Hardware Reviews

BMU 3000VA direct power

Russ Andrews BMU 3000VA

Russ Andrews BMU 3000VA install version

The Russ Andrews BMU 3000VA aims to bring pro standard power to your home. Recording studios and mastering suites have an obvious interest in achieving the cleanest mains possible and that’s why most, if not all of them, power their electronic equipment from balanced mains units. In domestic settings though, balanced mains remains widely misunderstood, under-appreciated, and thus not often encountered.

Balanced mains is not overly complicated. The normal UK mains supply is single-ended; nominally 230 Volts on the live wire and theoretically 0 Volts on the neutral wire. In a balanced mains unit a specially constructed 1:1 isolation transformer is fed a conventional 230 Volts, but outputs 115 Volts from each of two live taps on the secondary coil (the audio system side) with the centre-tap going to earth. The conductors that go from the mains unit to the audio system are therefore both live, 180 degrees out of phase with each other in respect to the earth. Noise on one phase is cancelled by the counter-phase noise on the other, and is drained to earth. This is essentially the same law of electro-physics exploited by balanced as opposed to single-ended signal interconnects.

The Russ Andrews Balanced Mains Unit 3000VA Install Version (BMU) is intended to allow balanced mains to be deployed in a typical domestic setting and if ever there was an audio product whose function very much trumps form, it might be the exemplar. Designed to be hard-wired into a dedicated audio circuit, and sited adjacent to the household electrical distribution panel, it’s a stout grey powder coated metal box with in and out cable glands and a dual-pole 16A MCB/on-off switch on one side. Inside the chassis is a pair of 1500VA precision-wound toroidal mains transformers using high quality steel cores, a choice of five input taps from 230 Volts to 260 Volts and a resettable thermal cut-out, all connected with Kimber mains wire.

Russ Andrews BMU 3000VA install version

For an additional £500 there’s a more glamorous sibling; a black chassis with five UK plug outlets and a 16Amp IEC inlet on the back. Otherwise electrically the same as the Install Version it’s intended to be sited on or near the system and used pretty much as any other plug-and-play mains distributor would be.

Under prevailing electrical regulations, fitting of the BMU 3000VA Install Version is not a DIY job, so I had a qualified electrician hard-wire it between the household distribution panel and dedicated 4mm twin and earth cable audio ring circuit. Incoming Voltage had been monitored for 24 hours beforehand and seen to vary between 225 and 232 Volts. My electrician therefore connected the BMU via its 230 Volt tap. There was a faint and momentary hum when the MCB was closed, but from that point on the unit remained completely silent when handling a total load that I calculated could top out at just shy of a kilowatt should system volume ever be wound up to maximum.

What’s the problem?

Power supplies in refrigerators, televisions and other appliances are not 100% efficient, and can waste some of the energy they consume by converting it to harmonic distortion which travels back down the conductors to pollute the mains supply. The unwanted noise gets transcribed and amplified by our audio system, resulting in a high noise floor that masks musical detail. Harmonics in the noise clash with notes in the recorded music, a corruption often heard as ringing or an unnatural emphasis. Our amplifier has less headroom for the music content that we do want to hear because at the same time it is amplifying the noise we don’t want to hear. Dynamic expression suffers, in particular at lower frequencies where bass may be heard as weak or thin.

Russ Andrews BMU 3000VA

BMU 3000 Balanced Mains Unit Mk II

The 50Hz AC sine wave of the mains supply can also be polluted by unwanted DC, and this causes a further drag on system performance by affecting the mains transformers in audio components. DC causes the inducted magnetic field that transfers energy from their primary to secondary windings to collapse and then restart twice every 50Hz cycle. We hear mechanical hum from the transformers, while the electrical noise generated is carried forward into the audio system.

Russ Andrews, founder and chairman of Russ Andrews Accessories, says that the biggest cause of DC on the mains is the cheap half-wave rectifiers in domestic appliances such as hair driers, microwave ovens, washing machines and tumble driers. “They do not need to be in your own house to cause problems. Any neighbour on the same phase as you, usually every third house on a housing estate, is directly connected to your mains at the input to your consumer unit.”

Another common culprit, at least in urban areas, is DC pollution from electrical systems operated by commercial or industrial users on the same phase as the household. Badly designed or faulty inverters in local domestic PV solar installations are also a common source of mains-born DC.

Russ Andrews BMU 3000VA

Specialist audio-grade DC blockers are available, but Russ Andrews is not a fan; he says he’s evaluated multiple designs, including ones made by his own in-house technicians from the most promising circuits and using the very best components, and he cannot recommend any of them to ‘anyone who values musicality.’ For now, he says a well-designed and specified balanced mains unit is the best compromise defence against DC. Such a device can be expected to reduce common-mode noise by up to 20dB, remove ‘most’ of any DC present on the incoming supply, and lower the impedance of the audio system mains feed to less than two Ohms.

Theory and practice

I had assumed that mains pollution in the hill-top village in Devon where I live was low. But some while back I installed a garden grounding array using a Russ Andrews SuperRouter and associated grounding cabling and heard a highly satisfactory drop in the background noise level. At the time I had fondly imagined that this meant ‘job done,’ but the BMU provided a further lesson in just how pervasive and destructive of audio quality locally generated and incoming pollution actually can be.

After installation of the BMU 3000VA a track that had been playing just before the electrician arrived initially sounded unfamiliar, so much deeper and more tuneful was the bass. The soundstage too had taken on deeper and wider dimensions with greater specificity, and the level of tonal and textural detail had increased too. But it was the increase in energy, right across the audio-band, that was most startling, both with large amplitude events such as orchestral kettle drum strikes, and more subtle moments such as gentle string plucks on pizzicato violin, and the phttt of a vocalist’s breath on the microphone, for example. Overall the system had gained notably more detail and air-moving power, an uplift of the kind of magnitude that might be expected from a swap to a higher quality more revealing source and much more powerful amplification.

Russ Andrews BMU 3000VA

Over the following weeks it also dawned on me that the time-of-day differences that I’d been used to hearing in the performance of my review system had become less marked. Daytime sound quality, when mains noise pollution might reasonably be assumed to be at its greatest, was now much closer to the higher levels previously experienced only late at night when mains noise pollution is at its lowest.

BMU 3000VA conclusion

These findings lead me to give the Russ Andrews Balanced Mains Unit – and I will make the safe, I think, assumption that the Install Version and its more costly socketed sibling perform identically from a sonic perspective – a strong recommendation.

That needs to be tempered with the caveat that results may be setting dependent. If an audio system is in a building in the middle of nowhere with effectively a dedicated feed from the nearest sub-station, and the system components all have uncommonly well-specified power supplies, then we might hear less impressive benefits than we do from a system in a city surrounded by hundreds of other homes. However, my own experience in a small village suggests this may not be as easy to second-guess as might be assumed. For just such audio dilemmas was the money-back product trial devised.


Type: balanced power unit
Output: 3000W
Protection: 2 stage magnetic and thermal
Case: IP66 install steel with acoustic damping
Internal wiring: Kimber Kable
Size HxWxD: 120 x 400 x 300mm
Weight: 22kg (packed)

Price when tested:
Manufacturer Details:

Russ Andrews
T 01539 797300



balanced power supply


Kevin Fiske

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