A linear power supply to replace the smps in the Behringer DCX2496
After the successful replacement of the analog i/o board of the DCX2496, I planned to replace the smps power supply. The idea was that getting rid of switching residue, whether radiated or conducted, would improve the DCX2496 performance even further.
Others had already done this and reported an improvement. But those solutions had a few shortcomings I didn't want; I really hate to use toroidal transformers for low level electronics. They are so wide-band that any interference, noise, pulses etc on the mains are transmitted to the secondary side almost un-attenuated. The advantages of toroids - smaller size and less stray fields - are secondary in my view, and those issues can be taken care of easier than trying to filter the mains junk.
I opted for a staged approach to get the clean power I want:
- First step: using an external transformer in the form of a 'wall wart'. This should give me 12VAC that I can run into the DCX. It is important to select one that has an EI-core as this is the first barrier against mains borne junk. More about the selection of the wall-wart here.
- Second step: Use a good quality mains filter on the secondary side of the wall wart. The 12VAC enters the DCX through the mains filter.
- Third step: use a separate 12V -> 18V transformer to provide the 18V center-tapped (CT) for the critical analog supplies. This should be a transformer with separate primary and secondary winding chambers to minimize HF coupling to the mains.
The raw 12VAC coming out of the mains filter will be used for the +5VDC and +3.3VDC digital supplies. The 18VAC CT transformer provides the +15VDC and -15VDC analog supplies. The use of the 18VAC CT transformer helps to keep the analog supplies isolated from the ground- and return currents of the digital supplies.
The rectified 12VAC also provides the input to a discrete regulator for the 8.2V to feed the DSP board 5V analog supply. This regulator uses a MOSFET as the pass element, for a very low dropout-voltage of a few 10's of mV only.
The analog +/- 15VDC supply the analog circuits of the input- and output boards. Ideally one would want to use a 'super regulator' as these suppress mains junk out to high audio frequencies. But you can't bring that advantage to the load circuits because you have several inches of wire between the regs and those load circuits, and this negates a large part of the super reg advantage.
In such a case it is better to suppress noise, EMI etc. as close as possible to the source, the mains, and use a local regulator, as on the DCX DSP board. That is also the reason I again use local regulators on my replacement I/O board.
Another issue is the rectifiers used. Often you see capacitive snubbers across rectifiers to suppress switch-off noise. But these have another side: they also provide a path for noise from the secondary to the supply. By using fast but soft turn-off rectifiers, snubbers are not needed and there is no switch-off noise.
An important issue with any linear supply is thermal management. There is a good reason why Behringer uses switched-mode power supplies: they run very efficient and therefore very cool. Stuff that is used for PA duty gets stacked up, placed in full sunlight and can be used in places that are very hot to begin with. Any additional watt increases the thermal load. But there is a disadvantage that is critical to hi-fi home use: switching residues remain on the supply lines, get into the circuits and lead to noise and intermodulation products. Other things being equal, linear supplies provide much cleaner supply voltages for lower noise, better imaging and clearer sound.
With a linear supply for the voltages and currents required by the DCX you can be looking at up to 10W dissipation you need to get rid of. The usual solution of using heat sinks is good if you can use heat sinks on the outside of the case. I didn't want to do a lot of mechanical mods to the DCX case, and internal heat sinks are out of the question. You'd only move the heat problem from the regulators to the rest of the DCX electronics. I had to find a way to get that heat out of the case.
The way I solved it was by using the existing mounting holes for that smps to thermally connect the regulators to the case bottom. The regs themselves still get warm (especially the +5VDC reg which goes up till 50 degrees Celsius at room temp), but the internal temperature does hardly rise. The case temperature does rise, but because of the large surface, also only a little.
The regulators and a power MOSFET are soldered on the PCB board edge and they mount though the holes of two aluminum strips in the holes used for the stock SMPS. I specifically opted for two aluminum strips instead of a single sheet to further minimize heat loss inside the case. So, there are 4 regulators, each firmly bolted to the case. There is a fifth bolt for a MOSFET that regulates the 8.2VDC for the +5V analog supply on the DSP board. This MOSFET is bolted on the strip with a counter-sunk bolt, but not directly on the case.
Selecting the wall-wart
Aside from the electrical specs for the wall-wart, there is the mechanical requirement that you want one with an EI-core transformer and not a toroid as explained above. Fortunately, they all seem to use EI cores.
In Europe, the overwhelming majority of available wall-warts provide a DC output. We can't use that; we need one that gives out AC so we can use the small on-board transformer of the new supply to get us to the 18V Center-Tapped (CT) for the analog supplies. The rectified voltage from the wall-wart has to supply the +5V, +3.3V and +8.2V regulators. From a voltage point of view, the 8.2V is critical: under no circumstances should the input to the reg drop below the drop-out value. The 8.2V reg is a discrete design with a drop-out of only a few 10's of mV, but still, if you figure in low mains and maximum ripple, you need a 12VAC output wall-wart. The total current required by the DCX, from all supplies, including the current for the displays of my modified I/O board, is about 1,5Amps. So, you're looking at something like a 20W supply.
It just so happens that one AC-type wall-wart you can get from several sources is a 12VAC-20W type sold for 20W halogen lights. For example, in Europe, IKEA furniture stores carry the FEMTON clamp-on 20W halogen light that sets you back a whopping 8.99 euro and includes a wall-wart. It connects with a small but sturdy plug at the wart. Recommended.
Other sources in Europe include Conrad Electronics. In the USA the choice is much greater, from Allied, Jameco or Mouser. Prices are mostly less than $ 10.
Then the on-board 12VAC -> 18VAC CT transformer. My efforts to use existing mains transformers in reverse direction were not successful. I had this transformer custom-made. (In fact, I had a bunch custom made. Single quantities are outrageously expensive).
Ward Maas from Pilgham Audio who also supplies the kits for my remote control I/O replacement board for the DCX also sources this Linear Supply kit including the custom transformer.
Update on the wall warts: It seems that IKEA and others are switching to miniature smps's for their LED and Halogen lights. These CANNOT be used for this project - you need one that delivers 12V AC, 50 or 60cycles!