Review: Pro-Ject Pre Box RS Digital – Chameleon

Sound

The biggest problem with the RS Digital is its chameleon options. Solid state or valve output stage? Fast and slow roll offs? Choice of output DACs? In one box? Insanity. On the one hand it is a testament to an engineering philosophy that leaves room for end-user choice; on the other, it is irresolute.

I’m torn. Overall, the RS Digital performs very well. Its headphone amp stands between super-powerful DAC/amps like the Telos HDA and the weaker outputs of the Lynx HILO. Unlike either, it shows small load effects, which makes perfect sense: a 5Ω output is enough to cause minor fluctuations in headphones with nominal impedances of 32Ω. Bump up the output and lose voltage. While I’d prefer a low option of 2Ω, the approach is measured. And immediate. As long as you’re listening to headphones of sufficiently low impedances, you can hear and feel the differences.

The RS digital doesn’t have a great amplifier for earphones. It is hard to nail L/R channel balance at low volumes with sensitive earphones. Both attenuator play and friction are behind some of this. Beyond that is that, at low volumes, I get a deviation of 0,3dB from left to right channels at low volumes, which is obvious. Beyond that, the headphone amp has a little bit more hiss than the Lynx HILO, that is audible through most earphones and some sensitive headphones. The valve output in the review loaner hisses way more than the solid state output. I’m not sure what’s up. Pro-Ject will get back to me on this.

That’s the ho-hum. If you’re an earphone person, you’ll want to hook up the RS Digital’s excellent pre-amp to your favourite headphone amp. As to headphones, I reckon that anything from the Grado RS series on up makes a good combination. It powers the 600Ω DT880 to painful levels with nary a hint of IMD distortion, and it puts stable signal into most portable headphones. It keeps a strong stereo signal. And, when utilizing either solid state or valve output stages, it has a lovely ring to the upper mids that reminds me in a small way of Phatlab Audio’s PHAntasy.

Next to the HILO, its upper midrange is more open and bright. Stereo detail favours the centre of the midrange. The RS Digital’s valve stage deadens high frequency stereo separation slightly, and outputs about 0,6dB more volume than its solid state comrade. Otherwise, it closely adheres to the solid state sound. In my opinion, the difference between DACs is nearly as pronounced. And the choice of DAC chips: DAC 1: 2x PCM1792 (being cleaner), and DAC 2: PCM5102 (being more centrally powerful), was a good one. Both are great. That both made it into a single unit and that either one can be used at the flip of a switch, boggles the brain.

With either selected, the RS Digital is capable of surpassing the -100dB signal to noise ratios stated in the manual. Its channels are also perfectly balanced. Interesting point: Pre Box hedged a bit with regards to THD levels. I measure 0,0006% (solid state) and 0,189% (valve) through its pre-amp. Impressive. Which is one of the reasons I love using it as a pre-amp. The RS boasts all the options you could want, is stable, easy to use, and, with a flip of the switch goes from mild-mannered and light to mildly weightier with greater channel bleed and softer edges typical of hybrid outputs.

While the Lynx HILO measures better, I prefer the sound of the RS Digital’s DACs. Still, Vivace is my favourite, though, where it comes to cleanly upper midrange decay and attack edges, RS Digital is the one to beat.

If its headphone output were a bit better, the RS Digital would be a killer one-box solution. As it is, it is unique in its feature set, and boasts an excellent DAC. If you’ve got a favourite outboard headphone amp, you’re in luck. Because it it spits both balanced output through XLR and single ended through RCA’s you can hook up a large variety of both professional and HiFi amps to it.

Because of its clear, bell-like upper midrange, I think it would be a shame to hook it up to an amp with a large low pass filter. Go as neutral as you can to enjoy the best this DAC offers. And if you’re keen on using the RS Digital with headphones, make sure that you’re phones have nominal impedances of 50Ω, and aren’t that sensitive. Anything DT. Anything HD. These are perfect companions.

End words

The RS Digital is too much DAC, too much amp, too much choice. I’m both smitten and exhausted. Smitten in that, as a DAC with a pre-amp, it has all its bases covered. If you’re plugged into the Pro-Ject world you added functionality with their CD players. Its headphone amp is good but not great. If you’re big into Planars and low sensitivity dynamic headphones, both output stages are very good. If you’ve got Grados or one of Final Audio’s hybrid headphones, the RS does better duty as a DAC and pre-amp. Exhausted because sussing each option took a while, and measuring each with a semblance of repeatability took hours.

You won’t find many DACs out there as functionally well-equipped as the RS Digital. You will find better headphone amps out there, sometimes in one-box solutions. But you’ll likely not find as well-balanced an amalgamation of the two. Nor is there a way in hell you’ll find one that offers two amps in one, and two DACs in one.

The RS Digital is a very well executed piece of equipment. It just needs better implementation of its headphone amp.

NOTE: Pro-Ject are looking into the headphone performance of the unit loaned me.