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By LEO SIMPSON & BOB FLYNN 200 Power Amplifier This rugged stereo amplifier puts out a genuine 100 watts per channel into 8-ohm loads and more than 160 watts per channel into 40. Every now and again we have a brainwave at SILICON CHIP. Truly! We were considering the design of a stereo power amplifier which had to have a genuine power output of at least 100 watts per channel into 8-ohm loads. It had to be rugged, easy to build and reasonably priced. While we discussed the pros and cons of various design approaches, we realised we already had the basis of a suitable design, the 50 and 100-watt modules described in our December issue. As it stands the circuit of the module described in our December issue cannot be pushed any harder to deliver more power. The supply rails cannot be increased because the 2N3055/MJ2955 output transistors would not stand it. Nor could the transistors for the first and se- cond differential amplifier stages. Then one of us had a flash of insight and after a minute or two of checking the concept on a plastic brain (calculator), we knew we had a simple solution: increase the power supply rails from ± 40 volts DC to ± 50V and swap the 2N3055/MJ2955 output transistors for the more highly rated MJl 5003/4 transistors. Naturally, we had to follow up with a lot more calculations, plotting load lines for drivers and output transistors and so on, but the concept was confirmed. We modified a power module to put the concept into practice, building a big power supply with a well regulated output of ± 50 volts DC and changing all the transistors which needed higher voltage ratings. Now, after considerable cooperation from Altronics of Perth in supplying key components, we present the "Studio 200" stereo power amplifier. The Studio 200 is a rugged stereo power amplifier housed in a large rack mounting case. It is eminently suitable for work in discos, bands and in the home. It is rugged but that does not mean it is not hifi. It has very good performance figures and excellent power output. Music power output, as measured by the IHF method (Institute of High Fidelity), is 120 watts per channel for 80 loads and 190 watts per channel for 40 loads. This gives a dynamic headroom figure of 0.8dB for both 40 and 80 loads. Typical harmonic distortion for powers of less than 90 watts into 80 loads is less than .01 % . Frequency response is 20Hz to lOOkHz ± ldB. Signal to noise ratio is better than 105dB unweighted with respect to 100 watts into 80. The Studio 200 stereo power amplifier features protection The Studio 200 stereo power amplifier is built into a large rack mounting case from Altronics. It has excellent specs and can be used for home hifi, or put to work in discos and bands. FEBRUARY1988 37 How the Circuit Works Thirteen transistors and three diodes make up each power amplifier. The input signal is coupled via a 1 JLF capacitor and 2.2k0 resistor to the base of 02 which together with 03 makes up a differential pair. 01 is a "constant current tail" which sets the current through 02 and 03 and renders the amplifier insensitive to variations in its supply rails (this is known as supply rejection). Signals from the collectors of 02 and 03 drive another differential pair, 04 and 05, which have a "current mirror" as their load. The current mirror, 06 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 05 is then used to drive the class-AS output stage consisting of drivers 08 and 09 and power transistors O 1 o, O 11 , 012 and 013 . 07 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 05. The voltage is adjusted by trimpot VR1. The function of 07 is to set the DC voltage applied between the bases of 08 and 09 . 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 .4 70 emitter resistor. These are included to ensure that the output current is shared more or less 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 03 via a 22k0, resistor. The level of feedback, and therefore the voltage gain , is set by the ratio of the 22k0 resistor to 1 kO . The low frequency rolloff is· set by the ratio of the impedance of the 1 kO resistor to the impedance of the 4 7 µF capacitor. This sets the - 3dB point at about 3Hz . The 1 J,tF input capacitor and the 22k0 base bias resistor feeding 02 have a more important effect and set a - 3dB point at about ?Hz . The two time-constants together give an overall - 3dB point at 1 OHz. The 330pF capacitor and the 2 .2k0 resistor feeding 02 forms a low pass filter which rolls off frequencies above 200kHz . The 68pF capacitor between base and collector of 05 rolls off the open-loop gain to ensure its in- against short circuits and against damaging loudspeakers if it is seriously overdriven (which often blows tweeters) or develops an internal fault. This protection is provided by the same PTC (positive temperature coefficient) thermistors used in our lOOW modules featured in the December issue of high power amplifiers used for disco and rock bands is an internal fault which not only blows the amplifier 's output transistors but burns out the speakers as well. Repairing the amplifier is relatively cheap but repairs to loudspeakers can be very expensive. By incorporating the Polyswitch PTC thermistors into each channel of the amplifier, you can effectively insure against amplifier and loudspeaker damage. SILICON CHIP. Apart from its generous power output capability and high fidelity performance, we regard the simple and effective protection incorporated in the Studio 200 as its most important feature. An expensive and common occurrence in 38 SILICON CHIP The circuit The circuit is shown in Fig.1. This shows the power supply and one herent stability with feedback applied . Another contributor to the amplifier's excellent stability is the output network consisting of a 6 .8JLH air-cored choke, a 6 .80 resistor and 0.15J,tF capacitor. Instead of using relays for loudspeaker protection each power amplifier uses a Polyswitch (made by Raychem Corporation, USA) . This device is a positive temperature coefficient thermistor with a very low resistance value, under normal operating conditions. When the current through a Polyswitch goes above a set value, 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 are applicable whether or not the Polyswitch is used . The Polyswitches give comprehensive protection . They allow the amplifier to deliver full power on program signals but the moment a short circuit is applied or the amplifier is seriously overdriven, 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 . channel, to save space. It is identical in configuration to the circuit of the lOOW module referred to previously but most of the semiconductors have been changed and so have some of the resistors. For those who have not read the abovementioned article, a brief description of the power amplifier circuit is given in one of the accompanying panels. Most of the changes we 've made relate to the higher supply voltages necessary to extract higher power from the amplifier. We'll discuss the main changes briefly. As already noted, the main 01 1N4002 • I 680 +49.5V 5A 0.1+ 48.1V 8 ! 8 0.1 J .012 INPUT 22k cr22k 1k 2x8F469 04 2.2k ~ 1.9V 2.2k _J B C 8 ' 48.3V :--, 1.9V :....J l 1.3V t - - 4 - - - ' - - + - - - - - - - - - -.....- - - 4 9 .5V 0.ll * ROE245A (80 LOAD) POWER RDE390A (4!1 LOAD) 240VAC - - - - - - - - - - - - - - - - - + 4 9.5V 8000 63VW N + 8000 _ 63VW + - 10k 1W ,--c B 0 ELJc 0.22 100V 0 8 VIEWED FROM BELOW .,. PLASTIC-0 CASE 8000 63VW - 8000 63VW - 10k 1W moE 0.22 100V LED1 111 ECB STUDIO 200 STEREO POWER AMPLIFIER SC1·1 -288 Fig.1: this diagram shows one powe.r amplifier and the power supply for the Studio 200. The circuit is essentially the same as the 100W module published in our December issue. Note that two of the 8000µF 63VW filter capacitors can be regarded as optional although they do give a worthwhile reduction in hum and a small increase in continuous power output. change is to the power output transistors which are now MJ15003 (NPN type) and MJ15004 (PNP type). These rugged bipolar transistors have a maximum power dissipation rating of 250 watts (at a case temperature of 25°C), a collector current rating of 20 amps and a collector voltage rating of 140 volts. So they are considerably more rugged than the MJ2955/2N3055s that they replace. They also have better high frequency gain which means that it is possible to obtain better high fre- Performance of Prototype Output Power (RMS) Music Power Frequency Response Input Sensitivity Harmonic Distortion Signal to Noise Ratio Protection Damping Factor Stability 1 00W into 8 ohms; 160W into 4 ohms 120W into 8 ohms; 190W into 4 ohms 20Hz-1 00kHz ± 1dB 1.25V < 0 .01 % (20Hz-20kHz) >105dB 5A fuses plus optional Polyswitch > 100 Unconditional FE BRUARY1988 39 600..----.---~-----.-------.---,----r------.----.-----, i < 300 i:l The input transistor pair is now 2 x BC556 instead of BC557s but apart from a higher collector voltage rating, these transistors (Vceo 65V instead of 45V for BC557) are otherwise identical. The second transistor pair and current mirror are now 2 x BF469 and BF470 instead of BC639s and BC640. The substituted transistors are normally used in high-voltage video output stages in TV sets and consequently have a very high collector voltage rating of 250 volts and excellent beta linearity which again improves performance. Power supply 10 20 30 60 40 70 80 90 VOLTAGE (VOLTS) Fig.2: this is the load line diagram for the MJE340/350 driver transistors. The straight line shows the resistive load reflected by the output transistors while the arched line shows a severe reactive load. The concave line to the right is the maximum power dissipation (20W) hyperbola of the transistors with SOAR derating included. 14--------.---.. . . . .-..-----,,---,-----,------r----.-----, f :E ~ i 61----4------+-------4----+-------il-_,-....,__ _ _ _ _ _ _ _--t = .., :::, The Studio 200 stereo amplifier has a very good power supply and this is the main reason it is able to deliver so much power. It uses a large toroidal power transformer with a centre tapped secondary winding of 35 volts a side, with a nominal rating of 300VA. This transformer has excellent regulation and very low hum radiation. It is also a lot more compact and weighs less than a conventional transformer of equivalent rating. Teamed with the transformer is a chassis mounting bridge rectifier and four B000µF 63VW electrolytic capacitors. These deliver balanced supply rails of ± 50V DC. Really, we are gilding the lily by putting in four B000µF electrolytics as it only makes a relatively small difference to the maximum continuous and music power capability. You can save about $40 by just using two B000µF 63V capacitors. Chassis 10 30 40 60 70 90 VOLTAGE (VOLTS) Fig.3: load line diagram for the MJ15003/4 output transistors. The straight line is for a 40 resistive load while the arched line is for a reactive load of 2.75 ± j2.750. The concave curves to the right are for the transistors' maximum power hyperbola (2 x 250W) with SOAR derating included. quency distortion performance from them. The driver transistors are unchanged but we have plotted load lines for both the drivers and output 40 SILICON CHIP transistors to check that they can safely deliver the much higher power output of this amplifier. The load line graphs are shown in Fig.2 and Fig.3. The chassis is a nicely finished 'three unit high' rack mounting case from Altronics (Cat No H-0418). This has heatsink extrusions at the sides on which we mounted the amplifier modules, one each side. To save chassis space we mounted the toroidal transformer directly to the front panel. This is more or less standard practice with rack amplifiers as the front panel is usually much thicker and stronger than the top or base panels. Order of assembly If you are buying a complete kit for this amplifier, no drilling of metalwork will be necessary but if mF and Music Power The music power rating of the Studio 200 was measured according to the method outlined in the Institute of High Fidelity's specification IHF-A-202. This uses a pulsed 1kHz waveform with a duration of 20 milliseconds . To conveniently measure power according to this method and those which use longer pulse durations, we have designed a self-contained test instrument. It will be described in a coming issue of SILICON CHIP. '· . f! » . . l's::d.f.i:t.s:Ll2:JEY.2:2:L:1t:i:J!S<at>E2:JER~D![Jl" M'f\$'. ,:,··. ''f&WtW<at>/;"»~f?&~: -v INPUT - - - T03 DEVICE . :__ ~ .i:?- +V Fig.4: here's how to assemble the power amplifier modules. Be sure to use the correct transistor at each location and take care with component polarity. For 80 loads, use the RDE245A thermistor; for 40 loads, use the RDE390A. 0 !r -INSULATING SLEEVES -~O-~ ~ II HEATSINK PCB I SHAKE-PROOF ·--~-WASHERS ~-- 9-NUTS Fig.5: mounting details for the TO-3 transistors. Trim the mica washers so that they do not overlap. The power amplifier modules are screwed to the heavy heatsink extrusions that make up the sides of the case. Use PC stakes to terminate all external wiring to the module. Note: the PTC thermistor has been replaced here by a wire link but should be included in all kit versions. you are working from scratch, any drilling of the case, heatsink brackets or whatever should be done before any assembly work gets under way. Kits for the Studio 200 stereo amplifier will be available shortly after this issue goes on sale from Altronics of Perth at $399. The first step in putting the amplifier together is to assemble the power supply into the chassis. You can fit all the other hardware at the same time. To prevent marking your workbench or table, we suggest that the case be fitted with four large rubber feet (32mm in diameter). The ones we used are sold in hardware stores as rubber door stops. Mount all the hardware bits on the rear panel and then mount the transformer and hardware on the FEBRUARY1988 41 All the power supply and amplifier output wiring should be run using heavy duty hookup wire. Take care when putting the lid on the case so as not to damage the outer insulation of the transformer. Note: some of the internal wiring in this photo differs slightly from the wiring diagram. The PTC protection thermistors are also not shown. front panel. We used a bolt with a blackened head to mount the transformer so that the bolt head would not be so obvious on the front panel. You can obtain one from a hardware store. The mains cord should be J-se•~ WASHER -10126 ~~~::;~. 0 -HEATSINK PCB ..l_ - - SHAKE-PROOF WASHER (8-NUT Fig.6: mounting details for the T0-126 transistors. Note that heatsink compound should be lightly smeared on the mounting surfaces. 42 SILICON CHIP secured in the rear panel of the chassis with a cord grip grommet. It is also anchored along the righthand side of the chassis (looking from the front) by two cord clamps. Wire up the transformer exactly as shown in the wiring diagram. Both the active and neutral wires from the power cord are terminated at the insulated terminal strip, as is the blue primary wire from the transformer (joined to the incoming neutral wire). Cut a short length off the brown wire from the transformer and strip both ends. One end goes to the incoming active wire while the other end goes to the power switch. The brown primary wire from the transformer also goes to the power switch. Before soldering these wires to the switch, slip a length of shrinkable sleeving over the pair. After soldering, push the tubing up over the switch and then apply heat from a hairdryer to shink it on. Wire up the bridge rectifier and filter capacitors using heavy duty hookup wire. Do not omit the lOkO 1W resistors wired across each supply rail. These are there to safely discharge the capacitors in the event the fuses to the amplifiers blow. Check all your wiring carefully at this stage and then apply power. The supply rails should be close to ± 50V DC. Switch off and allow all the capacitors to discharge. Now you can assemble both the amplifier modules. We suggest you assemble one module and test it before doing the other. That way, if you make a mistake on the first, you won't do it on the second. The procedure for each board is as follows. First mount all the small components leaving the power transistors and heatsink till last. The component overlay for the modules is shown in Fig.4. Note that the 68pF compensation capacitor associated with Q5 should have a voltage rating of at least 100 volts and so should the 0.15µF capacitor in the output filter network. The 6.BµH choke is wound with 24.5 turns of 0.8mm enamelled cop- HEATSINK HEATSINK BRACKET LEFT AMPUAER -V r-l-.1 . I I Q 22 0.22 '' ' POWER TRANSFORMER (MOUNTED ON FRONT PANEL) 1 I/I -V +v RIGHT AMPLIFIER v. /1 .... LED1~ 2 . <at>t - .- -.- - ,. - ~ \ HEATSINK BRACKET ' POWER CORD HEATSINK Fig.4: this diagram shows the wiring details of the amplifier and power supply. Note that for minimum hum output from both channels of the amplifier, the transformer must be rotated so that its leads exit as close as possible to the base of the case. Note the single point earth wiring which is essential for low hum output. per wire on a 13mm diameter plastic former . Alternatively, Jaycar Electronics supply the choke ready wound (Cat No EE-4030). Mount the four 0.470 5W wirewound resistors so that they are off ihe 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 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 FEBRUA RY1988 43 PARTS LIST 1 rack-mounting case with side heatsink extrusions (Altronics H-0418) 1 70V centre-tapped 300VA toroidal transformer (Altronics M-3092) 1 push on/push off mains switch with black button (Altronics S-1 090) 1 red LED and bezel 1 3-core mains cord and moulded 3 -pin plug 1 cord-grip grommet 2 plastic cord clamps 1 3 -way mains insulated terminal block 4 binding post terminals, 2 red, 2 black 1 2-way RCA panel socket 3 plastic cable ties 2 solder lugs 4 large rubber feet (as sold by hardware stores as door stops) 1 50mm of twin shielded cable 500mm red heavy duty hookup wire 500mm black heavy duty hookup wire 2 8000µF 63VW chassis mount capacitors (optional) 1 BR104 400V 10A bridge rectifier or equivalent 2 0 .33µF metallised polyester capacitors 2 1OkO 1W resistors 1 5 .6k0 1W resistor 4 2 2 2 4 Amplifier modules Capacitors 2 4 7 µF 16VW PC electrolytic 2 1µF metallised polyester (greencap or miniature) 2 0 .15µF metallised polyester 10 0.1 µF metallised polyester 2 0.012µF metallised polyester 2 330pF ceramic or miniature metallised polyester 2 68pF 1OOVW ceramic Power supply Semiconductors 2 B000µF 63VW chassis mount capacitors 6 BC556 PNP transistors 2 BF4 70 PNP transistors assembly to ensure reliable contact. Alternatively, if the nuts are nickel plated or stainless steel, use lockwashers. Depending on whether you intend to use the amplifier with 40 or 80 loudspeakers, you have a choice of PTC thermistors for protection. ,For 80 loads, use RDE245As. For 40 loads, use the higher current RDE390As. 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 one of the heatsink extrusions in the chassis. Heatsink compound should be used between the bracket and the heatsink to improve heat transfer. Powering up Before applying power remove 44 SILICON CHIP 2 printed circuit boards, code SC11-1287, 121 x 133mm 2 heatsink brackets 8 3AG fuse clips 4 5A 3 AG fuses 12 PC pins 2 plastic formers, 13mm dia x 10mm, plus 1-metre 0 .8mm enamelled copper wire (ECW) ; or 2 x 6.8µH aircored chokes 2 Raychem RDE245A polyswitch PTC thermistors for 80 loads or RDE390A for 40 loads 8 T0-3 transistor mounting kits 6 T0-126 transistor mounting kits the two fuses from the board clips and set VRl fully anticlockwise. 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 ZOOVDC range (or no lower than 50V DC if an analog meter). Now apply power and measure BF469 NPN transistors 8D139 NPN transistors MJE340 NPN transistors MJE350 PNP transistors MJ 15003 NPN power transistors 4 MJ 1 5004 PNP power transistors 6 1 N4002 diodes Resistors (0.25W, 5%) 2 x 56k0, 4 x 22k0, 2 x 18k0, 2 x 8.2k0 1W, 6 x 2.2k0, 2 X 1 kO, 2 x 4700, 2 x 2700, 6 x 1 ooo, 4 X 680, 2 X 6 .80 1 W, 8 X 0.470 5W wirewound, 2 x 5000 trimpots (Bourns cermet horizontal mount, 0 .2 x 0 .4-inch) Miscellaneous Screws, nuts, washers, solder, heatshrink tubing for mains switch insulation . the positive and negative supply rails. Again, they should be within a few volts of ± 50V. Now measure the other volta ges 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. continued on page 95 The rear panel carries a 2-way RCA panel socket for the inputs and two sets of binding post terminals for loudspeaker connections. deliver about three times the maximum current capability of the 7812 regulator. The power transistor should be mounted on a heatsink. +15 >'-+-----OUTPUT TO MODULE 1 0.1 Bridging 100W amplifier modules I am interested in the 100W amplifier module described in the second issue of SILICON CHIP. Over the next six months I propose to build at least six of these modules, incorporated into three stereo slave amplifiers, for public address applications. There are a couple of issues I would like to raise. Assuming PA applications, is it possible to easily switch such a stereo amplifier into a bridged mono mode? Second, is it proposed to develop a project along similar lines to this 100W project but with an output of the order of 300 to 500 watts into 40, so that it too can be incorporated into a stereo slave amplifier, with a bridging operation as well? Lastly, how about a versatile, high quality 8 - (or 12-) 4-2-1 mixer and a simpler 6-2 mixer, to complement the power amplifiers described above? (R.W., Scoresby, Vic). • Yes, it is quite easy to use the 100W power modules in bridge mode but there are a number of points to consider before you rush off to buy the parts. First, if you wish to use the 100W modules in bridge mode, you will only be able to do so if you use the bridged amplifier to drive an 80 load. You could not use it with a 40 load. The reason is that each amplifier in a bridge configuration "sees" half the real load impedance. Therefore, if you were to use a 40 load, each amplifier module would "see" a 20 load and would be overloaded. If you were using the PTC thermistors we specified for protection, they would operate prematurely and the effective power output would be low. Bridged power output into an 80 load would be close to 200 watts, depending on the regulation of the power supply. This general rule about bridging applies to all amplifiers, by the way, so there is no way around it. If INPUT C>-:-11--e--"" 220k 10k 1'/o -15V 10k 1% ... > - - - - OUTPUT TO MODULE 2 Fig. 2: this simple circuit allows two power amplifier modules to be driven in bridge mode. Be sure to use 8-ohm loads. we had been designing the amp modules to drive 40 loads in bridge mode we would have had to make sure that each module could safely handle a 20 load. That would have made them much more expensive. If you want more power, and you want to drive a 40 load, you would be better off considering the Studio 200 stereo power amplifier presented in this issue. It is essentially the same design but upgraded with a bigger power supply and higher-rated output transistors. By using one of these upgraded modules you get a lot more bang for your buck. With one module driving 40 you could get close to 200 watts, if you had a very well regulated power supply. That would mean using a transformer rated at around 400V A or more. Again, you could use the upgraded modules in bridge mode and probably get somewhere around 350 watts into an 80 load, depending on how big and how well regulated your power supply was. Again, you would need a big transformer, rated at 600VA or more. A suitable circuit to enable the modules to be driven in bridge mode is shown in Fig.2. This uses a dual op amp, type TL072. There is one more point to consider. You mention using the amplifiers for public address applications but you have not specified if you will be using them to drive line output transformers. If that is what you intend, you are suggesting a whole new ball game and one for which these modules were not designed. We'd be wary about using them for driving line transformers without at least incorporating flyback diodes across each of the paralleled output transistors and without a means of adjusting the DC output offset voltage close to zero. The offset voltage needs to be very low othenvise substantial DC will flow in the transformer. This would cause problems in the transformer and could cause excessive dissipation in one half of the power amplifier. We have not done any work along these lines and so we hesitate to recommend them in this more stringent application - they may work OK but we couldn't guarantee it. We'll have a look at your suggestion for a mixer project but it is likely to take some time to develop. In the meantime, have a look at the Jaycar 8002 8-channel mixer. It would take a lot of careful design to better its excellent performance. ~ Stereo Amplifier continued from page 44 Now switch your multimeter back to the 200V DC range and connect it across one of the 5600 resistors. Adjust VRl for a reading of 28 volts. This gives a total quiescent current of 50 milliamps. After five minutes or so, check the quiescent current and readjust VR 1 if necessary to get the correct voltage across the 5600 resistor. Now switch off, remove the two 5600 resistors and insert the fuses. If all is well, you can now assemble the second module and go through the same procedure. ic FEBRUARY1988 95