RME DPS-2 & LNI-2 DC

Need a better power supply? Then these boxes are worth checking out.

A poor power supply — that is, a noisy or poorly regulated one — can substantially degrade a product’s technical and audible performance. Thankfully, professional devices usually incorporate satisfactory power supplies (often supplementing low‑noise switch‑mode types with effective voltage regulation and noise filtering) and most op‑amps routinely used in audio circuitry are remarkably good at rejecting power line noise. But in parts of the world where the mains supply isn’t as clean or stable as we’d like, or for applications in which the lowest possible noise floor is essential, an ‘enhanced’ power supply system can be beneficial. RME now offer two interesting ‘enhanced’ external power supply devices to address this potential need. Though primarily intended as options for the company’s ADI‑2 series interfaces, they can be used to power other manufacturers’ devices too.

...in parts of the world where the mains supply isn’t as clean or stable as we’d like, or for applications in which the lowest possible noise floor is essential, an ‘enhanced’ power supply system can be beneficial.

DPS‑2

Marketed as a “linear, super‑low‑noise, dual audio power supply with µFilter and Sensor technology”, the quarter‑rack‑width DPS‑2 comes in steel housing and contains a beefy transformer, so it’s quite heavy (1.89kg). It accepts a mains AC input (factory set to 230 or 115 V ) via the usual IEC inlet, and generates a nominal 12V DC output with a 2.5A capability, via one of two output ports, of which more in a moment. Essentially, the DPS‑2 replaces the standard switch‑mode line‑lump power unit supplied with the ADI‑2 range and provides a more stable and lower‑noise DC supply. Its front panel carries a pair of LEDs (one for each output) that illuminate for the first 30s after power‑on to confirm operation. They also illuminate if a fault is detected, while a permanent display mode is accessed by pressing a button between the LEDs.

On the back are a mains on/off switch, fuse holder and mains inlet, and a slide‑switch provides a ground lift that separates the mains protective earth from the power supply output’s ground. There’s also a separate grounding terminal, to tie the output ground to a technical earth or other appropriate reference, and two Kycon KPP‑4P locking four‑pin sockets for the DC outputs. A pair of one‑metre output cables is included, both terminating in 5.5 x 2.1mm (centre‑positive) coaxial plugs, but one features RME’s usual bayonet‑locking coax plug, while the other has a standard plug for use with other manufacturers’ equipment.

Although both DC outputs provide a 12V DC power feed, they do so through different technologies, labelled Linear and µFilter. One or the other can power an attached device, according to preference, but you can’t simultaneously power two devices from different outputs; when a connection is detected at one output the other is disabled and its LED is extinguished.

The DPS‑2 accepts a mains AC input, while the LNI‑2 DC can run off 9‑18 V DC supplies.

Internally, the DPS‑2 features a traditional linear power supply but it’s built on a massive scale, with careful attention to detail throughout. For example, it employs a generously over‑specified 60VA toroidal mains transformer, with additional capacitive and magnetic shielding, and the mains input side employs an HF filter (to reduce mains noise above 3kHz) and a DC filter (to reduce asymmetry on the mains AC voltage). The transformer’s output feeds a bridge rectifier and then a reservoir with a capacitance of 20,000µF, with an intermediate choke inductor and a further 10,000µF reservoir, to create a raw rectified and smoothed DC voltage. This is further processed by the two output sections to reduce ripple and noise.

The Linear output routes the raw DC voltage through a conventional hybrid voltage regulator circuit to achieve a noise floor of less than 50µV, with load regulation of 3.63% and an output impedance of 0.22Ω. That’s a very low‑noise, well‑regulated power‑supply source — far better than any conventional line‑lump unit!

The µFilter output... is exceptionally quiet and tightly regulated. Astonishingly so, in fact.

The µFilter output employs an even more sophisticated voltage regulation system, in the form of a switch‑mode regulator, followed by RME’s µFilter (Micron filter) technology. In essence, this is a discrete, ultra‑low‑noise voltage regulator that reduces power supply noise to less than 2µV, with load regulation of 0.2% and an output impedance of 0.012Ω. That is exceptionally quiet and tightly regulated. Astonishingly so, in fact.

The 24‑page user manual explains the technologies and concepts behind this power supply design, and includes technical measurements and graphs. Clearly, RME’s designers are very proud of their achievements, and rightly so!

Both the Linear and µFilter outputs are rated for a nominal 2.5A, but the latter can manage 3.5A, while the Linear output can deliver up to 4A should that be needed. Full protection against overload, short‑circuit, and overheating is included on both outputs, as well as under‑voltage detection.

Given that the output cables terminate in a two‑pole coaxial connector, you might wonder why the hardware is fitted with four‑pin output sockets, and there’s a good engineering reason. All cables have some resistance and incur some losses that increase with current flow, so it’s common practice in laboratory test gear to use ‘remote sensing’, with additional wires in the cable allowing the device to monitor the actual voltage being delivered at the end connector. This ‘sensing’ arrangement neatly circumvents the potential losses by adjusting the power supply’s output to achieve an accurate voltage at the destination’s input. RME have adopted this solution (only on the µFilter output) to ensure that a perfect 12V DC supply reaches the connected device.

LNI‑2 DC

The second power supply option, the LNI‑2, is described as a “super‑low‑noise DC power filter and stabiliser”, with galvanic isolation between the input and output. Essentially, this is the µFilter regulator section from the DPS‑2, repackaged into a much smaller standalone unit, measuring just 80 x 30 x 123mm and weighing only 605g. The case is milled from aluminium, with two LEDs on the top indicating the presence of raw DC input and active, regulated DC output. Like the DNS‑2, the µFilter output voltage is presented on a four‑pin socket with the same remote sensing feature, and two 1m output cables are included, with locking and non‑locking coaxial plugs.

A (not supplied) 9‑18 V DC input source with a 2A capability (nominally 32W) is required, and connects via a coaxial plug (5.5 x 2.1mm, centre‑positive). RME’s bayonet or standard coax plugs can be used. The LNI‑2’s output is a rock‑solid 12V DC, rated at 2A but capable of 3A, with a noise level below 2µV, an output impedance of 0.012Ω, and a load regulation within 0.2% — in other words, it’s the same impressively low noise and consistent DC supply as the DPS‑2’s µFilter output.

Like the DPS‑2, a ground terminal allows the output ground to be connected to a protective or technical system earth. This is an important feature, as the switch‑mode regulator employed in the LNI‑2 uses a high‑frequency transformer as part of its DC‑DC conversion system, and that transformer provides the galvanic isolation between the external power supply source and the unit’s filtered DC output. Such isolation can be very useful for reducing ground‑loop problems and noise current leakage issues, but sometimes it’s helpful to be able to tie the DC supply to a reference ground, and the terminal provides that option. Again, a manual (17 pages) explains the theory of operation in useful but not overwhelming detail.

Impressions

Both the DPS‑2 and LNI‑2 power conditioners are superbly engineered products that achieve exactly what they are designed to do: provide the cleanest possible, solidly consistent DC power supplies for sensitive audio equipment. The technical specifications attest to that, and in use, while both units can get quite warm to the touch, they’re silent.

Of course, whether such power supply conditioning is genuinely required in practice is a more ambiguous matter, and one that’s heavily dependent on the detailed design of the equipment being powered, as well as the state of the local mains supply, and the grounding efficiency of the entire installation.

To test for any benefit in my studio, I compared the AES17 dynamic range performance of the ADI‑2/4 SE’s A‑D converter stage, substituting the standard line‑lump power supply for the DNS‑2. This product’s A‑D converter achieved the best AES17 score I’ve ever measured at 124.3dB (A‑weighted), outperforming its sibling (the ADI‑2 Pro) by a small but significant 0.3dB. To have scored so highly on its standard power supply indicates a very well‑designed converter with incredibly low‑noise internal power arrangements anyway, and for that reason I wasn’t entirely surprised that switching between the two power supplies didn’t reveal consistently reliable differences in the dynamic range figures.

However, in mitigation, the mains supply in my studio is unusually clean, symmetrical and with very low noise, coming from a substation that’s only a few hundred metres away, and feeds only domestic premises. In a location with a more contaminated mains supply, the DNS‑2 or LNI‑2 might well make a measurable or audible improvement, especially with audio equipment designed to a lower technical standard than RME’s ADI‑2 series products.