Noise-Trap passive mains filter
In most countries, high frequency noise on the AC power supply – often worse at certain times of the day – causes audio systems to dip in sonic performance. The high frequency noise seems to have a curious effect on some humans too, causing them to assure us that the variability in audio system sound is not real, but psychological, a result of how our changing perception and moods affects how we hear things. In full-on mode, they’ll dismiss all commercial solutions as mere foo, intended to part gullible punters from their money, and then we’ll get the fall-back position; audio designers know about mains noise, and build protection against it into their products, so everything’s just fine anyway.
In the real world, the impact of mains noise – both common-mode and differential – on audio system performance is no longer a contentious issue. It has gained more prominence over the last two decades, initially slowly as a result of the innate conservatism of the designer community, but more quickly of late as a result of the proliferation in homes and commercial settings of two relatively new primary causes of much of the mains pollution, the switch-mode power supply and LED lighting. They have made mains noise so widespread that few audiophiles can be unaware of the damage it does to sound quality. If you will tolerate the pun, they have made mains noise mains-stream (groan, Ed.).
What frustrated users of audio systems hear when the AC mains is carrying high frequency noise is a loss of dynamic energy, tonal density and detail. Sound-staging that should be pleasingly 3D now becomes disappointingly 2D with little front to back depth. What’s happening is that the pollution is making it past the power supplies in our system components which now have to transcribe and amplify both the music signal that we do want, and the pollution frequencies that we don’t want. We hear a loss of energy and detail in the music. We don’t need a glass of wine and comfy slippers to correct the fault; what we need is some form of technical intervention, preferably something that removes the unwanted pollution from the mains supply.
Corrupted power
A previous house in an isolated hamlet of four houses and a farm had given us blissful years of more or less unpolluted mains power, but when we moved to a village of 200 homes, the new and huge variability in system performance nearly drove me mad. In particular, system performance in the morning was markedly inferior to later in the day. With the help of a number of audio manufacturers, the multiple sources of the new level of pollution were identified – some of them by the regular time of day they operated (milking parlours) and some by the frequencies of the pollution that they injected into the mains supply (the school, the blacksmith with his power-hammer). The local grid operator would not even discuss the possibility of switching the household to an alternative phase supply, one not used by the major polluters, so the only alternatives were moving house again, or deploying mains treatment products. A brief household discussion settled that.
Alternatives
A search back through the archives here on The Ear.net will bring up reviews of mains treatment devices ranging from regenerators that seek to provide clean power by generating a perfect 50Hz sine wave from scratch, to balanced mains transformers that cancel common-mode noise on the mains feed, to passive mains filters that rely on inductance, capacitance, resistance (LCR) networks to prevent frequencies greater than 50Hz reaching the input socket of attached audio components.
None of these generic approaches is without its potential downside. Regeneration – it uses a powerful dedicated amplifier to create the alternative 240V 50Hz sine wave – is costly to do well. Balanced mains transformers work over a limited bandwidth, only on common-mode noise, and can be prone to generating their own noise if there is a DC component on the incoming power. Passive LCR filters need careful tuning, ideally for each individual location, if they are not to cause sonic problems of their own. Not only are the capacitors and inductors used in such filters prone to generating their own noise, but the filters can present so much more additional impedance to the mains signal that significant dynamic energy is lost. This is particularly a problem for power amplifiers where instant current is required to deliver big dynamic transients and it’s why LCR filters are a balancing act that not many manufacturers manage to pull off. Mains noise will undoubtedly be reduced by such a filter, but so too, more than likely, will dynamic expression.
The one mains noise mitigation technique that swerves all of the above is the shunt. It works by luring unwanted common and differential mode higher frequencies (the noise) down a low impedance path, either to earth or to some form of absorption that converts the unwanted energy into micro-heat. Crucially, and unlike LCR filtering, a shunt works in parallel rather than in series with the mains feed, so it cannot have any unhelpful impact on energy transfer.
Downsides
Richard Marsh, who readers with a long memory will remember for his days as designer for the formerly huge audio brand Monster, has decided that it is high time the shunt was given a make-over. He likes it for its relative simplicity compared to other noise mitigation techniques, and the fact that for all of its effectiveness, it has no sonic downside.
The result of his efforts is the Noise-Trap, a rather striking chrome and grey aluminium device that’s about a foot long. Costing £800, it has a blue LED to indicate being live, and a regular IEC mains inlet at one end that allows connection to a mains outlet via an ideally low-impedance power lead. When I sent the editor a photograph of the review sample, plugged in and sat on the carpet adjacent to the kit table, it caused him to remark ‘Interesting. I’ll take a look and check that it’s not a small Hoover.’
All joking aside, it’s not just the look of the Noise-Trap that’s unconventional but also its performance. Most mains shunts by other makers tend to be narrow-bandwidth affairs, working at the frequencies immediately adjacent to 50Hz. Marsh’s design is different – its size offering something of a clue – because the chassis contains not one shunt but a series of them, each working over an overlapping range of frequencies that in total cover from 50Hz to a claimed 100kHz. But that’s all we are told. Frustratingly for a reviewer wishing to present readers with a rounded view of the Noise-Trap, Marsh is unforthcoming about what’s else is inside. He referred me to his original ‘very old’ patent for a mains shunt, then added that Noise-Trap is more than that original idea with ‘new modifications and trade secrets ’.
Removing the lid of the Noise-Trap gets us no further. Visible is a circuit that discharges the capacitors if the Noise-Trap is unplugged from the wall socket so that users cannot sustain a shock if they grasp the pins of the plug , but the rest of the internals are potted in black resin, no doubt partly to hide them from casual view, but – and this is simply my guess – also in an effort to mitigate unhelpful mechanical excitation of the capacitors as they act as a conduit for the unwanted noise. Does the Noise-Trap shunt to earth or to absorption? It’s not clear, but neither does it matter that much to anyone but the technically curious. The fact is that it works, and works well.
Native DSD
Since changing the review platform DAC to one that handles DSD files in their original 1-bit form rather than converting them to something else, I’ve become addicted – ruinously for my wallet – to the shop-front operated as The Spirit of Turtle by Dutch recording engineer Bert Van Der Wolf. Here can be found a cornucopia of hi-res files in various formats, all of them scrupulously crafted recordings by Mr VDW himself. This is not so much a collection of yawn-inducing audiophile demonstration recordings, but a library of high musical art that just happens to be in hi-res.
The Beethoven Symphonies series by the Brugge orchestra Anima Eterna under Jos Van Immerseel is a case in point. The six volumes can be bought individually or as a set, and in a range of file formats. I bought the set in DSD256 and have been basking in world-class performances on period instruments that are recorded so expertly that the sometimes-thick veil between recorded and live sounds seems almost to no longer exist, so much vital presence has VDW managed to capture. This entire set is therefore an ideal diagnostic test of the Noise-Trap.
The evaluation was carried out in the morning, the time of day when raw AC mains power really makes its presence felt in the reference system. After stripping out the system’s regular mains-pollution defence measures and leaving the Noise-Trap to one side, I played the 6th Symphony in its entirety. There was no need of course. The first three minutes or so would have been more than enough to get a reference point, but the 6th is, in common with all the recordings on this remarkable set, just so satisfying from an artistic point of view that it would have been a crime to cut it so short.
Palpable
The Noise-Trap was introduced and the 6th was set playing again. Whatever the measured performance might be, and whether the Noise-Trap really is more sophisticated than a time-proven shunt to earth, it works extremely well. The two overriding immediate impressions were of an almost palpable sense of performance space that had been virtually absent only minutes before, and apparently stronger dynamic energy levels, almost as if the volume control had been wound up. Further attentive listening confirmed that the increase in detail retrieval extended to textural and tonal density.
Conclusion
None of those observations will come as a surprise to readers already well aware of the damage that mains noise does to the performance of their systems. The question is does the Noise-Trap return in sonic value the equivalent of its ticket price? As always with mains noise mitigation measures the answer to that question depends to a degree on how polluted our native mains supply is to start with, but in the context of the review system £800 for such a profound uplift in performance would make the Noise-Trap a complete no-brainer. Its extended bandwidth, enabling it to mitigate not only the primary frequencies of pollution from power supplies, LEDs and reactive loads, but also their harmonics, means in this instance that bigger and more expensive really does equal better.