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Stereo Class-A Amplifier; Pt.4 By JOHN CLARKE & GREG SWAIN P r ea m p l i f i er & Rem o t e Vo l u m e Con t r o l Mo d ul e In Pt.4 this month, we present a high-performance Stereo Preamplifier & Remote Volume Control module. It’s a lownoise, low-distortion design specifically engineered for the Class-A amplifier but which can also be used with other stereo power amplifiers. Depending on your requirements, you have several options when it comes to using the new Class-A Stereo Amplifier. Basically, the unit can be combined with a high-quality external preamplifier or used as a standalone unit. Typically, an external preamplifier will be necessary if you want to connect several signal sources and switch 14  Silicon Chip between them; eg, select between a CD player, DVD player and a tuner. The Class-A Stereo Amplifier would then function simply as a power amplifier, with the signal from the external preamp fed directly to the inputs of the power amplifier modules. In this case, all you would need to build into the chassis are the left and right-channel Class-A Power Amplifier modules (May & June 2007), plus the Loudspeaker Protector & Muting Module (July 2007). If you do elect to use an external preamplifier, then the SILICON CHIP Studio Series Stereo Preamplifier (October 2005, July 2006) makes the ideal companion unit. By the way, don’t be put off by the 102dB signal-to-noise ratio quoted for that unit in the July siliconchip.com.au Preamplifier Features & Performance Main Features • • • High performance design – very low noise and distortion Designed for the Class-A Stereo Amplifier but can also be used with other power amplifier modules On-board remote volume control circuit with motorised potentiometer and muting Measured Performance Frequency response............... flat from 10Hz to 20kHz, -3dB <at> 100kHz Input impedance.....................................................................~22kW Output impedance..............................................................100W Harmonic distortion................................. typically <.0005% Signal-to-noise ratio....... -125dB unweighted for 1V input Channel crosstalk................................... typically -125dB ultra-low noise and distortion, but with more than enough gain (with the “wick” wound right up) to drive the 20W Class-A Amplifier modules to full power output. In fact, if you were to wind the wick up too far, the amplifier will be driven well into clipping and horrible distortion. That pretty much defeats the purpose of building a high quality amplifier, so don’t do it! This preamplifier is almost identical in configuration to our Studio Series Stereo Preamplifier (October 2005). It’s a 2-chip design employing a dual op amp IC in each channel, the first stage providing the gain and the second stage acting as a buffer for the volume control, to present a constant low output impedance to the power amplifier modules. Low-noise op amps 2006 issue. That was a misprint – the correct figure is 110dB, so the Studio Series Stereo Preamplifier is an excellent performer that’s quite up to the job (especially considering its distortion is typically less than .0005%). Alternatively, many readers will want to use only one signal source, typically a CD or DVD player. In that case, the Class-A Stereo Amplifier can be used as a standalone unit but you do need to add a volume control. If your CD player is already fitted with an output level control, you may be tempted to dispense with a volume control on the amplifier but that could be a mistake. Just imagine what a blast you will siliconchip.com.au get from the amplifier and loudspeakers if you turn on the CD player and it has been inadvertently set to full output level. The result would not only be deafening but it could easily blow your tweeters. The simplest solution which we would recommend is to feed the signal in via a dual-gang 10kW log pot and we’ll show you how to do that next month, if you want to use that option. This simple scheme does have its problems though. First, the input signal level may be insufficient to drive the amplifiers to full power output, even when using a CD player. The amplifier modules have an input sensitivity of 625mV for full power but some recordings may give average output signal levels well below this. Second, using a simple volume control varies the input impedance to the power amplifiers, thereby slightly degrading the signal-to-noise ratio. Admittedly, we’re splitting hairs some­ what here but this is after all a true audiophile’s amplifier. So how do you eliminate those problems and achieve the level of performance we want? The answer is to incorporate a high-quality preamplifier module into the Class-A Stereo Amplifier chassis. This will result in an attractive self-contained package that we think will appeal to many people – particularly those who just want to use a single CD/DVD player. The preamplifier module described here meets the above criteria. It’s a minimalist design, which delivers The original Studio Series Preamplifier was based the high-performance OPA2134 op amps from Burr-Brown. These are specified at an extremely low 0.00008% harmonic distortion at 1kHz and the harmonic distortion (THD) for the completed preamplifier was typically less than 0.0005%. This time we’ve gone even one better and specified the National Semiconductor LM4562 dual op amp. This new device is specified at just 0.00003% harmonic distortion at 1kHz, which is even lower than for the OPA2134. In fact, it is far below the measurement capability of any commercially available equipment, including our own Audio Precision test gear. Just how the low distortion is verified is revealed elsewhere in this issue. As a result, the performance of the module on its own is actually far better than the completed stereo amplifier. Just running with its own regulated supplies (and not connected to the amplifier), the preamplifier module delivers harmonic that’s typically less than 0.0005%, a measurement which is more or less meaningless because it is about the same as the Audio Precision equipment. Furthermore, its signal-to-noise ratio with respect to a 1V input signal is around -125dB unweighted (22Hz to 22kHz bandwidth) and separation between channels is also around -125dB. Ultimately, it is not possible to get this fantastic performance from the completed stereo amplifier. And why not? The main reason is that residual August 2007  15 Fig.1: each channel of the preamp is based on a low-distortion LM4562A dual op amp (left channel only shown). IC1a has a gain of two while IC1b functions as a unity gain buffer to provide a constant low-impedance output. noise and hum from the power supply degrades the overall measured result, even though the finished amplifier is extremely quiet. Would it be possible to get a better measured distortion performance? The answer is a qualified yes, provided we had completely separate power supplies for both channels. The same comment applies to channel separation and residual noise. Such a solution would be a lot more expensive and would probably involve two separate power amplifiers – the so-called “mono block” solution. By the way, there’s no source selection built into the preamplifier, as we’re assuming that you will be us- ing it with just a single source. If you do want to switch between different sources, then you will need to use an external switch box (or an external preamp as indicated above). Remote volume control OK, we just couldn’t help ourselves – we just had to include remote volume control as part of the preamp design. After all, no sound system is complete these days without remote volume control and this one has all the “musthave” features. The design is similar to an earlier unit which we published back in June 2002 and subsequently used in the Ultra-LD Stereo Amplifier. How- Altronics Has The Complete Kit A complete kit of parts for the 20W Class-A Stereo Amplifier is available from Altronics, 174 Roe St, Perth, WA 6000, Australia. This kit comes with all the necessary parts, including a pre-punched custom metal chassis and front and rear panels with screened lettering. In addition, Altronics sells the various modules separately, for those who don’t need the complete amplifier. Browse to www.altronics.com.au for the details. 16  Silicon Chip ever, by using the recently-released PIC16F88-I/P chip, as opposed to the PIC16F84 used in the earlier design, we’ve been able to eliminate an LM393 comparator IC and the low-voltage reset circuitry. That aside, the features of this new unit are identical. Again it uses a motorised potentiometer. Press the “Volume Up” and “Volume Down” buttons on your remote and the pot rotates clockwise and anticlockwise. It takes about nine seconds for the pot to travel from one end to the other using these controls. For finer adjustment, the “Channel Up” and “Channel Down” buttons can be used instead. These cause the pot shaft to rotate only about 1° each time one of these buttons is pressed. Alternatively, holding one of these buttons down rotates the pot from one end to the other in about 28 seconds. If any of the buttons is held down when the pot reaches an end stop, a friction clutch in the motor’s gearbox slips so that no damage is done. Automatic muting is another handy feature. Press the “Mute” button” on siliconchip.com.au the remote and the pot automatically rotates to its minimum position and the motor stops. Hit the button again and it returns to its original position. Don’t want the pot to return all the way to its original setting? Easy – just hit one of the volume control buttons when the volume reaches the desired level. A couple of LED indicators – “Ack” and “Mute” – are used to indicate the status of the Remote Volume Control. The blue “Ack” (acknowledge) LED flashes whenever an infrared signal is being received from the remote, while the orange “Mute” LED flashes while the muting operation is in progress and then remains on when the pot reaches its minimum setting. So how does the unit remember its original setting during muting? Well, the microcontroller actually measures the time it takes the pot to reach its minimum setting. Then, when the Mute button is pressed again to restore the volume, power is applied to the motor drive for the same amount of time. By the way, some readers may wonder why we did not incorporate the Digital Remote Volume Control published earlier this year, in the January & February 2007 issues. After all, it works well and has the advantage of an attractive blue LED digital display. The simple answer is that its distortion performance is not good enough to match that of the class-A amplifier. Fig.2: the IR receiver module used in the Remote Volume Control circuit contains a lot more than just a photo diode. This block diagram of the internals reveals an amplifier plus discrimination and demodulation circuits, all in the 3-pin package. After the 38kHz carrier is removed, the data appears on the “OUT” pin (1) ready to be processed by the micro. Preamplifier circuit details OK, so much for the background stuff. Let’s see how it all works, starting with the audio preamplifier. Fig.1 shows the circuit details, with just the left channel preamp stages shown for clarity, along with the power supply. The right channel preamp circuitry is identical to the left. The audio signal from the source is AC-coupled to the input of the first op amp (IC1a) via a 4.7mF capacitor, while a 22kW resistor to ground provides input termination. In addition, the signal passes via a low-pass filter formed by a 100W resistor, a ferrite bead and a 560pF capacitor. This attenuates radio frequencies (RF) ahead of the op amp input. IC1a operates with a voltage gain of 2 (+6dB) by virtue of the two 4.7kW feedback resistors. The 4.7kW resistor and 220pF capacitor combination roll off the top end frequency response, with a siliconchip.com.au Fig.3: this graph shows the frequency response of the whole amplifier (including the preamplifier), taken at a power level of 1W into 8-ohms. It’s almost ruler flat from 10Hz to 20kHz and then rolls off gently to be -3dB down at about 100kHz. -3dB point at about 150kHz. This gives a flat response over the audio spectrum while eliminating the possibility of high-frequency instability. Note, however, that the -3dB highfrequency point for the entire amplifier is about 100kHz – see Fig.3. The output from IC1a (pin 1) drives one end of potentiometer VR1a (20kW) via a 22mF non-polarised coupling capacitor. The pot acts as a simple voltage divider and the signal on its wiper is fed to the input (pin 5) of op amp IC1b. The wiper of the pot is also AC-coupled, this time using a 1mF non-polarised capacitor. This is done to prevent any DC voltage appearing across the pot, which if present would cause an irritating sound during wiper movement. IC1b is used as a unity-gain buffer. This stage allows the preamp to provide a low-impedance output regardless of volume control setting. A 22mF non-polarised capacitor couples the audio signal to the output via a 100W resistor, which is included to ensure stability when driving the cable and amplifier input capacitance. This reAugust 2007  17 Fig.4: the Remote Volume Control section is based on a PIC16F88-I/P microcontroller (IC3). This processes the signal from infrared detector IRD1 and controls the pot motor via H-bridge transistors Q1-Q4. sistor, together with the ferrite bead in series with the output, also helps to attenuate RF noise that might otherwise find its way back into the preamp circuit. Power supply Power for the circuit is derived directly from the ±22V terminals on the power supply board (described in June 2007). Diodes D1 & D2 provide reverse polarity protection, after which each rail is further filtered using a 220mF electrolytic capacitor. Two 3-terminal regulators – REG1 and REG2 – then provide ±15V supply rails to power the op amps. In addition, +22V and 0V outputs are provided from the power supply (via a separate terminal block). These outputs are used to power the Loudspeaker Protector & Muting Module when the amplifier is finally assembled. Remote volume control circuit Now let’s take a look at the circuit for the Remote Volume Control – see 18  Silicon Chip Fig.4. The three critical components are the PIC16F88-I/P microcontroller (IC3), the motorised potentiometer and an infrared receiver/detector module (IRD1). In operation, the microcontroller monitors the demodulated infrared signal from IRD1. It then decodes this signal and drives the pot motor according to the RC5 code (see panel) sent by the handheld remote. IRD1 only has three leads but it is not a simple device; in reality, it is a complete infrared detector and processor – see Fig.2. First, it picks up the 38kHz infrared pulse signal from the remote and amplifies this to a constant level. This is then fed to a 38kHz bandpass filter and then demodulated to produce a serial data burst at IRD1’s pin 1 output. From there, the demodulated signal from IRD1 is fed into IC3’s RB0 input (pin 6). Operating under program control, the microcontroller then reconstitutes the demodulated data into byte-wide format using the Philips RC5 protocol specification. Basically, the Remote Volume Control can be operated on one of three modes within the RC5 Code. These are TV1, SAT1 and SAT2 and the desired code is selected using jumper links LK1 & LK2 at the RB7 & RB6 inputs of IC3. Normally, both these inputs are pulled high via internal resistors in IC3 but they can be pulled low using links LK1 & LK2. In operation, IC3 monitors these inputs and compares the selected code with the incoming serial data from IRD1. If the detected code is correct, the motorised potentiometer will be driven according to the pushbutton command sent by the remote control. Motor drive The motorised potentiometer is driven by four transistors (Q1-Q4) arranged in a H-bridge configuration. These in turn are driven via the RB2RB5 outputs of IC3 via 1kW resistors. The motor is off when the RB2-RB5 outputs are all set high. RB4 & RB5 turn PNP transistors Q1 & Q3 off, siliconchip.com.au while RB2 & RB3 turn NPN transistors Q2 & Q4 on. As a result, both terminals of the motor are pulled low and so the motor is off. Note that the emitters of Q2 & Q4 both connect to ground via a common 10W resistor (more on this shortly). The transistors operate in pairs so that the motor can be driven in either direction (to increase or decrease the volume). To drive the potentiometer clockwise, port RB3 goes low and turns off transistor Q2, while RB4 goes low and turns on Q1. This means that the lefthand terminal of the motor is taken to +5V via Q1, while the righthand terminal of the motor is held low via Q4. As a result, current flows through Q1, through the motor and then via Q4 and the 10W resistor to ground. Conversely, to spin the motor in the other direction, Q1 & Q4 are switched off and Q2 & Q3 are switched on. As a result, the righthand motor terminal is pulled to +5V via Q3, while the lefthand terminal is pulled low via Q2. The voltage across the motor depends on the voltage across the common 10W emitter resistor and that in turn depends on the current. Typically, the motor draws about 40mA when driving the potentiometer but this rises to over 50mA when the clutch is slipping. As a result, the motor voltage is around 4.5-4.6V due to the 0.4-0.5V drop across the 10W resistor (the rated motor voltage is 4.5V). Current sensing & muting Once the pot’s wiper reaches its fully clockwise or anti-clockwise position, a friction-type clutch in the gearbox begins to slip. This prevents the motor from stalling, while also allowing the user to manually rotate the pot shaft if necessary. The muting function depends on the microcontroller’s ability to detect when the wiper is “on the stops”. It does this by indirectly detecting the increase in the motor current. In operation, VR2 samples the voltage across the 10W resistor when the motor is running. The resulting signal at its wiper is then filtered using an 18kW resistor and a 100nF capacitor (to remove the commutator hash from the motor) and applied to IC3’s analog AN2 input (pin 1). This analog input is measured (by IC3) to a resolution of 10-bits, or about 5mV. Provided this input is below 200mV, the PIC microcontroller allows siliconchip.com.au Parts List 1 PC board, code 01208071, 201 x 63mm 1 Alpha dual-ganged 20kW log motorised pot (VR1) (Altronics Cat. R2000) 1 1kW (code 102) horizontal trimpot (VR2) 1 DIP 18-pin IC socket 2 DIP 8-pin IC sockets 5 2-way PC-mount screw terminal blocks, 5.08mm spacing (Altronics Cat. P2034A – do not substitute) 1 3-way PC-mount screw terminal block, 5.08mm spacing (Altronics Cat. P2035A – do not substitute) 1 4MHz crystal (X1) 4 ferrite beads (Altronics Cat. L5250A) 1 3-way SIL pin header, 2.54mm spacing 1 2-way SIL pin header, 2.54mm spacing 1 2-way DIL pin header, 2.54mm spacing 2 jumper links to suit headers 1 6.35mm panel-mount singleended spade connector 1 6.35mm spade connector 4 M3 x 25mm tapped standoffs 4 M3 x 6mm screws 1 M4 x 10mm screw 1 M4 nut 1 M4 flat washer 1 M4 star washer 1 250mm length of 0.8mm tinned copper wire 1 150mm length of red hookup wire the motor to run. However, as soon as voltage rises above this 200mV limit, the motor is stopped. When the motor is running normally, the current through it is about 40mA which produces 0.4V across the 10W resistor. VR2 is used to attenuate this voltage and is adjusted so that the voltage at AN2 is slightly below the 200mV limit. When the motor reaches the end of its travel, the extra load imposed by the slipping clutch increases the current and the voltage applied to the AN2 input rises above 200mV. This is detected by IC3 during muting and the microcontroller then switches the H- 1 150mm length of black hookup wire 2 100mm cable ties Semiconductors 2 LM4562 op amps (IC1, IC2) 1 PIC16F88-I/P programmed with “Low Noise Preamp Volume.hex” (lC3) 1 infrared decoder (IRD1) 1 7815 15V regulator (REG1) 1 7915 -15V regulator (REG2) 1 7805 5V regulator (REG3) 2 BC327 PNP transistors (Q1,Q3) 2 BC337 NPN transistors (Q2,Q4) 1 3mm red LED (LED1) 1 3mm blue LED (LED2) 1 3mm orange LED (LED3) Capacitors 2 220mF 25V PC electrolytic 1 100mF 25V PC electrolytic 4 100mF 16V PC electrolytic 4 22mF NP electrolytic 1 10mF 16V PC electrolytic 2 4.7mF NP electrolytic 2 1mF NP electrolytic or MKT polyester 5 100nF MKT polyester 1 10nF MKT polyester 2 560pF ceramic 2 270pF ceramic 2 22pF ceramic Resistors (0.25W, 1%) 4 100kW 7 1kW 2 22kW 6 100W 1 18kW 1 22W 1 10kW 1 10W 4 4.7kW bridge transistors (Q1-Q4) accordingly to immediately stop the motor. Note that AN2 is monitored only during the Muting operation. At other times, when the volume is being set by the Up or Down buttons on the remote, the voltage at AN2 is not monitored. As a result, the clutch in the motor’s gearbox assembly simply slips when the potentiometer reaches its clockwise or anticlockwise limits. Pressing the Mute button on the remote again after muting returns the volume control to its original setting. This is the “Mute Return” feature referred to earlier. Note also that connecting IC3’s RA4 August 2007  19 Fig.5: follow this parts layout diagram to build the Preamplifier & Remote Volume Control board. Be sure to use the correct part at each location and take care with components that are polarised. The leads to the motor are strapped to the underside of the board using cable ties. input to ground via LK4 disables this feature. Conversely, to enable Mute Return, LK3 is used to pull RA4 to +5V. Indicator LEDs LEDs 1-3 indicate the status of the 20  Silicon Chip circuit. The red Power LED (LED1) lights whenever power is applied to the circuit and provides power on/off indication for the entire amplifier. The other two LEDs – Ack (acknowledge) and Mute – light when their respective RB1 and RA1 outputs are Fig.6: bend the leads for IRD1 and the three LEDs as shown here before installing them on the PC board. The centre line of each lens must be 4mm above the board surface. pulled high (ie, to +5V). As indicated previously, the Ack LED flashes when ever the RB0 input receives an infrared signal from the remote, while the Mute LED flashes during the Mute operation and then stays lit while the volume remains muted. Crystal oscillator Pins 15 and 16 of IC3 are the oscillator inputs for 4MHz crystal X1, which is used to provide the clock signal. This oscillator runs when the circuit is first powered up for about 1.5 seconds. It also runs whenever an infrared signal is received at RB0 and then for a further 1.5 seconds after the last receipt of signal, after which the oscillator shuts down. Note, however, that this shut down does not occur if a Muting operation is still in process. siliconchip.com.au Make sure that the motorised pot is correctly seated against the PC board before soldering its terminals, otherwise its shaft won’t line up with the front panel clearance hole later on. This selects the TV1 infrared remote control code and this will be suitable for most applications. However, this code may also operate your TV and so we have provided options to select another code to prevent this from happening. The table in Fig.4 shows the linking options used to select either the SAT1 or SAT2 code. For example, installing LK2 (and leaving LK1 out) sets the code to SAT2. Power for the circuit is derived from the amplifier’s 22V DC supply and is fed in via a 22W resistor and a 100mF decoupling capacitor. The resulting rail is then applied to regulator REG3 which produces a +5V supply rail to power IC3, IRD1 and the H-bridge driver stage for the motor. A 10mF capacitor decouples the output of REG3, while the 100mF capacitor across IRD1 prevents this device from false triggering due to “hash” on the 5V rail. Construction Shutting down the oscillator in the absence of an infrared signal from the remote ensures that no noise is radiated into sensitive audio circuitry when the volume control is not being altered. Waking up again As just stated, when there is no IR signal from the remote, the circuit goes to “sleep” (ie, the oscillator shuts down) and so no noise is produced. However, as soon as it receives an IR signal, the circuit “wakes up” and drives the potentiometer. It then shuts down after about 1.5 seconds if it does not receive any further IR signals. In addition, the motor is enclosed by a Mumetal shield which reduces any radiated electrical hash from the commutator brushes. A 10nF capacitor connected directly across the motor terminals also prevents commutator hash from being transmitted along the connection leads, while further filtering is provided by a 100nF capacitor located at the motor output terminals on the PC board. Coding options Links LK1 & LK2 at RB7 and RB6 are used to program the different infrared coding options. The default selection is when both RB6 and RB7 are pulled high via their internal pull-up resistors – ie, when LK1 and LK2 are out. All the parts for the Preamp & Remote Volume Control Unit are installed on a single PC board coded 01208071 and measuring 201 x 63mm. The external connections to the power supply and to the audio input and output cables are run via insulated screw terminal blocks. Fig.5 shows the assembly details. As usual, begin by checking the board for defects and for the correct hole sizes. In particular, check that the motorised pot and the screw terminal blocks fit correctly and that the mounting holes are correct. That done, start the assembly by installing the six wire links. You can straighten the link wire by securing one end in a vyce and then pulling on the other end using a pair of pliers, to stretch it slightly. The resistors can Table 1: Resistor Colour Codes o o o o o o o o o o siliconchip.com.au No.   4   2   1   1   4   7   6   1   1 Value 100kW 22kW 18kW 10kW 4.7kW 1kW 100W 22W 10W 4-Band Code (1%) brown black yellow brown red red orange brown brown grey orange brown brown black orange brown yellow violet red brown brown black red brown brown black brown brown red red black brown brown black black brown 5-Band Code (1%) brown black black orange brown red red black red brown brown grey black red brown brown black black red brown yellow violet black brown brown brown black black brown brown brown black black black brown red red black gold brown brown black black gold brown August 2007  21 Avoiding An Earth Loop With IRD1 If the supplied infrared receiver (IRD1) includes an external metal shield (see photo), then steps must be taken to insulate it from the chassis when the preamplifier is installed. That’s because the shield is connected to the centre (GND) terminal of the device and a short between the shield and the metal chassis would create an earth loop. And that in turn would inject hum into the audio signal. One method is to attach a short strip of insulation tape to the inside of the front panel, with a hole cut out to match the hole in the panel. Alternatively, it should be possible to insulate the front of the device and arrange it so that it just stands clear of the front panel. then go in. Table 1 shows the resistor colour codes but you should also check them using a digital multimeter, as the colours can sometimes be difficult to decipher. Next on the list are the four ferrite beads. These each have a wire link run through them, which is then soldered to the board. Follow these with the two diodes (D1 & D2), then install sockets for the three ICs. Make sure that each socket is oriented correctly (IC3 faces in the opposite direction to ICs 1 & 2) and that it’s seated properly against the PC board. In fact, it’s best to solder two diagonally opposite pins of a socket first and then check it before soldering the remaining pins. The MKT and ceramic capacitors can now go in, followed by the nonpolarised capacitors and the polarised electrolytics. Make sure that the latter are all correctly oriented and note that the 100mF capacitor to the left of LED3 must be rated at 25V (the other 100mF capacitors can all be rated at 16V). Now install the transistors and 3-terminal regulators. Transistors Q1-Q4 all go in the remote volume control section and must be oriented as shown. Be sure to use the correct type at each location. Q1 & Q3 and both BC327s, while Q2 & Q4 are BC337s. Don’t get them mixed up. The same goes for the three regulators. REG1 is a 7815, REG2 a 7915 and REG3 a 7805 – again, don’t mix them up. These parts should all be inserted 22  Silicon Chip If your infrared receiver module has a metal shield like this one, then be sure to insulate it from the front panel as described in the accompanying text. Do not rely on the powder coating on the chassis to provide insulation! That’s asking for trouble. as far down as they will go, with their metal tabs facing towards the back of the board. No heatsinking is required for their metal tabs, since current requirements are only modest. The 2-way DIL (dual-in-line) pin header for LK1 & LK2 can now be installed, followed by the 3-way header for LK3 & LK4. A 2-way pin header is also used to terminate the motor leads (just to the right of Q1 & Q3). To install this header, first push its pins down so that their ends are flush with the top of the plastic, then install the header from the component side and solder the pins underneath. This will give about 7mm pin lengths to terminate the leads from the motor, which are run underneath the PC board. Crystal X1 (adjacent to IC3) can be installed either way around. Make sure it’s seated correctly before soldering its leads, then install trimpot VR2 and the six screw terminal blocks. Be sure to use the screw terminal blocks specified in the parts list – they give more reliable connections when terminating thin audio cable leads than the type used on our prototype. Mounting the motorised pot It’s absolutely critical to seat the motorised pot (VR1) correctly against the PC board before soldering its leads, If this is not done, it won’t line up correctly with its clearance hole in the amplifier’s front panel later on. In particular, note that the two lugs at the rear of the gearbox cover go through slotted holes in the PC board. Use a small jeweller’s file to enlarge these if necessary. Once the pot fits correctly, solder two diagonally opposite pot terminals and check that everything is correct before soldering the rest. The two gearbox cover lugs can then be soldered. Once the pot is in place, the motor terminals can be connected to the two pin header at the other end of the board using light-duty hook-up cable. These leads are twisted together to keep them tidy and pass through a hole in the board immediately behind the motor. As shown, they are then secured to the underside of the PC board using cable ties and connected to the header pins (watch the polarity). Don’t forget to solder the 10nF capacitor directly across the motor terminals. As previously stated, it’s there to suppress motor hash. Mounting the LEDs Fig.6 shows the mounting details for the infrared receiver (IRD1) and the three LEDs. As shown the centre line of each lens must be 4mm above the board surface. So how do you mount the LEDs accurately? Easy – just cut 11mmwide and 4mm-wide templates from thick cardboard. The 11mm template serves as a lead bending guide, while the 4mm template is used as a spacer when mounting the LEDs – just push each LED down onto the spacer and solder its leads. Hint: you can use sticky tape as a “third hand” to hold each LED and the template in place during soldering. IRD1’s leads should also be bent as shown in Fig.6 and the photos. This will allow a small amount of “give” in the leads when the lens later contacts the back of the front panel (ie, it will allow IRD1 to “spring” back slightly and keep the lens against the panel). Finally, complete the board assembly by installing the quick connector. As with previous boards, it’s held in place using an M4 screw, a flat washer, a shakeproof washer and a nut (see Fig.3 last month). Initial checks Before plugging in any of the ICs, it’s a good idea to check the supply voltages. However, if you don’t have the power supply running yet (or a suitable bench power supply), this siliconchip.com.au can wait until the final assembly in the chassis. Assuming you do have a power supply, connect the +22V, -22V & 0V leads to CON6 and switch on. Now check the voltages on pins 8 & 4 of the two 8-pin IC sockets (ie, between each of these pins and 0V). You should get readings of +15V (pin 8) and -15V (pin 5) respectively. Similarly, check the voltage on pin 14 of IC3’s socket – you should get a reading between 4.8V and 5.2V. If these voltages are correct, switch off and plug the ICs into their sockets, taking care not to zap them with static electricity. Note that IC1 & IC2 face one way while IC3 faces the other way. Remote volume control testing If you don’t have a dual power supply, then you can check out the remote volume control section only using a single rail 9-15V supply (connect this between the +22V and 0V terminals on CON6). As before, check the voltage on pin 14 of IC3’s socket (it must be between 4.8V and 5.2V), then switch off and plug IC3 into its socket. In addition, insert the jumper link for LK3, to enable the Mute return feature but leave LK1 & LK2 out for the time being (to accept the TV code from the remote). Further testing requires a universal remote control. These range from single TV remote controls with limited functions to elaborate models capable of operating many different types of equipment. Note, however, that simple TV remote controls will only operate this project using the TV code (026). That can cause problems if you have a Philips TV set located in the same vicinity as the amplifier, as the remote control will probably operate the TV as well. This is easy to solve – just use a multi-item remote control so that a different code can be used (either 424 for SAT1 or 425 for SAT2) An example of a TV-only remote control is the Jaycar AR-1703. Multiitem remote controls include the Altronics A-1009 and the Jaycar AR1714. Programming the remote The best approach here is to initially program the remote control for a Philips brand TV (just follow the instructions supplied with the unit). In most cases, programming involves siliconchip.com.au Universal Infrared Remote Controls The Remote Volume Control circuit is designed to work with most universal (“onefor-all”) infrared remotes. It recognises the RC5 protocol that was originally developed by Philips, so the remote must be programmed for a Philips (or compatible) appliance before use. Most universal remotes are provided with a long list of supported appliances and matching codes. To set the remote to work with a particular piece of gear, it’s usually just a matter of entering the code listed for the manufacturer (in this case, Philips), as detailed in the instructions. You’ll also note that different codes are provided for TV, CD, SAT, and so on. This allows two or more appliances from the same manufacturer to be operated in the same room and even from the same handpiece. This multiple addressing capability can be useful in our application, too. Normally, we’d program the remote to control a TV, as this works with the control module. But what if you already have a Philips TV (or some other model that uses the RC5 protocol)? Well, in that case, you simply use the SAT1 or SAT2 code instead, as the Remote Volume Control can also handle these! Typically, to set a remote to control a Philips TV, you first press and hold “SET” and then press “TV”. This puts the remote in programming mode, as indicated by a LED, which should remain illuminated. You then release both keys and punch in one of the listed Philips TV codes. For this project, code 026 works well. The red LED should then go out, after which the remote is ready for use. All universal remotes can be programmed in a similar manner but if in doubt, try reading the instructions! If the first code listed doesn’t work with the Remote Volume Control, then try another. Once the remote has been programmed, the Remote Volume Control must be set up to recognise the particular equipment address that you’ve chosen (either TV, SAT1 or SAT2). The details on how to do this are in the main text. Although this project should work with any universal remote, we’ve tested the following popular models: AIFA Y2E (Altronics A-1013), AIFA RA7 (Altronics A-1009) and Jaycar AR-1703. For all these models, the setup codes are as follows: TV = 026, SAT1 = 424 and SAT2 = 425. Note, however, that the AIFA Y2E doesn’t have a mute button. simultaneously pressing the “Set” button and the button for the item that is to be operated. In other words, press the “Set” and “TV” buttons together and enter a number for a Philips TV set. In this case, the Altronics A-1009 uses the number 026 or 191 and the Jaycar AR-1703 uses 11414. If you are using a different remote control, just select a number for a Philips TV set. If you later find that this doesn’t work, try another number for a Philips TV. Having programmed the remote, rotate trimpot VR2 fully anticlockwise. That done, check that the motor turns the potentiometer clockwise when the remote’s Volume Up and Channel Up buttons are pressed. It should travel fairly quickly when Volume Up is pressed and at a slower rate when Channel Up is used. Now check that the volume potentiometer runs anticlockwise using the Volume Down and Channel down buttons. If it turns in the wrong direction, simply reverse the leads to the motor. Check that the blue Acknowledge LED flashes each time you press a button on the remote. Next, set the pot to mid-position 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 August 2007  23 This view shows the Preamplifier & Remote Volume Control module mounted inside the completed Class-A Stereo Amplifier. The final assembly, wiring and adjustment details will be published next month. the pot reaches its minimum position. Note that a programmed timeout of 13-seconds will also stop the motor (if it hasn’t already stopped) after Mute is activated. This means that you have to adjust VR2 within this 13s period, otherwise the timeout will stop the motor. If it stops prematurely or fails to stop at all (ie, the motor runs for the full 13 seconds), try redoing the adjustment. Once the adjustment is correct, pressing the Mute button a second time should accurately return the potentiometer to its original position. As mentioned earlier, links LK1 & LK2 change the codes for the infrared transmission – see the table in Fig.4. 24  Silicon Chip You will only need to install one of these links (to select SAT1 or SAT2) if you have a Philips TV. Remove link LK3 and install link LK4 if the Mute return feature is not required. Note that with a new motorised potentiometer, the clutch will require a little “wearing in” to evenly spread the lubricant in the slipping sections. This can be done simply by turning the pot shaft by hand a few times before use. Readjust VR2 for best results after you do this. Avoiding a hum loop Finally, note that the power supply earth (0V) is not connected to the left and right channel earth tracks on the preamplifier PC board. This avoids a hum loop, since the two channels are normally earthed back through the power amplifiers via their signal leads. However, if you want to use the preamp on its own, both the left and right channel signal earths on the board must be connected to the 0V rail for the power supply. This can be done by connecting insulated wire links between the relevant screw terminal blocks. That’s all for this month. In Pt.5, we’ll show you how to assemble all the modules into a custom metal chassis to produce a complete high-quality class-A stereo audio amplifier. SC siliconchip.com.au