DC blockers are not a new thing but two new examples have been launched in the last month or so from mainstream brands, what gives? DC on the mains or DC offset causes transformers to vibrate and buzz, and when those transformers are large enough this buzzing becomes audible. I have had a few power amplifiers over the years that have done this, sometimes at a distracting level but it wasn’t until I started using the Moor Amps Angel 6 that I finally got to grips with the problem.
Image courtesy of Isol-8
Isol-8 discuss DC on the mains on their site and explain it thus: “As all transformers convert power through the medium of magnetism, any DC present will inhibit its capacity to transform AC by partially or completely saturating the magnetic circuit. It is this vulnerability to DC that causes acoustic hum and significantly reduces the transformer’s power capacity. Even measured at comparatively low levels we have found that this has a significant negative impact on the following power supply and circuit performance, which in turn can degrade sound quality.” They make two power distributors that incorporate DC blocking, the Powerline Axis at £650 and Substation Axis for very large amplifiers at £2,000.
Back in my system Tim Narramore from Moor Amps measured what was coming out of the wall and explained that there was more DC on the incoming power than is usual, his solution was to build a simple circuit around a large capacitor and put it in a box that sits in the powerline. This solved the issue immediately but is not something that Moor Amps manufacturers. Then an Auralic DAC started exhibiting the same symptoms, a buzz that ebbed and flowed and while not usually audible when playing music was quite clear when nothing was playing.
So when iFi and then Audiolab (DC Block, £100) launched electronic DC blockers recently I decided to investigate and got hold of the iFi DC Blocker which is a fairly compact, er, block with a power inlet at one end and an IEC plug at the other. Put it between power cable and amplifier and shazam, no more buzzing transformers.
The spec for the iFi DC Blocker (£129) suggests it’s good for a continuous 7 Amps which means that it’s safe to use with the majority of power amps, even big ones. I did wonder how it affects the sound of the amplifier however, after all if power cables make a difference putting something in the power line is likely to be audible too. I managed to find a period when the DC on the mains was low and not making the amplifier buzz and contrasted the sound with and without the DC Blocker. It’s presence turned out to be marked by a slight haze, a fine grain in the mids and highs that was more apparent with some pieces of music than others. We’re talking a small change here that will be more obvious on more revealing systems but which is more subtle than the buzz of the amplifier in the first place.
During a period when the amp was audibly buzzing I contrasted the DC Blocker with the capacitor based alternative just to see which was the least obvious and found in favour of the capacitor option which allowed the amplifier to deliver better timing through a slightly more relaxed presentation.
Looking around the net it seems that a few other companies make capacitor based DC blockers including MCRU (from £450) and ATL whose Trap Filter V3 looks like good value at under £100. You do need another power cable however which will increase cost if you want something decent, the benefit of the iFi DC Blocker is that it sits in-line with the existing cable and takes up very little space.