Power Supply and Enclosure update

The original plan for the Lunch Box was to have an external supply. One of the reasons for this was the relatively high dissipation in the 12V heater regulator and the corresponding size of heatsink required which meant it would not fit inside the rack. After a lot of discussion with Sven on groupdiy.com, this situation has changed. Sven was very keen to have a built in power supply option and wanted to squeeze it into a 28HP wide, 3U high module. I had already tried and failed to squeeze the power supply PCB layout into a 3U high Eurocard format mainly because of the size of the heat sinks. However, with a built in power supply you can only fit a further four modules into the lunch box. Even if all four are mic pre-amps, the 48V phantom power requirement is much reduced. This means the size of its heat sink can be reduced considerably. It also became clear that for good screening, fitting the PSU into a Fischer Electronik cassette would be a good idea. One advantage of using such a cassette is that it can also be used as a heat sink provided the regulator is electrically insulated from the cassette. This is easy to do with the TL783 regulator used for the 48V phantom supply so it was decided to delete its heat sink entirely and rely on the cassette.

Only having four modules to power also reduces the 12V heater supply requirements to just under 2 amps easing the burden its heatsink. To reduce this even further it was decided to replace the TO3 package LM338 regulator of the original 12V supply design with a TO220 package low drop out regulator the LD1084. This has a guaranteed maximum drop out voltage of 1.5V at 5 amps. Not only can this reduce the overall dissipation but it means we can reduce the transformer secondary ac voltage from 14V RMS to 12V RMS which is a much more convenient standard value. I had already built  a small bench supply to power the 6U sub-rack based test rig for the MK3 mixer using the original lunch box PSU board. So I removed the LM338 regulator from this board and replaced it with an LD1084 and changed the transformer secondary voltage to 12V. The temperature rise in the 12V heatsink was much reduced - with a four module load it was less than 20 degrees C which is quite satisfactory.

I later tried the same regulator on an external power supply using the same PCB and a much larger off board heat sink. With a 6 module load, the temperature rise was less than 15 degrees C.

By removing the 48V heatsink, enough room was made available to allow a Eurocard sized PSU PCB to be designed and still retain the existing 12V regulator heatsink as shown below:

As you can see, the TL783 regulator for the 48V supply on the left has no heat sink.

The next question was could it all be made to fit inside the 28HP cassette. First we needed to know how big the mains transformer would be. To allow it to also be used in an external power supply it was decided to rate the 12V heater supply for 6 modules and also to add a 12V utility supply winding. The specification we came up with was:

Primary 0-115, 0-115 @ 50/60Hz for series/parallel operation

Electrostatic Screen

Sec 1 240V @ 140mA

Sec 2 50V @ 70mA

Sec 3 12V @ 4.3A S

Sec 4 12V @ 1.6A GOSS band

Diameter not more than 95mm

Thickness not more than 50mm

Quotations from two manufacturers determined that this specification could be met and Sven and I each ordered a transformer. 

Extensive calculations and measurements then followed to establish if the toroid transformer and the 3U PSU board could all be fitted into a 28HP Fischer cassette along with an IEC mains input connector, a fuse and an on/off switch. I was also concerned about the heat dissipation in the 12V rectifier at higher current and you can see in the picture of the external power supply that I have replaced the on board DIL rectifier with an off board 25 amp rated type that is bolted to the chassis to act as a heat sink. Knowing the Fischer cassette could also be used as a heatsink, we looked at bolting a higher current rating bridge rectifier to it. Below is a sketch showing a possible layout of the major components:

In the last few days, prototype PCBs, the custom transformer and the 28HP cassette have arrived. Here they are fitted together:

So far it looks as though it will all fit in reasonably comfortably.Next step is to complete the build and test the power supply. In the meantime, a 3U rack has been obtained from SRS and filled with four 14HP modules and the 28HP power supply module to give an overall idea of what the lunch box will look like:

Introduction

The EZ Tube Mixer is an ambitious DIY project (eztubemixer.blogspot.co.uk). As I discovered when building the 4 into 2 demo mixer, there is a lot of cabling between modules and other components and within the modules themselves. The lessons learnt in building the EZ Tube Demo Mixer are being incorporated into the MKIII design which will hopefully be a lot easier to build (mark3vtm.blogspot.co.uk). However, this does not mean that the EZ Tube Mixer design is now obsolete. In the last year it has become clear that there is a lot of interest in much smaller tube projects consisting of perhaps a couple of channels of mic pre, or tube EQ, or compressor or even simple tube mixers. The EZ Tube Lunch Box is intended to fulfil this need.

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.