GainClone Amplifier

The GainClone amp described in Part I is reasonably easy to build if you have (or have access to) the necessary tools to cut aluminium.  At the very least, you will need a drill press (and a good assortment of drill bits), and a drop saw (also called a 'chop' saw) fitted with a blade suitable for cutting aluminium.  A compound saw (one that has vertical and horizontal travel) is an advantage for longer cuts, provided it is rigid enough to keep the cuts straight.  I don't recommend that you use material that has been guillotined to size, as the edges will be deformed.  This means that panels will not fit flush.

The drawings that follow have been colour-coded to make them as clear as possible.  The front panel and heatsink are made from 50 x 10mm aluminium bar, with the heatsink section cut down to 45mm.  The backing plate on the front panel may appear to be overkill, but it simplifies construction considerably.  The ability to have one panel that can be moved a little to ensure that everything is properly centred is critical, because countersunk screws impose a zero tolerance constraint.  There is no room for error, because the countersinking tries to force the two joined sections into alignment - right or wrong.

The design shown is only suitable for a small chassis - preferably no larger than that shown.  The 3mm panel thickness is enough to get nice edge joins without air-gaps in a small chassis, but you need additional mounting areas for larger constructions.

Figure 1 - Top (Plan) View of Chassis

Figure 1 shows the basic assembly.  There are many details that have not been included in these drawings because they would become too complex, and some details are left to the individual constructor.  The dimensions are only a suggestion, and you can adjust them to suit your application - as noted above, anything larger is not really recommended.

The drawing below shows the necessary detail of the front panel, its backing plate, the corner pillars and heatsink bracket.  Feel free to modify the construction to suit material you have on-hand or can obtain easily.  Note that the rear of the front panel is recessed in 4 places to accommodate the corner pillar securing screw heads.  You can use countersunk screws instead, but as noted above, you then have no scope for adjustment if anything is slightly out of square.

Figure 2 - Front Panel Details

Please note that the rear view is marked as 'X-Ray', so this is not what you actually see from the back.  The is a view straight through the front panel, so all holes and sub panels are where you expect to find them based on the other drawings.  To see what it looks like from the back, you can mirror the drawing.

You will see that there are no dimensions for the switch or LED holes.  These will be drilled or cut to suit the components that you obtain locally.  The LED hole isn't too hard - LEDs are commonly available in 3 or 5mm diameter.  While a dimension of 20mm is shown for the knob recesses, this may need to be larger or smaller depending on the knobs you want to use.  I suggest that the recess be a minimum of 2mm larger than the knobs.

Figure 3 - Back and Side Views of Chassis

The back and side views are above.  I have shown a row of holes for the side ventilation, and a simple pair of cuts at the top of the rear panel.  This combination is a great deal easier to achieve than the slots I used in the prototype, and will work just as well.

No holes for input and output connectors have been shown, as these will depend on the components you use.  Since many of them have different mounting requirements, you will need to arrange them to suit.  Likewise, the DC connector shown is only a suggestion, so this is another area that you will have to arrange to suit your needs (and based on what you can obtain from your normal supplier).

The layout shown here gives you a fair bit more room to locate everything than my prototype, mainly because the mounting pillars are only in the corners of the chassis.  I ended up restricting available rear panel space because of the way I assembled the original (although it seemed like a good idea at the time).

Figure 4 - Corner sections & Heatsink Bracket

Here is a detail drawing of the heatsink bracket, with all drilling details.  Also shown is the corner piece (4 pieces required) showing the drilling and tapping details. The heatsink bracket requires that you use recessed holes for the screws that go through the front panel.  Because it can be very difficult to get metal thread screws in fractional lengths, this allows you to use the same screw length for everything, but still get enough thread into the front panel to ensure a firm mechanical attachment.  If the angle is not recessed, the screws will probably be too short. Material for the bracket is 10 x 10 x 3mm aluminium angle, cut to 45mm in length. The corner sections are made from 10mm square aluminium rod, cut to 45mm length.  All holes are tapped, with through holes being tapped all the way through.  The end holes are not quite blind, since they intersect with the outermost through holes, and can be extended to intersect with the next set of holes as well.  This makes tapping much easier, since a blind hole thread is an invitation to broken taps unless you are very careful. Although I have given details for 3mm threads, you can use imperial taps instead if you can't get metric screws (or taps).  The closest imperial size is 1/8", and use the thread that is easiest to obtain locally.  Larger screws may be used, but they will look a bit out of place on a small chassis such as that described.

Self-tapping screws can also be used, but they are hard to screw into thick aluminium, and don't take kindly to being screwed in and out too many times (you will perform several trial assemblies as you go along).  I do recommend that self-tappers be used to attach the front panel backing plate to the front panel though - the blind (and shallow) holes in the panel are difficult to tap without breakage.  You need to choose the drill size very carefully to ensure that you get sufficient 'bite', but can still install the screws without risk of breaking them (or stripping the heads - very easy to do unless the screwdriver is a perfect fit.

Although most of the details can be figured out from the above drawings, we need to be very clear about the exact locations for drilling holes.  My recommendations for the panels are shown below.  Needless to say, you can make any changes you want as long as you know what you are doing.

Figure 5 - Top and Bottom Panels

The above drawing shows only the basic hole locations - not any countersinking details, and no holes for feet, internal parts such as the switch bracket and capacitor clamp.  Because these will change depending upon the components you use, the finer points need to be worked out to suit.

Figure 6 - Side Panels, Rear Panel, & Front Panel Backing Plate

Again, the rear panel holes will depend on your connectors, and the front panel backing plate needs holes for the pot bushes, power switch and LED.  The position of the heatsink and corner posts is indicated by the dotted lines to provide a reference.

Please be aware that the drawings above are intended as a guide only.  They are not to scale, and it is possible (probable?) that there are errors that may prevent easy assembly.  You must perform trial assemblies as you progress, to ensure that everything lines up and fits where it should.

Although countersunk screws are indicated for most panel fixing, pan-head screws can be used if you prefer.  To get a nice surface finish, 3mm aluminium plate has enough thickness to allow the screw holes to be recessed (as indicated for the heatsink bracket).  You will need to obtain a drill bit that has been re-ground as a sheet-metal (also called a 'lip and spur') drill bit, having a small spur in the centre and a relatively flat cutting surface.  These can be purchased, or you can grind your own if you are able.

The most desirable final finish is highly individual.  Options include painting, anodising (or 'anodizing'), and can include a brushed surface or sandblasting.  A brushed aluminium finish is usually obtained by abrasive paper wrapped around a wood block, and carefully drawn across the surface to give an even finish.  There are many other options of course, including polishing.

Paint has the advantage that it hides small imperfections, but aluminium requires specialised paints to get maximum adhesion.  Ultimately it is up to the constructor to decide (and in case you were wondering, my prototype was sandblasted).

Remember that the surface finish influences the thermal performance.  A matte black painted surface is a good choice for maximum heat radiation, closely followed by black anodising.  A polished finish may suit your tastes, but is the worst for heat transfer.

Part I shows photos of the finished amp, and has the schematics for each section.  All PCBs except for the DC connector board are available from ESP.  The extension shaft shown in Part I is also available.

Part I    

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