< Picture: Prototype Linear Supply
Current projects - 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 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 gives me 12VAC that runs 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.
[Spec sheet] [construction guide]
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.
[Note: The on-board transformer is custom made.
Pilgham Audio has a kit for this project].
The rectified 12VAC also provides the input to a discrete regulator for the 8.2V to feed the on-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.
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.
