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Studio Series Stereo Preamplifier In s t alli n g T h e Mo du le s In A Cas e At last! – here’s how to assemble your highperformance Studio Series preamplifier modules into a professional quality case! By PETER SMITH B ACK IN THE OCTOBER 2005 issue, we described a stereo preamp­ lifier module that sets new standards in low-cost, high-performance, buildit-yourself audio. The module boasts a minimalist design that typically produces less than .0005% total harmonic distortion and noise. Five relayswitched RCA inputs and a headphone output simplify wiring and minimise audio signal degradation. A matching headphone amplifier 64  Silicon Chip followed in the November issue. This has already proven quite popular and is undoubtedly the cleanest and quiet­ est we’ve ever described. Although designed for use with the preamp, the headphone amplifier also works as a standalone unit and can be connected directly to the line output of a CD or MP3 player. It has the ability to drive headphones down to 8W impedance with low distortion, while a second output socket allows connection of siliconchip.com.au Altronics are supplying a custom-built laser-cut case for this project, so that the assembly is dead easy. This is a prototype case - the final version has extra ventilation slots to ensure adequate cooling. siliconchip.com.au July 2006  65 Fig.2: follow this basic wiring layout to assemble your unit. Note in particular that all 240VAC mains terminations must be fully insulated and no low-voltage wiring is to be routed on the mains (left) side of the metal partition. The headphone amplifier is mounted on 10mm spacers, whereas all other modules use shorter 6mm types. 66  Silicon Chip siliconchip.com.au siliconchip.com.au July 2006  67 Fig.2: reproduced from the October 2005 issue, this graph reveals the extremely low total harmonic distortion and noise (THD+N) produced by the preamplifier module (the results are the same with the module both in and out of the case). This measurement was taken driving a 50kW load with a 600mV RMS input signal but the results are virtually identical when driving a 1kW load. two pairs of higher impedance units. Then in April this year, we provided details of an infrared remote control module to mate with the preamplifier. Using virtually any universal remote, this module allows you to take charge of the preamp from your lounge chair – an indispensable addition to any audio setup! As promised, this month we wrap up the series by showing you how to install these modules, along with a low-noise power supply, into a lowprofile steel case. To achieve a truly professional appearance, Altronics has produced a laser-cut steel chassis especially for this project. It is similar to their “1U” deluxe rack cases, featuring 2-piece construction and a bevelled front panel. Unlike the standard catalog item, Fig.4: if you’re making your own enclosure, here’s how to install the chassis earth point. Two nuts are used to permanently lock the assembly in place. Make sure that it forms a sound electrical contact with the steel base. 68  Silicon Chip Fig.3: here we’re measuring from one preamplifier input to the headphone output. The 32W and 600W cases exhibit slightly higher THD+N than the standalone headphone amplifier measurements, as we’re now using a smaller input signal to develop 200mW into 32W (ie, 382mV RMS) and 100mW into 600W (ie, 850mV RMS) – so noise (not distortion) becomes a larger overall factor. this custom design includes an internal divider for the 240VAC mains section and is finished in an attractive grey metallic paint. Naturally, the front and rear panels are screen printed with the necessary labelling, so you know what goes where. The finished product looks a lot like what you see in the photos – need we say more? Measuring up So how does the assembled unit perform? Measured individually, the performance of the preamplifier and headphone amplifier modules remain the same as described in their respective articles. This indicates that the layout within the case works well, with no additional noise induced from the mains transformer. The performance of the preamp input to headphone amplifier output is shown in Fig.3. As you can see, the 600W case in particular reveals slightly higher THD+N measurements than in the graph published in November 2005. This is to be expected, as we’re now using a considerably smaller input signal to develop our 100mW output – so noise (not distortion) becomes a larger overall factor in the measurements. Nevertheless, the results are excellent! Cooling down During testing of the completed assembly in a 1U rack case, we were initially concerned about the temperature rise of the power supply module in high ambient temperatures. This issue was resolved by machining additional ventilation slots above and below the supply. We recommend that you do the same if you decide to construct your own case. As noted in the headphone amplifier article, it’s imperative that all three of the voltage regulators are fitted with heatsinks. To maximise heat transfer, insulation pads must not be used between the regulators and their heatsinks. Instead, a thin smear of heat transfer compound is used on the mating surfaces before assembly. Note that without insulating pads, the heatsinks of the LM317 and LM337 regulators will be “live”. After tightening the securing screws, make sure that they are sitting squarely in position, so as not to contact nearby components. To maximise reliability, we also recommend that all of the electrolytic capacitors in the power supply module be upgraded to 105°C temperature rated types. Attention to these small details will ensure that your finished preamplifier provides long and reliable service. Mains wiring We’ll start by hooking up the 240V AC wiring in the unit – see Fig.1. As shown, all the mains wiring is located in the partitioned-off area in the lefthand side of the case. In fact, siliconchip.com.au this area is reserved exclusively for mains wiring! Fit the transformer first, noting that the large flexible washer supplied with the unit must be installed between the transformer and chassis. Orientate the transformer so that the wires exit at the top, with the primary (brown and blue) wires nearest to the rear of the case – see Fig.1. The secondary side terminations (orange, black, white and red wires on the Altronics transformer) can now be fed through the grommetted hole in the partition, ready for connection to the power supply board. Position all the wires so that there will be plenty of clearance to the lid when it is installed later. We expect that the Altronics case will already have a suitable chassis earth point, consisting of a doubleended 6.3mm spade lug fastened securely to the base. If you’re building your own enclosure, position the lug approximately as shown and attach with an M4 x 10mm screw, shakeproof washer and two nuts – see Fig.4 for details. Next, slip the mains rocker switch and IEC socket into their respective cutouts, noting that the earth pin side of the socket must be closest to the chassis earth point. You can now run and terminate the mains wiring. Use only 7.5A or 10A/250VAC mainsapproved cable for all connections. The Active (brown) lead from the transformer will be long enough to extend forward to the switch, with an additional length of brown and yel- low/green cable needed to complete the switch and earth point wiring, respectively. Use small cable ties where applicable to keep everything neat and tidy. Refer to Fig.1 and the photos for all the details. Do not solder the wires directly to the switch or socket pins! These devices are not designed to withstand the high temperatures seen during soldering and may be damaged. Instead, terminate each wire end in a fully insulated 6.3mm female spade crimp terminal. Note that a ratchet-driven crimping tool is required for this job. Low-cost automotive type crimpers are not suitable and their use will result in unsafe connections. Once the mains wiring is complete, go back and check that each connection is secure and well insulated. If necessary, use heatshrink tubing to completely cover any exposed terminations. That done, use your multimeter to check for continuity between the earth pin of the IEC socket and any convenient point on the chassis that is devoid of paint, such as the countersunk screws in the side panels. This test must be repeated later when the top panel of the case is fitted. At that time, use your meter to check that the top and both side panels are earthed. If not, carefully remove the paint from beneath the heads of the retaining screws to ensure a reliable connection. Module installation The modules may be installed into the case in any order, although you may find it easier to leave the headphone amplifier until last. The audio cable passes beneath this module on its way to the remote control module. To avoid hiccups, careful attention must be paid to the following points: • Adjustments to the lead bend of the LEDs and infrared receiver on the remote control module will be required to get everything in line. Ideally, the LEDs should all protrude through the panel by the same amount, while the body of the infrared receiver should just contact the rear of the panel. • If the infrared receiver includes an external metal shield (see photo), then steps must be taken to ensure that it is insulated from the chassis. We suggest a short strip of ordinary insulation tape on the inside of the front panel, with a hole cut out to match the hole in the panel. Do not rely on the paintwork to provide insulation! • All modules apart from the headphone amplifier are mounted atop 6mm untapped spacers and held in place using M3 x 10mm screws, shakeproof washers and nuts. Use taller 10mm spacers for the headphone amplifier only. • Although the preamplifier PC board has four 3mm corner mounting holes, only two of these are used to attach the PC board to the base of the chassis. The other end of the board is held firmly in place by the RCA sockets, which are attached to the rear panel via seven self-tapping screws. Similar mounting arrangements apply to the headphone amplifier and remote control modules soccer ball GO TO www.rsaustralia.com Easier access to over 150,000 electronic, electrical and industrial products siliconchip.com.au July 2006  69 Top: the supply wiring to the various modules is twisted together to improve the appearance and reduce noise. Note the dress of the flat IDC cable between the preamp module and the control module. Bottom: the self-tapping screws that secure the RCA sockets should be tightened before the two chassis-mounting screws for this module. – only the pair of holes furthest from the front panel should be used for mounting. • For the three main modules, fit the base-mounting screws first, winding on the nuts so that they’re only finger 70  Silicon Chip tight. Next, make sure that each module is in firm contact with the front/ rear panel and fit the nuts or screws on the pot shaft, jack sockets or RCA sockets, as appropriate. The idea is to tighten everything gradually, so that no stress is placed on board-mounted components. • Don’t tighten anything up just yet anyway – you’ll almost certainly need to remove one or more boards to run cabling underneath and to get access siliconchip.com.au Par t s Lis t For Complete Preamplifier 1 1U high steel case with internal divider 1 32mm black aluminium knob with grub screw (Altronics H 6236) 1 15V+15V 30VA (or 20VA) toroidal transformer (Altronics M 4915A) 1 SPST 6A 250VAC slimline rocker switch (Altronics S 3202) 1 snap-in fused male IEC socket (Altronics P 8325) 1 M205 500mA 250VAC slow-blow fuse 1 240VAC 3-pin IEC mains power lead 1 6.3mm double-ended chassis-mount spade lug 7 6.3mm female spade insulated crimp connectors Modules 1 preamplifier module (Altronics K 5502) 1 power supply module (Altronics K 5501, Jaycar KC-5418) 1 headphone amplifier module (Altronics K 5503, Jaycar KC-5417) 1 remote control module (Altronics K 5504) Wire & cable 470mm heavy-duty red hook-up wire 470mm heavy-duty blue hook-up wire 470mm heavy-duty brown hook-up wire 940mm heavy-duty black hook-up wire 400mm light-duty purple hook-up wire 650mm figure-8 shielded audio cable (Altronics W 3022) 250mm 7.5A 250VAC brown wire for mains cabling 650mm 7.5A 250VAC green/yellow wire for mains cabling 40mm length of 3mm bore heatshrink tubing 2 10-way IDC cable-mount sockets 390mm 10-way IDC ribbon cable 14 small nylon cable ties Mounting hardware 7 self-tapping screws (supplied with the RCA sockets) 8 M3 x 6mm untapped spacers 2 M3 x 10mm untapped spacers 10 M3 x 16mm pan head screws 10 M3 shakeproof washers 10 M3 nuts 1 M4 x 10mm screw 1 M4 shakeproof washer 2 M4 nuts Where To Buy Complete & Shortform Kits A complete kit of parts for the preamplifier – including the four modules (power supply, preamp, headphone amplifier and remote control) and a pre-punched steel chassis similar to that shown in the photographs – is available from Altronics, Cat. K-5500. The price at time of publication was $299.00. Alternatively, you can purchase a shortform kit that includes just the chassis, transformer and other hardware (but not the PC boards or on-board components) for $154.00 (Cat. K-5500S). Check them out at www.altronics.com.au or phone 1300 780 999 to order. Note: the infrared remote control handpiece must be purchased separately. We used an Altronics AIFA Y2E (Cat. A 1013) with the prototype but almost any universal remote should be suitable. to the terminal blocks for the audio cable wiring. Low-voltage wiring Now for the low-voltage wiring. First, trim the secondary leads of the siliconchip.com.au transformer to the right length, scrape the insulating enamel off the ends and tin them with solder. You should have about 8mm of nicely tinned wire protruding from the spaghetti tubing. That done, twist one start and one finish lead of each winding together to form a centre tap (the black & white leads on the Altronics transformer) and connect all three leads to the power supply module’s AC input (CON1). Before connecting anything to the July 2006  71 “SWITCH” terminal of CON7 on the preamp to the “JACK SW” pad on the headphone amplifier. This connection ensures that the audio signal is routed to either the RCA output or to the internal headphone output, as determined by the insertion and removal of a headphone jack. Important: if the headphone jack switch isn’t connected to the preamp board as described above, then you must insert a shorting link between the two terminals of CON7; otherwise, you’ll get no signal from the RCA out­ put (CON14)! Grounding The leads of the five red LEDs and the Acknowledge LED (we used blue) are bent at right angles, so that they go through their matching front panel holes. Similarly, make sure that the infrared receiver module lines up with its hole and is flush with the rear surface of the front panel. output of the supply, power up and measure the three rails at the supply outputs (CON2 & CON3). Assuming all is well, the +15V, -15V and +5V rails should all be within ±5% of the rated values. Now switch the power off and physically disconnect the 240VAC mains lead to prevent accidents while working under the hood! The ±15V and 5V cabling for all of the modules can be run next. Use heavy-duty, multi-strand hook-up wire for the job and twist the wires tightly together to reduce noise and improve appearance. Be sure to use the exact connection arrangement shown, with each module having its own set of power leads back to the power supply outputs. 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 text. 72  Silicon Chip Prepare each wire end by stripping off about 10mm of insulation. Tin the bare ends with solder and then trim them to about 8mm in length. If you’ve done it right, you should be able to fit two wires into one terminal block hole. Audio wiring All audio connections are made using twin (figure-8) shielded cable. Run the cable between the volume pot and CON2/CON3 on the preamplifier module first, positioning it beneath the headphone amplifier. The “left” volume cable should be fed through the large hole just to the rear of CON4 on the remote control board. We’ve labelled the terminal block connections “WIPER”, “GND” and “POT” to make identification easy. To prepare each wire end for connection to its terminal block, strip off about 18mm of the outer insulation, then twist the two shield wires tightly together and tin them with solder. An 8mm length of small heatshrink tubing can then be slipped over the shield wire to improve its appearance. Finally, strip about 8mm of insulation off the red and white wires and tin these as well. The connections to the headphone amplifier inputs are made with RCA plugs so fit these to the cable ends first before wiring the far ends to CON6 on the preamp module. Finally, run a single light-duty wire from the So far, you should have only one wire connected to the chassis earth point – the mains earth wire from the IEC socket. Now run an additional mains-rated green/yellow earth wire from the point marked “EARTH” on the remote control module to the chassis earth point. This solidly earths the body of the volume control pot to prevent motor hash or mains hum finding its way into the audio path. Next, use your meter to make sure that mains earth is not connected to the 0V (GND) rail of the power supply. If it is, you’ll need to find the source of the problem before continuing. Even though we intend to earth the audio input as the next step, it is very important that this occurs only via the provided earth point marked on the preamplifier board. To earth the audio ground, run a second wire from the chassis earth point to the pad marked “EARTH” on the preamp module, again using mainsrated green/yellow wire. Both earth wires should fit into a single spade crimp terminal to mate with the free end of the chassis-mounted lug. This earthing method will reduce the chances of creating an audible “earth loop” in your system but success is not guaranteed! For example, if your power amplifier also earths the audio signal, an earth loop will exist once the two are hooked together. This may or may not be a problem. If you notice more hum in your audio system after connecting the preamp, then try disconnecting the earth wire to the preamp module. Never, ever disconnect the mains earth from the chassis! Well that’s about it. We hope you enjoy listening to your new preamp­ SC lifier! siliconchip.com.au