I just read an article about Audiolab’s DC Block 6:
There seems to be a lot of products on the market:
and probably even more products.
But are these products dealing with a real problem? Would it make sense to invest 500-700 EURO in a DC-Blocker?
Torben
What problem are you trying to solve exactly? Reading ‘what hifi’ would be an easy one to fix… 
DC on the mains causes toroidal transformers to hum, sometimes loudly if it rattles the device casing. That type of transformer is often used in linear PSUs. SMPS don’t have issues with DC on the mains AFAIK.
DC offset / bias on AC can be a real problem if you experience transformer hum caused by it.
There are a few relatively inexpensive DC blocking devices to try. Note that an ordinary power conditioner that does not state the capability of blocking DC likely does not offer this capability.
Not for the OP (those asking whether they need it do not need it), but for people searching for solutions dealing with transformer hum, there are also products like the USD89 (at the time of this post) Emotiva CMX-2 (important: not CMX-6), and similar products from iFi and avahifi Humdinger.
It varies from setup to setup in terms of matching. Some people need to get through different brands of DC blockers to find one that matches their setup / AC environment. Some people find using a DC blocker reduces but not totally eliminates transformer hum in their environment.
If you’re competent with a soldering iron and don’t mind a bit of DIY, it’s super easy and cheap to build a DC blocker. A fistful of big electrolytic caps and a big bridge rectifier is all you need. Lots of circuit designs available online.
If you needed a DC blocker it would be self evident. You would have audible mechanical transformer hum and your affected transformers could get excessively hot at idle. However it only becomes a serious issue with toroidal transformers of 500VA and more. So in reality most of the time DC is just an annoyance if it is there.
I built one for my previous amp. It was a big dual-mono hybrid with 2 x 800VA transformers. Without the DC block, they could buzz very annoyingly at times.
I have two DC blockers which are an earlier version of these:
Highly recommended, not silly pricing, trustworthy seller (yes he’s in Bulgaria), and they work. That I use each blocker with a pair of Meridian DSP8000SEs and a pair of DSP5200SEs should give an idea of my confidence in them.
I went through a bad period of hum. DC offset is one thing which does vary during the day (washing machines etc.), but running toroids off the UK’s (often) 245V mains when the toroids are Euro-standardized at 230V really doesn’t help from a hum standpoint either.
I’ve just received a DC blocker from ATL Audio. I decided on their model, using discrete diodes, over the Audiolab version because it looked like it used better quality components.
In fact, it cost less than the Audiolab–no duty on this product.
And, no more toroidal hum.
Wow, that’s crazy cheap considering the build quality and components used! My DIY build wasn’t a lot cheaper.
I had considered building my own, but by the time I’d sourced the components and a case, this wasn’t significantly more.
It’s really well built. I opted for male / female IEC sockets as that’s neater.
Yes, to successfully remove toroidal hum caused by a DC offset on the mains. That’s the sole purpose.
Typically, the DC offset varies from day to day, and, for me was more pronounced during the working week. I also checked if the source was in my home first: it wasn’t.
There are two versions of the blocker: one with a bridge rectifier, and the other, with discrete diodes. The latter handles higher offsets.
How? By powering off everything electrical or using some measurement? Thanks.
I simply powered down the house circuits. If I hadn’t given my son my scope, I would have used that to measure the offset.