Starting with a general view of the pre-amp, which at this point was very nearly finished - just needed the front panel to be repainted and labelled:
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I chose an all-aluminium case that features extruded front and rear panels, resulting in no screw heads on the front panel. These are much more solid than the cheaper cases, and being made from 1.6mm aluminium instead of thin steel meant that all the screws on the bottom panel can be neatly countersunk (unlike on my NICAM Tuner, which was built in a cheap Maplin case.
Despite buying some really nice panel-mount gold-plated phono sockets a while ago (as seen on the original box and some very expensive hi-fi), I decided to use PCB-mount sockets. These aren't as nice as the separate sockets, but have lots of advantages for the Production dept. There is much less assembly - they are very hard to line up neatly, and you need to attach lengths of wire and terminate them in Molex connectors (which are another connection to potentially upset your music!). Another point in favour of the PCB-mount sockets is that they can support the rear of the PCBs which makes assembly easier...
The front panel has a sub-panel to hold the switch PCBs and the display. This was an important detail to get right, as a thin panel will flex and resonate when switches are pressed. We've all seen products that are built like this - press a switch and notice the others move in sympathy!
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Sub-panel made from 1/8 inch "L" stock - neatly notched to clear the bolt heads. That was done because I'm no good at tapping aluminium!
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The front view, showing how the various PCBs are mounted.
This panel performs an important "cable management" function - the 10 and 14 way ribbon cables travel between the PCBs and the sub-panel. For neatness, I couldn't resist making a neat little guide in each end of the panel - of course, I took quite a lot of time making sure the radius is nice and smooth...
The cable-clamp shown here is made from a strip of stainless steel from an old windscreen wiper. Those metal strips are well worth saving - I've found countless jobs for them!
As you can see, the M3 bolts and spacers that hold the VFD display have been replaced with press-fit inserts - these make the final assembly neater and simpler... Similar comments apply to the slightly different M3 inserts that have been installed on the horizontal part of the bracket - this section ends up underneath the power supply and control PCBs, so if I'd used conventional nuts I'd have to remove the PSU and control PCBs before I was able to remove this sub-panel.
This image shows the sub-panel mounted on the chassis, along with the rotary encoder and headphone socket PCB.
From the position of the spacers, you can see how the PCBs overlap the bottom of the bracket as mentioned above. You can also see that more material had to be removed from the bottom of the L-bracket to accommodate these spacers. I could have just made quick cut-outs to clear them, but I think it looks nicer to clean the opening out properly. There isn't any significant loss of rigidity as the bracket is 3mm stock...
Here is a close-up of the rotary encoder and its bracket:
This was quite tricky to get right as it needed to be accurately aligned in every dimension! The bolts securing it are countersunk on the back of the bottom panel and I used M3 inserts again, so there wasn't much room for 'engineering adjustments' afterwards. So, while final assembly is easier, the need for accuracy was greater than usual.
The length of the encoder shaft meant that the bracket had to be mounted quite close to the rear of the front panel. To enable this, material had to be removed from the front panel extrusion. This also applied to the two switch PCBs, though this was only a millimetre or so.
This image shows the detail of the Store switch:
This button is much smaller than the rest of the switches, and came from an old car stereo - these can be good sources of switches if they need to be illuminated.The clear perspex is required because the switch is offset to allow maximum light from the LED to get to the button. It's held by a simple M3 spacer and bolt - simple, but effective.
This shows the final assembly of this part of the unit:
You can see the headphone PCB, the rotary encoder, right-hand switches PCB, part of the VFD display, the control PCB, and bits of the phono PCB and main analogue PCB. I was very pleased with how it all came together - not bad for a prototype!
Moving to the opposite corner, here's how that worked out:
The wiring to the D-type sockets had been worrying me a bit, but as you can see, it all made sense in the end. When planning ribbon-cable runs, it's easy to get things confused and you end up having to make extra folds and twists in the run. I last made that mistake with my NICAM tuner - when I was planning the layout of the PSU/analogue board, I was viewing the PCB from the rear of the unit, so pin 1 of the connector ended up in the wrong corner, requiring a twist to correct the error.
You might have spotted that I've mounted the mains socket on the inside of the rear panel. This is a temporary measure that will be corrected once I've finished the rear panel [nope, still not done that!]. Otherwise, I'd have to unsolder the mains connections each time I need to remove it.
Note that I've maintained the length of all of the primary connections so that they can reach the mains inlet. This is good practice because you have the option to re-wire the input easily for 120V operation if required. Also, the joins between purple and grey are double-insulated - there's heat shrink underneath the Hellermann sleeve.
Note that the earth connection has its own bolt that is not used for anything else. Not just good practice, but quite likely a mandatory safety requirement in most parts of the world.
You can just about see the corner of a piece of aluminium that I added between the bottom panel and the transformers. This is needed because the bolts used are M4 - it would otherwise be impossible to countersink them into a 1.6mm panel. Rather than using the long a long bolts supplied with the transformers, I used a pair of short screws into 20mm spacers. These securely hold the extra plate, and allow you to use another short bolt to fasten the dished washer that holds the transformer. This also saves you having to accurately cut short the supplied bolt, minimising the risk of the top panel coming into contact with the bolt and causing a shorted turn.
Although the transformer is close to the surround PCB, experiments show that this is not a problem. I did a lot of testing with the workshop hifi, and I could only hear mains-induced hum if I set the amplifier to maximum gain. In this condition, it has a sensitivity of some 150mV - a typical power amplifier would expect around 1V, which is some 16.5dB less sensitive. I found that I could minimise the hum by carefully rotating the transformer - this is normal, of course, and it's wise to allow for this during construction. I also found that placing a small piece of mu-metal between the transformer and PCB cancelled the hum completely, so this is an option should the slight hum be audible when used with real power amplifiers.