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Ultra-low-distortion Preamplifier with Tone Controls Part 2 by John Clarke Last month, we introduced our state-ofthe-art stereo Preamplifier. Along with almost unmeasurable noise and distortion – typically 0.0003% THD+N! – it sports remote volume control, input selection and muting, plus bass and treble adjustment knobs on the front panel. Now let’s build the input selection boards and power supply. T he circuits of the optional input selector board and frontpanel pushbutton board were shown in Figs.8 and Fig.9 last month. We also listed the parts required to build those two boards in that article. Fig.10 and Fig.11 show the PCB overlay diagrams for these two boards, so you can see how those parts are fitted. By the way, you don’t have to build either of these boards if you don’t need the ability to select between more than one set of stereo inputs. In that case, you would connect the chassis-mounted input sockets directly to CON1 and CON3 on the main Preamplifier board. Constructors – please read note in the April 2020 issue Parts List before purchasing components. 38 If you do want the Input Selector but only need the remote control feature, and don’t want front panel pushbuttons/input indicators, you could build the input selector board (Fig.10) but not the front panel pushbutton board (Fig.11). You can then use the remote control to select between the three inputs, although it won’t show which is selected – you will have to remember the last selection you made. Incidentally, we haven’t listed all the features and specifications again – refer to the April issue for these and performance graphs. (We think you’ll agree, it’s outstanding!) Input selector construction The Input Selector board is easy to assemble. It’s built on a double-sided PCB, coded 01111112, measuring 110 × 85mm and available from the PE PCB Service). Start by fitting the resistors where shown. Remember to always check resistor values with a DMM set to measure resistance to make sure they’re going in the right places. Follow with diodes D1-D3, ensuring that their cathode stripes face as shown, then feed some resistor lead offcuts through the ferrite beads and solder them in place. Features  Bass and treble controls  Motorised volume control  Infrared remote or manual control  Relay input switching and isolation  Suits practically any amplifier module. Practical Electronics | May | 2020 SC INPUT 1 20 1 9 INPUT 2 CON11 INPUT 3 CON 1 2 CON13 RELAY2 2.2kΩ 100kΩ 2.2kΩ 2.2kΩ 4004 100Ω D3 2.2kΩ Q7 2.2kΩ Q6 2.2kΩ Q5 9 10 10 µF 10kΩ 2.2kΩ 2.2kΩ LEFT TUP NI REIFOUTPUT ILP MAERP 10kΩ RIGHT 2 1 1 1 1 1 1 0OUTPUT 2.2kΩ 100kΩ 100kΩ 1 2 Fig.11 (below): the three switches are mounted on the front of the pushbutton board while the header socket goes on the back (keyway towards S2). Take care with the switch orientation (see text): the six pins for each switch are for the switch contacts themselves (four) plus two for the integral LEDs. TO CON 9 ON INPUT SELECTOR BOARD CON8 10 µF IC4 LM393 CON14 100nF CON15 2.2kΩ 100nF 470pF 100Ω D2 1 2 2.2kΩ BEAD 470pF 4004 100Ω RELAY2 D1 100Ω 100Ω 4004 BEAD 100Ω RELAY1 CON9 13 14 Fig.10 (left): follow this diagram to build the input selector PCB. Make sure that the two header sockets are correctly oriented and note that Q5-Q7 are BC327 PNP transistors, while Q8 is a BC337 NPN transistor. 14 13 2 1 CON10 (ON BACK) Q8 S1 +LED1 We recommend you solder IC4 directly to the board, although you can use a socket if you really want to. Either way, make sure its pin 1 dot/notch faces to the left, as shown. Fit the MKT/MKP/ceramic capacitors next. We explained in detail last month why there are three different options for the 470pF capacitors, and that if you use ceramics, they must be NP0/ C0G types for good performance. We used MKTs on our prototype. Mount them, plus the two 100nF MKTs now. Next, solder the four transistors, noting that Q5-Q7 are BC327s, while Q8 is a BC337. The two electrolytics can then go in, with the longer positive leads through the holes marked ‘+’, followed by the 10-way and 14-way header sockets, CON8 and CON9. These sockets must be installed with their slotted key-ways towards the top. Finally, complete the assembly by installing the relays, the three stereo RCA input sockets and the two vertical RCA output sockets. Note the left and right labelling for the output sockets – this is not a mistake and arranging them this way gives the optimum layout for the PCB. Front panel pushbutton board assembly There just four parts on the pushbutton board – the three pushbutton switches on one side and the 14-way IDC header socket on the other (see Fig.11 above). The board is coded 01111113, measures 66 × 25mm and is available from the PE PCB Service. The three pushbuttons can go on first, but note that they must be installed the right way around. These have ‘kinked’ pins at each corner plus two straight pins for the integral blue LED. The anode pin is the longer of the two and this must go in the hole marked ‘A’ on the PCB (towards the header). S2+LED2 S3 +LED3 Once the pins are in, push the buttons all the way down so that they sit flush against the PCB before soldering their leads. The IDC header socket can then be installed on the other side of the board, with its key-way notch towards the bottom. Choice of power supply If you’re building this Preamplifier as part of a full amplifier, chances are you already have a suitable power supply producing the required ±15V DC rails. Otherwise, we mentioned a few suitable power supply designs last month – see the box at the end of the article. If you’re building a standalone Preamplifier, or building the Preamplifier into an amplifier which already has a power supply but doesn’t have ±15V DC rails then you could use the the 4-Output Universal Voltage Regulator from our May 2016 issue. The parts are easy to source and the PCB (code 18105151) is available from the PE PCB Service. However, there are two other options. You could build a simple low-power Universal Regulator. The parts are easy to source and we can sell you the board (code 18103111). You could choose These views show the completed Input Selector and (at right) both sides of the pushbutton board assemblies. Note the orientation of the header sockets on the two modules – check that these sockets, the relays, the RCA sockets and the button switches are all sitting flush against their respective PCBs before soldering their leads. Practical Electronics | May | 2020 39 this supply for the same reasons as the 4-Output model described above. However, if you are also building a power amplifier from scratch, then it is worth considering the Ultra-LD Power Supply board. It is available as a kit (code K5168) from Altronics, or you can buy its PCB (code 01109111) from the PE PCB Service and source your own parts. As noted above, the the 4-Output Universal Voltage Regulator was covered in our May 2016 issue, so we’ll only explain building the other two supplies here. Building the Universal Regulator Fig.12 shows the circuit of the Universal Regulator while Fig.13 is the PCB overlay. You can power it from a 30V centretapped transformer secondary (15-015V) or a single 15V winding. The centre-tapped option is better if you can swing it, since it results in a lower ripple at the regulator inputs. The transformer’s AC output is rectified by a diode bridge (D1-D4) and filtered by a pair of 2200µF capacitors. It’s then regulated to +15V by REG1 and −15V by REG2. These regulated rails are available from terminal block CON2, which is wired to the Preamplifier. Its built on a PCB measuring 71 × 35.5mm; see opposite for the list of parts you’ll need. Start assembly by fitting the the two resistors and then the six diodes (with the polarity shown in Fig.13). Next, mount the LEDs with the longer (anode) leads towards the bottom of the board. Follow with the two MKT capacitors. You can then fit the two 3-way terminal blocks, with the wire entry holes facing the nearest edge of the board. The Universal Regulator power supply board can handle a wide range of inputs and outputs.With a 15-0-15V AC transformer you will get a regulated +15, 0V and −15V DC supply, perfect for the Ultra Low Noise Preamplifer (and many other projects!). Now solder REG1 and REG2 with the tabs towards the board edge as shown, taking care not to get the two mixed up. Finally, solder the four capacitors in place, ensuring that their longer (positive) leads go into the pads marked with a ‘+’ symbol. The photo above shows two flag heatsinks (and they are mentioned in the parts list). It won’t hurt to fit these, but if you’re only going to be powering the Preamplifier and your transformer secondary voltage is the recommended value, they should not be necessary since the Preamplifier doesn’t draw a lot of current. Building the Ultra-LD Power Supply The circuit of the Ultra-LD Power Supply is shown in Fig.14. The bottom section is similar to the Universal Regulator described above and operates in the same manner. A chassismount bridge rectifier is used for the high-voltage AC secondaries of the power transformer, which are shown as 40-0-40V here, but lower voltages can be used with this board too. The resulting DC rails are then filtered by three 4700µF capacitors each and made available at CON1 and CON2, to be fed to the amplifier modules. Fig.15 is the PCB overlay for this supply. The two wire links should not be necessary as our boards are doublesided and have copper strips on the top layer connecting these points, but if you DIY etch a single-sided board, you will need to fit the two links using 1mm-diameter tinned or enamelled copper wire. Next, mount the diodes with the orientation shown, then the LEDs, with the longer (anode) leads towards the top of the board. You can then bend the regulator leads to fit the hole pattern on the PCB and attach their tabs to the board securely using M3 machine screws and nuts. Once you’ve checked that they are straight, solder and trim the leads. The terminal blocks go in next. Dovetail CON4 with CON5 and CON3 with CON6 before soldering them in place, with the wire entry holes facing towards the nearest edge of the board. Then you can mount the two 5W resistors, with their bodies a few millimetres above the PCB surface to allow cooling air to circulate. Next, install the spade connectors. If using the vertical type with two pins, push these into the board and solder them in place – you will need a hot iron to do this. If using the chassis-mounting REG1 7815 D1 A T1 INPUT 15V 230V 0V 15V N 1 K K IN D4 A A K K OUT A GND C1 2200 µF 25V 20.5V K  LED1 100 µF 25V 100nF D5 K OUTPUT 3 +15V 2 2 0V 3 1 –15V D2 CON1 A A A 1.5k D3 1.5k C2 2200 µF 25V 20.5V 100 µF 25V 100nF K K A 78 1 5 7 91 5 LEDS D1-D6: 1N4004 K A OUT REG2 7915 A CON2 D6  LED2 GND IN K A IN GND IN OUT GND IN GND OUT SC UNIVERSAL Universal Regulator REGULATOR TAPPED TRANSFORMER SECONDARY, DUAL OUTPUT CONFIGURATION 2011 Fig.12: the Universal Regulator circuit generates ±15V rails. Diodes D1-D4 form a bridge rectifier, while capacitors C1 and C2 filter the rectified AC. Regulators REG1 and REG2 provide a steady output voltage, while LED1 and LED2 indicate operation. You can also use a transformer with a single secondary (or a plugpack) connected between pins 1 and 2, or 2 and 3 of CON1. 40 Practical Electronics | May | 2020 4004 4004 SC 100nF 4004 + 100 µF D2 D3 + D4 C2 2200 µF 1102 © REG1 - + 100 µF REG2 rotalug eR lasr evinU type, attach them to the board using the specified M4 machine screws and nuts, as shown in Fig.16. Now all that’s left is to solder the small electrolytics capacitors in place, followed by the large ones. In both cases, the longer (positive) leads must go into the pads marked with a ‘+’ on the PCB. Initial checks Before installing the three ICs on the preamp board, it’s a good idea to check the supply voltages. You will need to wire up a transformer to your power supply, then connect the power supply’s +15V, 0V and −15V outputs to the relevant inputs on the main preamplifier PCB. It’s safer to use a 15VAC plugpack for testing if you don’t already have the transformer and power supply installed in an earthed metal chassis. Just connect one wire from the plugpack output to either of the low-voltage AC input terminals on the power supply board, and the other wire to the centre tap transformer connection point. Plug the plugpack into a mains socket and switch on. Now check the voltages on pins 8 and 4 of the D5 C1 2200 µF 1.5k + 100nF LED2 3 2 – 1.5k + DC OUTPUT 3 D1 LED1 CON2 4004 2 CON1 AC INPUT 1 4004 11130181 + + D6 CS 4004 18103111 n© I 2011 G 0V 1 – Fig.13: this PCB overlay corresponds with the circuit of Fig.12. You could fit flag heatsinks to REG1 and REG2 but they are not strictly necessary for use with the Preamplifier, as it doesn’t draw a lot of current. four 8-pin IC sockets (IC1-IC4) on the preamp board; ie, between each of these pins and the 0V (centre) terminal of CON6. You should get readings of +15V and −15V respectively. Similarly, check the voltage on pin 14 of IC5’s socket. It should be between +4.8V and +5.2V. If these voltages are correct, switch off and install the ICs. Note that IC1IC4 face one way, while microcontroller IC5 faces the other way. Remote control/switch testing The remote control functions can now be tested using a suitable universal remote, eg, Altronics A1012. As stated earlier, the default device mode programmed into the micro is TV, but if this conflicts with other gear you can choose SAT1 or SAT2 as the device instead. Whichever mode is chosen, you must also program the correct code into the remote (see panel below). Note that if you don’t have a split rail power supply ready yet, you can still check the remote control functions by using a single 9-15V DC supply connected between the +15V and 0V terminals of CON6 (watch the polarity). Selecting The Mode and Programming The Remote As stated in the text, it’s necessary to program the universal remote control correctly. By default, the microcontroller’s RC5 code is set to TV but SAT1 or SAT2 can also be selected. Just press and hold button S1 on the pushbutton board during power-up for SAT1 or button S2 for SAT2. Pressing S3 at power-up reverts to TV mode. Once you’ve chosen the mode or ‘device’, the correct code must be programmed into the remote. This involves selecting TV, SAT1 or SAT2 on the remote (to agree with the microcontroller set-up) and then programming in a three or 4-digit number for a Philips device. That’s because most Philips devices (but not all) use the RC5 code standard that’s expected by the Preamplifier. Practical Electronics | May | 2020 Most universal remote controls can be used, including the model shown above, the Altronics A1012 and the Jaycar AR1955 or AR1954. For the Altronics A1012, use a code of 023 or 089 for TV mode, 242 for SAT1 or 245 for SAT2. Similarly, for the Jaycar remotes, use code 1506 for TV, 0200 for SAT1 or 1100 for SAT2. In the case of other universal remotes, it’s just a matter of testing the various codes until you find one that works. There are usually no more than 15 codes (and usually fewer) listed for each Philips device, so it shouldn’t take long to find the correct one. Note that some codes may only partially work, eg, they might control the volume but not the input selection. In that case, try a different code. Also, some remotes may only work in one mode (eg, TV but not SAT). Parts list Universal Regulator (±15V outputs) 1 PCB, code 18103111, 71 x 35.5mm, available from the PE PCB Service. 1 transformer, 230V AC primary, 15-0-15V AC or 230V AC to 15V AC plugpack to suit (see text) 2 3-way terminal blocks, 5.08mm pitch 4 tapped spacers 4 M3 x 6mm machine screws 2 TO-220 heatsinks (optional) 2 M3 x 10mm machine screws, nuts and shakeproof washers for heatsinks Semiconductors 1 7815 +15V linear regulator 1 7915 −15V linear regulator 6 1N4004 diodes 1 red 5mm LED 1 green 5mm LED Capacitors 2 2200µF 25V electrolytics 2 100µF 25V electrolytics 2 100nF MKT Resistors (all 0.25W 1% metal film) 2 1.5kΩ Ultra-LD amp/preamp power supply (±57V and ±15V outputs) 1 PCB, code 01109111, 141 x 80mm, available from the PE PCB Service. 1 transformer, 40-0-40V and 15-0-15V AC secondaries (see text) 4 3-way PCB-mount terminal blocks, 5.08mm pitch (Altronics P2035A or equivalent) (CON1-4) 2 2-way PCB-mount terminal blocks, 5.08mm pitch (Altronics P2034A) (CON5-6) 3 PCB-mount or chassis-mount spade connectors [Altronics H2094] 3 M4 x 10mm screws, nuts, flat washers and shakeproof washers (if using chassis-mount spade connectors) 4 M3 x 9mm tapped nylon spacers 6 M3 x 6mm machine screws 2 M3 shakeproof washers and nuts 150mm 0.7mm dia. tinned copper wire Semiconductors 1 35A 400W chassis-mounting bridge rectifier (BR1) 1 7815 1A 15V positive linear regulator (REG1) 1 7915 1A 15V negative linear regulator (REG2) 4 1N4004 1A diodes (D1-D4) 1 5mm green LED (LED1) 1 5mm red LED (LED2) Capacitors 6 4700µF 63V electrolytic 2 2200µF 25V electrolytic 2 220µF 16V electrolytic Resistors 2 3.3kΩ 5W 41 ~ T1 CON1 TERM1 BR1 35A/600V + +57V A ~ 4700 µF 63V 4700 µF 63V 4700 µF 63V 40V POWER S1 F1 5A A TERM2 – 0V 0V 3.3k 5W –57V A 40V 0V 4700 µF 63V TERM3 4700 µF 63V 4700 µF 63V CON2  LED2 +57V 3.3k 5W 0V K 15V N  LED1 K 0V –57V CON4 15V CON5 30V AC 0V E T1: 240V TO 2x 40V/300VA, 2x 15V/7.5VA CON6 D1 –D4 : 1N4004 K +20V 0V K 1N4004 A K A A K K LEDS A REG1 7815 GND 2200 µF 25V +15V OUT IN A CON3 100 µF 16V K A 0V 78 1 5 7 91 5 IN OUT GND IN GND IN GND 2200 µF 25V IN GND OUT 100 µF 16V –15V REG2 7915 OUT SC Power amplifier and&preamplifier power AMPLIFIER PREAMPLIFIERpower POWERsupply SUPPLY 2019 Fig.14: the Ultra-LD Power Supply is based on a toroidal transformer (T1) with two 40V windings and two 15V windings, but you could use two separate transformers if necessary. You can use a transformer with lower voltage main secondary windings (ie, less than 40V) to achieve a lower amplifier supply voltage, without making any changes to the board. A s before, check the voltage on pin 14 of IC5’s socket (it must be between +4.8V and +5.2V), then switch off and install IC5 (pin 1 towards IRD1). Also, insert the jumper link for LK3 to enable the mute return function Now connect the three boards using the ribbon cable assemblies. The connectors are all keyed, so as long as you CON4 CON1 A LED2 – 4700 µF 63V + 4700 µF 63V + NI- + TERM3 –IN TC TERM2 4700 µF 63V 4700 µF 63V + 4700 µF 63V + + CT NI+ TERM1 +IN LED1 + CON2 AMPLIFIER POWER 2 tuptu O–57V 0V +57V OUTPUT 2 - + 3.3k 5W DC INPUT FROM BRIDGE 4700 µF 63V A 4004 4004 CON5 K A K A 4004 K K 4004 2200 µF 2200 µF 25V 25V REG2 7915 REG1 7815 220 µF 16V D3–D6 11190110 3.3k 5W 220 µF 16V CON3 CON6 +20V –15V V 5 1- 00 +15V V 5 1 + 00 V 02+ PREAMP DC OUTPUT 42 uS r e woP reifilpmA 2.k M DL-artlU 0110 9 111 AMPLIFIER 1 tuptu O POWER OUTPUT 1 2 buttons on the pushbutton board. Each time a button is pressed, you should hear a ‘click’ as its relay switches on and the blue LED in the corresponding switch button should light. Also, the orange Acknowledge (ACK) LED should flash each time you press a button on the remote. If the ACK LED doesn’t flash, make sure the code programmed 15V AC INPUT into the remote matchCA V 5 1 TCT C 15V CAV 0 3 ~ 5 1 30VAC 15V es the device mode (ie, TV, SAT1 or SAT2). The Ultra-LD Mk.3 Power Supply +57V + 0V 0 –57V - plug the 10-wire cable into the 10-pin sockets and the 14-wire cable into the 14-pin sockets, everything should be connected properly. Next, rotate VR2 fully anticlockwise and use the remote to check the various functions. First, check that the inputs can be selected using the 1, 2 and 3 buttons on the remote and the S1-S3 Fig.15: install the parts on the Ultra-LD Power Supply board as shown here, taking care to ensure that all the electrolytic capacitors are mounted with the correct polarity. Be sure also to use the correct regulator at each location. The two LEDs indicate when power is applied and remain lit until the 4700µF capacitors discharge after switch-off. Practical Electronics | May | 2020 The Ultra-LD Power Supply can handle both an amplifier and this Preamplifier. Be sure to watch the polarity of electrolytic capacitors, diodes, LEDs and regulators. ACK LED won’t flash at all unless the code is correct. Now check that the volume pot turns clockwise when the Volume Up and Channel Up buttons are pressed and anti-clockwise when Volume Down and Channel Down are pressed. It should travel fairly quickly when Volume Up/Down buttons are pressed and at a slower rate when the Channel Up/Down buttons are used. If it QUICK CONNECT PC BOARD M4 FLAT WASHER M4 STAR WASHER M4 x 10mm SCREW & NUT Fig.16: here’s how the single-ended male spade quick connectors are secured to the power supply PCB. Vertical spade terminals with solderable pins can also be used. turns in the wrong direction, reverse the leads to the motor. Adjusting trimpot VR2 Next, set the volume control to midposition, set VR2 fully anti-clockwise and hit the Mute button. The pot will rotate anti-clockwise, and as soon as it hits the stops, the clutch will start to slip. While this is happening, slowly adjust VR2 clockwise until the motor stops. Now press Volume Up to turn the potentiometer clockwise for a few seconds and press Mute again. This time, the motor should stop as soon as the pot reaches its anticlockwise limit. A programmed time-out of 13 seconds will also stop the motor if it continues to run after Mute is activated. This means that you have to adjust VR2 within this 13s period. If the motor stops prematurely or runs for the full 13s after the limit is reached, try redoing the adjustment. Making the interconnecting cables To connect the three boards, you need to make two IDC cables. These diagrams show how these cables are made. Pin 1 on the header sockets is indicated by a small triangle in the plastic moulding and the red stripe of the cable must always go to these pins. You can either crimp the IDC headers to the cable in a vice or use an IDC crimping tool (eg, Altronics T1540 or Jaycar TH1941). Don’t forget to fit the locking bars to the headers after crimping, to secure the cable in place. Having completed the cables, it’s a good idea to check that they have been correctly terminated. The best way to do this is to plug them into the matching sockets on the PCB assemblies and then check for continuity between the corresponding pins at either end using a multimeter. Practical Electronics | May | 2020 1 0 -WAY IDC SOCKET Troubleshooting If the unit fails to respond to remote control signals, check that the remote is in the correct mode (TV, SAT1 or SAT2) and has been correctly programmed. If you’re using a remote other than those listed in the panel, work through the different codes until you find one that works. Start with codes listed under the Philips brand as these are the most likely to work. If the unit responds to the 1, 2 and 3 buttons on the remote but the button switches don’t work, check that the ribbon cable to the pushbutton board has been crimped properly. Similarly, if the remote volume function works but not the remote input selection, check the cable from the Preamplifier board to the input selector board. Note that the cable from the Preamplifier board also supplies power to the other two boards. So it’s worthwhile checking that there is 5V between pins 8 and 4 of IC4 on the Selector board and again check the ribbon cable if this supply rail is missing. Audio testing If you are using a ±15V supply for testing, you can test the Preamplifier further by connecting its outputs to a stereo amplifier and feeding in audio signals from a mobile phone, tablet, iPod, CD/DVD/Blu-Ray player or just about any other source. Depending on your device, you may need a cable with a 3.5mm stereo plug at one end and red/white RCA plugs at the other end to make the connection. These are commonly available. Reproduced by arrangement with SILICON CHIP magazine 2020. www.siliconchip.com.au 1 0 -WAY IDC SOCKET LOCATING SPIGOT UNDER 200mm x 1 0-WAY IDC RIBBON CABLE CABLE EDGE STRIPE LOCATING SPIGOT UNDER 300mm x 14 -WAY IDC RIBBON CABLE 14-WAY IDC SOCKET CABLE EDGE STRIPE 14-WAY IDC SOCKET 43