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Build a 50W/channel stereo amplifier; Pt.2 Last month, we introduced our new high performance 50W/channel stereo amplifier & described the circuit operation. This month, we conclude with the presentation of the construction details. By LEO SIMPSON & BOB FLYNN Most of the construction of the new amplifier is quite straightforward. The work mainly involves mounting components on the five printed circuit board assemblies. These are the power amplifier board, the input selector board, the selector switch board, the tone control board and the optional RIAA preamp board. The first job is to assemble the input selector board which is shown in Fig.7(a). This board is coded 01103951 and carries the RCA input and output sockets. Before mounting any of the parts, it is a good idea to carefully check the copper pattern on the underside of the board. You should especially check for shorts between the long parallel tracks to the selector switch. 42  Silicon Chip Don’t just rely on a visual check here – switch your multi­meter to a high Ohms range and use it to confirm that the tracks are isolated from each other. This test will quickly locate faults on any board that has not been correctly etched. You will need to go through a similar checking procedure with each of the other boards when you come to them. Now install the parts as shown in Fig.7(a). The first job is to install the 25 PC pins. Fourteen of these support the selector switch assembly and these should be in­stalled from the copper side of the PC board; ie, so that the shoulder of each pin sits against its respective copper pad. The remaining pins are located at the left and right channel outputs, the tape inputs and the optional RIAA preamp inputs. If you’re not building this latter board, you can forget the pins for the preamp inputs but install a couple of links instead. These links are shown dotted on the diagram. This board is completed by soldering in the three 3 x 2-way RCA socket panels. One of these, at the end adjacent to the selector switch, is cut down to a 2 x 2-way, so that a total of 16 RCA sockets is provided. Fig.8(a) shows the selector switch board (code 01103952). Position the switch with the locating spigot towards the top and push the body of the switch all the way down onto the board before soldering the terminals. The pads along the bottom edge of the switch board can now be soldered to the 14 PC pins on the input selector board. Tone control board Fig.9(a) shows the parts layout on the tone control PC board (code 0110­ 3953). Commence assembly by installing PC pins at the external wiring points, then fit the wire links, resistors, capacitors and semiconductors. Check the orientation of polarised Fig.7(a): the input selector board. Note that if the optional RIAA preamp is not included in the amplifier, the two links shown dotted should be included & the associated PC pins omitted. TAPE IN TAPE OUT AUX2 AUX 1 VCR TUNER CD PHONO GND IF RIAA PREAMPLIFIER IS FITTED: REMOVE LINKS SHOWN DOTTED. R AND L CONNECT TO INPUTS AND RR AND LL CONNECT TO OUTPUTS OF PREAMP BOARD LL R L RR 1k 1k PCB PINS SOLDERED TO TRACKS OF SWITCH BOARD LEFT GND RIGHT TO TAPE INPUT OF CONTROL BOARD LEFT GND RIGHT TO SOURCE INPUT OF CONTROL BOARD Fig.7(b): this is the fullsize etching pattern for the input selector board. parts carefully when installing them on the board. These include the ICs, diodes, transistors and electrolytic capacitors. The 6.8µF and 22µF capacitors are bipolar types and can be installed either way around. The headphone socket, pots and pushbutton switches should be left till last. Be sure to push them all the way down onto the board but don’t solder all the leads at this stage. Instead, tack solder diagonally opposite pins at either end of each component. The tone control assembly can now be tested in the chassis to ensure that everything aligns properly. Adjust the alignment of the pots and switches as necessary before soldering the remaining pins. Balance control Fig.10 shows the wiring of the switch for the balance con­trol. The resistors are wired around the switch pins together with three short lengths of hook-up wire. These are soldered to the tone control board which can now be mounted in the chassis. It is mounted to the front panel using the pot nuts and lockwash­ers. The rear of the tone control board is secured using two 12mm tapped spacers and screws. Don’t fit the dress panel to the chassis at this stage. It should be left in its protective wrapping for as long as possi­ble, to protect it from scratches. When all the pot nuts are secured, use your multimeter to check that all the pot cases are electrically connected together, via the chassis. If not, it might be necessary to remove the board from S1 Fig.8(a): the selector switch board. This mates up to the 14 pins on the input selector board & is soldered at right angles to it. Fig.8(b): the etching pattern for the selector switch board. Check that it has been trimmed correctly along the bottom, so that there are no shorts. April 1995  43 MONITOR S2 3 2 BALANCE S4 1 1k 1k MODE S3 1k SOURCE INPUT L GND R TAPE INPUT L GND R 1k 1uF 1 22uF 22uF 100uF 100pF VOLUME VR1 100pF 100uF 1uF 15k 4.7k 1k 100k IC1 LM833 15k 4.7k 1k 100k 4.7k 4.7k TREBLE VR3 .0047 33pF 22k 22k .0047 100uF 4.7k 100uF 1 33pF IC2 LM833 .0047 .0047 4.7k -15V LED K 22k GND .01 0.1 3.9k .01 +15V LED A BASS VR2 22k 0.1 22k 82  82  Q2 5.6k Q1 6.8uF HEADPHONES D2 5.6k Q2 6.8uF 1 D1 5.6k 47k 33pF 15  15  100  100  Q1 22k 10k 10k 15  15  OUTPUT TO POWER AMPLIFIERS R GND L 22k D1 5.6k D2 47k 33pF 10k 10k IC3 TLO72 TONE DEFEAT S5 100uA 100uF 22k 44  Silicon Chip Fig.9(a) (left): the tone control board. Note that while the balance con­trol (S4) looks like a single potentiometer, it is actually a rotary switch, as shown in Fig.10 on the facing page. Fig.9(b) (above) shows the PC pattern for this board. This is shown 70% of actual size & may be reproduced full size by enlarging it by a factor of 1.41 on a photostat machine. RIAA preamp board As noted previously, this preamp board is optional and we assume that many readers will not need it. The parts layout is shown in Fig.11(a). It’s best to start with the smaller parts (resistors and wire links) first. Take care with the orientation of the LM833 IC and the electrolytic capacitors. The two input inductors (L1) are each made by winding four turns of 0.4mm enamelled copper wire on a ferrite bead (Philips type 4330 030 3218). Power amplifier board This board is identical to that presented in the February 1995 issue but we are repeating the assembly instructions here for the sake of completeness. The component layout is shown in Fig.13(a). To begin, first install the PC pins and links, followed 91k 7 6 7k 4. 4. 7k 91k 5 1.6k 4 9 1.6k 8 3 S4 82 0  A 10 2 11 6k 1. 12 82 0  the chassis and then take a round file to lightly clean off any paint or anodising from around the pot mounting holes. The reason for making sure that the pots are properly earthed is to keep hum and noise to a minimum. Don’t forget to strip the enamel off the ends of the lead wires before the inductors are soldered into the PC board. 1 1. 6k This photo gives a good general view of the tone control board and the power amplifier board. 1 3 2 CONNECT TO PINS 1, 2 AND 3 ON CONTROL BOARD The balance control is an 11-position rotary switch with resistors wired around its terminals. This arrangement gives much better separation between channels than a potentiometer. Fig.10: here’s how the rotary switch is wired with the resistors to provide the balance control. April 1995  45 10uF 22uF 1M RIGHT OUTPUT GND 150  RIGHT INPUT .015 390W 100pF 100k 100  100k GND 200k 16k .0047 1 L1 100k LEFT OUTPUT 100pF 1M 10uF 390  GND 100  100k 0V -15V 16k 150  GND 0.1 47uF .0047 0.1 IC1 LM833 LEFT INPUT +15V 47uF L1 200k .015 22uF Fig.11(a): the optional RIAA preamplifier board. The large electrolytic capacitors are bipolar types & can be installed either way around. by the resistors and capacitors. Make sure that you install the electrolytic capacitors with correct polarity. This done, install the fuse clips and note that there is a trick to this task. The clips have little lugs at one end which stop the fuse from moving longitudinally. If you install the clips the wrong way around, you won’t be able to fit the fuses. HEATSINK 3mm SCREW DEVICE MICA WASHER INSULATING BUSH Fig.11(b): the full-size etching pattern for the optional RIAA preamplifier board. In most cases, this board will not be needed. 3mm WASHER 3mm NUT Fig.12: each LM3886 is insulated from its heatsink using a mica washer & insulating bush. Smear the mating surfaces lightly with heatsink compound before bolting the assembly together. The mains switch should have its lugs sleeved with heatshrink tubing to avoid the possibility of electric shock. 46  Silicon Chip L1, the loudspeaker filter inductor, consists of 15 turns of 0.5mm enamelled copper wire wound onto a 10Ω 1W resistor and soldered at both ends. To wind it, first scrape the enamel off the start of the copper wire and solder it to one end of the resis­tor. Now neatly wind 15 turns onto the resistor body, then scrape the enamel off the end of the wire and solder it to the other end of the resistor. Finally, install and solder the assembly into the PC board. The positive and negative power supply connections to the right channel should be made with heavy duty hook-up wire (32 x 0.2mm or better) which should be twisted as shown on Fig.13(a). The 0V connections should be made via the same sort of hook-up wire but underneath the board. Finally, you can install the power ICs. Make sure that the tabs of the devices line up precisely with the back edge of the PC board so that they can be properly secured to the heatsinks. Next, fit 15mm metal standoffs to the board and line up the heat­sinks against the ICs so that the positions of the mounting screws can be marked. After drilling these holes, use standard TO-3P mounting kits to secure the ICs to the heatsinks – see Fig.12. Use your multimeter (switched to a high “Ohms” range) to make sure that the IC mounting tabs are isolated from the heat­sinks. The heatsinks we used are supplied by Altronics (Cat H-0522). To mount them into the chassis, you could use small L-shaped brackets or, April 1995  47 -15V 100uF 47uF 0V 4700uF 0V +35V -35V F3 32 x 0.2 INSULATED WIRE ON COPPER SIDE OF BOARD +15V 100uF REG1 330  1W 47uF 4700uF SPEAKER GND 1 10 / L1 0.1 SPEAKER 47uF 1k 1uF GND INPUT (NC) 1k 22uF 22k 5.6 1W F2 Fig.13(b): this is the full-size artwork for the power amplifier PC board. Check all PC boards carefully for possible etching defects (compare them with the published patterns) before installing any of the parts. -35V F3 +35V 0.1 100uF SPEAKER GND 1 47uF GND INPUT (NC) 1k 22uF 22k IC1 3886 SPEAKER 0.1 5.6 1W 0.1 100uF 39k 100uF 10 / L1 IC1 3886 220pF 22k Fig.13(a): this is the parts layout on the power amplifier board. Use PC stakes to terminate external connections & note the twisted supply con­nections for the righthand channel. The leads shown dotted are underneath the board. The two LM3886 audio amplifier ICs must be insulated from the heatsinks, as shown in Fig.12. REG2 25VAC BR1 330  1W 100uF 39k CT 330  1W 330  1W 0.1 220pF 22k 25VAC 0.1 1k 1uF F2 LEFT F1 1A RIGHT SPEAKER OUTPUTS CORD GRIP GROMMET EARTH A (BROWN) POWER TRANSFORMER EARTH  LUE) N (B TRANSFORMER SECONDARIES GREY BLACK, BLUE AND GROUND CONNECTION ACTIVE RED POWER AMPLIFIER BOARD -35V WHITE +15V 0V G (NC) L -15V WHITE GND (0V) -35V LEFT OUTPUT DO NOT EARTH SIGNAL BRAIDS AT POWER AMPLIFIER +35V +35V R G L OUTPUT TO POWER AMPLIFIER EARTHED TO CASE A LED1 K -15V  TWO SOLDER LUGS +15V 0V .01 250VAC MAINS TERMINAL STRIP CONTROL BOARD TONE DEFEAT S5 HEADPHONES BASS VR2 POWER S7 K LED1 48  Silicon Chip A TREBLE VR3 VOLUME VR1 as we did, blind-tap holes into the edge to secure them directly. EXTERNAL EQUIPMENT GROUND PHONO CD TUNER VCR AUX 1 AUX 2 TAPE IN TAPE OUT Chassis wiring GND R L L INPUT BOARD R SELECTOR S1 G (NC) R RIAA-IEC PREAMPLIFIER BOARD BALANCE S4 GND RIGHT OUTPUT +15V 0V MONITOR S2 -15V MODE S3 GND RIGHT INPUT LEFT INPUT L G R SOURCE INPUT GND GND L G R TAPE INPUT LEFT OUTPUT RIGHT OUTPUT Fig.14 shows the chassis wiring details. The mains cord enters through a hole in the rear panel and is securely clamped using a cord-grip grommet. Strip back the outer sheath of the mains cord by about 80mm. The Active (brown) lead goes to the fuseholder while the Neutral (blue) lead goes to the mains terminal block. The other side of the fuseholder goes to the mains termi­nal block and then to the mains switch. The Earth lead (green/yellow) is soldered to one of the adjacent solder lugs. The second solder lug terminates the earth lead which is run along the rear panel from the binding post terminal adjacent to the RCA input sockets. Don’t alter the earth wiring – you may get a hum loop if you do. The primary leads of the transformer are connected to the mains terminal block, as shown, while the 25V secondary leads are connected to the screw terminal block on the power amplifier board. Be careful to use the correct phasing of the secondary leads, otherwise you will not get any DC output from the bridge rectifier. Be sure to use mains-rated 250VAC cable for the connections to the power switch. We used heatshrink tubing to cover the switch lugs after the wires had been soldered on. We also sleeved the connections to the fuseholder. This avoids the possibility of an electric shock from the switch terminals. Note that the .01µF 250VAC “anti-thump” capacitor con­nected at the mains terminal block must be rated at 250VAC. Do not install the shielded signal cables at this stage. The next step is to power up each board in turn and check that it is operating correctly. We start with the power amplifier board, since it the most involved. But first, Fig.14 (left): the chassis wiring details. Take care when installing the mains wiring & sleeve all exposed terminals on the fuseholder & mains switch with heatshrink tubing to avoid accidental contact. Make sure also that the mains cord is securely clamped by the cord grip grommet. April 1995  49 RESISTOR COLOUR CODES ❏ No. ❏   2 ❏   2 ❏   2 ❏   2 ❏ 12 ❏   2 ❏   4 ❏   4 ❏   8 ❏   1 ❏   4 ❏ 12 ❏   2 ❏   4 ❏   2 ❏   2 ❏   4 ❏   2 ❏   2 Value 100kΩ 91kΩ 47kΩ 39kΩ 22kΩ 15kΩ 10kΩ 5.6kΩ 4.7kΩ 3.9kΩ 1.6kΩ 1kΩ 820Ω 330Ω 100Ω 82Ω 15Ω 10Ω 5.6Ω 4-Band Code (1%) brown black yellow brown white brown orange brown yellow violet orange brown orange white orange brown red red orange brown brown green orange brown brown black orange brown green blue red brown yellow violet red brown orange white red brown brown blue red brown brown black red brown grey red brown brown orange orange brown brown brown black brown brown grey red black brown brown green black brown brown black black brown green blue gold brown 5-Band Code (1%) brown black black orange brown white brown black red brown yellow violet black red brown orange white black red brown red red black red brown brown green black red brown brown black black red brown green blue black brown brown yellow violet black brown brown orange white black brown brown brown blue black brown brown brown black black brown brown grey red black black brown orange orange black black brown brown black black black brown grey red black gold brown brown green black gold brown brown black black gold brown green blue black silver brown OPTIONAL RIAA PREAMP ❏ No. ❏   2 ❏   2 ❏   4 ❏   2 ❏   2 ❏   2 ❏   2 Value 1MΩ 200kΩ 100kΩ 16kΩ 390Ω 150Ω 100Ω check all your work carefully against the associated wiring diagrams of Fig.13(a) and Fig.14. Power amplifier testing Before checking the power amplifier board, connect a 1kΩ 0.5W resistor between the +15V and 0V rails at the 3-way terminal block (adjacent to the 3-terminal regulators). This 1kΩ resistor will draw a 15mA current from the +15V supply rail and thus ensure that the input voltage to the 7815 regulator does not exceed the ratings (ie, 35V). Now apply power and check the supply rails. They will nor­ m ally be around ±37V, depending on the value of the AC mains voltage. Now check the quiescent current in each channel. This can be done in one of two ways. The first is to remove 50  Silicon Chip 4-Band Code (1%) brown black green brown red black yellow brown brown black yellow brown brown blue orange brown orange white brown brown brown green brown brown brown black brown brown one fuse (while the power is off) and connect your multimeter, switched to an “Amps” range, across the fuse clips. With no input signal and no load, the quiescent current should typically be around 30mA but may range up to 70mA. Alternatively, you can connect a 100Ω 1W resistor across the positive rail fuse clips and measure the voltage across it. For a current of 30mA, the voltage across the 100Ω resistor should be 3V DC. The DC voltage at the output of each channel should be within ±15mV of 0V DC. Next connect suitably rated loudspeakers and check that you can get an output. With no signal, both channels should be very quiet. If you touch the input PC pins on the PC board you should get an audible “blurt” from the relevant loudspeaker. 5-Band Code (1%) brown black black yellow brown red black black orange brown brown black black orange brown brown blue black red brown orange white black black brown brown green black black brown brown black black black brown If the circuit isn’t working, check all the audio paths from the input through to the output for continuity. You should also check that the PC pins are well soldered into position, as is link LK1. If LK1 is open circuit, the amplifier will be muted. If all is well, switch off, connect the ±15V supply wires to the tone control board and check the voltages on it. This done, connect the supply wires to the RIAA preamp (if fit­ ted) and check the voltages on it. If all these checks are OK, you can complete the wiring of the amplifier by running all the shielded cable, as shown in Fig.14. You will also need to fit the extension shaft to the selector switch. Troubleshooting If the above measurements are not OK, the most likely causes are broken Compare this photo of the amplifier with the chassis wiring diagram of Fig.14. Note that the RIAA preamp in the righthand front corner is optional & if left out, it leaves an extra pair of high level inputs. tracks or solder bridges between IC pins. For example, if you have the correct supply voltages on an IC but its output is close to +15V or -15V, it is most likely that there is a break in the feedback network or to the inputs to that IC. You can follow this up by measuring the voltage at the input pins of the ICs. Again, these should all be very close to 0V. If not, check for breaks in the copper track, poor solder joints, and that the IC is not in the wrong way around. Note: if you’ve put the IC in the right way around, it is most unlikely that any malfunction will be due to a faulty IC. So don’t immediately rush out and buy new ICs if you strike problems. What happens if one of the power amplifiers is not working? If the other channel is working correctly, then you have an ideal crosscheck. Check the voltages in the good channel and then in the bad channel and you can usually get a fair idea of what the problem is. It is unlikely that you will get the same fault in both channels, unless you have made the same assembly mistake in both! Listening tests No, we’re not going to listen to music – yet. The idea of the next few checks is to make sure that everything is really working as it should. You’ll need a pair of headphones. Plug them into the headphone socket, turn on the power and listen. With the Volume at minimum you shouldn’t be able to hear anything. If you now select the phono input and wind up the Volume to max­imum, you will hear some hiss and a small amount of hum. That is normal. If you now switch to the other inputs (CD, Tuner, etc), the noise should drop to extremely low levels (we doubt you’ll be able to hear anything, even in a very quiet room). Now wind the Volume control back, switch to the CD inputs and try poking a small screwdriver into the left channel input socket. You should hear a “blurt” in the left channel. Now try the test for the right channel. If you repeat this test for extreme CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ Value 1µF .01µF .0047µF 220pF 100pF 33pF IEC 1u0 10n 4n7 220p 100p 33p EIA 105 103 472 221 101 33 OPTIONAL RIAA PREAMP ❏ ❏ ❏ ❏ ❏ Value 1µF .015µF .0047µF 100pF IEC 1u0 15n 4n7 100p EIA 105 153 472 101 settings of the tone controls (eg, full bass boost, full bass cut, etc) you can confirm that they are working as well. Similarly, you can check the operation of the Mono/Stereo switch and the Balance control. If all is well, the front panel can now be mounted but be careful – one scratch and you’ll ruin the appearance of the whole project. Fit the lid to the SC case and the job is finished. April 1995  51