF 16VW electrolytic 4 47µF 50VW NP electrolytic Resistors (1%, 0,25W) 1 220kΩ 2 22kΩ 1W 2 56kΩ 1 2.2kΩ 4 22kΩ 1 39Ω 0.5W (RX) Extra parts for built-in version 1 BD649 NPN transistor (Q9) 1 13V 0.5W or 1W zener diode (ZD1) 1 100µF/100VW electrolytic capacitor 1 2.7kΩ 1W resistor 1 220Ω 5W wirewound (RY; see text) 1 U-shaped TO-220 heatsink (Altronics Cat H-0502 or equiv). par­ticularly if you are using heavygauge cables. Note that we have shown a resistor in series with the supply from the April 1997  59 Bridged Amplifiers In Car Audio Systems Fig.6: this is the wiring diagram for the bridged version of the ULP, as shown in Fig.3. M ANY HIGH-POWERED amplifiers in cars operate in bridged mode and they are often run at high power for extended periods. When they fail, the speakers are just as likely to be damaged as the speakers in a home stereo system. And the possibility of a fire is just as high. So to protect valuable loudspeakers in cars, the ULP is a wise investment. You will need one ULP for each stereo amplifier and one for each bridged amplifier. In each case, the ULP can be powered directly from the +12V battery line. The circuit for this bridged amplifier version is shown in Fig.3 while the wiring diagram is shown in Fig.6. plugpack. It is marked RX on Fig.4. If you use a 9V plugpack, this resistor should not be necessary. However, the unloaded voltage of a typical 12V DC plugpack can easily be +15V or even higher and that could cause an increase in power dissipa­tion in the relay. Therefore, the series resistor is necessary. We suggest that RX be a 39Ω 0.5W resistor. If the plugpack vol­tage is higher still, increase RX to 47Ω. Testing When all the wiring is complete, it is time for a power test. Do not connect any wires from your speakers or amplifier at this stage. Just connect the plugpack and apply power. The relay should close after a short delay of about two seconds. If that happens, you are almost home and hosed. Next, you can simulate a fault condition with a 6V or 9V battery (or even two 1.5V cells in series). Connect the battery across each of the inputs in turn, first with one polarity and then the other. In each case, the relay should immediately open and then close as soon as the battery is removed. 60  Silicon Chip Fig.7: here is the full size etching pattern for the PC board. If you strike trouble, switch off and check the circuit for errors. Normally, you can expect the unit to work as soon as you switch it on so now it should be merely a matter of wiring the unit in series with your loudspeakers and then you can rest easy. Fig.5 shows the wiring of the builtin version. This is the same as for Fig.4 except that the regulator components involving transistor Q9 are included. Note that Q9 is mounted on a U-shaped heatsink. In addition, if the amplifier’s DC supply is above 40V, it will be necessary to connect an external 5W wire­ wound resistor (RY) in series with the collector of Q9. This resistor is shown on Fig.5 and a table of values is shown on Fig.2. For example, if the amplifier’s DC supply is around 60V, resistor RY SC should be 220Ω 5W.