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DIY Digital Tweak: SQ&D Iso Trans

Posted by Jon Risch (B) on October 13, 2001 at 17:11:03

Several people have asked me about low cost isolation transformer, to isolate their digital gear from the rest of the system. Most transformers advertised as isolation types command a premium, and are not readily available.

Now there is a solution:
You go to RS, and get two of their 25 volt 2A transformers, part number 273-1512 at5 $10 each. Measure them and mark which one has the higher output voltage (they may be the same, in which case, don't worry about it), Connect the AC power to the primary of the one with the higher voltage on the secondary, then connect the secondary of the other one to the secondary of the first, and then wire in an outlet acrss the primary of the second one to plug the CD player/DAC into.

A huge honking capacitor (I suggest a large AC voltage safety margin here, even though it is only 25 VAC) can be placed across the secondaries, effectively killing any digital hash coming out of the player, or coming in from the line. Placing the cap on the secondaries avoids any leakage currents, etc. Several uF is not too much, say at a minimum of 63 VAC (it still needs to be AC rated), although if you use a big cap, also use a smaller one, say 0.1 uF or so, to assure that RF frequiencies get shorted, and will help filter things nicely.

The end result on a decent system can be astonishing, the combination of the two transformer cores, and the capacitive short at HF's far exceeds anythig that can safely be acheived with just a cap directly on the AC line.

The only down side is that these particular transformers will only pass about 50W or so, which covers many of the modern CDP's and DVD's, except for the higher end models that draw a lot of current.

Of course, you are not limited to the RS units, you can do this with any step down transformers, and still have isolation, and use the capacitor filtering trick.

Overall rating of the composite transformer aassembly would be the secondary voltage times the current rating to give the amount of wattage it can handle. I would NOT use anythig lower in voltage than around 25VAC for the secondary, as losses through both transformers would be higher, as would the possibility of core saturation, etc.

If you find a higher voltage secondary unit, size the voltage of the caps appropriately. Obviously, an X or Y rated across the full AC line cap would be fine with just about any step down voltage.

This trick with the cap should also be able to be used with a pair of one to one transformers, providing excellent filtering, with a much higher level of leakage current safety.

As always, when dealing with the AC line voltages, take all necessary precuations, use properr build technique and safety measures (breakers, fuses, etc.) and place the entire assembly in some kind of box.

Jon Risch

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