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New stereo module delivers up to 160 watts into 4-ohm loads By LEO SIMPSON & BOB FLYNN The last time we published a power module using bipolar transistors was back in December 198 7. That single channel design used 2N2955/2N3055 TO-3 metal-pack transistors to deliver either 50 watts into an or 100 watts into 4Q, depending on whether two or four output transistors were used. Later, in February 1988, we upgraded the module by substituting the more rugged (and more expensive) MJ15003/4 TO-3 power transistors. Both designs have been very popular and are still available but recently we have seen the need for a more compact, multi-purpose amplifier module which would drive 4Q or 8Q loads without having to change the design. In fact, the real stimulus for the design was that we wanted to produce a new integrated stereo amplifier which would fit into a midisized chassis; ie, about 340mm wide. We had a target of 50 watts per 34 SILICON CHIP Say goodbye to tin lid transistors and hello to f antas tic plastic. This new stereo power module uses four big plastic Darlington transistors in each channel, making a rugged and compact design incorporating full protection. channel for the new design and initially we intended to base it on one of the Japanese-made stereo modules. These are used in very large numbers in today's lower cost stereo amplifiers and particularly in the all-in-one rack systems. In fact, we went ahead with a design based on such a module , capable of delivering 50 watts per channel. But after building several prototypes we had to give the game away. There were just too many compromises in the design. Those modules looked great ,on paper but, in practice, they have drawbacks which cannot be cured, since the core circuit is contained in moulded black plastic. So it was back to the drawing board. OK then, having spent a great deal of development time which had so far come to nought, what was to be the next approach? We did not want to use TO-3 power transistors (hence our into 4Q loads, has low distortion and very low residual noise. The total power output will depend to a great extent on the regulation of the power supply. We will have more to say about this later. Protection reference to tin lid transistors at the start of this article). Sure, TO-3 power transistors give plenty of power for their size (particularly the MJ15003/ 4s) but after 30 years or more, they are a bit old hat and are becoming more expensive as time goes by. By contrast, plastic encapsulated power transistors are becoming more rugged and cheaper. And they are much easier to design into PC boards and have single screw mounting. So the transistors we have selected for the new design are the TIP142 (NPN) and TIP147 (PNP) plastic Darlingtons. Made by Philips, Motorola and a number of other companies, they are housed in the so-called plastic TO-3 encapsulation (ie, TO3P) although Philips list it as the SOT93 pack and Motorola as the TO-218. Either way, these new Darlingtons (first listed by Philips in July 1988) are quite rugged, with the following ratings: power dissipation 125 watts (TMB = 25°C); collector-emitter voltage 100 volts (V CEO - open base); collector current 10 amps DC, 15 amps peak; DC current gain (hFEl >1000<at> 5 amps; and a maximum junction temperature of 150°C. In fact, they compare very favourably with the old favourite 2N3055s which have a power dissipation of 115 watts, 15A maximum collector current and VCER of 70 volts. They also compare quite well in the critical area of "second breakdown". Performance The performance of this new stereo power module is very respectable and certainly better than the vast majority of the low to midrange amplifiers. In particular, it delivers lots of power This aspect is most important in any medium to high power amplifier design and we have followed the same approach as we have used in our designs of December 1987 and February 1988 - fuses in the positive and negative supply rails and a Polyswitch PTC (positive temperature coefficient) thermistor in series with the output. While some readers may regard PTC thermistors as a needless option they will bless them if ever they are called into operation. They are much cheaper than having to replace the drivers in your loudspeakers and they are excellent insurance against loudspeaker fires which can happen in some cases of amplifier failure. The trouble with today's power amplifiers is that they use big power supplies which can deliver a great deal of current. If just one of the transistors in the amplifier fails, the result can be that the circuit applies the full positive or negative supply rail to the loudspeaker. Typically, an 8Q loudspeaker will have a voice coil resistance of about 5.5Q. When that cops the full positive supply of this amplifier, it will have to dissipate around 150 watts or more. Now maybe the fuses will blow and save it but the most likely result is that the voice coil will be burnt out. That's not all. In some cases, the red hot voice coil sets the loudspeaker on fire which then generates huge quantities of acrid smoke from the acetate filling material in the cabinet. There have been documented cases of this happening - and big insurance payouts for smoke and fire damage to homes. So the PTC thermistor in this amplifier is highly desirable. Note that you cannot rely on the fuses to give protection to the loudspeaker. They are selected to protect the amplifier and its supply more than for output protection. And in the case of the fault condition outlined in the preceding paragraphs, they may not What Is A Darlington Transistor? The TIP142/147 plastic power transistors specified in this new amplifier design are referred to as Darlingtons, after S. Darlington, who first proposed the Darlington transistor pair in the early 1960s. Effectively, they are a compound transistor pair, with the emitter of the first transistor connected to the base of the second. Hence, the current gain of the pair is approximately equal to the product of the two transistor gains. The internal circuit configuration of the TIP142 is shown in Fig.1. It includes a reverse diode from collector to emitter of the second transistor. This diode is very handy for reverse voltage protection in amplifier and switching circuits. These days, these integrated transistors are shown on circuit diagrams by the conventional transistor symbol, with nothing to distinguish them from ordinary discrete transistors. NPN TIP140 TIP141 TIP142 COLLECTOR r--_ - -_ -- - 7I _......., 1 I I BASE I I I I I L ___ - - - - __ _j EMITTER Fig.1: the equivalent circuit of the TIP142 NPN Darlington which has inbuilt base-emitter shunt resistors and a reverse protection diode. The collector is connected to the metal tab of the plastic package. However, while Darlington transistors have the advantage of high current gain and space saving on printed circuit boards, they are at a disadvantage in switching circuits where their saturation voltage (V CEsat) is usually not as low as can be achieved with discrete transistor switching circuits. FEBRUARY1992 35 +38.5V 01 f1 0.22! 5A 02 0.22! 010 03 l BC556 C .,. .033 _j L1 6.8uH D.4m 3W 0.1 1 B o--1 INPUT OUTPUT 22k i 2.2pF 0.15! ,. o.4m 3W 1k 1.,. 2xTIP147 + 47 16VW+ 8 68pf 011 4.7k 1 2.2V 4.7k C 05 04 8 2xBF469 37.2V E F2 SA 0.22J 2.2V 1000 i 1.6V -38.SV 0.221 A PLASTIC SIDE YELLOW 8 + 240VAC +38.SV 4700 N + sovw YELLOW ELJc VIEWED FROM BELOW ~ ECB BF-,BD139 ~ BCE TIP- GND . 4700 + sovw -38.SV TWIN SOW POWER AMPLIFIER Fig.2: the voltages shown on the circuit diagram are nominal values & are what can be expected if you have a 240V mains supply. If your mains supply is higher, you can expect most of the voltages to be higher in proportion. Note that the DC voltage on the output should be within ±30mV. blow soon enough, if at all. Note that this loudspeaker (and fire) hazard is common to all modern amplifier designs , commercial and do-ityourself, not just the design under discussion here. PTC protection thermistors were first used in loudspeakers from the UK about seven or eight years ago. To our knowledge, we were the first to incorporate them in the output of an amplifier design although they are also used in the high voltage rails of amplifiers such as the NAD which have very high music power output. Normally, PTC thermistors have a 36 SILICON CHIP very low resistance, a mere 0. H1 or less in the case of the ones specified for this amplifier. They stay that way while ever the current through them is below their cutoff rating (around 5 amps in this case). If the current rises above this value, the PTC thermistor's resistance suddenly rises to a high value, around several hundred ohms, effectively disconnecting the amplifier from the load and thus protecting it and the loudspeaker. Thus, the PTC thermistor is very effective in protecting the loudspeaker against overdrive from the amplifier or worse, a catastrophic failure in the amplifier. When the fault condition is removed, the PTC thermistor's resistance gradually returns to normal although this may take several minutes or more to fully recover. The PTC thermistor will also protect the amplifier against short circuit loads1although in this respect the supply fuses give backup protection. The circuit Now let's have a look at the circuit of Fig.2. This is very similar to the design featured in December 1987 except that we have substituted the TIP142/147 Darlingtons for the 2N2955/2N3055 output transistors and their MJE340/350 driver transistors. The input signal is coupled via a lµF metallised polyester capacitor and 1.8kQ resistor to the base of Ql which together with Q2 makes up a differential pair. Q3 is a "constant current tail" which sets the current though Ql and Q2 and thus renders the amplifier largely insensitive to variations in its supply rails (known as power supply rejection). Diodes D1 and D2 provide a voltage reference of about 1.2V for Q3 so that it applies a constant voltage to its 680Q emitter resistor. This sets the current through Q3 to close to lmA. This means that Ql and Q2 each operate with a collector current of about 0.5 milliamps. Signals from the collectors of Ql and Q2 drive another differential pair, Q4 and Q5, which have a "current mirror" as their load. The main advantage of the current mirror, D3 and Q6, is that it makes the second differential pair highly linear and therefore low in distortion. The output of Q5 drives the classAB output stage consisting of Darlingtons Q8-Q11. By class-AB we mean an amplifier which is essentially classB (ie, each half of the output stage conducts for only half the signal) but which has a small current bias to minimise cross-over distortion. PARTS LIST 1 PC board, code SC01102921, 80 x 233mm 1 60 x 60 x 290mm 3mm-thick angle aluminium 19 PC stakes 8 SOT-93 (T0-218) transistor mounting kits 2 SOT-32 (T0-126) transistor mounting kits 8 20mm fuse clips 4 5A M205 20mm fuses 2 Philips 4322-021-30330 or Neosid 60-601-72 coil formers 2 RDE245A polyswitches 2.5 metres 0.8mm enamelled copper wire 1 power transformer 2 x 25V, 160VA (from Harbuch, Altronics or Torrtech) 2 500Q horizontal mount trimpots 2 560Q 5W resistoris (for setting quiescent current) Semiconductors 4 TIP142 NPN Darlington transistors (08 ,010) 4 TIP147 PNP Darlington transistors (09,011) 4 BF469 NPN transistors (04,05) 2 BF470 PNP transistors (06) 2 BO139 NPN transistors (07) 6 BC556 PNP transistors (01,02,03) 6 1N4148 signal diodes (01 ,02,03) 1 P04 6A bridge rectifier Capacitors 2 4700µF 50VW electrolytics 2 47µF 16VW electrolytics 2 1µ,F 63VW 5mm pitch metallised polyester 8 0.22µF 63VW 5mm pitch metallised polyester 2 0.15µF 10% 100VDC 10mm pitch metallised polycarbonate (Philips 2222 344 21154) 2 0.1µF 63VW 5mm pitch metallised polyester 2 .033µF 100VW 5mm pitch metallised polyester 2 820pF 50V ceramic 2 68pF 100V ceramic 2 2.2pF 50V ceramic Resistors (0.25W, 5%) 6 22kQ 4 680Q 215kQ 0.5W 4180Q 44.7kQ 101000 24.7k01W 2 6.8Q 1W 2 1.8k0 8 0.47Q 3W 10% 21kQ Vbe multiplier The current bias in the output stage is controlled by transistor Q7 and trimpot VR1 . Q7 is a Vbe multiplier, so called because the voltage between its base and emitter is multiplied by the ratio of the resistors between base and collector and base and emitter, respectively. VR1 adjusts this voltage to give a voltage between the collector and emitter of Q7 of about 2 volts. In practice, it is adjusted to give an output stage current of 40 milliamps. Q7 is a BD139, a transistor normally used in amplifier driver stages and video circuits. It has a dissipation rating of 8 watts and so is only doing light duty. However, it is specified here because it gives better Vbe tracking with the output stage transistors and therefore better stability for the quiescent current. The Darlington transistors Q8 and Q10 and Q9 and Ql 1 are connected as parallel pairs to share the output current. Each Darlington has a 0.47Q emitter resistor which helps ensure equal current sharing. As well, the emitter resistors improve the output stage bandwidth and the stability of the quiescent current. The value is a compromise though; bigger emitter resistors would give better stability and current sharing but would reduce the maximum output power capability. The 100Q base resistors for the Darlingtons serve a number of functions. First, they reduce any tendency for the output stage to oscillate; always a possibility with emitter follower stages. Second, they limit the base current in the event of a short circuit in the output and thereby reduce the possibility of damage to the Darlingtons. Negative feedback is applied from the output stage back to the base ofQ2 via a 22kQ resistor. This resistor, and the lkQ resistor also connected to the base of Q2, sets the voltage gain to 23. The low frequency rolloff of the voltage gain is set by the 47µF capacitor in series with the lkQ resistor. This sets the -3dB point at about 3Hz. However, the lµF input capacitor is the main factor in the low frequency response of the amplifier and sets a -3dB point at 7Hz. The overall effect of the two time constants is a -3dB point at 10Hz. The 820pF capacitor and the 1.8kQ input resistor feeding Ql form a low pass filter which rolls off frequencies above l00kHz. This filter is a little more savage than we have used in previous designs but we have done this to give a greater margin of safety in the output in case the preamplifier stages have any tendency to high frequency instability. The 68pF capacitor between base and collector of Q5 and the 2.2pF capacitor between base and collector ofQ2 are used to roll off the open loop gain to ensure stability with feedback applied. We have also used our standard RLC network in the output stage. A configuration originally proposed by Australian engineer Neville Thiele, it uses a 6.8µH air-cored choke, a 6.8Q resistor and a 0.15µF capacitor. FEBRUARY1992 37 The beauty of this network is that it effectively isolates the amplifier output stage from any nasty impedance dips which may occur at high frequencies and which could cause the amplifier to be unstable. It also has another favourable effect because it kills any RF signal pickup by long speaker leads. Power supply The power supply for the amplifier is shown in Fig.2. This uses a 160VA transformer with a centre-tapped 50V winding feeding a 6-amp bridge rectifier and two 4700µF 50VW electrolytic capacitors. PC board design The PC board for this stereo amplifier has been designed so that it can be built as two separate modules. The stereo pair can be built with an onboard power supply which will also feed a preamp stage. Alternatively, if you want to use a bigger bank of filter capacitors, a higher rated bridge rectifier and the capacitors would be mounted off the board. Do not substitute a transformer with a higher secondary voltage. If you do so, you run the risk of blowing the Darlington transistors, particularly if you are driving 4Q loudspeakers. The specified heatsink is a 3mm-thick aluminium angle extrusion, 60 x 60 x 290mm long. This heatsink is adequate where the amplifier is intended for normal program material. If you envisage using it with a bigger capacitor bank and more onerous signal conditions such as a guitar amplifier, then a bigger heatsink or thermal cutouts would be desirable. For the remainder of this article though, we will assume that the reader is building a stereo module on the specified heatsink. Note that the parts list specifies all the components for a stereo amplifier and makes reference to transistors such as Ql, Q2 etc. Transistors Q1-Q11 are shown on the circuit diagram (Fig. 2) and these are duplicated in the second channel. The same goes for the diodes. Assembling the board Fig.3: the parts layout for a complete stereo amplifier power module with on-board power supply. The 6.8µH output inductors (Ll) are each wound on a Philips 4322021-30330 or Neosid 60-601-72 coil former using 24.5 turns of 0.8mm enamelled copper wire. Fig.2 shows the pinout details for the transistors. 38 SILICON CHIP We suggest that you mount the PC pins, resistors , diodes and wire links first , followed by the capacitors. There are only two electrolytics on the board, apart from those in the power supply. Make sure they are mounted with correct polarity. Most of the remaining capacitors are moulded metallised polyester capacitors which have a standard lead spacing (pitch) of 5mm. We recommend against greencaps as they won't fit. The fuses specified are M205 20mm-long types as widely used in commercial amplifiers. The main reason we have specified them is that they take up less board space than the larger 32mm 3AG fuses and cost no more. All the TO-5 transistors (Q1-Q3 , etc) are mounted with the flat side facing towards the front; ie, away from the heatsink. Similarly, the TO-126 transistors (Q4-Q6, etc) mount with the metal side facing to the front. The exception is Q7 which naturally mounts with its metal face to the heatsink (with a mica washer, of course). The 0.47Q 3-watt resistors are made by Philips and again have been specified to save board space, being a lot more compact than the common 5W cement "bathtub" types. Mount them so that they clear the board by about 3-4mm. The 6.8µH output inductors are each wound on a Philips 4322-02130330 or Neosid 60-601-72 coil former using 24.5 turns of 0.8mm enamelled copper wire. Clean and tin the ends of the inductors before installing them on the board. Output transistors There are several ways of mounting the output transistors but the way we did it is as follows. First, all the Darlingtons and the two BD139s were mounted on the aluminium heatsink. In each case, they were mounted using the specified mounting kit consisting of a mica ~asher and plastic insulating bush for the screw. Heatsink compound is applied sparingly to both sides of the mica washer before it is set between the transistor and heatsink. The details are shown in the diagram of Fig.4. With all the transistors mounted on the heatsink, set your multimeter to a low ohms range and check that the transistor collectors are isolated (ie, infinite resistance) from the heatsink. That done, set the heatsink upside down on your workbench and fit the PC board over the transistor leads. You may need to adjust some of the transistors so that their leads line up with the board holes. Tack soider a couple of Darlington transistor leads at each end so that the top board surface is about 8mm from the bottom edge of the heatsink. You will also need to slightly crank the leads of the BD139s (Q7) to line them up with their respective PC board holes. When you are satisfied with the lining up of the board, soldE;Jr Performance of Prototype Output power .. .. .......... .. ..................... 55W into 8 ohms, 80 watts into 4 ohms (one channel driven) Frequency response ......................... 15Hz - 35kHz ±1 dB Input sensitivity .................................. 900mV (for clip point into 8 ohms) Harmonic distortion ........................... typically less .05% from 20Hz to 20kHz) Signal to noise ratio ........................... 105d8 unweighted; 11 ?dB Aweighted Separation between channels ........... 84d8 or greater (1 00Hz - 10kHz) Protection .......................................... 5A fuses plus RDE245A Polyswitch Damping factor .................................. <50 (for 8 ohm loads) Stability ............................ ................. unconditional all the transistor leads to the board pattern. Power up Before applying power, check all your work very carefully against the wiring diagram of Fig.3. This done, remove the four SA fuses and solder a CAPACITOR CODES Value IEC Code EIA Code 1µF 105 0.15µF 0.1µF 1u0 220n 150n 100n 820pF 68pF 2.2pF 820p 68p 2p2 0.22µF 224 154 104 821 68 2.2 560Q 5 watt resistor across each of the on-board fuseholders. These are current limiting resistors which reduce the likelihood of any damage to the output transistors in case you have done something silly like swapped a TIP142 for TIP147. But of course you have already checked to see that nothing like that has happened, haven't you? Now connect the positive and negative supply leads to one channel of the amplifier. Set trimpot VRl fully anticlockwise - this gives the minimum setting for quiescent current through the output transistors. Set your multimeter to the 200VDC range (or no lower than S0VDC if an analog meter). Do not connect a loudspeaker or output load at this stage. Now apply power and measure the positive and negative supply rails. RESISTOR COLOUR CODES D D D D D D D D D D D No. Value 4-Band Code (5%) 5-Band Code (1%) 6 2 4 2 2 2 4 4 10 2 22kQ 15kQ 4.?kQ 4.?kQ 1.8kQ 1kQ 680Q 180Q 100Q 6.8Q red red orange gold brown green orange gold yellow violet red gold yellow violet red gold brown grey red gold brown black red gold blue grey brown gold brown grey brown gold brown black brown gold blue grey gold gold red red black red brown brown green black red brown yellow viole! black brown brown yellow violet black brown brown brown grey black brown brown brown black black brown brown blue grey black black brown brown grey black black brown brown black black black brown blue grey black silver brown FEBRUARY1992 39 The stereo power amplifier module will form part of a complete stereo amplifier to be described in a future issue of SILICON CHIP. This photo shows the unit in company with its companion tone control board at bottom left & the input preamplifier board at right. temporarily short out the PTC thermistors. Troubleshooting They should be within a few volts of ±38.5 volts. Now measure the other voltages on the circuit. They should all be within ±10% of the nominal values, depending also on whether your 240VAC mains supply is high or low (it is above 240VAC more often then not). The voltage at the output should be within ±30mV of 0V. Now switch your multimeter back to the 200VDC range and connect it acl'oss one of the 5600 5W resistors. Adjust trimpot VR1 for a reading of 22.4 volts. This gives a total quiescent current of 40 milliamps. After 5 minutes or so, check the quiescent current again and readjust VR1 if necessary to get the correct voltage across the 5600 resistor. (If you are doing power tests on the amplifier and the heatsink becomes very hot, you can expect the quiescent current to at least double. When it cools down though, the quiescent current should drop back to around 40mA). Measure the voltage across each 0.470 3W emitter resistor. They should all be about 9- lOmV, which means that each Darlington transistor is getting its rightful share of the quiescent current. Now switch off and connect the positive and negative supply rails to the other channel. Measure the voltages as before and adjust VRi in 40 SILICON CHIP that channel for the correct quiescent current. If all is well, switch off, remove the 5600 5W resistors and fit the 5A fuses. The amplifier module is now ready for work. Nate that if you intend running continuous power tests on the module, the PTC thermistor will operate before you can get full power into a 40 load. They will let full power be delivered on music signals but not for continuous sinewave signals. To do such full power tests, you will have to INSULATING MICA WASHER -~~JI 'SCREW r mnmfs --.._ HEATSINK 1 T0220 DEVICE Fig.4: transistors Q7-Qll are each isolated from the L-shaped heatsink using a mica washer & insulating bush. Smear the transistor tabs & mica washers with heatsink compound before bolting the assemblies together & use your DMM to check each transistor as it is mounted to ensure correct isolation. What happens if one of the amplifiers is not working? If the other channel is working correctly then you have an ideal cross-check. Check the voltages in the good channel and then in the bad channel and you can usually get a fair idea of what the problem is. It is unlikely that you will get the same fault in both channels, unless you have made the same assembly mistake in both! And now we'll give you a few clues which may help you solve any problems. First of all, let's assume that most of the amplifier voltages are correct but you have zero quiescent current. Look for a short across VR1 or Q7. If you have lots of current through the 5600 resistors and cannot control it with VR1, look for an open circuit in the 6800 base resistor to Q7 or a defect in that transistor. What if the output of the amplifier is fully latched up at either +38V or -38V? The most likely cause of this is a defect in the first or second differential pair of transistors, or something silly like the wrong transistor, say a BF469 where a BF470 should be. Solder bridges between tracks can also cause this fault. The above are the more common problems with build-it-yourself amplifiers. Most times though, you can expect the modules to work perfectly at switch on. SC