Lunch Box Enclosure update:
I have been having a discussion with Frank Rollen (frontpanels.de) and one or two group members about an alternative lunch box enclosure. The reasons for this and the results of those discussions are set out below:
The Eurorack enclosure for the lunch box and the power supply have always been a bit of a problem. Although all makes of Eurorack meet the same specification, they are all slightly different and often this means DIYers have to hunt for a local supplier, find all the parts they need and then order them. The built in power supply does work, but it uses up two modules and it is a bit of a tight squeeze inside, which some DIYers might struggle with.
With these thoughts in mind began a discussion with Frank Rollen (frontpanels.de) who I use for all my custom front and rear panels. He has recently introduced a new range of 19inch rack mounting enclosures
http://www.frontpanels.de/nrg-case-19-enclosures/. This is very interesting to me for all kinds of projects because they have some very neat features and they are not very expensive. For example, the sides are made from steel and have plenty of slots in them. Although these are intended principally for ventilation, they are perfect anchor points for output transformers (a VTB2291 fits perfectly). This saves a lot of measuring, drilling and de-burring. The sides also include useful earth bonding points. The top and bottom are aluminium which will minimise magnetic conduction throughout the case. The front and back are aluminium and Frank provides Front Panel Designer outlines of both so you can easily add in the specifics for your project. This may sound like an advert for Frank's products but I am really impressed how appropriate these are for the DIYer.
One particularly interesting feature is the mounting bars he offers as accessories. These have 500 series module mounting points at 1.5 inch intervals. You can also fit the same mounting bars further back to mount backplane PCBs. There is space for 10 standard 500 series modules.
Now I don't do 500 series modules; my modules are 2.8 inches wide. However, it occurred to me if I made front panels that were 3 inches wide instead, they would exactly fit two 500 series slots. Five of them would fit in Franks enclosure.
The other good thing about Frank's enclosure is that it is 300mm deep - this is deeper than most of the standard Euro racks I use and it means it should be possible to squeeze in a power supply behind the backplane PCB instead of having to use module space for it. It will therefore be more open and accessible and easier to build and maintain.
The bottom line is I could see the possibility of a standard enclosure with built in power supply that could hold up to five 3 inch modules. It also happens to be at least 20% cheaper than the Euro racks I currently use.
Frank sent me a 3U case to try this out. I have designed a 5 slot 3 inch pitch motherboard, with fixings compatible with Frank's mounting bars, and a new power supply PCB both of which are now being made. To save space, the power supply board only contains phantom power and HT supplies as I plan to run the heaters from 12VAC instead of dc (I recently did some tests in the prototype lunch box that showed using AC heaters did not affect noise or hum performance of my mic pre boards). This means the power supply PCB is only 100mm by 100mm and should fit behind the motherboard in Frank's enclosure
The other day I did a trial assembly yesterday to see if everything would fit in the box. The attached picture shows a top view of the results. You can see there is room behind the motherboard on the left side wall for two output transformers. On the right wall can go the mains transformer. The PCB next to it is an old spare 100mm by 100mm PCB that I used to represent the power
supply PCB. The mains inlet socket can go behind it on the rear panel. I used one of the 6 module motherboards for the test.
So far it looks pretty good.
This does not affect the original Lunch Box concept. You can still use a Euro rack to build a 6 slot version with external power supply or a 4 slot version with internal power supply. However, I suspect the 5 slot version with built in power supply will be very attractive and I can see it eventually replacing the other versions simply because it standardises a hole range of things like fixing holes and will make it more of a DIY exercise than a design and purchasing exercise for most constructors.
Also, since there should be just enough room to get the power supply board in behind the motherboard in a Eurorack, then the 4 slot version could use this power supply instead of the two module wide power supply as at present. This then provides a 2 slot wide front panel space which can be used for meters, faders, XLRS etc.
In theory, the new power supply will also fit behind a 6 slot motherboard in a Eurorack. However, this leaves very little space if you need a full complement of output transformers for example. I am sure there will be some occasions where it will work, but if it does not then the external power supply is always an option.
As always, thoughts and comments are appreciated.
Cheers
Ian
