BUILDING THE MJR7-Mk5. Board Layout


Here is a full resolution image of the latest PCB (May-2011), as viewed from the component side. The blue lines are at 1/10 inch (2.54 mm) intervals, the board size being about 4 x 6 inches. Boards are commonly available in sizes 4 x 6 inches and 100 x 160 mm, and either can be used.

There are several ways to make the boards, two suitable for DIY constructors are described, the first using a UV light box, the second an etch resist pen.

This layout can be printed on transparent film (I use tracing paper myself, Goldline 112gsm, with a Canon S600 inkjet printer, which gives a good opaque image, but takes a while to dry. I use the high quality, plain paper, greyscale print settings.) The transparency can then be used with an ultraviolet light box and photo-sensitive boards to produce the PCB, a typical exposure time is 3 minutes followed by about 30 sec development time. This is not too difficult, and the equipment need not be expensive. I bought my own light box second-hand on eBay. A small hand drill is adequate, but for more frequent use a good drill with speed control and stand is invaluable. I have the FBS 240/E drill and MB 140/S stand made by Proxxon. A good article about making PCBs is How to make really really good homemade PCBs. I used MGI Photosuite to print the image the correct size, this has a 'print preview' which allows adjustment of the printed image size and gives a numerical print size which can be used with a little trial and error to get the right printed dimensions, i.e. a board size 4 x 6 inches. If using this program remember to check the 'aspect ratio' box when adjusting print size, then height and width will stay in the correct ratio. The transparency is used on the UV box with the printed side up so that the printing is in contact with the UV sensitive side of the board to give the clearest image. When developed and etched the name 'MJR7-Mk5' should be printed the right way.

An alternative method avoiding the light box is to print the layout on paper and stick this onto a piece of plain board, e.g. with double-sided sellotape or some other non-permanent method, then drill through the diagram and the board. After cleaning the copper surface use an etch resist pen to draw in the connections. Printing a mirror-image of the board diagram above to copy from helps. Then etch in ferric chloride solution, then clean off the etch-resist, e.g. with 'wire wool' cleaning pads. I sometimes use this method to make a single board, but of course the UV method is neater and quicker if many boards are to be made. I usually drill from the non-copper side using this method, but then there are raised ridges round the holes on the copper side. A better idea may be to use a layer of cardboard, about 1mm thick, on the copper side and drill through this to stop the drill slipping around too much on the copper, but I never tried this yet.

All holes can initially be drilled at 1mm, then the few larger sizes are easier to drill accurately. The three holes for the fixing bolts are 3.2mm or 1/8 inch. The Panasonic TSUP 4700uF output capacitors need 2mm holes. The only other sizes needed are 1.2mm, which is needed for the fuse holder, and the inductors, and 1.1mm for the terminal blocks. The 1mm drill bits should preferably be tungsten carbide if fibreglass boards are used, but cheaper 'HSS' (high speed steel) are adequate for the few larger size holes, although they will soon become blunt. The carbide drills are rather fragile, and liable to break with a hand-drill unless a good stand is used. It takes a little practice to avoid breaking drills, I broke 3 drilling 5 boards, but then did another 10 without breaking any. The type with a thicker top section are apparently easier to break than those with a constant diameter, which can flex a little more before breaking. Reground carbide drills are available for about 12.50 (UK) for a pack of 10. Using the laminated type boards with just a thin surface layer of fibreglass the drills will last longer. There is no real advantage to using the full fibreglass boards, the difference in dielectric properties has practically no effect for a single sided board with widely spaced tracks, and the greater physical strength has little if any benefit in this application. I have used these laminated boards myself many times with no problems, including my distortion extraction circuit, which I have used to measure distortion components down to -140dB and beyond.

The next diagram shows the resistors and presets added to the board. The 4k7 presets can initially be set to their mid point and the 220R set to minimum. After completion of the whole amplifier each 4k7 is adjusted so that the mosfet sources are at about half the total supply voltage, and the 220R is adjusted to set the quiescent current of the mosfets to 100mA. This is easily done by checking the total amplifier current with both 220R set to minimum and then increasing one 220R to give a 100mA increase in the total current, then adjusting the other 220R to give a further 100mA increase in current.

The capacitors are added in the next diagram. The polarised electrolytics must be connected with the correct polarity, the negative terminal is usually indicated by a light coloured band down the side of the case and a row of minus signs. Take extra care when soldering the 10p ceramic, these are apparently easy to damage with excess heat, and they are a vital part of the high frequency stabilisation, without these the amplifier will almost certainly be unstable. Using some sort of heat-shunt clip close to the capacitor body when soldering is recommended, e.g. one of the clips sometimes used on test leads.

The last diagram shows the transistors. The 2SA1209 and 2SC2911 are shown with a thick black line to indicate the back metal part of the case. If a supply voltage much more than the recommended 60V is used it may be a good idea to attach small heatsinks to the 2SA1209 current sources (these are the ones furthest from the centre of the board). Even at 60V they get quite hot, so a heatsink will at least increase their lifetime even though it is not essential.

The leds must have cathode, c, and anode, a, the right way round, otherwise serious damage may result when switched on after the transistors are added. It is a good idea to add the leds first before the transistors and try connecting the power supply to check that the leds all have voltage drop around 1.7V to 1.8V, or just check that they all light up. (Actually only 3 in each channel will light at this stage, the mosfet bias led only works when the transistors are included). For most leds the cathode is identified by either a flat on the body or by a shorter lead, but there are a few exceptions to this rule, so the data sheet of any different types used should be checked if there is any doubt.

A separate mains earth in the signal source should be avoided if this can be done safely, and most cd players etc. use double insulation power supplies to allow earth links to be omitted without danger. Keeping everything simple is a good idea, and I prefer a direct connection from a 10k log volume control to amplifier input, and plug a single input lead from the volume control into the signal source. Pre-amps and input selectors are best avoided, unless the pre-amp solves some impedance or signal level matching problem. If an input selector switch is really necessary one possible improvement is switching both signal and earth inputs so that there are not a whole array of input earths connected at the same time, each with possible interference pickup problems.


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