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Special Feature Project . . . Ultra-LD 2 x 100W stereo amplifier Finally, we have produced a rack-mounting version of our ground-breaking Ultra-LD amplifier. It’s taken many months but now it is here. It is rated at 100 watts RMS per channel, at vanishingly low levels of distortion – typically below .002%. It is ultra quiet too and it looks the part. Part 1: By GREG SWAIN & JOHN CLARKE 20  Silicon Chip www.siliconchip.com.au T HE NEW ULTRA-LD Stereo Amplifier incorporates a preampli­fier stage, LED bargraph power meters, fan-forced cooling, gold-plated heavy-duty speaker terminals and a host of internal engi­neering features that make it easy to build. The external finish and presentation Fig.1: the block diagram for the Ultra-LD Stereo Amplifier (one channel shown only). IC1 amplifies the selected input signal and drives a 100W power amplifier stage via volume control VR1. It also drives the LED bargraph circuitry (IC2 & IC3). www.siliconchip.com.au is to a professional standard. This is an ampli­fier that you will be proud to say, “I built this one myself!” If you could buy the equivalent of this amplifier from one of the big name brands you would have to pay lots more dollars and even then you wouldn’t get the extremely low dis- tortion and noise, very high damping factor and so on. Oh, and just a word about overall performance – these days there are lots of Dolby surround sound amplifiers and surround systems which are available at relatively low prices and some have quite high power outputs, up to 100W per channel from five channels. Are these comparable to the Ultra-LD? Let’s just put it nicely. For home theatre they are great value but most are not hifi. The Ultra-LD is a purist’s hifi amplifier. It’s an amplifier for audiophiles; it has no tone controls, no loudness control, no balance control and no switch­ing for multiple speaker systems, all of which can add to distor­ tion and noise. By the way, we’re not alone in adopting this purist con­cept. Take a look at some of the really expensive “audio­phile” amplifiers. Many don’t include tone controls or balance controls, or any other unnecessary features. Instead, the aim is to offer the best possible performance for your dollar and that’s what we’ve done with this unit. It also makes the amplifier delightfully easy to operate – you just switch it on, select the signal source and adjust the volume control to your liking. Design concept The original version of this amplifier was published in the March, May & August 2000 issues of SILICON CHIP. To save kit buyers a lot of money, we presented that version in standard PC tower case. This had the virtues of low cost, plenty of internal space for all the modules, different levels for the power supply and amplifier modules, and even inbuilt shielding for signal wires. All told, it was an effective although bulky package. So why are we now presenting this conventional rack-mounting version? There are several reasons but perhaps the main one is that many readers just did not like the amplifier-in-a-PC-case concept. They reckoned it was ugly, too bulky and pushed the recycling angle too far. Furthermore, the styling clashed with their existing hifi equipment. Still, we were not keen to revisit the Ultra-LD amplifier concept until Altronics recently indicated that they were seriously interested in producing a rack-mounting version. They weren’t so much interested in November 2001  21 the project as a kit but more as an addition to their existing professional equipment. And since it was to be a “professional amplifier” it would need to be substan­tially redesigned to meet new criteria. In essence, the new version of the amplifier would have to be easier to assemble. That meant that all soldered wire connec­ tions to the PC boards would have to go. Instead, all connections were to be made via crimped “quick connects” and board-mounted spade lugs. In addition, Altronics wanted LED bargraphs to match the styling of other amplifiers in their range and wanted us to adapt the design to a custom-made rack chassis which features an integral tunnel heatsink with fan cooling, slotted front panel and so on. All of that meant that we had to redesign the PC boards for the amplifier modules and the loudspeaker muting and protection module, as well as design a PC board for the power supply. In addition, we have incorporated a very low distortion preamplifier based on the Philips 5534 op amp and this incorporates the LED bargraph display circuitry. So there is a new board for the preamplifier, plus another board for the stereo RCA socket pairs on the back panel. The RCA input board and the pre­ amplifier board are connect­ed together using a flat 26-way ribbon cable fitted with header sockets at either end. This eliminates messy wiring to the source switch – you just plug the header sockets at each end of the cable into the matching pin headers on the PC boards and the job is done. All told then, this version of the Ultra-LD has had to be completely re-engineered to suit the rack case and it is now a lot easier to assemble. Even so, this is not a project for anyone new to electron­ics. Apart from having six PC boards to assemble, there is a lot of wiring to run in the chassis. We expect that the average construc­ tor, working carefully, will take around 50-60 hours to build and test it completely. The result will be an amplifier that you can be proud of and one which will deliver superb sound quality for many years to come. Operating features As already noted, the Ultra-LD is a very simple amplifier. On the front panel, it has just two knobs, one for 22  Silicon Chip the input selector and the other for the volume control. Apart from the On/Off rocker switch, the only other features are the LED bar­graphs for both channels and the headphone socket. Plugging in your headphones mutes the speakers via the relays on the muting and protection board. As well as being pretty, the LED bargraphs in this amplifi­er do serve a useful purpose. They display a signal range of 30dB so that as each extra LED lights, it indicates a signal increase of 3dB. When the orange 0dB LED lights, the amplifier is on the verge of clipping and so if the topmost red +3dB LED lights, you know the amplifier is clipping and the volume control should be reduced to give the best sound quality and also to protect your speakers against possible damage. On the rear panel, there are six pairs of RCA sockets, to cater for five stereo inputs (CD, DVD, tuner, etc) and a tape monitor output. The heavy-duty gold-plated loudspeaker terminals can accept the heaviest speaker cables available and while they may look a bit over the top, they are essential in keeping the distortion as low as possible. As in all high-power amplifiers, ventilation and cooling are important. This is achieved using a fan-cooled internal tunnel heatsink for the output transistors. Air flows in through slots in the base of the case and is blown out through slots in the sides – these must be kept clear. By the way, the fan only kicks in for heatsink temperatures above 60°C, so most of the time it will not be operating, keeping noise to an absolute minimum. After all, there’s not much point in having only the smallest whisper of residual noise from the speakers if the fan is noisy. Block diagram Let’s take a look now at the block diagram of the new stereo amplifier – see Fig.1. To keep things simple, this shows only one channel – the other channel is identical. S1 is a 5-position rotary switch and this selects the audio input signal – either CD, DVD, Tuner, Auxiliary or Tape. From there, the signal is fed to a non-inverting op amp stage (IC1) which operates with a gain of 3.6. Its output is fed to the Tape Out socket via a stopper resistor and is also fed to volume control VR1. The output from the volume control has two signal paths. First, the signal on the wiper is fed to a power amplifier which operates with a gain of 16. This then drives the loudspeaker via a loudspeaker protection/muting circuit. It is also fed directly to one side www.siliconchip.com.au of the headphones socket. Second, the signal from VR1 is also fed to IC2 which is a precision halfwave rectifier. It’s output is filtered and then fed to a display driver circuit based on IC3. IC3 in turn drives a 10LED bargraph display. This display operates over a 30dB range and is set www.siliconchip.com.au Above: the Preamplifier & LED Display module mounts vertically on the front panel while behind it are the two Power Amplifier modules, attached to a large tunnel heatsink. On the other side of the heatsink are the Power Supply module, the Loudspeaker Protector & Fan Controller module (mounted on the rear panel), and an RCA input-socket module (mounted upside down at top, left). up so that the top LED in the bargraph indicates signal clipping. OK, so much for the basics. We’ll now look at each of the main circuit modules and describe its operation in some detail. November 2001  23 How It Works: Preamp & LED Display Module This is the prototype Preamplifier & LED Display module. The LED displays operate over a 30dB range and are set up so that the topmost LED (red) indicates clipping. F IG.2 SHOWS the circuit for the Preamplifier & LED Display module. For the sake of simplicity, only one channel is shown – the other channel is identical. The preamplifier section uses a single 5534 low-noise op amp (IC1). As shown on Fig.2, the incoming audio source signals (CD, DVD, Tuner, Auxiliary & Tape) are selected by rotary switch S1 and fed to pin 3 of IC1 via a 150Ω “stopper” resistor and a 10µF bipolar capacitor. Note that the CD input is attenuated by feeding it through a voltage divider before it is fed to S1, so that it more or less matches the perceived levels from other sources. The 150Ω stopper resistor and the 10pF capacitor together form a lowpass filter to eliminate RF interference. Additional RF suppression is provided by a ferrite bead which is slipped over one of the leads of the 150Ω resistor. The 100kΩ resistor on pin 3 of IC1 sets the input impedance and also sets the input bias current for the op amp. IC1 is wired as a non-inverting amplifier and operates with a gain of 3.6, as set by the 4.7kΩ and 1.8kΩ feedback resistors (ie, Gain = 1 + 4.7/1.8 = 3.6). The 10pF compensation capacitor between pins 5 & 8 ensures stability, while the 390pF feedback capacitor rolls off the response above 100kHz. IC1’s output appears at pin 6 and is fed to volume control VR1 via a 10µF capacitor and 100Ω stopper resistor. In 24  Silicon Chip addition, the pin 6 output is fed to the TAPE OUT sockets via a 10µF bipolar capacitor and another stopper resistor (150Ω). The stopper resistors prevent instability by decoupling the output of IC1 from the capaci­tive effects of long cables. Following the volume control, the audio signal is fed di­rectly to the corresponding power amplifier. It’s also fed to pin 2 of IC2 (TL072) via a 220kΩ resistor and 0.22µF capacitor. LED display circuit IC2, D3 & D4 together form a precision half-wave rectifi­er. It works like this: when the input signal swings negative, pin 1 of IC2 goes high and forward biases D3. As a result, the op amp operates with a gain of -1.5 as set by the ratio of the 330kΩ feedback resistor to the 220kΩ input resistor. Conversely, when the input swings positive, pin 1 goes low. D4 is now forward biased and clamps pin 1 to 0.6V (ie, one diode drop) above ground. This effectively disables IC2. As a result, IC2 half-wave rectifies and inverts the nega­tive portion of the audio signal applied to its pin 2. This half-wave rectified signal is then filtered using a 680kΩ resistor and .01µF capacitor and applied to pin 5 of IC3. IC3 is an LM3915 display driver. As wired here, it operates here in bargraph mode (pin 9 tied high) and drives the 10 LEDs in 3dB steps. The 1.2kΩ resistor between pins 7 & 8 sets the display brightness, while the full-scale reading is set to 1.25V by connecting pin 8 to ground. This display circuit operates with a 30dB range and the gain of the precision rectifier is set so that the last LED (+3dB) lights at the point of clipping. Inevitably, this leads to some compromises in the display, since most of the action takes place over the second half of the volume control. At low-to-moderate listening levels, only the bottom two or three LEDs will flash on and off according to the signal peaks. Howev­er, that’s the way it has to be if you want the last LED to accurately indicate clipping. Alternatively, by increasing the gain of the precision rectifier (IC2), we could get more LEDs lighting up at “normal” listening levels. However, the last LED in the bargraph would then come on before the onset of clipping, so the display wouldn’t mean much – it would just look pretty. Power supply The preamplifier and precision rectifier circuits (IC1 & IC2) are powered from ±15V DC supply rails. As shown on Fig.2, half-wave rectifiers D1 & D2 are fed with 15V AC from the power transformer to derive unregulated supply rails of about ±20V DC. These rails are then filtered using 1000µF electrolytic capacitors and fed to 3-terminal regulators REG1 and REG2 to obtain the +15V DC and -15V DC rails respectively. Two 100µF 25VW capacitors are www.siliconchip.com.au Fig.2: the preamplifier and LED display circuit. S1 selects the signal source, while IC1 amplifies the selected signal and feeds volume control VR1. The signal then goes direct to the corresponding power amplifier stage. It also goes to precision half-wave rectifier IC2 which in turn drives an LM3915 LED display driver (IC3). used to filter the outputs of REG1 & REG2. In addition, two 10µF 35VW capacitors are connected between the +15V and -15V rails. These two capacitors provide additional supply www.siliconchip.com.au line filtering and are installed close to the supply pins of IC1 and IC2. IC3 is powered from a 12V rail derived from the Loudspeaker Protector & Fan Controller board (see Fig.5). Note that the LM3915 display circuitry is earthed via a 10Ω resistor, as is the Loudspeaker Protector circuit. This is a precaution to stop induced hum currents flowing in the earth path. November 2001  25 How It Works: Power Amplifier Module The power amplifier modules now feature heavy-duty quick connect terminals, as do all the other modules in the amplifier. Each amplifier module delivers up to 100W RMS with very low distortion. F IG.3 SHOWS the circuit for the two power amplifier mod­ules. It’s virtually identical to the “Ultra-LD 100W Amplifier” published in the March 2000 issue of SILICON CHIP. The input signal is coupled via a 2.2µF capacitor and 1kΩ resistor to the base of Q1 which together with Q2 makes up a differential pair. Q3 & Q4 act as a constant current tail to set the current through Q1 & Q2 and this makes the amplifier insensi­tive to variations in the power supply rails. The collector loads of Q1 & Q2 are provided by current mirror transistors Q5 & Q6. Commonly used in operational amplifi­er ICs, current mirrors provide increased gain and improved linearity in differential amplifier stages. In a conventional direct-coupled amplifier, the signal from the collector of Q1 would be fed directly to the base of the following class-A driver stage transistor (Q8). In our circuit though, the signal from Q1’s collector is fed to 26  Silicon Chip the base of Q7 which forms a cascode stage with Q8. Q9 provides a constant current load to Q8. Q4 does double-duty, providing the base voltage reference for constant current sources Q3 & Q9. A 3.3V zener diode, ZD1, provides the reference bias to the base of Q8. In effect, Q8 acts like an emitter follower and applies a constant voltage (+2.7V) to the collector of Q7 and this improves its linearity. The output signal from the cascode stage appears at Q8’s collector. Note the 100pF capacitor between Q8’s collector and Q7’s base. This rolls off the open-loop gain of the amplifier to ensure a good margin of stability. The output signal from the Q7-Q8 cascode stage is direct-coupled to the output stage. This comprises driver transistors Q11 & Q12 and the four output transistors, Q13-Q16. Actually, it may look as though Q9’s collector drives Q11 and that Q8 drives Q12 but in reality Q8 drives both; the signals to the bases of Q11 and Q12 are identical, apart from the DC voltage offset provided by Q10. Vbe multiplier Q10 is a “Vbe multiplier”. It can be thought of as a tem­ p eraturecompensated floating voltage source of about 1V. Q10 “multiplies” the voltage between its base and emitter, as set by trimpot VR1, by the ratio of the total resistance between its collector and emitter (330Ω + 390Ω + VR1) to the resistance between its base and emitter (390Ω + VR1). In a typical setting, if VR1 is 100Ω (note: VR1 is wired as a variable resistor), the voltage between collector and emitter will be: Vce = Vbe x 820/490 = (0.6 x 820)/490 = 1.004V In practice, VR1 is adjusted not to produce a particular voltage across Q10 but to set the quiescent current through the output stage transistors. Because Q10 is mounted on the same heatsink as the driver and output www.siliconchip.com.au Fig.3: the power amplifier circuit uses differential input pair Q1 & Q2 to drive cascode pair Q7 & Q8. This stage in turn feeds driver stages Q11 & Q12 which then drive the output stages (Q13 & Q14 and Q15 & Q16). transistors, its temperature is much the same as the output devices. This means that its base-emitter voltage drops as the temperature of the output devices rises and so it throttles back the quiescent current if the devices become very hot and vice versa. Driver & output stages Q11 & Q12 are the driver stages and they, like the output transistors, operate in class-AB mode (ie, class B with a small quiescent current). Note the 100Ω resistors connected in series with the bases of these transistors. These act as “stoppers” and they reduce any tendency for the output stages to oscillate supersonically. The output stages are connected www.siliconchip.com.au as compound current feed­back (CFB) transistors. This configuration acts like a very linear power transistor but with only one base-emitter junction rather than two as in a Darlington-connected power transistor. In this circuit, two paralleled power transistors, Q13 & Q14, are connected to NPN driver transistor Q11, while power transistors Q15 & Q16 are connected to PNP driver transistor Q12. The four paralleled 1.5Ω emitter resistors for each CFB transis­tor pair are there to help to stabilise the quiescent current. They also slightly improve the frequency response of the output stage by adding local current feedback. Note, however, that there is no in- trinsic means in the circuit for ensuring even current sharing between Q13 & Q14 and between Q15 & Q16. What current sharing there is will depend on the inherent matching (or lack of it) between the transistors. By the way, we did try the effect of small emitter resis­tors for each of the power transistors but these had the effect of worsening the distortion performance – so we left them out. Note that the current and power ratings of the output transistors are such that even if the current sharing is quite poor, it won’t cause problems. Feedback Negative feedback is applied from the output stage to the base of Q2 via an 18kΩ resistor. The amount of feedback – and therefore the gain – is set by the ratio of this 18kΩ resistor to the 1.2kΩ resistor at the base of Q2. This gives November 2001  27 a gain of 16 (ie, for a non-inverting amplifier, gain = 1 + 18kΩ/1.2kΩ). This means that an input signal of close to 1.8V RMS is required for full power. The low frequency rolloff of the amplifier is partly set by the ratio of the 1.2kΩ feedback resistor to the impedance of the associated 100µF capacitor. This has a -3dB point of about 1.3Hz. The 2.2µF input capacitor and 18kΩ base bias resistor feeding Q1 have a more important effect, with a -3dB point at about 4Hz. The two time-constants combined give an overall rolloff of -3dB at about 5Hz. At the high frequency end, the .0012µF capacitor and the 1kΩ resistor feeding the base of Q1 form a low pass filter which rolls off frequencies above 130kHz (-3dB). An output RLC filter comprising a 6.8µH choke, a 6.8Ω resistor and a 0.15µF capacitor couples the output signal of the amplifier to the loudspeaker (via the relay contacts in the Loud­speaker Protector). It isolates the amplifier from any large capacitive reactances in the load and thus ensures stability. It also helps attenuate EMI (electromagnetic interference) signals picked up by the loudspeaker leads How It Works: Power Supply Module and stops them being fed back to the early stages of the amplifier where they could cause RF breakthrough. The low-pass filter at the input is also there to prevent RF signal breakthrough. Finally, the output of the amplifier is attenuated using a 330Ω 1W resistor and fed to the headphone socket. The loudspeak­ ers are automatically switched off when the headphones are plugged in, by using the headphone switch to disconnect the drive to the relay driver transistors on the Loudspeaker Protector & Fan Controller module – see Fig.5. fully-regulated supply improves the separation between channels. Second, with a class-AB amplifier such as this, the very high asymmetrical signal currents flowing in each half of the output stage cause a distorted signal voltage to be superimposed on the supply rails. By using a fully regulated supply, we avoid the possibility of these signals being fed back into the input stages. Circuit details This easy-to-assemble module provides the ±52.5V and ±55V rails to the power amplifiers. The power transformer leads plug straight into the quick connect terminals on this board. T HE POWER SUPPLY is based on a 300VA toroidal transformer that’s been specially sourced for this amplifier by Altronics. This transformer has six windings: 2 x 35V; 2 x 50V; and 2 x 15V. Fig.4 shows the circuit of the power supply for the ampli­fier modules. There are two sets of supply rails: ±52.5V (nomi­nal) and fully-regulated ±55V. The unregulated ±52.5V rails 28  Silicon Chip feed the class-AB output stages and nothing else. The fully-regulated ±55V rails feed the input stages and the class-A driver stages of the amplifier. So why have we used these regulated supply rails when just about every commercial domestic stereo amplifier uses unregulated supply rails for the whole power amplifier circuit? The reasons are twofold. First, the The power supply circuit for the amplifier modules can be split into two parts. First, the two 35V windings are connected together and drive bridge rectifier BR1. This then feeds four 8000µF 63VW electrolytic capacitors to provide unregulated supply rails of around ±52.5V (at no signal) to power the output stages of the amplifier. The two 8.2kΩ 1W resistors are there to discharge the filter capacitors when the amplifier is switched off. The two 50V windings are also connected together (to give 100V AC centre-tapped) and these drive bridge rectifier BR2 and two 470µF capacitors to derive unregulated supplies of about ±71V. These rails are then fed to positive and negative 3-terminal regulators REG1 & REG2 to derive the ±55V rails. It’s not what it seems At first sight, this may appear like a conventional 3-terminal regulator plus booster transistor arrangement, with the power transistor being slaved to the regulator. But that’s not how this circuit works. www.siliconchip.com.au Fig.4: the power supply module produces ±52.5V (nominal) rails for the class-AB output stages of the power amplifiers, plus regulated ±55V rails for the input and class-A driver stages. Looking at the positive regulator for the moment, REG1 in fact carries all the current, which is only around 40mA, so there is no need for a booster transistor. However, the 3-terminal regulator cannot do the whole job. That’s because its input voltage is about 71V and when the power is first ap­plied, this would appear directly across the regulator, causing it to blow (its maximum input-output differential is only 40V). This is where power transistor Q1 comes into play. When the voltage www.siliconchip.com.au across REG1 exceeds 33V, zener diode ZD2 is biased on via the associated 47Ω resistor. This causes Q17 to turn on and this limits the voltage across REG1 to around 35V or so. The current through Q17 is limited to around 6.5A peak by the three paralleled 15Ω resistors in its emitter circuit. This peak current is very brief and occurs only while the 100µF ca­ pacitor at the output of REG1 is charged up to around 40V. From there on, the LM317 takes over and Q17 switches off. The same process occurs for the negative regulator REG2, with NPN transistor Q18 taking care of the charging current for the associated 100µF output capacitor. Low voltage windings As shown on Fig.4, the transformer also includes two 15V secondary windings. One of these windings feeds two half-wave rectifier circuits on the preamplifier module (see Fig.2), while the other feeds a full-wave bridge rectifier on the loudspeaker protector module (see Fig.5). The rectifier outputs in turn feed 3-terminal regulators to derive ±15V & +12V rails. November 2001  29 How It Works: Loudspeaker Protector & Fan Controller The Loudspeaker Protector module protects the loudspeakers in the event of a catastrophic amplifier failure. It also mutes the loudspeakers at switch-off & switch-on to prevent thumps and controls the heatsink fan. F IG.5 SHOWS the Loudspeaker Protec­tor & Fan Controller circuit. This has several functions: (1) it provides muting at switch-on and switch-off, to prevent thumps from the loudspeakers; (2) it protects the speakers against catastrophic failure in the amplifier; and (3) it provides temperature control for the fan-cooled heatsink, switching the fan on if the heatsink temperature rises above 60°C. The main reason for incorporating speaker protection into an amplifier is to prevent further damage in the case of a serious amplifier fault. In the Ultra-LD amplifier, the main supply rails are ±55V DC. This means that if one of the output transistors fails and there’s no loudspeaker protection, more than 50V DC would be applied to the speak­er’s voice coil. In a nominal 8Ω speaker, the voice coil has a DC resistance of around 6Ω and so the power dissipa­tion would be around 400W 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 around 8.5A may not blow a 5A fuse straight away), the voice coil would quickly become red-hot and could set fire to the speaker cone material. 30  Silicon Chip This risk applies to any audio power amplifier of more than about 40W per channel. So a loudspeaker protection circuit is a good idea. Circuit details As shown in Fig.5, 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 – Q1, Q2 & Q3 for the left channel and Q4, Q5 & Q6 for the right channel. We’ll just talk about the left channel here, since the circuit for the right channel is identi­cal. In operation, the active signal from the amplifier’s left channel is fed to a low-pass filter consisting of three 22kΩ resistors and two 47µF bipolar (BP or NP) electrolytic capaci­tors. This filter network removes any audio frequencies and just leaves DC to 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 Q1 and to the base of Q3. Q1 monitors the amplifi­er output for negative DC signals while Q3 monitors for positive DC signals. Q3 turns on if a DC signal of more than +0.6V is present. Similarly, Q1 turns on if a DC signal of more than -0.6V is present on its emitter. This, in turn, pulls Q2’s base low and so Q2 also turns on. Q2 & Q3 have a common 56kΩ collector load resistor (R1) which normally feeds base current to Q7. If the headphone socket switch is closed, this means that Q7 normally is on. And that means that LED1, Q8 and relays RLY1 & RLY2 are also on. However, if Q1 or Q3 is turned on by an amplifier fault condition, Q7’s base is pulled low and so Q7, Q8 and the relays all turn off, disconnecting the speakers. Diodes D5 & D6 protect Q8 by quenching any back-EMF spikes that are generated when the relays are switched off. Q4, Q5 and Q6 monitor the right channel of the amplifier and they switch Q7, Q8 and the relays in exactly the same manner. The relays selected for the job have contacts rated at 10A and there are two reasons for this. First, we want the contact resistance in the relays to be as low as possible so that it has negligible effect on the amplifier’s per­formance, in respect of distortion, damping factor and so on. Second, the relay contacts have to pass and break the heavy DC currents which would otherwise flow through the loudspeaker if a fault occurs in the amplifier. Note that we don’t merely use the relays to disconnect the ampliwww.siliconchip.com.au Fig.5: each channel of the amplifier is monitored for DC faults by three transistors – Q1, Q2 & Q3 for the left channel and Q4, Q5 & Q6 for the right channel. If a DC signal is detected, Q7’s base is pulled low and this turns off Q8 and the relays. Another function of this circuit is to turn off the loudspeakers when the headphones are plugged into the head­phone socket. When this happens, the normally closed contacts (pins 6 & 7) in the headphone socket are opened and this removes the drive to Q8. As a result, Q8 turns off and so the relays also turn off and disconnect the speakers. open and Q9’s base is pulled high by a 2.2kΩ resistor. This turns Q9 on to run the fan. The fan is fed via a 22Ω 5W resistor so that it does not run at full speed. This makes it quieter but it still pumps a fair amount of air through the tunnel heatsink. When the heatsink subsequently cools down to around 40°C, the thermal cutout closes again and the fan is switched off. Note that 40°C is relatively cool, so the fan will usually run for a long time after it comes on. At normal listening levels, the heatsink only rises a few degrees above ambient and so the fan should rarely (if ever) come on. And even if it does, it operates so quietly that you won’t be aware that it is running. Fan control Power supply Temperature sensing for the fan control is based on a 60°C thermal cutout which is bolted to the centre of the main heat­sink, between the two power amplifiers. This thermal cutout controls transistor Q9 which in turn switches the fan on and off. The thermal cutout has a set of normal­ly closed contacts and so Q9’s base is normally low. This means that both Q9 and the fan are normally off. However, if the heatsink temperature rises above 60°C, TH1’s contacts Power for the Loudspeaker Protection circuit is derived from a 15V AC winding on the power transformer. This feeds a bridge rectifier (D1-D4) and the resulting 20V DC rail is then filtered using a 1000µF capacitor and fed to 12V 3-terminal regulator REG1. Finally, the regulated +12V rail from REG1 is filtered using a 10µF capacitor. This rail powers the Loudspeaker Protector board, as well as the LM3915 display drivers and the LED bargraphs SC on the preamplifier board. fier’s output from the loud­speakers. 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. tection circuit, the relay opens within less than 0.5s and this prevents any turn-off thump from being heard. Muting delay The muting function is achieved using resistors R1 & R3 and capacitor C1 (220µF). When power is first applied, C1 is discharged and so no base current can flow to Q7 via R1. C1 then charges via resistor R3 (220kΩ) until, after about three seconds, enough voltage is present to allow base current to pass via R1 to Q7. This turns on Q7 which then turns on Q8 and the relay to connect the loudspeakers. If power is removed from the prowww.siliconchip.com.au Loudspeaker switching November 2001  31