Where  does the DC voltage come from and what is the result of it?  Please note that houses are normally connected to a three-phases cable, and each phase  is used to build a so-called "group"  in your house.  Each group is connected to a series of AC outlets.   So, the basement is connected to one group, and the first floor to another,  etc.  The good part is that if one group has a blown fuse,  the lights won't turn off in the whole house. You must have noticed that.  Now.... what is the bad part of this? You will never have the same load on each group. If your washing machine is running on one group, using  a LOT of  current,  and your  stereo equipment on another group, just using A LITTE BIT,  the groups are unevenly loaded.  Your neighbour is doing some electric welding, and the small factory near by is just running some heavy machines on two groups.   The result is, all houses in the block have their own uneven load of the three phases.  Additional to this come the many electronically loads, builded by PC's switched power supplies,  electronically regulated power supplies from washing machines,  airconditioners etc.    These draw artificially some DC current out of the AC network.  The result of this is a big MESS  on the mains line, coming into your house. These "phase cutting"  electronics will load the mains with a DC current when the electronics is not so perfect. ( and that's just what they are - not so perfect)  So.... Most  mains outlets in your house will  have a small DC Voltage on it, depending on the hour of the day. It is also depending on what the your neighbour is doing at that moment - he may not even know.  How much you "think"  this is, and how much you  think this matters,  is one thing.   Only,  by thinking you can't measure it.  What is reality in your house,  and what happens in your stereo equipment is another thing.   Take a normal voltmeter, and measure the mains  voltage.  Put in on DC,  and  if the number you see now is 0,000 then your mains is not DC loaded.  So you won't need a DC-proof mains transformer, and you can  forget about this  page.    HOWEVER.... when you  see some DC, please read the rest.

What happens to HiFi Equipment when connecting to a mains voltage with a few volts DC on the AC line?  Most mains transformers use a low cost E-Core, because they're cheap to make.  These do NOT like DC voltage at all because they have no air gap.   Just put 1Volt DC on a transformer like this,  and try it, and you will see how much DC current will flow.   It is a few amperes.  So, you can understand now that also 0.1 Volt DC can not be just  neglected.   This DC current  will  pre-magnetize the core, because they're just not made for DC current.   The AC voltage peak values will lead the core into saturation, causing mechanical,  audible hum.  There are bad waveshape with lots of harmonics on it, adding 100Hz, 150Hz, 200Hz, etc to the signal.    Hopefully there is a special mains filter for this  in your amplifier.  When not,  you will hear a sound best described  as "hummmmmm".   Of course you will check up all audio cables, but never find where it comes from.

What can you do against it?    You can buy special DC- filtering equipment.    They start from  1000 Euro.   Upper limits don't excist in HiFi.   These create an artifical DC voltage, which is opposite  polarity of the unwanted voltage, add it up,  and remove it like this.  Technically this method is called "compensation".   Here is a great product from the well know company Burmester.    Please click the link, and read their technical explanations.  They are very right about what they say.  It is explained very nice.

Is there a better way for DIY?  I think the most  elegant way is to use an air-gapped mains transformer.    These by nature have a certain DC capabilty, and simply filter out the DC voltage as if it was never there.     All Lundahl mains transformers have a core air gap, and can deal with the DC voltage on the AC line.