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This view shows the new preamplifier board with its motorised pot in position. The Remote Volume Control board mounts on the back, behind the LED displays. Do you want remote volume control for your Ultra-LD 2 x 100W RMS Stereo Amplifier? The remote volume control described last month fits neatly into the chassis, along with a re-designed preamplifier PC board. By JOHN CLARKE & GREG SWAIN T HE ULTRA-LD STEREO Amplifier described in the November 2001 to February 2002 issues has proven very popular. It delivers superb audio performance but there was one thing that was lacking – infrared remote control. That’s not usually important for such functions as power on/off switching and input selection but there’s 56  Silicon Chip one thing you really do miss – remote volume control. As it stands, if you want to adjust the volume, you have to get out of your “comfy” chair, walk over to the amplifier, adjust the control and then walk back and sit down again. And that’s just terrible – a real imposition for anyone. Seriously though, remote volume control is a very conveni­ent feature. Some CDs (or even individual tracks) can sound louder than others and a remote control allows you to quickly nudge the volume up or down at the press of a button. Our approach here has been to fit the Remote Volume Control unit described last month to the Ultra-LD Stereo Amplifier. This unit not only provides remote volume control but also lets you to quickly mute the amplifier; eg, if the phone rings. Fitting it to the Ultra-LD Actually, we had the Ultra-LD Stereo Amplifier very much in mind when we designed the Remote Volume Control. Unfortunately, there was just no way that we could fit the motorised pot into the chassis with the existing preamplifier. The pot’s motor and gearbox take www.siliconchip.com.au Fig.1: this is the new preamplifier and LED display circuit. It’s virtually identical to the original circuit published in November 2001 but now includes buffer stage IC6 (TL072). This new stage isolates the audio signal on VR1’s wiper from the precision rectifier stage based on IC2, thus allowing us to substantially reduce the resistor values in this part of the circuit to shunt leakage currents in humid weather. www.siliconchip.com.au July 2002  57 PARTS LIST ULTRA-LD PREAMP (REVISED) 1 PC board, code 01107021, 246 x 74mm 1 PC board, code 15106023, 26 x 23mm (to mount pot) 1 26-way DIL pin header 1 2-way polarised locking pin header & plug (2.54mm pitch) 2 2-way mini PC terminal blocks (Altronics P 2038) - 5mm pitch 1 2-pole 6-position switch (Altronics S 3022) (S1) 1 20kΩ motorised stereo log pot (VR1) – as used with Remote Volume Control 2 F29 ferrite beads 4 25mm-long M3 tapped spacers 4 double-ended male quick connects (Altronics H2261) Semiconductors 2 NE5534AN op amps (IC1,IC2) (Altronics Z2792 – do not substitute NE5534N) 2 TL072 dual op amps (IC2, IC6) 2 LM3915 display driver (IC3,IC5) 1 7815 3-terminal regulator (REG1) 1 7915 3-terminal regulator (REG2) 2 1N4004 diodes (D1,D2) 4 1N914 diodes (D3-D6) 16 green thru-panel LEDs (LEDs 1-8, 11-18) (Altronics Z0711) 2 yellow thru-panel LEDs (LED9, LED19) (Altronics Z0713) 4 red thru-panel LEDs (LED10, LEDs20-22) (Altronics Z0710) 2 33pF ceramic 6 10pF ceramic Resistors (0.25W, 1%) 2 100kΩ 2 1.8kΩ 2 68kΩ 1 1.5kΩ 2 33kΩ 2 1.2kΩ 2 22kΩ 4 150Ω 2 15kΩ 4 100Ω 2 6.8kΩ 2 33Ω 2 4.7kΩ 1 10Ω 2 2.2kΩ SATELLITE BOARD 1 PC board code, 15106022, 46 x 23mm 1 8-way polarised locking pin header & plug (2.54mm pitch) 1 250mm-length of 8-way rainbow cable 1 infrared receiver/decoder (IRD1; transferred from Remote Volume Control board) 2 red thru-panel LEDs (LEDs1-2; transferred from Remote Volume Control board) 1 100µF 16VW PC electrolytic capacitor 1 2.2kΩ 0.25W 1% resistor (see text) MOUNTING HARDWARE Capacitors 2 1000µF 25VW PC-mount electrolytics 2 100µF 25VW PC-mount electrolytics 10 10µF 35VW PC-mount electrolytics 4 10µF 50VW bipolar electrolytics 2 2.2µF 50VW bipolar electrolytics 2 0.1µF MKT polyester 2 390pF ceramic 1 3-way polarised locking pin header & plug (2.54mm pitch) – to replace IRD1 on Remote Volume Control board 2 2-way polarised locking pin headers & plugs (2.54mm pitch) – to replace LEDs 1& 2 on Remote Volume Control board 4 10mm-long tapped Nylon spacers (to mount Remote Volume Control board) 12 M3 x 6mm screws 4 M3 nuts & washers 4 10mm-long tapped brass spacers 1 aluminium plate, 46 x 50mm (1mm thick) – see Fig.6 up a fair amount of space and it would have meant butchering the preamp board to get it all to fit. In the end, there was nothing for it but to design a new Preamplifier & LED Display board to accommodate the motorised pot. And while we were at it, we decided to provide mounting holes for the Remote Volume Control PC board and to make a few circuit improvements. Importantly, the new preamp board is a drop-in replacement for the old one. All the mounting holes, the LED displays and the controls are in exactly the same positions as before, so there are no extra holes to drill in the chassis. As can be seen from the photos, the Remote Volume Control’s PC board mounts on the back of the new preamp board, while the infrared receiver (IRD1) and the two LEDs (Acknowledge & Mute) have been transferred to a small satellite board. This satellite board connects back to the main board via a cable and matching pin headers. As with the preamp board, you don’t have to drill any extra holes to mount the satellite board. Instead, it’s attached via a simple bracket to an existing mounting point for the lefthand power amplifier, so that IRD1 “peers” out through one of the vertical slots in the front panel. The Acknowledge and Mute LEDs sit back some way behind the slots but are still quite visible. Easier to build The need to re-design the preamplifier board also gave us an opportunity to simplify the construction. In particular, the volume control pot is much easier to mount than before. The pot now mounts on its own PC board and this is soldered at right angles to a 5-way pin header on the preamp board. In addition, we’ve now mounted the Speakers LED (LED22) on the preamp board, together with a pin header to accept the exter­ nal wiring connections from the loudspeaker protector board. This does away with the old mounting method, which involved gluing the LED to a cable-tie mount attached to the front panel. By the way, although the new preamp board has been designed to accommodate a motorised pot, you don’t have to use a motorised pot if you don’t want to. If you don’t want remote volume con­trol, just install a conventional pot instead. Basically, this new preamplifier board supersedes the original design, whether you use a motorised pot or not. It differs from the earlier unit mainly in terms of layout, although there are also a few circuit changes which we’ll detail below. Circuit details 58  Silicon Chip Fig.1 shows the circuit for new pre­ amplifier. It’s virtually identical to the circuit published back in November 2001, so we won’t repeat all the details. Instead, we’ll concen­trate mainly on the circuit changes. As before, the audio signal is selected by switch S1 and fed to op amp IC1 which operates with a gain www.siliconchip.com.au The Remote Volume Control PC board is secured to the back of the revised preamp board using 10mm Nylon spacers and machine screws and nuts. Note that IRD1 and the Mute & Acknowledge LEDs have been replaced with polarised pin headers. of 3.6. Its output appears at pin 6 and is fed to the volume control, to the Tape Out socket and to the LED display circuitry. IC2 is the precision rectifier and this drives IC3 (the LED display driver) exactly as before. The big difference here is that we no longer drive the precision rectifier directly from the volume control. Instead, it is now driven via a unity-gain buffer stage based on IC6. This buffer stage has a high input impedance and isolates the audio signal at the volume control from the precision recti­ fier. This in turn eliminates the need to use high value resis­tors in this section of the circuit. To explain further, high-value resistors were previously necessary to prevent switching “spikes” generated by the preci­sion rectifier from being fed back into the volume control. But although this was very effective, it could have one undesirable side effect – in humid weather, one or more LEDs in the bargraph displays could light for several minutes after switch on, due to moisture on the PC board. The “cure” at the time was to connect 82kΩ resistors from the cathodes of D3 & D5 to ground, to shunt this leakage resist­ance. However, it wasn’t a complete cure (as we subsequently discovered), at least not on the prototype – some LEDs could still light for a minute or so on very humid days. www.siliconchip.com.au On the plus side, this hasn’t been a problem with kit ver­ sions of the Ultra-LD Stereo Amplifier. The PC boards supplied by Altronics are solder-masked and so are unaffected by mois­ture. We could have fixed our prototype board by cleaning and spraying it with a protective lacquer but we never quite got around to it. The new buffer stage based on IC6 completely eliminates this problem once and for all. It’s inclusion allows us to reduce the feedback resistors associated with IC2 (the precision recti­fier) by a factor of 10, which means that any leakage currents are shunted to ground. In particular, we’ve reduced the 220kΩ input resistor to 22kΩ, the 330kΩ feedback resistor to 33kΩ and the 680kΩ input resistor to IC3 to 68kΩ. At the same time, the .01µF capacitor on pin 5 (SIG) of IC3 has been increased to 0.1µF so that the time constant for the LED drive signal remains the same. Apart from that, the circuit is exactly the same as before except for just one minor tweak – the resistor in series with the Power LED (LED21) has been increased from 1.2kΩ to 1.5kΩ, so that it more closely matches the brightness of the Speaker LED. So are there any audible benefits from the new preamplifier board? A small satellite board now carries IRD1 and the Mute & Acknowledge LEDs. This sits vertically behind the slots at one end of the case and is attached to a bracket which is secured by one of the power amplifier mounting screws. Make sure that IRD1 (arrowed) lines up with one of the slots. July 2002  59 Fig.3: the motorised pot is mounted on a small interface board to make the connections easy. Fig.2: install the parts on the Preamplifier & LED Display board as shown here. The motorised pot sits flat against the board and is connected to the adjacent 5-way pin header via a small interface board – see Fig.3. Fig.4: the satellite board carries IRD1 and the two LEDs. This board connects to the main Remote Volume Control board via pin-headers and a 7-way ribbon cable. Note that pin 3 on the 8-way header is unused. 60  Silicon Chip Fig.5: here are the full-size etching patterns for the satellite board and the interface board for the motorised pot. This view shows how the metal tabs on the bottom of the gearbox cover are bent up and soldered to the interface board. www.siliconchip.com.au This is the fully-assembled preamplifier board. The switch, volume control pot and mounting holes are all in the same positions as before, so there are no holes to drill. Make sure that all polarised parts are installed the right way around. from the earlier design are as follows: A 5-way pin header is installed to accept the pot connec­tions; • The new board includes buffer stage IC6 plus LED22 and its adjacent 2-pin header; • Double-ended quick connect terminals are now used at the 0V and +12V positions. These terminals accept the supply connections from the Loudspeaker Protector board and also supply power to the new Remote Volume Control board. • The Preamplifier and LED Display module is now attached to the front of the chassis using 25mm spacers (the • Nope – there are none! It’s audio performance with regard to noise and distortion are almost exactly as before. If you already have an Ultra-LD Stereo Amplifier, there’s no need to rush out and replace the preamplifier board with this new design – unless you want the remote volume control facility, that is. Construction Fig.2 shows the parts layout on the revised Preamplifier & LED Display board. This board is coded 01107021 and has a large hole near the centre to accept the pot’s motor. The pot itself pot is soldered to a small interface PC board coded 15106023 – see Fig.3. Basically, it’s just a matter of installing the parts on the preamp board as shown in Fig.2 and as set out in the December 2001 issue (be sure to refer to this article). The main differ­ences Table 2: Capacitor Codes      This close-up view shows how the motorised pot is connected to the 5-way pin header. Note that the back of the gearbox cover sits flat against the PC board. Value IEC Code EIA Code 0.1µF   100n   104 390pF   390p   390 33pF   33p   33 10pF   10p   10 Table 1: Resistor Colour Codes  No.   2   2   2   2   2   2   2   2   2   1   2   4   4   2   1 www.siliconchip.com.au Value 100kΩ 68kΩ 33kΩ 22kΩ 15kΩ 6.8kΩ 4.7kΩ 2.2kΩ 1.8kΩ 1.5kΩ 1.2kΩ 150Ω 100Ω 33Ω 10Ω 4-Band Code (1%) brown black yellow brown blue grey orange brown orange orange orange brown red red orange brown brown green orange brown blue grey red brown yellow violet red brown red red red brown brown grey red brown brown green red brown brown red red brown brown green brown brown brown black brown brown orange orange black brown brown black black brown 5-Band Code (1%) brown black black orange brown blue grey black red brown orange orange black red brown red red black red brown brown green black red brown blue grey black brown brown yellow violet black brown brown red red black brown brown brown grey black brown brown brown green black brown brown brown red black brown brown brown green black black brown brown black black black brown orange orange black gold brown brown black black gold brown July 2002  61 Fig.6: here’s how to make the metal bracket that’s used to support the satellite PC board. It’s made from light-gauge aluminium sheet. previous module used 20mm spacers plus a spacer nut – ie, about 22mm overall). As before, the LEDs must all be stood off the PC board so that they later protrude through their matching holes in the front panel when the PC board is mounted in the chassis. This is done by first inserting the LEDs into the PC board, then mounting the board to the front of the chassis on 25mm spacers and attaching the front panel. The LEDs are then pushed through their matching front panel holes and their leads soldered. Actually, it will probably be easier to quickly tack-solder the longer of the two leads for each LED, then remove the pream­plifier board and complete Fig.7: the satellite board is attached to the bracket using 10mm tapped spacers & M3 x 6mm screws. Note that IRD1 and the two LEDs should be installed with their centres about 7mm above the board surface. the soldering. You will find that the LED leads are just long enough. Make sure that the LEDs are all correctly oriented (the anode lead is the longer of the two) when installing them on the PC board. Similarly, make sure that you install the rotary switch the right way around. Cut its shaft length to 26mm (use a small hacksaw) before installing it on the PC board with pin 1 located exactly as shown in Fig.2 (ie, at bottom right). Push the switch all the way down onto the board so that it is properly seated before soldering its pins. Once the switch is in, rotate its shaft fully anticlockwise, then move its indexing collar one position anticlock­ wise so that it operates as a 5-position switch (see December 2001 issue). Mounting the pot Fig.3 shows how the motorised pot is soldered to its PC board (code 15106023). Make sure that it is properly seated before soldering its six pins. Once these pins have been soldered, bend up the two metal tags on the bottom edge of the gearbox cover and solder them to the thick copper tracks at either end of the board (this provides extra rigidity). The motorised pot board can then be soldered to the matching 5-way header pins on the preamp board. Make sure that the back of the pot’s gearbox cover is rest­ing flat against The view above shows the aluminium bracket with the two spacers attached while at right is the bracket with the satellite board fitted. Note that the Mute and Acknowledge LEDs sit back behind the front of IRD1. 62  Silicon Chip www.siliconchip.com.au Here’s how the satellite board is mounted inside the chassis. IRD1 must sit directly behind one of the slots. the preamp board before soldering the tracks to the header pins. Note that you may have to bend the header pins slightly to ensure contact with the tracks on the pot board. Remote control boards The main Remote Volume Control board is built exactly as described last month, except that IRD1, LED1 and LED2 are re­placed by pin headers -–see Fig.4. Be sure to install the pin headers the right way around, with their “backs” towards the edge of the board. Fig.4 also shows how to build the satellite board. Install the parts exactly as shown, with the two LEDs aligned close to the edge of the PC board but with their centres about 7mm above the board surface – see Fig.7. You will have to bend their leads down by 90° about 3mm from their bodies before installing them. IRD1 is installed at full lead length and its leads then bent by 90 (at both ends) so that it faces in the same direction as the LEDs (see photo). Adjust IRD1 for height so that it lines up with the LEDs but note that the front of its lens sits well forward of the two LEDs. The 8-way pin header mounts with its back towards the edge of the board, as shown. The 2.2kΩ resistor (shown dotted) pulls the output of IRD1 high and may be necessary for long cable distances between the satellite board and the main board; eg, for distances over about 400mm. You can safely leave it out for cable distances less than this. A length of 7-way rainbow cable is used to connect the two boards together. This is fitted with an 8-way header socket at one end and with matching 2-way and 3-way header sockets at the other end to plug into the main board. Note that pin 3 on the 8-way header is unused. The Remote Volume Control should be tested before install­ing it into the Ultra-LD Stereo Amplifier. It’s just a matter of connecting the pot motor and the satellite board, applying power and pressing a “volume” button on the remote to see if it works. Reverse the connections to the motor if the pot travels in the wrong direction. If the Acknowledge LED flashes when you press the button but there’s no action from the motor, check the coding for the remote control unit (see last month’s article). Assuming it all works, the Remote Volume Control board can now be mounted on the rear of the preamplifier board using 10mm tapped nylon spacers and eight M3 x 6mm screws. Note that it may be necessary to first fit four of the screws with nuts (done all the way up), to provide the necessary clearance in the middle of the spacer. Once the board is in place, plug the leads from the motor into the 2-pin header and install the power supply leads. The latter run from the screw terminal block and are soldered to the bottom lugs of the +12V and 0V quick connects on the preamp board. Installing the preamp board You will have to remove the front panel and the power amplifier/heatsink assembly if you are retrofitting the new preamplifier into an existing Ultra-LD Stereo Amplifier. This is best done by first removing the side panels from the chassis. It’s then basically a matter of removing the old board and slip­ping the new board into position MINI SUPER DRILL KIT IN HANDY CARRY CASE. SUPPLIED WITH DRILLBITS AND GRINDING ACCESSORIES $61.60 GST INC. www.siliconchip.com.au July 2002  63 You will have to remove the power amplifier board/heatsink assembly (see text) before installing the new preamp board in the chassis. There’s plenty of room to accommodate the extra parts, including the pot motor. (although it’s hardly a 5-minute job). Don’t forget to fit the shielded audio leads to the preamp board before installing it in the chassis. There’s nothing more frustrating than reassembling everything and then discovering that you’ve forgotten to connect these leads (yes, it’s happened to us). The same goes for the flat-ribbon input cable – be sure to plug it into it’s header on the preamp board. Provided that you’ve fitted the preamp board with 25mm spacers, you will find that the motorised pot is an exact fit (ie, the front of the pot sits against the chassis). It should be secured to the chassis using the supplied washer and mounting nut. Note that you may have to slightly elongate the hole for the anti-rotation spigot, to suit the motorised pot. Don’t try to bend the ant-rotation spigot on the pot – it will simply snap off. The existing Speakers LED can either be left in place on the front panel or you can remove it and use the new one fitted to the preamp board. If you choose the latter, the leads from the Loudspeaker Protector have to be fitted with a 2-way header socket. Remove the Speakers LED from the preamp board if you intend leaving the existing Speakers LED in place. Mounting the satellite board Fig.6 shows how to make the mounting bracket for the satel­lite PC board. It can be made from 0.8-1mm thick aluminium sheet (as used for the lids of project cases). Cut out the bracket to the dimensions indicated before marking out and drilling the 3mm holes. That done, make a 12mm-long saw cut as shown The connections from the satellite board to the main Remote Volume Control PC board are made via a 7-way flat ribbon cable and several pin headers. 64  Silicon Chip (use a hacksaw with a fine blade), then bend up the bottom-right section along the dashed line. The bracket can now be cleaned up using a light file to remove any burrs and by scrubbing it with steel wool. Finally, the satellite board can be secured to the bracket on 10mm tapped spacers (see Fig.7) and installed in the amplifi­er. As shown in the photos, the foot of the bracket is secured using an existing mounting screw for the left­ hand power amplifi­er. Be sure to adjust IRD1 so that its lens is in line with one of the vertical slots in the front panel and don’t forget to plug in the connecting cable. If the cable’s too long, it can be tidied up by folding it back on itself and securing it with some cable ties. Testing Now comes the best bit but first make sure that the volume control is set to minimum (otherwise you could frighten the living daylights out of yourself). OK, fire up your favourite CD, sit back in your chair and smugly press the Volume Up button of the remote. The volume should smoothly increase and the Acknowl­edge LED should flash. Finally, check that the Volume Down and Mute functions work as well and that the Mute LED lights correctly. In practice, you will find that the Volume Up and Down but­tons provide all the control you need. The Channel Up and Down buttons can be used to make very fine volume adjustments, SC if necessary. www.siliconchip.com.au