Sunday, 16 June 2013
Power Supply Assembly
In my larger mixer designs I use an external power supply as it avoids any possible interference of mains transformers with sensitive microphone input circuits and their transformers. For this smaller mixer I wanted to try building the power supply inside the mixer as much as anything to see if it could be done without compromising performance. From past experience I generally avoid steel enclosures because they can easily conduct interference magnetically from mains transformers to input transformers. Unfortunately the Rackz enclosure is entirely steel so magnetically conducted interference remains a possibility. The best way to minimise it is to increase the distance between mains and input transformers.
The microphone transformers are towards the rear of the case as are the input connectors and the output transformers so mounting the power supply at the back of the case seems like a bad idea. It turns out there is not really enough room there anyway. The only other space with sufficient volume is right at the front of the case. This would be directly below the channel and master faders but these will be connected using screened cable so they should not be susceptible to interference. So I decided to fit the power supply at the front. Since the SMPSU heater experiments were inconclusive I have decided to use the big old International Power linear 12V 5.1A PSU for the heater supply. This has a rather large transformer so I decided this had better be fitted on the right hand side at the front, as far away as possible from the microphone transformers. Initially I tried fixing it directly to the floor of the Rackz case but this proved very awkward as access to mark drilling points is restricted by both the case and the power supply itself. Even if it could be done that way, access for wiring would also be limited. All in all not an easy solution. What I really needed was a neat way to build the entire power supply as a separate assembly.
After some thought it occurred to me that the 3U panel at the bottom front of the mixer is unused (see Sub-Rack Build post). The entire power supply, heaters, HT and phantom, could be built and wired together as a complete and fully functional assembly on this panel. I could even fit a mains on/off switch there too. After laying the components on the panel it became clear they would just fit so I drilled the holes and fitted the parts I have to hand:
The International Power heater supply is on the right and just below it will fit the mains on/off switch (this will be at the top of the panel when it is fitted to the Rackz case). In the centre is the HT350 PCB for the HT supply and on the far left is the phantom power supply PCB. Between these two PCBs will fit the custom toroid transformer for the HT and phantom supplies. Mains will entier via a fused IEC connector at the rear and be routed along the right hand side of the case to the mains switch from where it will be wired to the two transformers. Heater, HT and phantom supplies will all exit to the left and be routed to the rear of the backplane. The picture below shows the power supply panel fitted into the case. You can see it is quite close to the bottom of the sub-rack:
You might think having these two large transformers would make the mixer front heavy. However, there are six quite large output transformers to be mounted on the rear input/output panel which I expect will largely balance them out.