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HIGH p The new amplifier can deliver up to 120 watts RMS into a 100V AC line. The large heatsink keeps the power Mosfets cool. This rugged 120W Mosfet amplifier module is designed specifically to drive a 100V line transformer for public address applications. It can also be used as a stand alone module for guitar amplifiers and domestic stereo amplifiers. By LEO SIMPSON & BOB FLYNN Why is there any need to design an amplifier specifically to drive a line transformer? The answer is that a transformer presents a much more difficult load for an amplifier to drive than a normal loudspeaker. For a start, the primary of a typical transformer will have a very low DC resistance which may be of the order of 100 milliohms (0. H2] or less. Second, transformers do not like direct current flowing in their windings; the resultant magnetisation causes distortion of the output waveform. 14 SILICON CHTP This means that any amplifier designed to drive a transformer must have a very low DC voltage at its output. Just consider what happens with a typical direct coupled amplifier which has a DC voltage at its output of say + 20mV. When a transformer with a primary resistance of 0.10 is connected, a current of 100 milliamps will flow through it. This will cause substantial magnetisation of the transformer core as well as increased power dissipation in the output transistors of the amplifier. To solve this problem , the amplifier circuit must include a nulling adjustment so that the DC output voltage can be set to a very low value, say less than ± 5mV. The input differential transistor pair must also be thermally bonded together to ensure that the nulled output DC voltage does not drift away from zero as the temperature changes. Another problem with a line transformer is that if the load on the secondary winding is disconnected or switched in level while substantial power is being delivered, very high spike voltages can be delivered by the transformer. These high energy spikes can easily destroy bipolar transistors unless "flyback" diodes are connected across both halves of the output stage. Even if amplifiers do include these diodes and the spike voltages are thereby limited to the supply rails, the output stage is still not safe if bipolar transistors are used. Second breakdown The particular problem with bipolar transistors is " second breakdown". This is a mechanism whereby the current passing POWER PA UFIER MODULE +51V 5A 09 0.22! 2SK134 D10 1N5404 0.47 16VW o--=t INPUT ADJUST ZERO OUTPUT + 2.2k 4.3uH 100V LINE TRANSFORMER VR1 2000 f..,. / 0.27I 3x120 1W IN PARALLEL -,LOAD D11 1N5404 04 BC556 I. 45.5V 22k L__; r-- 1.4V 3.9 5A 3.9k L____: 0.22 t-i L---+---~--+-..:.....---___.,_______________________ 51v 120W PA MODULE ~ ECB s, 0 0 G VIEWED FROM BElOW Fig.2: the first two stages of the amplifier operate in cascode mode to give greater open loop bandwidth and improved linearity. The ouput transformer is required only if you intend running a 100V line for PA work. through a power transistor is "squeezed" into narrow channels and thus causes hot spots. These can destroy the transistor. The transistor manufacturers get around this problem by specifying the "safe operating area" for each bipolar device. What this amounts to is that the transistor is derated when higher voltages are present between its collector and emitter. To give a specific example, consider the MJ15003 NPN transistor (one of the output transistors used NOVEMBER 1988 15 Cascode Operation Explained Vee A cascade stage is one where two transistors are connected in series across the supply rail. In our diagram (Fig.2) we show an idealised schematic of a cascade stage. A reference voltage of, say, 4 volts is fed to the base of 02. By emitter follower action, its emitter will be 3.4 volts and this will be the collector supply for O1 . Thus 02 maintains a constant collector voltage on 01 and so eliminates any variations in gain which would otherwise occur if the collector voltage was able to fluctuate. The varying collector current drawn by 01 is the emitter current of 02 which converts it to a voltage signal at its collector. 02 can be regarded as a "grounded-base" stage because of the constant voltage at its base. · The combined effect of operating 01 with a constant collector voltage and 02 in a groundedbase mode gives a stage with much improved linearity and band- in the Studio 200 power amplifier). This device has a power rating of 250 watts, a maximum collector current rating of 20 amps and a collector voltage of 140 volts. With a collector voltage of 50 volts you can pass 5 amps through the transistor (provided the case temperature is maintained at 25°C) and so obtain a power dissipation of 200 watts. However, at a collector voltage of 100 volts you can only pass 1 amp safely through the transistor and thus it is derated to 100 watts dissipation. Designers can cope with this situation provided they know what sort of load the amplifier is intended to drive. They can draw the load lines and select the transistor operating conditions so that the limits of the "safe operating area" are not exceeded. The problem is, when the amplifier is intended to be used with a line transformer, it is much more difficult to predict the load characteristics. This means it is 16 SILICON CHIP CASCDDE STAGE Vref. : :!:J... i .,. Fig.2: a cascode stage is formed by connecting two transistors in series across the supply rail. Note the reference voltage fed to the base of Q2. width compared with a single common-emitter stage. In the past, cascade stages have been a feature of RF circuitry. Cascade stages were originally designed around valves. The word "cascode" is derived from the phrase "cascaded via the cathode", a reference to the cathode in a valve. much more difficult to guarantee that the transistors will not be damaged by unsafe operating conditions. This is where power Mosfets come into their own. They don't have to be derated for "safe operating area" because they don't have any tendency to internal hot spots. And if they are driven hard so that their temperatures are unduly elevated, they compensate automatically by reducing their transconductance. In effect, they are just about unburstable. Their only weakness is that they can be damaged by excessive gate-to-source voltage. This can be prevented by connecting a suitable zener diode between gate and source. New circuit design With these thoughts in mind, we set out to design a new amplifier module which would be suitable for driving a 100V line transformer. It would use power Mosfets, have provision for nulling the DC output, flyback diodes across the output devices and so on. The circuit is as shown in Fig, 1. It is suitable for use with or without a 100V line transformer. When used without a transformer, it will deliver 90 watts into 80 loads and 125 watts into 40 loads. The performance is fully detailed in the specification panel elsewhere in this article. While most of the circuit is fairly standard, it does incorporate a feature which has not been seen in many published circuits to date . This involves cascade operation for the first two stages, a feature which gives improved linearity and better bandwidth. Let us now describe the circuit in detail. Ql and Q2 are PNP transistors connected as a differential input stage. The input signal is fed to the base of Ql while the negative feedback signal is fed to the base of Q2. The total current through Ql and Q2 is set by constant current source Q3. Q3 is biased by diodes Dl and DZ so that it applies close to 0.67 volts across its 6800 emitter resistor. This sets the current through Q3 at close to lmA and so the current through Ql and Q2 is 0.5 milliamps for each. In a conventional direct-coupled amplifier, the signal from the collector of Ql would be connected directly to the base of the following class-A driver stage transistor. In our circuit though, the signal from Ql connects to the emitter of cascade transistor Q4 while the output signal appears at its collector and is then fed to the base of Q5. Q5 and Q6 form a cascade class-A stage with Q7 operating as a constant current load. The base of Q7 is biased by diodes Dl and DZ (which also serve as reference voltage for Q3). With this bias voltage, the current through Q7, Q6 and Q5 is just over 10 milliamps. Five diodes, D3 to D7, provide the base reference voltage for Q6 and thus set the collector-emitter voltage for Q5 at close to 2.6 volts. (To read how a cascade stage works, see the panel at the top of this page). The output signal from the cascade stage is coupled directly to Delving Into the Mysteries of the 100V Line Why do public address amplifiers use 1 00V lines for speaker distribution? Does the speaker line operate at a constant 1 00 volts AC? How do you match a speaker to a 1 00V line? · These and other related questions cause a lot of confusion to people in and out of the public address field. The big problem with public address systems is that the very long speaker leads can have considerable voltage losses if conventional low impedance speakers are used. Imagine the voltage loss in a pair of speaker lines 200 metres long with an 80 speaker. A 200 metre length of such cable will have a DC resistance of about 5.50. This means that 40% of the power would be lost in the cable. When you consider that a PA system in a large building may have tens of kilometres of speaker wiring running back to the amplifier, the resistance losses with conventional low impedance speakers would be intolerable. The way around this problem is to feed the amplifier's output into a step-up transformer and then into the long speaker lines. Each speaker is then coupled to the line via a ~AC .,.. ~ 5W 2W 1W 100VAC LINE srnOJ 0.5W (b) Fig.3: how the amplifier output is connected to give a 100V line. Each loudspeaker is connected to the line using a separate stepdown transformer. step-down transformer which usually has tap connections to vary the loudness from the speaker. So how does the figure of 1 00 volts come into the picture? The assumption is that when the amplifier is running at full power, it will be delivering 1 00V AC to the speaker lines . This makes it easy for the PA system installer. Instead of having to calculate the total load impedance when all loudspeakers are connected, all he has to do is add up the power settings for every speaker connected and see that it is equal to or less than the power rating of the amplifier. Consider a 1 00 watt power amplifier (with 1 00V line output) . The installer can install any combination of speakers which give up to 1 00 watts . For example, he may have 50 loudspeakers all of which are connected via the " 2 watt" primary tap on their individual step-down transformers. So under the worst case conditions, when the attenuators (if fitted) on all speakers are set to maximum loudness, the maximum power delivered to each speaker will be 2 watts. Thus, provided no one modifies the installation, the loading on the amplifier will never be excessive (ie, too low in impedance). Remember that 1 00VAC will rarely, if ever, be present on the speaker lines. That only happens when the amplifier is driven to its maximum output. In the past, many PA systems used 70V lines. This is exactly the same principle as the 1 00V line except that for a given power level, resistance losses in a 70V system will be twice that in a 1 00V installation . the output stage. A 39pF capacitor from the collector of Q6 to the base of Q5 rolls off the open-loop gain of the amplifier to ensure a good margin of stability. Output stage Four Hitachi power Mosfets are used in the output stage. They are connected in source follower mode (similar to emitter follower mode with bipolar transistors). The signals to the output stage are fed via 2200 resistors to the gates of the Mosfets. These resistors also Right: the pen is pointing to the two input transistors (Ql and Q2) which must be thermally bonded together. This is done to minimise temperature drift of the output DC voltage. NOVEMBER 1988 17 During assembly, push the small signal transistors down onto the board as far as they will comfortably go before soldering the leads. The metal faces of the BF469/470 transistors (Q6 and Q7) go towards the heatsink. function as "stoppers" to prevent spurious RF oscillation. Zener diodes ZDl and ZDZ plus diodes DB and D9 prevent overdrive to the gates of the Mosfets. When the load is short circuited, these diodes limit the voltage between the gates and sources of the Mosfets to about ± 11.6 volts. The quiescent current through the output stage is determined by the setting of the 5000 trimpot, VRZ. The lOmA current (via Q7 etc) through the resistance of the trimpot provides a voltage between the gates of the Mosfets to bias them on slightly when no signal is present. This is a normal feature of all class-B amplifiers and is used to minimise crossover distortion. With Mosfets though, it is usual to set the quiescent current much higher than in an equivalent bipolar amplifier. The reason for this is twofold. First, Mosfets are even more non-linear at low currents than bi polars (contrary to what is 18 SILICON CHIP 1A M3092 BROWN +51V 240VAC 8000 63VW BLUE + GND .,. 8000 63VW -51V Fig.4: suggested power supply for the 120W PA module. Check the output rails before connecting them to the module. written in some journals). Second, at a current of 100 milliamps, the transconductance (measured in amps per volt or "mhos ") of the Hitachi Mosfets has a zero temperature coefficient. Hence, the total quiescent current for the output stage is set at ZOOmA. This relatively high current also reduces any tendency to RF instability which can be a problem with power Mosfets if their quiescent current is set too low. The high quiescent current is one disadvantage of Mosfets. It means they need a bigger heatsink and that they waste more power than an equivalent bipolar transistor amplifier. Output stage protection Apart from the zener diodes •D 010 2SJ49 J,•G 1g '---- I I '-------;11:l <at>® ~ ~ F -o.5!,Jfil~ ® ~ <at>£ ~ 1 l N <at> ~ ~ ¥ l OUTPUT (;)=+-22µF ~ ~ -GrND --mo-..DIJ,e eo3lt ~ ~ II Oll _ _ _ _ _Q_6_~_9-F-0-3-0-7----,r---.-l~ Z01 • ---;--u- e GNO ~ *a; ..-m_o..~3~ c:).001 0 0.47µF .-aru,. +.-{Ifil-e INPUT 04 ~ .-(illJ-e = -51V +5lV Fig.5: here's how to install the parts on the printed circuit board. Keep the component leads short and make sure that the Mosfet output transistors (Q8-Q11) are electrically isolated from the heatsink. The 2200 resistors shown dotted are mounted on the copper side of the board. already mentioned, the Mosfets also have have diodes D10 and Dl 1 for flyback protection. These diodes safely clamp any spike voltages, generated by the load, to the positive and negative supply rails. To protect against short circuit loads, a 5A fuse is connected in series with the supply rails to the Mosfets. An RLC network is connected between the amplifier output and the line transformer (or loudspeaker load if the transformer is not used). This network serves two purposes. First, it ensures stability of the amplifier under all loading conditions, including large shunt capacitances. Second, it effectively decouples the amplifier from the load and connecting lines at very high frequencies. This stops large RF signals picked up by the loudspeaker lines from being fed back to the input of the amplifier (via the feedback components) and being detected in Q2. Thus, it helps stop RF breakthrough. Similarly, at the input of the amplifier, there is quite a savage low pass filter which attenuates any extraneous RF signals before they get to the base of Ql. The voltage gain of the amplifier is set by the 22k0 and lkO resistors at the base of Q2. These set the voltage gain to 23. The low frequency response is set mainly by the 0.47 µF input coupling capacitor, giving a - 3dB point at about 20Hz. This could have been set for a lower frequency but this would cause problems of distortion and loading with the output transformer. DC nulling As mentioned before, it is most important that the DC voltage at the output of the amplifier be as close to zero as possible. To ensure this, the two input transistors are thermally bonded together so that any temperature drift will be minimised. As well, the 2000 trimpot VRl allows the output voltage to be set close to zero; ie, to less than ± lmV. Power supply The suggested power supply (Fig.4) for the amplifier module uses a 300VA toroidal transformer with a centre-tapped 70V secondary winding (ie, 35 volts a side). This feeds a 400V 35 amp bridge rectifier and two 8000µF 63VW electrolytic capacitors. 100V line transformer The recommended 100V line transformer was supplied by Altronics of Perth, as were the supply components. The transformer is a toroid with a rating of 160VA. It has been designed to present a 40 load to the amplifier. It has two prinmary windings and two secondary windings. The method of connection to the amplifier is shown on the circuit of Fig.1. No feedback is applied around the transformer but even so the performance is very good, both as far as frequency response and harmonic distortion are concerned. Full details are shown in the specifications panel. Construction The wiring layout of Mosfet amplifiers is very critical so the printed board is a crucial feature of the design. The printed board NOVEMBER 1988 19 PARTS LIST 1 PCB, code SC01111881, 95 x 163mm 1 cast aluminium heatsink with integral bracket, 1 95mm wide by 66mm high; Jaycar Cat. No. HH-8550 or equivalent L-shaped bracket and heatsink 4 3AG fuseclips 2 5A 3AG fuses 6 PC pins 1 plastic coil bobbin, 1 2mm diameter x 11 mm long; Siemens B65672-B-T1 or equivalent (or 4 .3µH aircored choke; see text) 4 T0-3 transistor mounting kits Semiconductors 2 2SK134 Mosfet transistors 2 2SJ49 Mosfet transistors measures 163 x 95mm and is coded SC 01111881. It is meant to be used with a large heatsink. The one shown in our illustrations is from Jaycar (Cat. HH-8550). As an alternative, the board could be used with a heavy gauge aluminium angle bracket and a large extruded heatsink. The heatsink must be reasonably large to keep the amplifier as cool as possible, for long term reliability. 4 1 1 1 9 BC556 PNP transistors BC548 NPN transistor BF470 PNP transistor BF469 NPN transistor 1 N4148, 1 N914 small signal diodes 2 11 V 400mW zener diodes 2 1 N5404 3A silicon diodes Capacitors 1 22µF 16VW PC electroyltic 1 0.47µF 16VW PC electrolytic 1 0 .27 µF metallised polyester (greencap) 4 0.22µF metallised polyester (greencap) 1 .001 µF metallised polyester 1 39pF ceramic Resistors (0.25W, 5%) 1 x 27k0, 3 x 22k0, 2 x 18k0 Assembly of the board is a straightforward matter but it should not be hurried. First, you should closely inspect the board to see if there are any shorted tracks or open circuits in the copper pattern. These should be fixed before proceeding further. The PCB component diagram is shown in Fig.5. Fit the small components first, such as the resistors and diodes. Make sure that you don't confuse -0 0 I SHAKE-PROOF •~--- e.-~- WASHERS ~--NUTS Fig.6: this diagram shows how the Mosfet output transistors are mounted on the heatsink. Use your multimeter to check for shorts between the case and heatsink as each transistor is mounted. The nuts should be soldered to the PC pattern after assembly to ensure reliable contact. 20 SILICON CHIP Transformers and Power Supply Parts 1 300VA power transformer, 70V centre-tapped, Altronics Cat. M-3092 or equivalent 1 160VA 1 OOV line transformer, Altronics Cat. M-1124 1 35-amp bridge rectifier, Altronics Cat. FB-3504 2 8000µF 63VW electrolytic capacitors 1 1 A fuse and fuseholder the small diodes (1N914s} with the 11 V zeners. The fuse clips, trimpots and small transistors can be mounted next. Ql and Q2 should be mounted so that their flat faces are touching. When you have soldered them in place, put of drop of superglue between them and squeeze them together. Note that all the transistors should be pushed close down onto the PCB before soldering (see photo). The 4.3µH choke at the output of the amplifier is wound with 19.5 turns of 0.8mm enamelled copper wire on an 11mm plastic bobbin. Two layers of wire are wound on so that the start is at one side and the finish is at the other side of the bobbin. Bend the start and finish leads at 90° and scrape off the enamel coating before soldering the choke to the board. Heatsink assembly PCB I 0.5W, 2 X 3.9k0, 2 x 2.2k0, 1 x 6800, 4 X 2200, 2 X 680, 3 X 120 1W, 1 x 5000 trimpot (Bourns Cermet horizontal mount, 0.2 x 0.4-inch), 1 x 2000 trimpot (Bourns Cermet horizontal mount) The four Mosfet power transistors are mounted on the heatsink but with their leads soldered to the printed board. The assembly is as shown in Fig.6. We used 5mm fibreglass tubing for the insulating bushes. Smear all the mounting surfaces of the Mosfets and the heatsink with heatsink compound before assembly. The transistors are mounted to the heatsink using 12mm 6BA PAPST AC and DC FANS PAPST manufactures the largest line of equipment fans in the world providing the most efficient, reliable and versatile solutions in both ac and de applications. In addition to models with sleeve bearings, proven in millions of installations, Papst offers ball bearing versions for high environmental temperatures, enhanced performance models that deliver increased airflow, quiet types with particularly favourable airflow/noise characteristics for sensitive audibility requirements , flat fans for restricted installation space and unconventional mechanical designs for special mounting requirements . Papst first for Reliability- Versatility-Availability 4 MELBOURNE: Adilam Electronics Pty Ltd Incorporated 1n V ICTORIA Suite 7 , 145 Parker Stree t, Templestowe 3106 . PO Box 13 1, Bulleen 3105 Telephone: (03) 846 251 1 (4 lines) . Telex : AA 151369 . Fax: (0 3) 846 1467 . SYDNEY: Suite 1, Ramsgate Plaza, 19 1 Ramsgate Road, Sans Souci 22 19. Telephone : (02) 529 2277. Fax : (02 ) 52 9 5893 . DISTRIBUTORS: ADEL AIDE: NS Elec tro nics (08) 46 8531. BRISBANE: St Lucia Electronics (07) 252 7 4 66. CANBERRA: Electronic Components (062) 80 4654 . PERTH: Pro-Spee Distributors (09) 362 50 11 . .•. The pen points to one of the four 22011 resistors mounted on the copper side of the board. Mount the resistors so that they are 2-3mm proud of the board while keeping their leads as short as possible. Performance of Prototype Frequency Response 125 watts into 4 ohms ; 90 watts into 8 ohms; 120 watts into 1 OOV AC line 20Hz to 50kHz (-3dB) without line transformer; 20Hz to 23kHz (-3dB) with line transformer Input Sensitivity 1 .1 V into 22k0 input impedance Harmonic Distortion ( 0.1 % from 20Hz to 20kHz -111 dB unweighted (20Hz to 20kHz); -11 9dB A-weighted )50 for 80 loads Unconditional Power Output (RMS) Signal-to-Noise Ratio Damping Factor Stability screws and nuts. Solder the nuts to the PCB pattern after assembly to ensure reliable contact. Alternatively, if the nuts are nickel plated or stainless steel, use lockwashers. As each transistor is mounted, 22 SILICON CHIP use your multimeter (set to a low "ohms" range) to check that its case is insulated from the heatsink. If the meter does indicate a short, remove the transistor and check carefully for metal swarf around the mounting holes. After the nuts have been tightened and soldered, the gate and source leads of the Mosfets can be soldered to the PCB pattern. The four gate resistors are then soldered in place, on the copper pattern side of the PC board. Install these four resistors so that they just sit proud of the PCB (see photo for details). Now closely inspect all your work for correct assembly and soldering. Make sure there are no blobs of solder bridging out tracks. As a final check on your work, connect your multimeter (set to a low "ohms" range) and test for shorts between the supply rails and the OV rail. There is a trap here - flyback diodes DlO and Dl 1 will show a low resistance for one connection of the multimeter and a high resistance for the reverse connection. Assuming that you have wired up the power supply, check the voltage on the two supply rails before mak- Fig.7: the full size PC pattern. The wiring layout is critical so the PCB pattern is a crucial feature of the design. ing connections to the amplifier board. The supply rails should be close to ± 50V DC. Switch off and wait for the 8000µF capacitors to discharge to below 5V before hooking the supply up to the amplifier. Caution: remember that the power supply puts out a total of 100 volts DC. This is a potentially dangerous voltage. Make sure you don't come in contact with it. Setting up Now remove the negative supply fuse from its clip and connect a multimeter set to measure up to 1A across it. Do not connect a load or the output transformer to the amplifier at this stage. The output stage quiescent current can now be set. Rotate the 5000 trimpot (VR2) fully anticlockwise and apply power. Now rotate the 5000 trimpot for a current of 200mA. Switch off the power supply and wait for the voltage across the supply capacitor to drop. The multimeter can now be removed from circuit and the 5A fuse replaced in the negative supply line. Reapply What is Transconductance? While the gain of bipolar transistors is specified as a simple ratio of collector current to base current (and known as beta or htel, the gain of Mosfets and V-fets is referred to as transconductance. This is because a fet (field effect transistor) is a voltage controlled device; a signal of several volts into the gate gives a drain-source current of several amps. Hence, a Mosfet varies its conductance (the reciprocal of resistance) in proportion to its gate signal. power and measure the DC voltage at the output of the amplifier. Rotate trimpot VR1 to set it to zero; ie, to less than ± lmV. Leave the amplifier with power connected for an hour or so and then check the settings for DC output and quiescent current. Reset if necessary. It is normal for both settings to drift slightly. Now you can connect the 100V line transformer or the loud- The gain of a Mosfet is specified in terms of amps per volt or in the old unit of conductance, Mho (which is "Ohm" spelt backwards and still used by American semiconductor manufacturers). The new unit for conductance is the Siemen (used by European and Japanese manufacturers) . For the 2SK134 (N-channel) and 2SJ49 (P-channel) devices, the transconductance is typically 1 Siemen (ie, 1 amp per volt) at a drain current of 3 amps and a drain-source voltage of 1 0V. speakers and check for the presence of hum or any other signal. With no signal applied the amplifier should be absolutely quiet. Touching your finger to the input should cause the speaker to emit a small "blurt" . With that, your amplifier is ready for work. Footnote: a complete PA amplifier based on this new module will be published in a future issue of ~ SILICON CHIP. NOVEM BER 1988 23