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MUDL ARK A 205 a u d i o s y s t e m s A 20-watt/channel class A2 triode amplifier In last month’s issue, we described the innovative design of the Mudlark A205 stereo valve amplifier which is based on parallel single-ended triodes and a “parafeed” output transformer arrangement. This month, we give the construction details and the parts list. Pt.2: By DAVID WHITBY B EFORE WE GO ON to discuss the assembly of the Mudlark amplifier, we should mention some of the chassis hardware and componentry in the design. While the output transformer configuration is unusual and was described last month, the 100V line output transformers employed are not cheap or inexpensive by any means. They employ quality M6 grain-orientated silicon steel laminations and interleaved windings, as used in the best conventional output transformers. As WARNING! High and possibly lethal voltages are present in the external power supply, underneath the chassis on the PC board and on the choke board assembly. DO NOT touch any parts with power applied and exercise extreme caution at all times while testing the amplifier. 76  Silicon Chip well, the transformers are mounted with their cores at right-angles to each other and have been fitted with copper straps. Both these measures have the effect of cancelling or greatly reducing residual leakage flux from the transformers which could otherwise prejudice the amplifier’s operation. The extruded aluminium chassis gives several advantages apart from having an attractive appearance, with a fine-grooved finish on the top section. It is very rigid and strong, providing a stable platform for the large PC board which mounts underneath. One major change which has been made to the under-chassis arrangement of the PC board involves the octal socket and matching plug which connects all the wiring to the chokes and transformers. In last month’s issue, this was shown with a cable which threaded under the board and into the transformer compartment on top of the chassis. Now the wiring has been greatly simplified because the ceramic octal socket is mounted on the topside of the PC board, the same as the octal sockets for the output valves. Then, inside the transformer compartment, a short cable connects from the octal socket to the vertical PC board which accommodates the nine chokes. The main PC board is double-sided with plated-through holes and 4-ounce plated copper tracks. It measures 180 x 233mm. As noted last month, the kit is available in two versions. Version 1 is a complete set of components and contains everything you need to build the amplifier down to the last detail. There is no drilling or cutting required and all you will need are general electronic assembly tools such as soldering/desoldering tools, pliers, side-cutters, screw & nut drivers, Allen keys, etc. By contrast, version 2 comes with a fully built and tested main PC board which makes it much easier to get the amplifier up and running. Main board assembly Construction starts with the main PC board assembly (Fig.5). First, remove the small RCA connector-mounting siliconchip.com.au 3W resistors. The 1W and 2W resistors are fitted first and all lie flush with the board. The 1W 3W resistor is mounted vertically to aid cooling and the four 27kW 3W resistors are mounted horizontally but raised 20mm up off the board, again to aid cooling. If you discover a mistake after soldering, carefully remove any wrongly placed components using a “solder sucker” or “solder wick”. Component removal is more difficult on a double-sided PC board than on a single-sided board due to the plated-through holes; all the solder must be completely removed from the holes before the component leads will easily pull out. The main thing is to take your time and not rush the job! Polarised components board from the corner of the main PC board and then familiarise yourself with the front edge (switches and pot), the back edge (terminal block connector pads), the component side (which has the most printing) and the underside (with the least printing). Next, fit the seven 16mm hex spacers to the outer hex marked holes on the component side along the righthand and lefthand edges of the board. These are secured with the supplied M3 x 8mm screws and star washers, fitted from the underside of the board. That done, fit two of the 12mm hex spacers supplied to the two hex marked holes in the underside of the board on either side of the central octal (8pin) socket. The remaining 12mm hex mounting spacers are fitted later, to the underside of the chassis. Next, fit the five 8-pin and two 9-pin sockets on the underside of the board, taking care to observe their orientation as shown on Fig.5. Note that the central 8-pin socket is oriented differently from the four other octal sockets. It is most important that you orient each octal socket correctly. The central siliconchip.com.au keyway must line up with the keyway shown on the PC overlay. The smaller 9-pin sockets are polarised and will only fit into the PC board one way around. All the sockets must be pushed as far as they will go into the PC board and held level/parallel with the board surface while they are soldered. The resistors are divided up into three separate packs of 1W, 2W and All the electrolytic capacitors in the kit are separately packed according to value and are clearly marked with voltage polarity. Take great care to orientate them strictly according to the overlay diagram of Fig.5. Since the voltages are high, reversal of polarity will quickly destroy them at switch-on, so take every precaution to get the polarity right according to the component overlay. It is also most important to install the diodes and zener diodes correctly. Don’t get them mixed up. All are critical to polarity but the ones which can cause the most fireworks if reversed are the two main power doubler diodes (D1 & D2) near the relay – so take care to double-check all the diodes and zener diodes before soldering. A socket is supplied in the kit for Measured Performance Output power ....................................................20 watts per channel into 8W Frequency response .......................................... 12Hz to 57kHz within ±3dB; -1dB at 50kHz (at 1W/8W – see Fig.10) Input sensitivity ......................................................... 360mV for 10W into 8W Harmonic distortion ......................typically less than 1.5% at listening levels (see Figs 11, 12 & 13). Separation between channels .........................................................see Fig.9 Signal-to-noise ratio ................... -67dB unweighted (22Hz to 22kHz); -91dB A-weighted, both with respect to 20W into 8W Damping factor...................... >6 with feedback applied; >2 with feedback off September 2005  77 Above: inside the finished amplifier. The octal sockets mount on the rear of the PC board and protrude through matching holes in the chassis. the 555 timer IC and the overlay clearly indicates the mounting direction. Do not fit the 555 timer into the socket at this stage. Mount the BC337 as shown on the overlay diagram. The two BF469 transistors must be mounted so that the metallic side of each transistor body faces towards the back of the PC board (ie, away from the switches and potentiometer). Having mounted all the polarised components, you can then install all the non-polarised capacitors. Board hardware Next, fit the two pushbutton switches to the underside of the PC board, making sure they are pushed firmly into the board and are as parallel as possible, before they are soldered. That done, fit the potentiometer, the relay and the three 3-way terminal blocks which together make up the 9-way connector at the relay end of the board. Next, fit and solder the PC pins for 78  Silicon Chip the feedback wires (either side of the 220kW 1W resistors at the front righthand corner of the board) and to the pads marked “LEDS” near to centrefront of the board. Finally, fit and solder the RCA input connector block to the component side of the small PC board which you previously broke off the main board. You can then connect the 4-way flat shielded cable as shown in Fig.5. The other end of this cable will need to be terminated on the underside of the main PC board, at the PC pins near switch S1. Preparing the chassis Six 12mm-long tapped spacers need to be to the fitted to the underside of the chassis using M3 x 8mm-long countersunk screws through the countersunk holes on the top of the chassis. No star washers are fitted and the spacers need to be held by a nut driver and tightened firmly from above. The front and back panels are Fig.5 (facing page): this diagram shows the component layout on the main PC board as well as the wiring to the RCA input connector board at the top lefthand corner. Note that the two 10mF 400V capacitors should have a bead of silicone sealant under them to anchor them securely to the board. packed with brackets and screws for mounting to the chassis .The panel positions are obvious due to their shape and the printing. You need to attach the three small right-angle brackets for each panel using slotted countersunk screws and nuts, then use countersunk Allen screws and nuts to attach the panels to the chassis. At the same time as you mount the central right-angle brackets under the chassis ends, you can also attach the top cover mounting brackets, on the top of the chassis. Before tightening the screws, position each panel as symmetrically as possible over the end of the chassis. Fit the power switch to the left-hand siliconchip.com.au 10k 2W CON1 V5.21 D2 V52 25V (THIS BOARD MOUNTS UPSIDE DOWN ON BACK PANEL) 1000F/63V 1000F/63V SH D1 D4 NC C 220k 2W 10nF 250V 100F 25V 220k 2W 330k 2W 100F/400V FIT UNDERNEATH 10nF 100V 100 1W 100F/400V 100 1W 220uF/50V 100nF 100V 100 2W FIT UNDERNEATH SKT1 V3 IC1 LM555 180 2W Q3 BC337 330k 2W 27k 2W 56 2W 220F/50V ZD1 12V 1W NO 220k 1W 1000F/63V 1000F/63V RELAY1 270F/200V D7 D6 D5 220 2W D3 56 2W 270F/200V 270F/200V 1000F/63V 470k 1W 270F/200V 22k 1W SH 10k 2W 12.5V 47F 25V RBLL V6 1000F/63V FLAT 4-WAY SHIELDED CABLE UNDER PC BOARD FIT UNDERNEATH 47 1W 47 1W 10F/400V 10F/400V 47 1W 1 3W 47 1W 10k 2W 1N4007 VOLUME CONTROL siliconchip.com.au 220 1W 22nF/400V TO R&L SPEAKER TERMINALS 470k 1W ZD2 NFB (L) R47 PRE/DRIVER VALVE HEATER VOLTAGE WIRE (UNDER BOARD) 220k 1W + – BLUE LEDS UNDER BOARD (CONNECTED IN SERIES, PUSHED INTO HOLES PROVIDED) 220k 1W 1000F/63V NFB (R) 22k 2W V4 Q2 BF469 220k 1W 10k 1W 10k 1W 10k 1W 22k 1W 50k+50k 100k 1W MUDLARK A205 502A Rev1 KRALDLUM 10k 1W MULDLARK A205 S2 – FIT UNDERNEATH 100k 1W 470k 1W 470nF/630V 22nF/400V YEL WH SH RED BLK SH Q1 BF469 22k 1W 220 1W FIT UNDERNEATH 220k 1W 27k 3W 100k 1W 27k 3W 10F/450V KLINK NIL 27V/1W 27k 3W 470 2W 27k 3W 1000F/63V 10F/450V 22k 1W 47k 1W 10k 2W C17 S1 – FIT UNDERNEATH V5 470nF/630V 10k 2W 100k 1W 100k 2W 1000F/63V 22k 2W V1 1000F/63V 10k 2W 470nF/630V 220F 50V 1000F/63V 1000F/63V 10F/450V FIT UNDERNEATH 1N4007 1N4007 1000F/63V 47k 1W 10F/450V 220k 1W 680 1W 470nF 630V 10F/450V 4.7k 2W 220 1W V2 D10 1N4007 D8 FIT UNDERNEATH D11 D9 4.7k 2W FIT UNDERNEATH 10k 2W 100 2W 100 1W 220F/50V WARNING: LETHAL VOLTAGES ARE PRESENT ON THIS BOARD WHILE IT IS OPERATING! SHIELDED WIRE (UNDER PC BOARD) September 2005  79 TO PINS ON OCTAL PLUG 3 4 8 7 6 1 2 5 WARNING: LETHAL VOLTAGES ARE PRESENT ON THIS BOARD WHILE IT IS OPERATING! FROM PRIMARY OF LEFT CH. OUTPUT TRANSFORMER 3 4 7 6 1 2 5 OIDUA SSELG CON1 OUTPUT TRANS FROM PRIMARY OF RIGHT OUTPUT CH. TRANSFORMER LEFT CHANNEL PLATE CHOKE 1 RIGHT CHANNEL PLATE CHOKE 1 LEFT CHANNEL HT FILTER CHOKE LEFT CHANNEL PLATE CHOKE 2 RIGHT CHANNEL PLATE CHOKE 2 RIGHT CHANNEL HT FILTER CHOKE LEFT CHANNEL PLATE CHOKE 3 RIGHT CHANNEL PLATE CHOKE 3 8 502HC Fig.6: here’s how the eight chokes are installed on the PC board which sits on top of the chassis. The output leads go to an octal plug – see Fig.7. side of the front panel and the speaker terminals to the appropriate holes in the back panel. The terminals with the red rings mount close to the top of the chassis. Line up the lead holes so that they are horizontal before you tighten the nuts (this makes it easier to insert and attach the speaker leads when you finally listen to the amplifier). Two rubber grommets are also fitted to the two holes at the rear of the chas- sis to take the wires from the output transformers to the speaker terminals. Choke assembly We now move to the top of the chassis and proceed with the choke assembly which fits under the central cover. Fig.6 shows the wiring diagram for the choke board but you have to follow the strict assembly procedure set out below. This view shows the rear of the choke PC board before the protective plastic cover is fitted. This cover provides shock protection. 80  Silicon Chip Begin by fitting six stacked pairs of 30mm male/female spacers to the top of the chassis, at the rear (ie, away from the valve socket holes). Fasten these firmly with 3mm nuts and star washers from the underside of the chassis. Next, affix the longer of the two 27mm self-adhesive foam strips to the chassis, centrally between the two rows of spacers. The next step is to locate six of the eight EC9 chokes between the spacers as shown in the photo. They are stacked as three pairs of chokes. Fit the choke PC board over the terminal pins to make sure everything lines up, then secure the larger of the two supplied aluminium plates with four 30mm male/female spacers and two M3 x 8mm screws to the tops of the spacers. This clamps the six chokes into position as shown in the photos. Make sure that this aluminium plate is the right way around – the long edge of the plate with the holes further in goes towards the terminal pin side of the chokes. You should now affix the smaller 27mm self-adhesive foam strip to the top of larger aluminium plate, located siliconchip.com.au OUTPUT TRANSFORMERS ALUMINIUM PLATE 30MM SPACERS ALUMINIUM PLATE EC9 CHOKES This photo shows the top view of the chassis with the metal transformer cover removed. This reveals the stacked array of EC9 chokes and the two output transformers which have copper straps around their windings to reduce leakage flux. 3 centrally between the spacers just fitted. Once it’s in place, fit the remaining two EC9 chokes between the spacers on the larger aluminium plate. That done, check that the choke pins line up with the corresponding choke PC board holes, then attach the smaller aluminium plate with the M3 x 8mm screws and tighten firmly to clamp the chokes in place. You can now solder all the choke pins to the board and fit the 3-way terminal block, as shown in the photo, followed by the short preassembled 8-way cable and octal plug assembly. The eight leads are soldered to the choke PC board, as shown in the diagram of Fig.6. As a final step to the choke assembly, fit the small aluminium plate and the protective plastic over the exposed choke PC board, to avoid shock hazard. Four screws secure this aluminium plate and the cover. 2 4 Fig.7 (right): the pin numbering scheme for the octal plug (viewed from the top). This plug and its 8-way cable are supplied pre-assembled and is wired to the choke board. The plug then connects to the central octal socket on the chassis, as shown below. 1 5 8 6 7 TOP (REAR) VIEW OF OCTAL PLUG Output transformers Each output transformer is fitted siliconchip.com.au September 2005  81 RED BLK SH YEL WH SH (RIGHT SPEAKER TERMINALS) (LEFT SPEAKER TERMINALS) E E + + CABLE FROM POWER UNIT SHIELDED CABLE FROM FEEDBACK RESISTORS (REAR PANEL) TO POWER SWITCH FROM OUTPUT TRANSFORMERS (4-WAY SHIELDED CABLE FROM FRONT OF PC BOARD) 9 (MAIN BOARD) 8 7 6 5 4 3 2 1 25V 12.5V Fig.8: follow this wiring diagram to connect the main board to the rear of the chassis and to connect the power cable and power switch wiring. The wiring from the output transformers is also shown. to the chassis, using four M3 x 8mm screws from the top and secured with four 3mm nuts and star washers from underneath. Note the positions of the leads from the transformers and check that the orientations are as shown on the photos. The leads from the transformers are supplied at the correct length and all you need to do is to strip the ends Compare this view inside the finished unit with the wiring diagram above. 82  Silicon Chip for connection and soldering. Strip about 12mm of insulation from the transformer wire leads and terminate them as follows: (1). The Blue and Brown leads are the primary (high-impedance) windings and the Black and White leads are the secondary (low impedance) windings. Twist and solder the two blue primary leads together and then trim the end to about 6mm long and terminate it in the lower terminal of the 3-way terminal block on the choke board. (2). Twist, fold back and tin the ends of each brown wire to produce a 6mm thicker end and then terminate the left-channel brown lead (from the transformer nearest the front of the chassis) to the upper terminal of the 3-way terminal block. Follow this with the right-channel brown lead from the other transformer to the central terminal of the terminal block. (3). Run the black and white secondary leads along either side of the choke assembly, as shown in the photo. The secondary leads from the left transformer are run along the choke assembly behind the choke board and siliconchip.com.au The twisted brown & blue wires to the power switch are run through the inside channel of the chassis, as shown here. Note that the two yellow 10mF 400V polypropylene capacitors sit on a bed of silicone sealant to hold them in place. the leads from the right transformer are run along the other side and held in place with tape. Feed the ends of the secondary wires down through their associated rubber grommets at the rear of the choke assembly for later soldering to the speaker terminals. Perspex panels & blue LEDs. Now for the Perspex panels and the blue LEDs. First, secure the top central cover using its two Allen screws, then place the chassis upside down on a soft surface (to protect the paint) for this operation. The Perspex panels come with a protective coating and are predrilled with blind holes to match the holes on the top front of the chassis. The Perspex panels are located so that the ends with two holes are located near the chassis centre. Fit the panels to the chassis using the special screws for plastic fastening and tighten gently until the panels are firm. Take care not to over-tighten the screws and do not use ordinary selftapping screws, as they are likely to crack the Perspex. Two blue LEDs and a twisted pair of wires, with small connectors to mate with the PC pins on the main board, are provided. The LEDs are later connected in series after fitting them into the panels, so note that the longest lead is the positive (anode) terminal. Orientate the LEDs so that the positive lead of one LED faces the negative lead of the other at the chassis centre siliconchip.com.au and then push them firmly through the chassis holes and into the blind holes in the Perspex panels. Bend the two central LED leads towards each other until they are horizontal and no more than about 8mm from the chassis. Next, trim them so they just overlap and solder them together. Cut the remaining LED positive lead to 8mm and solder the red wire of the twisted pair to this lead and slide the sleeving from the red wire fully down over the soldered joint. Repeat this procedure for the remaining LED lead and the white wire of the twisted pair. Finally, push the sleeve-covered LED leads and the twisted pair down flush with the chassis, with the leads directed to the right (when facing the front of the upside-down chassis). Fitting the assembled board Before fitting the main board to the chassis, we suggest that you spend more time double-checking the component placement and soldering. Any errors are much easier to fix now than after the board is fitted to the chassis and connected to the wiring. To fit the main board to the chassis, first face the back of the upside-down chassis towards you. That done, begin to load the board in component side up and as close to the inside of the chassis as possible, but slightly to the right of centre at first. When the volume control shaft is near the inside of the front panel, move the PC board to the left while putting some downward pressure on the back area of the board near the 9-way connector block. Now push forward until the volume control and switch shafts go through their front panel holes and the ceramic valve sockets locate into their respective holes in the chassis Next, secure the board to the six 12mm spacers on the chassis using six M3 x 8mm screws. From the top of the chassis near the central ceramic octal socket, fit two M3 x 8mm screws into the two 12mm spacers previously mounted on the underside of the PC board. Now fasten the 16mm mounting spacers which are on the component Where To Buy A Kit The price for the full kit (including finished metalwork, panels and all components) is $870.00 (including GST) or with a fully assembled and tested PC board $970.00 (including GST). All metalwork and panels in the kit are fully finished and no drilling, cutting or punching is required. Note: this design is copyright to Gless Audio. Mudlark A205 kits, fully built amplifiers and other components including valves are available from: Gless Audio, 26 Park St, Seaford, Vic 3198. Phone (03) 9776 8703; Mobile 0403 055 374; email glesstron<at>msn.com September 2005  83 The power supply box houses the two large toroid transformers, which are connected back to back– see Fig.9. Be sure to sleeve all exposed connections on the fuseholder and the BP connector that terminates the mains Neutral lead using heatshrink insulation. side edges of the PC board to the inside flanges of the chassis with 12mm clearance spacers and M3 x 20mm countersunk head screws. You can then fit the knob to the potentiometer. Internal wiring to the board The RCA input connectors should already be wired to the main board via the supplied cable. Fit them to their holes in the back panel, using an M3 x 15mm countersunk head screw. Most of the underchassis wiring is shown in the diagram of Fig.8. The 6-way power cable is supplied with the octal plug fitted and the ends of the cable stripped and tinned ready to fit to the 9-way terminal block on the main board. Fit the end of the cable through the hole in the back panel using the supplied rubber boot and terminate the tinned ends into the terminal block as follows (terminal block numbered from outside edge of PC board): 2 brown; 4 blue; 5 orange; 6 white; 7 red and 8 black. Speaker terminal wiring The next step is to connect the two supplied 100mm black leads to termi84  Silicon Chip nals 8 & 9. Twist their bared ends with the existing wires at these terminals before securing them. That done, twist the other ends of these wires to the black leads from the output transformer secondaries (as shown in Fig.8), then place the supplied blue sleeving over the joined ends and solder them to the correct black-marked speaker terminals. Now carefully identify the right and left secondary (speaker) wires and separate out the white wires. Twist their bared ends to the supplied feedback cable leads – red wire to the right and the yellow wire to the left – and place the supplied red sleeving over each. Solder each to the appropriate right or left red-marked speaker terminal. Next, feed the connector end of the feedback cable back along the inside outer edge of the chassis (see photo) and connect the yellow wire to the pin nearest to the edge of the PC board (ie, adjacent to the 220kW 1W resistors) and the red wire to the other pin. Power wiring The first step here is to terminate the previously fitted blue wire from the link pad near valve socket V4 under This close-up view shows the wiring to the octal socket that’s inside the power supply – see Fig.9. the PC board to terminal block pin 3 (ie, with the blue power lead) for the 14GW8/PCL86 valves supplied with the kit. Alternatively, if you want to use 6GW8/ECL86 valves, terminate this lead to terminal block pin 5 (with the orange power lead). The wiring for the power switch is supplied as a blue and brown twisted pair fitted with spade connectors compatible with the power switch. Terminate the brown wire to terminal block pin 1 and the blue wire to terminal block pin 3. That done, feed the spade connector ends back through the inside channel of the chassis and siliconchip.com.au MOUNTING PLATE OCTAL SOCKET FOR AMPLIFIER POWER CORD T2 4 4 5 3 6 2 7 1 3 6 7 8 1 REAR VIEW (NC TO PINS 2 & 5) 8 INSULATED WIRE JOINERS ('BP' CONNECTORS) HOW TO ATTACH THE EARTH LUG STAR WASHERS LUG EARTH LUG T1 METAL BASEPLATE FIT HEATSHRINK INSULATION OVER REAR OF FUSEHOLDER M4 x 10mm SCREW, NUTS AND STAR WASHERS N UE BL GRN/YEL BR FUSE 1 CORDGRIP GROMMET Fig.9: here’s how to wire up the external power supply. With the exception of the primary leads for transformer T1, all the transformer leads are connected to the octal plug. siliconchip.com.au MAINS POWER CORD WARNING: THIS POWER SUPPLY CIRCUIT OPERATES AT LETHAL VOLTAGES September 2005  85 AUDIO PRECISION SCFREQRE AMPL(dBr) & AMPL(dBr) vs FREQ(Hz) 10.000 27 JUL 05 12:26:56 AUDIO PRECISION SCTHD-HZ THD+N(%) vs FREQ(Hz) 10 27 JUL 05 13:01:33 5.0000 0.0 -5.000 -10.00 1 -15.00 -20.00 -25.00 -30.00 0.1 10 100 1k 10k 100k Fig.10: this is the frequency response of the amplifier at a power level of 1W into an 8-ohm load. It has a peak of +5dB at 11Hz. connect them to the spade lugs on the power switch – see photo. External power supply The parts line-up for the external power supply comprises a drilled polycarbonate box, two 160VA toroidal transformers, a laser-cut transformer mounting plate, mains cord, fuse-holder and 2.5A fuse, ceramic Here’s another view of the chassis with the metal transformer cover removed. Don’t operate the amplifier without this cover – it’s necessary to protect against dangerous voltages. 86  Silicon Chip 20 100 1k 10k 20k Fig.11: this graph shows the total harmonic distortion versus frequency at a power level of 1W into an 8-ohm load. octal socket and all necessary screws and hardware. Fig.9 shows the assembly details. Take special care with the mains wiring and the pin connections to the octal socket. Make sure that the mains cord is tightly secured with the cordgrip grommet – you must not be able to pull it out. Check also that the earth lug is securely fastened to the baseplate (see Fig.9) and insulate all exposed mains connections – ie, on the fuseholder & BP connector. Testing the amplifier Caution! – the Mudlark A205 operates at high voltages and the utmost care must be exercised in checking the internal circuitry when it is powered up (see the warning panel p.76). It’s possible to get the A205 going without any test gear as long as no faults exist but it is preferable to have at least a reasonable quality multimeter with maximum voltage range of 500V or more, to determine if all is OK before you fully power up the amplifier. The first job is to test the external power supply, before it is connected to the amplifier. Make measurements at the octal socket with a multimeter on the AC voltage ranges. To do this, you will need to link pins 7 & 8 of the socket with a short length of wire – this takes the place of the power switch on the amplifier chassis. Next, insert a 2.5A fuse into the fuseholder and apply power. Check that the following nominal voltages are present: between pin 1 & pins 7 or 8, 25VAC; between pin 1 & pin 6, 12.5VAC; between pins 3 & 4, 160VAC (take care!). If all is OK, unplug the mains cable before proceeding. siliconchip.com.au AUDIO PRECISION SCTHD-W THD+N(%) vs measured LEVEL(W) 10 27 JUL 05 12:15:16 27 JUL 05 12:19:27 1 1 0.1 50m AUDIO PRECISION SCTHD-W THD+N(%) vs measured LEVEL(W) 10 0.1 1 10 50 Fig.12: this graph shows the total harmonic distortion versus power at 1kHz. It is less than 1.5% for listening levels (less than 2W) but rises rapidly above 14W as the circuit goes into fairly soft clipping. Before testing the main amplifier, make sure that the 555 timer IC is not in its socket. We DO NOT want the main HT voltage present while we are checking the other voltages. Next, install all the valves, making sure that the spigots on the output valves line up with the central keyways in the octal sockets. That done, rotate the volume control to minimum and stand the amplifier upside-down on a soft surface, ready for measurement. With the front panel switch off, plug the cable from the amplifier into the power supply and then plug in the mains cable and switch on the mains power. Switch on the front panel power switch and the blue LEDs should light up the Perspex panels. After a minute of so, check that the valve filaments are lighting up. You may have to peer closely to see the output tube filaments. Now connect the positive lead of your multimeter to one of the screw heads near the centre back of the PC board and measure the negative DC bias voltage on ZD2 which is up near the front of the board. This should be about 26V DC. If this is OK, it is now safe to apply HT voltages to the output valves. Without negative bias, the valves could be damaged when the HT is applied. Switch off the power and wait for a few minutes before inserting the 555 timer in its socket and then switch on again. If all looks OK after a few minutes reverse the multimeter polarity, ready to measure positive voltages. These should be reasonably close to the values shown on the circuit diagram of last month’s issue. The voltage across the 56W cathode resistor of the power valves is a good indicator of correct operation. This should be around +6V a few minutes after switch-on, rising to about +7V after about 20 minutes of operation at average mains voltages. If all seems well, you can fit the baseplate to the amplifier and it is ready to test with some audio input signals. Connect your speakers and a CD player and enjoy! Finally, note that the power supply box runs quite warm during normal operation (the case is a high-temperature type, so this isn’t a problem). And watch out for the valves SC – they get very hot, so don’t touch them. siliconchip.com.au 0.1 50m 0.1 1 10 50 Fig.13: this graph shows the same test as for Fig.7 but this time the feedback from the output transformer secondary is disconnected, giving rise to about the twice the distortion. AUDIO PRECISION SCCRSTK XTALK(dBr) & XTALK(dBr) vs FREQ(Hz) 0.0 27 JUL 05 12:31:07 -20.00 -40.00 -60.00 -80.00 -100.0 -120.0 20 100 1k 10k 20k Fig.14: this graph shows the separation between channels at a power level of 1W into 8W. Fig.15: the distortion from the A205 is predominantly second harmonic, as demonstrated by these scope waveforms. The lower trace shows a 1kHz sinewave at 10W into an 8-ohm load while the upper trace shows the distortion products which have a frequency of 2kHz. September 2005  87