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Hugged design has full output protection This high-power amplifier module gives very low distortion and noise. It also features foolproof protection against short-circuits and loudspeaker damage. By LEO SIMPSON & BOB FLYNN Build this 100W amplifier module Over the years there have been many power amplifier designs and most have have had one or more drawbacks in terms of expensive components, unreliability and a tendency to damage expensive speakers when they give up the ghost. We've had a close look at these past designs and have come up with the definitive solution for those who need a rugged reliable design. It gives high quality sound without breaking the bank. The four power transistors mount on a small right-angle 10 SILICON CHIP bracket which has been used in a number of the designs we have just referred to. On that basis this new module can be regarded as a high quality drop-in replacement. The right-angle bracket can be mounted on a vertical heatsink of your choice so it can be used as the basis for a high quality guitar amplifier, in public address applications or, naturally, in a high fidelity stereo amplifier. All of the transistors and other parts are readily available from just about any electronic corn- ponents supplier, with the exception of the special protection component which we'll get to in a moment. The amp module can built in two versions. The larger version, which we think most people will build as a matter of course, will deliver just over 100 watts continuous into 4-ohm loads. It uses the full complement of four output transistors. The smaller module which uses just two output transistors, will deliver 50 watts into 8-ohm loads. It can also drive 4-ohm loads but in +40V D1 1N4148 t 01+ 39V 02 1N4148 8 06 BC640 l .012 22k VR1 500r! 1k 04 c 8 t ROE245A (4P. LOAD) RDE115A (B!l LOAD) C O 1OOW AMPLIFIER MODULE ELJc BC557 8 E BC639. BC640 011-1287 0 ~ 0 8 VIEWED FROM BELOW ECB Fig.1: foolproof loudpeaker protection against component failure and over-drive is the big feature of this circuit. This is provided by the PTC thermistor in the output network of the amplifier. this case it must be used with lower supply rails if long term reliability is to be obtained. Performance of Prototype Performance A particular feature of this amplifier is its low distortion. For the 100W version, the harmonic distortion is less than 0.1 % for the entire frequency range from 20Hz to 20kHz at all power levels up to maximum. And if you can get' access to a distortion analyser, the harmonic distortion can be reduced considerably below this level by optimising the routing of the supply leads. Signal to noise ratio is better than - 100dB with respect to full power. Frequency response is flat within ± ldB from 20Hz to l00kHz. The remaining performance details are shown in the spec. panel. Protection The trouble with all high-power amplifier designs is that, if they have a transistor failure, there is a big chance that they will burn out the loudspeaker system too, despite having fuses in the power supply. Output power Frequency response (at 1 W) Input sensitivity Harmonic distortion (20Hz-20kHz) Signal to noise ratio Protection SOW version 1 OOW version 50W into 8 ohms 1 00W into 4 ohms 20Hz-1 00kHz ± 1 dB 870mV 20Hz-50kHz ± 1 dB 870mV .05% 100dB 2A fuses plus RDE115 Polyswitch 0.1% 100dB 3A fuses plus RDE245A Polyswitch Damping factor (without Polyswitches) 50 (with Polyswitches) 30 Unconditional Stability There have been documented cases in the past where such catastrophic failures have led to serious fires. What can happen is that the voice coil gets red hot because of the high fault current from the amplifier. If not detected in time, the red hot voice coil can set the 100 100 Unconditional speaker cone on fire. After that, you can have a raging fire on your hands, with enormous volumes of smoke being generated by the filling material in the cabinet. In view of this risk, many designers incorporate relay protection circuits which disconnect the DECEMBER1987 11 1000,-----r-~--"T"'""--"T"'""---,-----r----,---...,....------ 8oot---t-----+---+---+---+-----+----1---_j__ __J 10 40 70 50 80 9o VOLTAGE (VOLTS) Fig.2: this diagram shows the load lines "seen" by the driver transistors Q8 and Q9, when the output drives a 40 resistive or reactive load and the output transistors have a beta of 20. This gives a straight load line of 800 and a curved line of (56.6 + j56.6)0. Note that the curved line exceeds the dissipation ratings of BD139/140 but not the more rugged MJE340/350. loudspeaker .in the event of a large DC voltage appearing across the output. These work OK but they add up to more circuit complication and expense. Relays are not the answer when the amplifier is shorted out though. In this case most designers rely on fuse protection and hope that the output transistors will be rugged enough to withstand the heavy currents until the fuses blow. Sometimes they do, sometimes they don't. If the output transistors do blow, there is a strong chance that they will take out the driver transistors too. Nor are relays the answer if the amplifier is seriously over-driven. Turn up the volume control too far and you may drive the amplifier well into clipping. The amplifier then delivers a square wave signal to the loudspeaker which can be three or four times the maximum power it is supposed to deliver under normal conditions. This can burn out the voice coil of a tweeter or dislodge one of the turns of a voice coil on a larger speaker. Either way, this momentary event can cause expensive damage to speakers. Up to now, there has been no really effective protection against amplifier overdrive, whether deliberate or inadvertent. The protection solution Fuses and relays are not the answer. Nor are transistor protection circuits which switch off the drive in the event of an overload condition. The latter can cause quite serious audible distortion and have now gone out of vogue with amplifier designers who know what they are a bout. ;t8it++ OV 6800 + "' 50VW _ .___ _ _ _ _.,___ _ _ -40V Fig.3: this is the suggested power supply for the amplifier. Note that the ultimate power output will depend on the transformer regulation. 12 SILICON CHIP The answer is the Polyswitch, made by the US company, Raychem Corporation. This is a positive temperature coefficient thermistor with a very low resistance value, under normal operating conditions. When the current through a Polyswitch goes high it immediately switches to a high resistance state and stays in that state until the fault condition is removed. It's like a fuse which can repair itself. The resistance of the Polyswitch is so low (typically much less than 0.10) that it has a negligible effect on amplifier performance. The distortion figures we quote above are applicable whether or not the Polyswitch is used. As far as we know, this is the first time that PTC thermistors have been incorporated into an amplifier design to give comprehensive protection. It works extremely well. It allows you to drive the amplifier to full power on program signals but the moment a short circuit is applied or the amplifier is seriously over-driven the Polyswitch goes high in resistance to give protection. If a transistor fails, and causes the amplifier to deliver a large DC voltage to the speaker, again the Polyswitch goes high to give protection. After the Polyswitch has switched to its high state, it takes some time to fully revert to its low resistance condition. This depends on how much current is passing through it. If the drive level is maintained after a fault has occurred, the Polyswitch will stay high in resistance. Polyswitches are more expensive than fuses but less expensive then relay protection circuits. We think that some readers will regard the Polyswitch protection as an optional feature. That's OK; put a wire link in instead. But for complete peace of mind, put in the Polyswitches. They are very cheap insurance. The circuit Now let's have a look at the circuit of Fig.1. This is a straightforward design which is based on applications literature produced by Hitachi some years ago. Originally it was intended for use with power INPUT +40V Fig.4: four output transistors are required for the 100W version of the module. For the 50W version, leave out Rl and R2, Q12 and Q13 and change THl and the fuses, as specified in the parts list. Mosfets but these are too expensive and dissipate too much power for this application. We have adapted the circuit for use with bipolar transistors. It has proved to be very reliable. Thirteen transistors and three diodes make up the semiconductor count. The input signal is coupled via a lµF capacitor and 2.2k0 resistor to the base of Q2 which together with Q3 makes up a differential pair. Ql is a "constant current tail" which sets the current through Q2 and Q3 and renders the amplifier insensitive to variations in its supply rails (this is known as supply rejection). Signals from the collectors of Q2 and Q3 drive another differential pair, Q4 and Q5, which have a "current mirror" as their load. The current mirror, Q6 and D3, does not give this second stage a particularly high gain but it does make it very linear (ie, relatively distortion free). The output of Q5 is then used to drive the class-AB output stage consisting of drivers QB and Q9 and power transistors QlO, Ql 1, Q12 and Q13. PARTS LIST 1 OOW VERSION 1 printed circuit board, code SC11-1287, 121 x 133mm 1 heatsink bracket (Jaycar Cat. No EE-3630) 1 large single sided heatsink (Jaycar Cat. No HH-8572 or bigger) 4 3AG fuse clips 2 3A 3AG fuses 6 PC pins 1 plastic coil former, 13mm diameter x 1 0mm long; or 1, 6.8µH air-cored choke (Jaycar Cat. No EE-4030) 1 Raychem ROE 245A Polyswitch PTC thermistor 4 T0-3 transistor mounting kits 3 T0-126 transistor mounting kits Semiconductors 2 BC557 PNP silicon transistors 1 BC557, 2N2907 PNP silicon transistor 1 BC640 PNP silicon transistor 2 BC639 NPN silicon transistors 1 B01 39 NPN silicon transistor 1 MJE340 NPN silicon transistor 1 MJE350 PNP silicon transistor 2 2N3055 NPN silicon transistors 2 MJ2955 PNP silicon transistors 3 1N914, 1 N4148 silicon diodes Capacitors 1 4 7 µF 1 6VW PC electrolytic 1 1µF metallised polyester (greencap or minature) 1 0.15µF metallised polyester (greencap or miniature) 5 0 . 1µF metallised polyester (greencap or miniature) 1 .012 metallised polyester 1 330pF ceramic or miniature metallised polyester 1 68pF 1 OOVW ceramic 1 2.2pF 1 OOV ceramic Resistors (0.25W, 5%) 1 X 4 7k!l, 2 x 22k!l, 1 X 18k0, 1 x 6.8k0 0 .5W, 2 X 4 . 7k0, 1 X 2.2k0, 1 X 1 kO, 1 X 6800, 1 X 4700, 5 X 1000, 1X6.801W, 4 x 0.220 5W wirewound, 1 X 5000 trimpot (Bourns Cermet horizontal mount, 0 .2 x 0 .4-inch) SOW VERSION Delete: 1 2N3055 NPN power transistor 1 MJ2955 PNP power transistor 2 T0-3 transistor mounting kits 1 RDE245 PTC thermistor 2 0 .220 5W wirewound resistors 2 3A 3AG fuses Add: 1 RDE115 Polyswitch PTC thermistor 2 2A 3AG fuses DECEMBER 1987 13 Q7 is a Vbe multiplier, so called because it multiplies the voltage between its base and emitter by the ratio of the resistors between its base and collector and base and emitter, respectively. It effectively maintains a fixed voltage between its collector and emitter, regardless of the drive current delivered to the output stage by Q5. The voltage is adjusted by trimpot VR1. The function of Q7 is to set the DC voltage applied between the bases of QB and Q9. By doing this it sets the "quiescent current" through the output stage (ie, the current when no signal is present). This minimises crossover distortion. The complementary output transistors are connected in parallel to give high current output capability. Each output transistor has its own 0.220 emitter resistor. These are included to ensure that the output current is shared more or less -SCREWS ! _r --O-~ ---r-1--PCB -INSULATING SLEEVES 0 HEATSINK I I SHAKE-PROOF •-----~-WASHERS ~- ~---NUTS Fig.5: mounting details of the T0-3 transistors. Trim the mica washers so that they do not overlap. equally between the output transistors and to help stabilise the quiescent current. Negative feedback is applied from the output stage back to the base of Q3 via a 22k0 resistor. The level of feedback, and therefore the voltage gain, is set by the ratio of the 22k0 resistor to 1k0. The low frequency rolloff is set by the ratio of the impedance of the 1k0 resistor to the impedance of the 47 µF capacitor. This sets the - 3dB point at about 3Hz. This is not the only determinant of low frequency response though. The 1µF input capacitor and the 22k0 base bias resistor feeding Q2 have a more important effect and set a - 3dB point at about 7Hz. The two time-constants together give an overall - 3dB point at lOHz. The 330pF capacitor in conjunction with the 2.2k0 resistor feeding Q2 form a low pass filter which rolls off frequencies above 200kHz. The 68pF capacitor between base and collector of Q5 and the 2.2pF capacitor between base and collector of Q3 rolls off the openloop gain to ensure its inherent SC11-1287 The PC artwork must not be altered otherwise the high performance of the amplifier cannot be guaranteed. 14 SILICON CHIP ratings will be considerably exceeded. There is therefore a high risk of amplifier failure when driven hard into typical 4-ohm loudspeaker loads. For this reason we have specified MJE340/350 transistors as drivers. They are probably the most rugged driver transistors available. Polyswitches Two devices are specified, depending on whether you want the 100W or 50W version. For the 100W version use the RDE245A Polyswitch. For the 50W version, use the RDEl 15 Polyswitch. Both these devices will be available from Jaycar Electronics stores. Power supply The suggested circuit is shown in Fig.3 and is a centre-tapped transformer driving a bridge rectifier and two 6800µ,F capacitors. The specified transformer has a 56V centre-tapped winding rated at 2 amps. This can be obtained from Dick Smith Electronics (Cat No M-0144) or Jaycar Electronics (Cat No MF-1095). Putting it together The 50W version of the amplifier module uses only two output transistors. Note that two wirewound resistors have been omitted and the PTC thermistor is smaller than in the 100W version. stability with feedback applied. Another contributor to the amplifier's excellent stability is the output network consisting of a 6.8µ.H air-cored choke, a 6.80 resistor and 0.15µ,F capacitor. Second breakdown protection A feature of this amplifier which is not evident from the circuit diagram is the careful selection of driver transistors to prevent second-breakdown. The 2N3055s and MJ2955 transistors used as output devices are inherently rugged (and cheap) but a number of amplifier modules published in Australian electronics magazines over the last ten years or so have specified BD139/140s as driver transistors. These are plainly not suitable for an amplifier intended to deliver 100 watts into 4-ohm loads. Assuming that the output transistors have the minimum beta (current gain) of 20 and with the amplifier driving a reactive load (ie, a typical 40 loudspeaker) of (2.83 + j2.83)0 the driver transistors will "see" a complex load impedance of (56.6 + 56.60. With this load, as depicted in Fig.2, the second breakdown Assembling the board should be done as follows. First mount all the small components leaving the power transistors and heatsink till last. Note that miniature polyester capacitors can be used instead of the larger greencaps if you wish since we have made provision for both types. The 68pF compensation capacitor associated with Q5 should have rating of at least 100 volts and so should the 0.15µ,F capacitor in the output filter network. The 6.8µ,H choke is wound with 24.5 turns of 0.8mm enamelled copper wire on a 13mm diameter plastic former. Alternatively, Jaycar Electronics supply the choke ready wound (Cat No EE-4030). Mount the four 5W wirewound resistors so that they are off the board by about 1mm or so. This aids power dissipation. Now mount the heatsink bracket. It is secured to the board by the mounting screws for the four output continued on page 96 DECEMBER1987 15 CEN Cash in your surplus gear. Advertise it here in Silicon Chip. Advertising rates for this page: Classified ads - $7 .00 for up to 15 words plus 40 cents for each additional word; Display ads (casual rate) - $20 per column centimetre (max. 10cm). Closing date: five weeks prior to month of sale . If you use a PO Box number, you must include your permanent address and phone number for our files. We cannot accept ads submitted without this information. To run your own classified ad, put one word on each of the lines below and send this form with your payment to: Silicon Chip Classifieds, PO Box 139 , Collaroy Beach , NSW 2097 . PLEASE PRINT EACH WORD SEPARATELY, IN BLOCK LETTERS Advertisers Index Our advertisers are vital to the success of SILICON CHIP. Please give them your support. Altronics ............. .. 36,37,44, 45,60 ,61 Arista Electronics .... .. .. .. .. ... 4 7 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ($7.00) Name Postcode Enclosed is my cheque or money order for$ ....... ...... ..... ..... .... ..... . .. or please debit my Bankcard □ Visa □ Marantz Australia . . ..... . .. . ... IFC Microbee .. ....... ....... .. ...... IBC RCS Radio .. ........... ........ .. 49 Card No .. ... Scan Audio Pty Ltd .......... .. Signature .. Tandy Electronics ....... .. .. OBC High-power amplifier module transistors and the driver transistors. Mount the power transistors first. These must all be isolated from the heatsink by using mica washers and insulating bushes, as depicted in Fig.5. Smear all mounting surfaces with heatsink compound before assembly. Solder the mounting nuts to the PCB pattern after assembly to ensure reliable contact. Alternatively, if the nuts are nickel plated or stainless steel, use lockwashers. The two driver transistors and the Vbe multiplier (Q7) are bent over and also attached to the heatsink bracket using T0-126 mounting kits. (See Fig.6). When the whole assembly is completed, the heatsink bracket should be attached to a suitably large heatsink, preferably with vertical fins. Heatsink compound should be used between the bracket and the heatsink to improve heat transfer. Before applying power remove the two fuses from the board clips and set VR1 fully anticlockwise. 96 Dick Smith Electronics ..... Jemal Products ................. 59 Address ..... .. ... .......... ......... .... .... .. ......... .. ....... ... ........ ... ........ . . Suburb/Town ....... ....... .... ... .......... ........ .. ..... ......... ... ..... ..... 67 8,9 , 16,48,54,55 Emona . .. .. ... ........ .. .. .... .... . 43 Jaycar Electronics .. ... .. . 20-27 Crusader ..... ...... .. .. ........ .. . SILICON CHIP 89 ctd from page 15 This gives the setting for minimum quiescent current through the output transistors. Solder a 5600 5W wirewound resistor across each fuseholder. Set your multimeter to the 200VDC range (or no lower than 50V DC if an analog meter). Now apply power and measure the positive and negative supply rails. They should be within a few volts of ± 40V. Now measure the other voltages on the circuit. They should all be within ± 10% of the nominal values. The voltage at the output should be within ± 30mV of OV. No load should be connected at this stage, by the way. Now switch your multimeter back to the 200V DC range and connect it across one of the 5600 resistors. Adjust VR1 for a reading of 28 volts. This gives a total quiescent current of 50 milliamps. For the 50W version which uses only two output transistors, VR1 should be adjusted for a reading of 14 volts. This gives a total quies cent current of 25 milliamps. After five minutes or so, check {-·~" WASHER -T0126 i(F~ r~ ___ 11 O O:~::E WASHER - - HEATSINK PCB ..l_ ...- SHAKE·PR00F WASHER f;f!!J-NUT Fig.6: mounting details for the T0-126 transistors. Note that heatsink compound should be lightly smeared on the mounting surfaces. the quiescent current and readjust VR1 if necessary to get the correct voltage across the 5600 resistor. Now switch off, remove the two 5600 resistors and insert the fuses. The module is now ready for use.