MJR Phono Pre-amp. Board Layout.


Update 13-Nov-2016:
Another error, in the original board layout diagram one of the electrolytics was reversed, if anyone used that board layout they will need to replace the capacitor, it would certainly be damaged by reverse bias. I have marked the corrected capacitor with a red star.


Here is the final PCB (March 2012), as viewed from the component side. The black dots along the top and side are at 1inch intervals, the board size being about 4 x 5 inches.

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. The advice to not use sodium hydroxide for developing is particularly important, its use almost guarantees failure, I only use it for stripping off all the photoresist after etching. 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 5 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 'MJR-PHONO-1' 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. 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.
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. Note that some 'etch resist pens' are just ordinary marker pens. In my experience some give very poor results. The ones made by Dalo are more expensive but work reasonably well, they use an ink valve so that you need to press the point in to release ink. Some different opinions can be found here.
Next 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.

All holes can initially be drilled at 0.8mm or 1mm, I usually use the 1mm size because the drills survive longer. The four holes for the fixing bolts, if required, are 3.2mm or 1/8 inch. The 1mm drill bits should preferably be tungsten carbide if fibreglass boards are used, but cheaper 'HSS' (high speed steel) are adequate for the 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, 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.

The next diagram shows the resistors and wire links added to the board.

The capacitors and presets are added in the next diagram. There is a space near the input for optional cartridge loading capacitors, which may be needed for some cartridges, but remember that the cartridge loading capacitance includes the capacitance of the connecting cables used. 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 68p ceramics, 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, LEDs and ICs. The leds must have cathode, c, and anode, a, the right way round. 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. The components shown in red are mounted on the copper side of the board. The surface mount jfets are very small and need a pointed soldering bit, and some sort of clip to hold them in position while soldering. One OPA134 op-amp needs to be on the copper side to preserve layout symmetry between the channels. The BF862s have interchangeable source and drain, so they can both be on the copper side as shown.

I have shown T3 as a BC560C, but I used a 2SA1085E and recommend these if available, but there are probably a lot of fakes about. Also note that this type has a different lead order, BC560C is C-B-E but 2SA1085E is E-C-B, so some adjustment is needed compared to the diagram.


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