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20W Class-A Amplifier; Pt.3 Universal Speaker Protection & Muting Module Designed for use in our new Class-A Stereo Amplifier, this Speaker Protection & Muting Module is really a universal unit. It can be used with other SILICON CHIP amplifier modules and commercial stereo amplifiers and protects the loudspeakers in the event of a catastrophic amplifier failure. A second function of the unit is to mute the loudspeakers at switch-on & switchoff to prevent thumps. By Greg Swain & Peter Smith siliconchip.com.au July 2007  69 Fig.1: each channel of the amplifier is monitored for DC faults by three transistors – Q5, Q6 & Q7 for the right channel and Q8, Q9 & Q10 for the left channel. If a DC signal is detected, Q3’s base is pulled low, turning it off along with Q4 and the relay. Q1 & Q2 provide the switch-off muting feature. If the “AC Sense” input voltage ceases, Q1 turns off, and Q2 turns on which again pulls Q3’s base low and turns off Q4 and the relay. A LT H O U G H DESIGNED specifically for our new Class-A Stereo Amplifier, this unit can actually be used with any audio amplifier with supply rails up to about 70V DC simply by selecting two resistor values to suit. Basically, the unit provides the following features: (1) it protects the loudspeakers against catastrophic failure in the amplifier – eg, if an output transistor goes short 70  Silicon Chip circuit; (2) it provides muting at switch-on and switch-off, to prevent thumps from the loudspeakers; and (3) it provides an input for an overtemperature switch to disconnect the loudspeakers if the output stage heatsink rises above a certain temperature. Note, however, that this last feature is not used in the Class-A Stereo Amp­ lifier. That’s because the heatsinks run hot all the time (about 30°C above ambient) and disconnecting the loudspeakers does nothing to cool them since the output stage in each amplifier module draws a constant 1.12A – equivalent to a power dissipation of just under 50W. By contrast, disconnecting the loudspeaker from a class-B amplifier immediately reduces the current through the output stage to the quiescent current setting – typically around 50mA (assuming that there’s siliconchip.com.au This view shows how the unit is mounted in the rear lefthand corner of the Class-A Stereo Amplifier. no fault in the amplifier). So for a class-B amplifier, it makes sense to use over-temperature sensing. If the heatsink to which the output transistors are attached gets too hot, disconnecting the loudspeakers immediately reduces the dissipation to just a few watts, which allows the heatsink to cool. Note that the loudspeakers are connected (and disconnected) using a heavy-duty double-pole relay. We’ll have more to say about that later. Protecting the loudspeakers By far the biggest reason for incorporating speaker protection into an amplifier is to prevent further damage in the case of a serious amplifier fault. In the Studio 350 Audio Amplifier, for example, the main supply rails are ±70V DC. This means that if one of the output transistors fails and there’s no loudspeaker protection, more than 70V DC would be applied to the speak­er’s voice coil. In a nominal 8W speaker, the voice coil has a DC resistance of around 6W and so the power dissipa­tion would be around 800W until the supply fuse blew. In the meantime, this amount of applied DC power is likely to push the voice coil out of its gap, damaging the voice coil and suspension in the process. And if the on-board supply fuse didn’t blow fairly quickly (a strong possibility since a current of siliconchip.com.au around 11.7A may not blow a 7A fuse straight away), the voice coil would quickly become red-hot and could set fire to the speaker cone material. This risk applies to any audio power amplifier of more than about 40W per channel. So a loudspeaker protection circuit is a good idea. The risk of setting fire to the loudspeaker is nowhere near as great with the Class-A Stereo Amplifier because the supply rails are just ±22V. In this case, a shorted output transistor would result in a dissipation of about 80W in the speaker’s voice coil. It might not be enough to cause a fire but it’s certainly high enough to damage the loudspeaker – ie, by burning out the voice coil. Muting the thumps Muting switch-on and switch-off thumps is another important function of this unit. Switch-on thumps are eliminated by using a simple circuit to delay the relay from turning on when power is first applied. That way, the amplifier modules are able to power up and settle down before the relay switches on (after about five seconds) to connect the speakers. By contrast, switch-off thumps are eliminated by using an “AC Sense” input to monitor the secondary AC voltage from the transformer (up to 50V AC max.). When this AC voltage disappears (ie, at switch-off), the circuit switches the relay off in less than 100ms. This is much faster than simply relying on the collapsing DC supply rail to turn to the relay off. In practice, this could take half a second or more as the main filter capacitors discharge – more than long enough for a any switch-off thumps to be audible. Circuit details Refer now to Fig.1 for the circuit details. As shown, each channel of the amplifier is con­nected to the NC & NO (normally closed & normally open) con­tacts of a relay. The relay wipers and NC contacts then each respectively connect to the positive and negative loudspeaker terminals. Each channel of the amplifier is monitored for DC faults by a triplet of transistors – Q5, Q6 & Q7 for the right channel and Q8, Q9 & Q10 for the left channel. We’ll describe the operation of the right channel only, as the circuit for the left channel is identi­cal. As shown, the active signal from the amplifier’s right channel is fed to a low-pass filter consisting of three 22kW resistors and two 47µF 50V bipolar (BP or NP) electrolytic capaci­tors. This network removes any audio frequencies and just leaves DC (if present under fault conditions) to July 2007  71 B C B R1 Q4 BC556 E C E ZD1 100k D4 63V 12V/1W 100  B C Q8 BC546 E C C E B 1N4004 D3 D1 1N4004 B BC546 1N4148 100  E 470nF + B 100k Q10 BC546 22k 33k C 1747 07 02 F 10 Q9 BC556 E B Q2 C BC546 C B E Q1 22k 50V NP 47 F 50V NP 47 F 7002 Q3 BC546 10  R2 5W/10W OPERATION LINK FOR 22-24V D5 1N4004 33k CON3 Fig.2: install the parts on the PC board as shown here, taking care to ensure that all polarised parts are correctly oriented. Be sure also to use the correct transistor type at each location. Below is the completed PC board. Q6 BC556 10k E 10k C 22k E C D2 1N4004 22k BC546 50V NP 47 F 50V NP 47 F Q5 BC546 C 2.7k 22k 1W +TUOKPSL LEFT SPEAKER OUT+ TO LEFT SPEAKER B B 1N 4148 LEFT SPEAKER IN+ E Q7 22k 1W 24VDC 10A +NIKPSL FROM LEFT AMP SPEAKER OUTPUTS -NIKPSL LEFT SPEAKER IN/OUT– RLY1 -NIKPSR RIGHT SPEAKER IN/OUT– +NIKPSR RIGHT SPEAKER IN+ FROM RIGHT AMP SPEAKER OUTPUTS +TUOKPSR RIGHT SPEAKER OUT+ TO RIGHT SPEAKER CON2 OVER-TEMP AC SENSE SWITCH INPUT INPUT (50V MAX.) CON1 + – 22V DC POWER INPUT be monitored by the three transistors. This is done because we don’t want audio signals to trip the protection cir­cuit. The low-pass filter output is connected to the emitter of Q5 and to the base of Q7. Q5 monitors the amplifi­ e r output for negative DC signals while Q7 monitors for positive DC signals. In operation, transistor Q7 turns on if a DC signal of more than +0.6V is present on its base. Similarly, Q5 turns on if a DC signal of more than -0.6V is present on its emitter. This in turn pulls transistor Q6’s base low and so Q6 also turns on. Normally, in the absence of any amplifier faults, transistors Q5-Q7 are all off and Q3 is biased on via the 100kW resistor connected between its base and the positive supply rail Table 1: Resistor Colour Codes o o o o o o o o o No. 2 2 4 2 2 1 2 1 72  Silicon Chip Value 100kW 33kW 22kW 22kW 1W, 5% 10kW 2.7kW 100W 10W 4-Band Code (1%) brown black yellow brown orange orange orange brown red red orange brown red red orange gold brown black orange brown red violet red brown brown black brown brown brown black black brown 5-Band Code (1%) brown black black orange brown orange orange black red brown red red black red brown not applicable brown black black red brown red violet black brown brown brown black black black brown brown black black gold brown siliconchip.com.au it reaches 13.2V. This now forward biases Q3 which then turns on Q4 and the relay to connect the loudspeakers. This is more than sufficient time for the amplifier modules to settle down and achieve stable operating conditions. Why 13.2V on Q3’s base? Well, that’s the sum of the voltages across ZD1, diode D4 and Q3’s base-emitter junction when the transistor is on. Switch-off muting This prototype board (also shown in the other photos) used an MJE350 transistor for Q4 but this has now been changed to a BC556. (ignore Q1 & Q2 for the time being). As a result, Q3 pulls Q4’s base down (via resistor R1) to just over 12.6V, as set by diode D4 and zener diode ZD1, and so Q4 and relay RLY1 are also on. Now let’s consider what happens if an amplifier fault condition results in DC being present at its output. In this case, either Q6 or Q7 turns on and pulls Q3’s base low via a 10W resistor. And when that happens, Q3, Q4 and the relay all immediately turn off, disconnecting the speakers. Diode D5 protects Q4 by quenching any back-EMF spikes that are generated when the relay is switched off. Transistors Q8, Q9 and Q10 monitor the left channel of the amplifier and they switch Q3, Q4 and the relay in exactly the same manner. Relay specifications The relay selected for the job is a 24V DPDT type with contacts rated at 10A. There are two reasons for this high contact rating. First, we want the contact resistance in the relay to be as low as possible so that it has negligible effect on the amplifier’s per­formance, as regards to distortion, damping factor and so on. Second, the relay contacts have to pass and break the heavy DC current which would otherwise flow through the loudspeaker if a fault occurs in the amplifier. However, we don’t merely use the relay to disconnect the siliconchip.com.au amplifier’s output from the speakers. If we simply did this, it’s possible that the contacts would just arc across and so the heavy DC current would continue to flow through the loudspeaker. That might seem unlikely but when you have a heavy DC cur­rent and a high DC voltage pushing it along, it can be quite hard to break the circuit. This problem is solved by shorting the moving relay contacts to the loudspeaker ground lines (via the otherwise unused NC contacts) when the relays turn off. This diverts the arc current to chassis and ensures that the fuses blow on the amplifier. By the way, the relay specified in the parts list (ie, the Altronics S-4313) has an in-built green LED that lights when the relay turns on. It’s a nice feature that lets you quickly check the status of the relay during testing but is not really necessary. Muting delay at switch-on Muting at switch-on is achieved using a delay circuit. This consists of the 100kW resistor and the 47µF capacitor connected to Q3’s base, along with diode D4 and zener diode ZD1. When power is first applied, the 47µF capacitor is discharged and Q3’s base is held low. As a result Q3, Q4 and the relay all remain off. The 47mF capacitor then charges via the 100kW resistor until, after about five seconds, Transistors Q1 & Q2, together with diodes D2 & D3, provide the switch-off muting function. D2 & D3 rectify the AC voltage that’s fed to the “AC Sense” input (at connector CON2) from a transformer secondary winding (up to 50V AC max.). Provided this AC input voltage is present, the rectified output forward biases Q1 and so keeps it turned on. This in turn holds Q2’s base low and so Q2 is off and Q3 functions normally. The 100kW resistor and the 470nF capacitor form a time constant that’s long enough to ensure that Q2 remains off when Q1 very briefly turns off during the AC zero crossing points. However, if the AC signal ceases (ie, at switch off), Q1 immediately turns off and Q2 turns on and quickly discharges (within a millisecond or so) the 47mF timing capacitor via a 100W resistor. As a result, Q3, Q4 and the relay all turn off and the loudspeakers are disconnected, effectively eliminating any switch-off thumps. Over-temperature input Connector CON3 is the temperature sensor input. It relies on the use of a normally-open (NO) thermal switch that’s normally bolted to the heatsink used for the amplifier’s output transistors. Basically, this input is wired in parallel with transistors Q6 & Q7 (and Q9 & Q10) and it controls transistor Q3 is exactly the same manner. When the temperature reaches a preset level (set by the switch itself), the contacts inside the thermal switch close and pull Q3’s base low via the associated 10W resistor. As a result, Q3 turns off and this switches off Q4 and the relay. When the heatsink subsequently cools down, the thermal cutout opens again and Q3, Q4 and the relay turn on again to reconnect the loudspeakers. As previously stated, the overtemperature sense feature is not used July 2007  73 Parts List 1 PC board, code 01207071, 112.5mm x 80mm 1 10A 24V DPDT PC-mount relay (Altronics S-4313) 3 2-way 5mm or 5.08mm pitch terminal blocks (CON1-CON3) 4 M3 x 10mm tapped spacers 4 M3 x 6mm pan head screws 6 M4 x 10mm pan head screws 6 M4 flat washers 6 M4 shakeproof washers 6 M4 nuts 6 6.3mm double-ended 45° or 90° chassis-mount spade lugs (Altronics H-2261, Jaycar PT-4905). 0.7mm diameter tinned copper wire for link Semiconductors 7 BC546 NPN transistors (Q1-Q3, Q5, Q7,Q8 & Q10) 3 BC556 PNP transistors (Q4, Q6 & Q9) 4 1N4004 diodes (D1-D3, D5) 1 1N4148 diode (D4) 1 12V 1W zener diode (ZD1) Capacitors 1 47mF 63V PC electrolytic 4 47mF 50V non-polarised (bipolar) electrolytic (Altronics R-6580, Jaycar RY-6820) 1 470nF 50V metallised polyester (MKT) Resistors (0.25W, 1%) 2 100kΩ 1 10kΩ 0.5W 2 33kΩ 1 2.7kΩ 4 22kΩ 2 100Ω 1 10kΩ 1 10Ω 2 22kΩ 1W 5% Attaching The Spade Connectors It’s important that the double-ended spade lugs are fitted correctly to the PC board. Fig.3 (right) shows how they are mount­ed. Each lug is secured using an M4 x 10mm screw, a flat washer (which goes against the PC board pad), an M4 star lockwasher and an M4 nut. The trick to installing them is to first do the nut up finger-tight, then rotate the assembly so that it is at a rightangle to the PC board. A screwdriver is then used to hold the M4 screw and the spade lug stationary while the nut is tightened from below using an M4 socket and ratchet. Do the nut up nice and tight to ensure directly from the AC terminals on the bridge rectifier (more on this in a future issue). Note that the values shown for R1 & R2 on Figs.1 & 2 assume a 2224V supply rail. If the available DC supply rail is higher than this, then resistors R1 and R2 must be changed accordingly to ensure a base current of about 3-5mA for Q4 (as set by R1) and to ensure that no more than about 24V DC is applied to the relay (R2). In the latter case, it’s just a matter of selecting R2 so that the relay current is about 37mA (assuming that the relay has a coil resistance of about 650W). The table included with Fig.1 shows resistor values to suit the supply rails used to power the SC480, Ultra-LD, Plastic Power and Studio 350 power amplifier modules. Construction with the Class-A Stereo Amplifier because the heatsinks run hot all the time and disconnecting the loudspeakers does nothing to cool them. Power supply Power for the Loudspeaker Protection circuit is derived from a suitable DC rail within the amplifier. This can range anywhere from about 20V DC up to 70V DC. In the case of the Class-A Stereo Amplifier, we use the +22V and 0V rails from the power supply board. The “AC Sense” signal is picked up 74  Silicon Chip The parts for the Speaker Protection & Muting Module are all mounted on a PC board coded 01207071. Fig.2 shows the assembly details. Mount the resistors and diodes first, taking care to ensure that the diodes are all oriented correctly. Table 1 shows the resistor colour codes but you should also check each resistor using a digital multimeter before installing it, just to be sure. Install a 2.7kW 0.25W resistor for R1 and a link for R2 if you are building the unit for the Stereo Class-A Amplifier. Alternatively, select these resistors from the table shown in Fig.1 if you intend using a supply rail greater a good connection and to ensure that the assembly does not rotate. Don’t be too heavy-handed though, otherwise you could crack the PC board. The exact same mounting method should also be used for the spade lug terminals attached to the power amplifier modules and to the power supply board described last month. than 24V. If the supply rail is between the values shown in the table, then simply scale the resistor values accordingly and use the nearest preferred value. The six double-ended spade lugs for the speaker input and output terminals are next on the list. These are attached using M4 x 10mm screws, flat washers, star washers and nuts – see Fig.3. Note that, ideally, the double-ended spade lugs supplied should be 90° types. However, if you are supplied with 45° types, just bend the lugs to 90° before installing them on the board. The transistors, the electrolytic capacitor and the bipolar capacitors can now be installed, taking care to ensure that the correct tran­ sistor type is fitted to each location. The four 47µF bipolar capacitors can go in either way around but watch the orientation of the polarised 47µF 63V electrolytic capacitor. Finally, you can complete the board assembly by fitting the three 2-way terminal blocks and the DPDT relay. Testing If you have a suitable DC supply, you can test the unit prior to installing it. To do this, first connect the supply to screw terminal block CON1 and install a wire link between one of the CON2 “AC Sense” input terminals and the positive supply rail (this is done to ensure transistor Q1 turns on). Do not connect anything to the temperature switch input or to the speaker terminals at this stage. siliconchip.com.au This rear view shows the Loudspeaker Protection Module installed in the 20W Class-A Stereo Amplifier (ie, at bottom right). The full wiring details will be published in a following issue. Next, apply power and check that the relay turns on after about five seconds. If it does, temporarily short the temperature switch input – the relay should immediately switch off. Similarly, the relay should immediately switch off if you disconnect the link to the “AC Sense” input. The next step is to check that the Temperature Sensors Temperature sensors are variously called “thermostat switches”, “thermal cutouts” and “thermal circuit breakers” and are available in a range of trip temperatures from 50-100°C. Note that the temperature sensor used with this unit must be a normally open (NO) type. Both NO and NC (normally-closed) temperature sensors are available from Altronics and Jaycar Electronics. siliconchip.com.au relay switches off if a DC voltage is applied to the loudspeaker terminals (this simulates an amplifier fault condition). To do this, apply power, wait until the relay switches on, then connect a 3V (2 x 1.5V cells in series) or 9V battery (either way around) between the ground terminal of CON1 and the LSPKIN+ terminal. The relay should immediately switch off. Repeat this test for the RSPKIN+ terminal, then reverse the battery polarity and perform the above two tests again. The relay should switch off each time the battery is connected. Note that we don’t connect to the LSPKIN- or RSPKIN- terminal for this test because these two inputs are fully floating at this stage. That changes when the Speaker Protector module is installed in a chassis and the loudspeaker leads connected, because the negative loudspeaker terminals on the amplifier are connected to chassis (via the power supply). Troubleshooting If the relay doesn’t activate when power is first applied, switch off immediately and check for wiring errors – eg, incorrect supply polarity, a transistor in the wrong location, etc. If this doesn’t locate the fault, switch on and check the supply voltage, then check the voltages around the transistors. Q3’s emitter should be at about 12.6V and its collector at 12.8V, while both Q3 and Q4 should have base-emitter voltages of 0.6V. Similarly, Q1 should have a baseemitter voltage of 0.6V (provided the link between the “AC Sense” Input and the positive supply terminal is in place) but the other transistors (Q2 & Q5-Q10) should all be off – ie, they should have base-emitter voltages of 0.2V or less. If Q3’s base voltage is low (around 0.2V), then it could mean that Q2 is on and Q1 is off, possibly due to no voltage being applied to Q1’s base. Alternatively, one of the transistors in the speaker input monitoring circuits (ie, Q5-Q10) could be faulty (short circuit). You can quickly isolate which circuit section is at fault by disconnecting the 10W and 100W resistors to Q3’s base. Just remember that all transistors that are turned on will have a baseemitter voltage of about 0.6V. This should enable you to quickly locate where the trouble lies. That’s all for now. Next month, we’ll describe the low-noise Preamplifier & Remote Volume Control Module. SC July 2007  75