Just over a year ago I put out a feeler for the EZ Tube Lunch Box project in groupdiy.com. Early discussion centred around the two key issues of power supply and the mechanics. The question with the power supply was whether it should be internal or external to the Lunch Box. After a lot of discussion and some trial PCB layouts it was decided it should be external. This avoids any possible interference problems and gives some flexibility in configuring it for different numbers of modules. A PCB was designed that includes 12V heaters, 300V HT and 48V phantom supplies and a prototype was built and tested:
On testing, it was discovered that the 12V heatsink was not as effective as expected and its temperature rise when powering 4 modules was too high. The 12V rectifier also got hotter than expected. The PCB layout was modified to allow the 12V regulator and its heatsink to be mounted off board for larger projects. For these projects a separate off board rectifier should also be used. The schematic of the power supply is shown below:
The mechanics discussion centred on the overall size of the lunch box, the ability to expand it and how to make it for a reasonable price. There are plenty of off-the-shelf 3U high 19-inch sub-racks available that are not too expensive. One of these will hold up to six modules. However, making anything smaller, for example a three module lunch box, involves doing something special which generally ends up meaning more expensive. On balance, the general consensus was that a standard sub-rack was the best solution and one that everyone could live with. If someone really needs one that is three modules wide they could always cut down a standard sub-rack.
During the discussion, Holger revealed he had already built something very similar to the lunch box. It looks like this:
The thing that first struck me about this is that Holger has placed the input and output connectors on the front rather than at the rear as is more usual. My original thoughts were that the sub-rack would need to be quite deep. The reason is that the EZ Tube PCBs do not contain any output transformers so, if used, they need to be mounted externally. Since I had expected the input/output connectors would be at the back of the sub-rack it made sense to put the transformers there also. So this means you need sufficient depth in the rack to mount them from the rear panel. However, if instead you assign a couple of module widths to input output connectors on the front, then there will be space behind them to fit any output transformers. This means the sub-rack can be a little less deep and also a little cheaper. Once you allow connectors at the front you realise there are other possibilities, like fitting meters and faders, for example, to the front. This configuration is ideal for location recording where it can be a real nuisance to have to keep going round the back of a rack to plug things in.
As a result of this I have decided to develop two versions of the mechanics. One will have the lesser depth rack and will be intended for use with front panel mounted input and output connectors. The second will be deeper and have room for rear mounted transformers. Connectors would also normally be rear mounted. In both cases, the dc supply would enter though the rear panel. I have ordered a rack of each type from SRS in the UK and I will build both of them as part of this project.
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