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Part 3: building the 500W Audio Power Amplifier In this final article on the 500W audio power amplifier, we present the details of the loudspeaker protector module and the thermal switch for the fan. By LEO SIMPSON & BOB FLYNN As we left the power amplifier last month, supposing you were building it, you had just had the module on “heat soak” for about an hour to check the quiescent current setting. This is set by adjusting trimpot VR2 so that the voltage across the 390Ω 5W resistors (temporarily installed across fuses F1 and F2) is 30V. After the initial 66  Silicon Chip setting, the voltage will creep up quite a bit, perhaps to 45V or more, so it is necessary, to readjust trimpot VR1 to bring the voltage back to 30V. It is important to note that the thermal compensation provided by the Vbe multiplier transistor (Q9) does not give perfect compensation for the drift in quiescent current. Even after you have tweaked it a number of times, it will still drift about. Of course, if the thermal compensation wasn’t included in the circuit, the quiescent current would rapidly go out of con­trol as soon as the amplifier was called upon to deliver signifi­cant power. Having set the quiescent current to your satisfaction, you can now set the DC offset current at the output, by adjusting trimpot VR1. You need a digital multimeter for this test. Set it to the lowest available DC voltage range, probably 200mV, and connect it directly across the output terminals on the PC board. Adjust VR1 to obtain zero volts. You should be able to get it to within ±1mV although again, it will tend to drift about. Fig.1: the circuit of the Loudspeaker Protector is changed from that presented in the April 1997 issue and employs a thermal cutout to operate the relay if the heatsink temperature exceeds 80°C. In practice, the DC offset voltage or its drift is not important if you are driving a 4Ω or 8Ω load. Even if the DC output offset went as high as ±50mV, the current would still be less than 20mA through a 4Ω loudspeaker and that is negligible in the overall scheme of things. The main reason we have included the offset adjustment trimpot (VR1) is so that the amplifier can be used to drive one or more 100V line transformers. Because such transformers have a very low primary resistance, the resulting DC offset current from a 50mV DC offset being present at the output could be consider­ able. The current could lead to substantial power dissipation in the amplifier and could lead to premature saturation of the transformer itself, resulting in less power delivered and higher distortion. Loudspeaker protector Now that we have come this far, we can turn our attention to the Loudspeaker Protector PC board. The circuit of this is shown in Fig.1. This is based on the Universal Loudspeaker Pro­tector we presented in the April 1997 issue but inevitably, we have modified it. The original circuit was designed to suit a stereo amplifier and since this is a mono amplifier we have omitted three transistors and the other components needed for the second channel. Second, we have changed the method of powering the board. The method we had intended using involved run- ning the module from the +80V DC rail via a 470Ω 10W wire­wound resistor and using the on-board regulator circuit to obtain 12V for the relay and so on. This method works but there is a problem with the time delay between the amplifier being turned off and the relay actually disconnecting the loudspeakers. This problem arises because of the large amount of ca­pacitance in the filter bank – 40,000µF on each rail. This ca­pacitance takes quite a long time to discharge, particularly if the Warning! The 160V DC supply across the capacitor bank in the power supply is potentially lethal. As well, high voltages exist on the bridge rectifier and on many components in the amplifier module, including the fuse­ holders. The following rules should be observed: (1). Do not operate the amplifier without the Perspex shield covering the filter capacitors. (2). Disconnect the mains plug and allow the filter capacitors time to fully discharge before working on the circuit. The LEDs will go out when the capacitors have discharged to a few volts. amplifier is not delivering any power at the time it is turned off. The solution is to power the Loudspeaker Protector module from the 57VAC supply via a 270Ω 10W wire­ wound resistor. This feeds the AC supply to the PC board and to a diode and 470µF filter capacitor to provide a DC supply. This is shown on Fig.1. With this arrangement, the derived DC supply drops rapidly to zero as soon as the amplifier is turned off and so the speaker is disconnected almost immediately. The other difference between the circuit presented here and the original circuits shown in the April 1997 issue is that we use a thermal cutout switch to control the relay. This is a different arrangement to that shown on the proto­type amplifier module in the photograph on pages 24 & 25 of the August 1997 issue. That showed the thermal cutout switch mounted on the heatsink and connected in series with the loudspeaker output. Having the thermal cutout in series with the amplifier’s output would be appropriate if the Loudspeaker Protector module was not being used but we don’t want two sets of contacts in series with the loudspeaker; ie, the thermal cutout and the relay contacts. Therefore we connect the thermal cutout so that it operates the relay and that means that only the relay contacts are in series with the loudspeaker circuit. Note that the thermal cutout switch has a pair of “normally closed” conOctober 1997  67 Fig.2: the component overlay for the Loudspeaker Protector PC board. Note that some component positions are vacant. tacts. When the temperature of its mounting base (ie, the heatsink in this case) rises above 80°C, the contacts open and interrupt the base current to transistor Q4 on the Loudspeaker Protector module. For the sake of completeness, let’s now give a brief de­scription of the Loudspeaker Protector circuit in Fig.1. The amplifier’s output is connected to the three-transistor monitoring circuit via two 22kΩ series resistors and two 47µF bipolar capacitors. This network is a low-pass filter which removes virtually all audio signal. From there, any DC signal is fed directly to the emitter of transistor Q1 and the base of Q3. If a positive DC signal of more than 0.6V is present (indi­cating an amplifier fault), Q3 will turn on. In the same way, if a negative DC signal Capacitor Codes ❏ ❏ ❏ ❏ ❏ ❏ Value IEC Code EIA Code 0.15µF   150n   154 0.1µF   100n   104 .01µF   10n  103 820pF   820p   821 100pF   100p   101 of more than 0.6V is present (again, an amplifier fault condition), Q1 will turn on and this will turn on Q2. Q2 & Q3 have a common 56kΩ load resistor and this normally feeds base current to Q4. Q4 feeds base current to Q5 and so both of these transistors and the relay are on. But when an amplifier DC fault occurs, either Q2 or Q3 is turned on to shunt the base current away from Q4. Thus Q5 and the relay are turned off and the speaker is disconnected. Because we are dealing with such a high power amplifier, both sets of relay contacts are connected in parallel, to handle the high currents involved. To give some idea of the size of the fault current, that can occur, consider what happens if one of the output transistors suffers a “punch-through” failure and goes short circuit. This connects the 80V rail directly to the loudspeaker and if it is a nominal 4Ω speaker it will have a voice coil resistance of about 3Ω. Thus, a peak current of around 25 amps or more will initially flow. With any luck, the relevant supply fuse will blow but then the amplifier is likely to “latch” in the opposite direction and connect the other 80V rail across the speaker, to give it a double whammy, if it hasn’t already been burnt out by the peak dissipation of more than two kilowatts! As you can see, it is important for the relay to disconnect the speaker very rapidly, before it is damaged. These very high fault currents will form an arc across the relay as it tries to break the circuit. For this reason, the moving contacts of the relay are shorted to the loudspeaker ground lines. Thus the current is shunted away from the speaker and the fuse(s) blow. As already noted, the DC supply rail for the Loudspeaker Protector circuit is derived from one of the 57V AC lines from the power transform­ er. This feeds diode D2 via the 270Ω 10W resistor. The resulting DC rail from the 470µF filter capacitor is fed to transistor Q9 which functions as a voltage regulator to provide +12V to the circuit. Resistor Colour Codes: Loudspeaker Protector Module ❏ ❏ ❏ ❏ ❏ ❏ No. 1 2 2 1 1 68  Silicon Chip Value 220kΩ 56kΩ 22kΩ 2.7kΩ 2.2kΩ 4-Band Code (1%) red red yellow brown green blue orange brown red red orange brown red violet red brown red red red brown 5-Band Code (1%) red red black orange brown green blue black red brown red red black red brown red violet black brown brown red red black brown brown This view inside the chassis shows the thermal switch for the fan (right) and the thermal cutout (left) which interrupts the load. The fan operates when the heatsink temperature reaches 60°C, while the load is disconnected at 80°C. Note that while the relay is off, for example, just after power is applied, the voltage across the 470µF filter capacitor will rise to +80V. That is why we have specified a rating of 100V for this capacitor. In addition to monitoring DC faults in the power amplifier, the Loudspeaker Protector also provides a turn-on delay for the loudspeaker. This prevents audible turn-on thumps from the ampli­fier itself or any preamplifier circuitry preceding it. This is achieved with resistors R1 & R3 and capacitor C1. When power is first applied, C1 is discharged and no base current can flow via the 56kΩ resistor R1 and so Q4 & Q5 and the relay are held off. C1 then charges via the 220kΩ resistor R3 and eventually sufficient voltage is present to allow resistor R1 to bias on Q4. This turns on transistor Q5 and the relay and so the loudspeaker is connected to the amplifier. The delay is several seconds. PC board assembly All the parts, with the exception of the 270Ω 10W resistor, are mounted on the PC board which is coded 01104971. The wiring diagram is shown in Fig.2. Note that some transistor and other component positions are vacant. Fit the PC pins first and then the resistors. The two 47µF electrolytic capacitors can go in either way around since they are non-polarised (NP or BP). The other electrolytics are polar­ ised and must be inserted the correct way around. Next insert the transistors, diodes and zener diode and make sure that you put the correct type in each position. Finally, the relay can be installed. We mounted ours by soldering short lengths of stout tinned copper wire to each relay pin. These wire leads are then pushed through the relay mounting holes on the board and soldered. We understand that some kitset suppliers may provide a PC board with slotted holes so that the tinned copper wire may not be necessary. When the board is complete, check your work carefully and then install it in the case. The chassis wiring diagram of Fig.3 shows the details. Make the connections for the power supply and the thermal cutout but do not make the speaker connections yet. Resistor Colour Codes: Power Amplifier Module ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 4 2 1 1 1 1 2 4 2 1 1 5 2 3 Value 22kΩ 18kΩ 8.2kΩ 1.2kΩ 560Ω 470Ω 330Ω 270Ω 220Ω 180Ω 120Ω 100Ω 30Ω 18Ω 4-Band Code (1%) red red orange brown brown grey orange brown grey red red brown brown red red brown green blue brown brown yellow violet brown brown orange orange brown brown red violet brown brown red red brown brown brown grey brown brown brown red brown brown b brown black brown brown orange black black brown brown grey black brown 5-Band Code (1%) red red black red brown brown grey black red brown grey red black brown brown brown red black brown brown green blue black black brown yellow violet black black brown orange orange black black brown red violet black black brown red red black black brown brown grey black black brown rown red black black brown brown black black black brown orange black black gold brown brown grey black gold brown October 1997  69 Fig.3: this is the complete wiring diagram for the 500W power amplifier. Note that it differs in detail from that presented last month. Note also that the full DC supply is potentially lethal and that high DC voltages exist on the amplifier supply rails and on may components, including the fuseholders. 70  Silicon Chip Fig.4: actual size artwork for the Loudspeaker Protector PC board. DANGER! High Voltage Switch off and allow the filter capacitors to completely discharge before working on the circuit. Fig.7: this diagram shows how the fan is wired to the mains via the optional thermal switch. Apply power and the relay should operate after about two seconds. Next, try simulating an amplifier fault condition with a 6V or 9V battery. Connect the battery across the inputs, first with one polarity and then the other way around. In each case, the relay should immediately open and then close again as soon as the battery is removed. Fig.8 (below): this is the artwork for the amplifier PC board, reduced to 0.707 times actual size. To bring it up to full size, you will need a photocopier which can enlarge by a factor of 1.414. Fig.5: this warning label should be affixed to the Perspex cover over the filter capacitors in the power supply. October 1997  71 Parts List 1 500W amplifier module (see parts list, August 1997) 1 toroidal transformer, 2 x 57V, 800VA 1 240VAC 17W 140mm fan 1 3AG panel mount fuseholder 1 5A slow-blow 3AG fuse 1 15A, 2-pole mains rocker switch with neon indicator 1 3-way mains terminal strip 1 80°C thermal cutout (TH1) (Altronics S-5610) 1 60°C thermal switch (TH2; for fan switching) 1 Perspex sheet, 332 x 100mm 4 100mm-long brackets plus machine screws & nuts (to mount Perspex cover) 1 metre, 1mm dia. tinned copper wire 1 metre, 14 x 0.2mm hook-up wire, red 1 metre, 14 x 0.2mm hook-up wire, black 2 metres, 32 x 0.2mm hook-up wire, red 2 metres, 32 x 0.2mm hook-up wire, black 0.5 metre, 32 x 0.2mm hook-up wire, white 8 capacitor mounting clips 24 3M x 10mm CSK screws 24 3M nuts 24 3mm shake proof washers 1 4M x 20mm CSK screw 1 4M nut 1 4mm steel washer Semiconductors 1 KBPC3504 400V 35A bridge rectifier 2 red LEDs If these checks are OK, you are ready to complete the wir­ing. If not, check the circuit for errors. Now make the speaker and amplifier connections to the Loud­speaker Protector board, using heavy duty hookup wire. This should be twisted and oriented as shown in the photos. Fan wiring With that done, it is time to wire in the fan. This is switched by a thermal switch similar to that used for controlling the Loudspeaker Protector. However, the thermal switch used to 72  Silicon Chip Capacitors 8 10,000µF 100VW electrolytic 1 .01µF 275VAC polypropylene Resistors 6 15kΩ 1W Loudspeaker Protector 1 PC board, code 01104971, 107 x 55mm 8 PC pins 1 relay with 240VAC 10A DPDT contact, 12V coil <at> 75mA, Jaycar SY-4065 or similar 4 3mm x 20mm screws 4 3mm nuts 4 6mm spacers 1 U-shaped heatsink (Altronics Cat H-0502 or equivalent) Semiconductors 3 BC547 NPN transistors (Q1, Q3, Q4) 1 BC557 PNP transistor (Q2) 1 BC327 PNP transistor (Q5) 1 BD649 NPN Darlington transistor (Q6) 1 13V 500mW zener diode (ZD1) 2 1N4004 silicon diodes (D1,D2) Capacitors 1 470µF 100VW electrolytic 1 470µF 25VW electrolytic 1 220µF 16VW electrolytic 2 47µF 50VW NP (non-polarised) electrolytic Resistors (0.25W, 1%) 1 220kΩ 1 2.2kΩ 2 56kΩ 1 2.7kΩ 1 22kΩ 1W 2 22kΩ 1 270Ω 10W wirewound control the fan has “normally open” contacts and operates at a temperature of 60°C. Hence, until the heatsink rises to that temperature, the fan does not operate. When the heatsink tempera­ ture does rise above 60°C, the thermal switch will operate and its contacts will stay closed until the temperature drops below 35°C. The 240VAC supply to the fan comes from the same insulated terminal block which is used to connect the transformer primary winding. The wiring to the thermal switch and the fan should be run in 250VAC-rated hook­up wire. It should be twisted as shown in the photos. Fit heatshrink tubing over the terminals of the thermal switch, to avoid the possibility of accidental contact with the 240VAC mains supply. When all the wiring is complete, apply power and recheck the voltages in the amplifier. Assuming everything is OK, disconnect the power and wait until the filter capacitors in the power supply have completely discharged (ie, when the LEDs go out). Now unsolder the 390Ω 5W resistors across the fuses, F1 & F2, and fit the fuses. These should be 5A for an 8Ω load and 7.5A for a 4Ω load. Do not make the mistake of leaving the 390Ω resistors on the board. If the amplifier does blow the fuses at some stage, the resistors will be back in circuit and may contribute to further damage in the amplifier, before they themselves burn out. You are now ready for a listening test. Connect a loud­ s peaker and This internal view of the completed prototype shows the finalised wiring to the Loudspeaker Protector and the thermal switches on the heatsink. Note that these details are different to the chassis photo in last month’s issue. The transparent Perspex shield over the bank of filter capacitors is a worth­while safety measure in view of the high supply voltage – 160V in total. program source and prepare to be impressed. Finally, a few omissions and errors crept into the parts list published for the amplifier module, on page 32 of the August 1997 issue. Two 300Ω 0.25W resistors were omitted, a 6.8kΩ 1W resistor was specified instead of 8.2kΩ 1W and only five 0.1µF MKT polyester capacitors were specified while 11 are required. Also, the 100pF 500V ceramic capacitor should be an NPO type, Philips 2222 650 10101. Note that on the PC board component overlay diagram on page 57 of the September 1997 issue, the unlabelled transistor adjacent to trimpot VR1 is SC Q3, a BC556. October 1997  73