Without suggesting that the Z-Systems RDP-1 Reference Digital Pre-amplifier is the world's first all-digital pre-amp, it's the first device used which has absolutely no analogue inputs or outputs. Its first threat to convention? The RDP-1 fits the digital source, e.g. a CD transport, and the D/A converter. So let's take this slowly, and with upper-case assistance: You Connect Your D/A Converter Directly To Your Power Amplifier.

As you've deduced, the RDP-1 performs all pre-amp functions in the digital domain, a control unit which treats digital sources in an all-digital manner, rather than compromising their performance by controlling them in the analogue domain. Unlike other pre-amps offering, say, digital volume and balance operation, the RDP-1 leaves the signal in its digital state right up to the unit's AES/EBU, coaxial S/PDIF and TOSlink optical outputs because there's no on-board digital-to-analogue conversion. That's why a DAC follow an RDP-1 in the system chain.

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Looking backwards, it's the same for source inputs. (Before you panic, note that Z-Systems has announced that it will be producing an A/D converter to enable analogue sources to be used with the RDP-1.) For the time being, the RDP-1 accepts signals through a pair of AES/EBU and three S/PDIF inputs, or one optical input; you can specify TOSlink or ST. What you soon learn after mere moments with an RDP-1 is that its behaviour is wholly determined by the D/A converter into which it's fed. This device - more than any CD transport you care to employ - is the ultimate acid test for D/A converters.

Moreover, the way the RDP-1 exposes the character of transports and DACs only serves to reinforce how utterly transparent is its own behaviour. This in turn confirms what every digital guru has been preaching for years: that signals suffer less degradation when they're manipulated in the digital domain. At the very least, all of us need to control playback level, the most basic form of signal manipulation possible, so it's revelatory to acquire performance gains even with such a fundamental operations.

To illustrate this phenomenon, I tried the RDP-1 with as many DACs as possible, noting swiftly that - while evident - transport differences imposed less on the sound than did DACs. With signals from the Theta Data III transport, the Meridian 586.2 DVD player and Roksan Caspian and Marantz CD63SE CD players, I used the RDP-1 in tandem with both stand-alone DACs and pre-amps with on-board DACs. The latter included the Meridian 861 and Lexicon DC-1, while the former consisted of the Theta Pro Gen Va and Chroma and Musical Fidelity's X-DAC. Additionally, I heard the RDP-1 in a day-long session at Sound Components in Florida through a $300,000 system made up of a Mark Levinson No. 37 CD transport and three No. 36S DACs, a Faroudja DVD and Runco laser disc players, the Meridian 861 processor/pre-amp, two Mark Levinson No. 33H power amplifiers, one No. 333 amplifier and one No. 331 amplifier, a pair of Wilson Audio Grand SLAMM and three WATT/Puppy System 5.1 speakers, all wired with Transparent Reference cables.

Why the hardware litany? Because the true purpose of the RDP-1 is to act as a digital tone control, not just a pre-amp controlling volume, balance and source selection. This £???? baby features the most sophisticated parametric equaliser available for domestic audio purposes, and the only way you can assess its worth is if it 'improves' even a system which you felt was above reproach.

After 're-learning' controls, you soon come to grips with a front-panel array which includes 10 push-buttons that combine to provide multiple roles for the three rotaries. Three facilities assist you in eluding technofear: a lucid display telling you what's happening, a remote control which duplicates all of the fascia operations and an owner's manual which identifies otherwise alien labels. The best way to set up the RDP-1 is with a spectrum analyser and an SPL meter, but most audiophiles would rather do it by 'ear', using the remote control from the listening seat.

Read more about the RDP-1 on Page 2.