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.

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.

First, the RDP-1 allows you to adjust dither so it best suits your DAC: 16-bit, for older units, 20-bit for most current products and 24-bit for forthcoming designs. You can adjust channels individually, or use a stereo-linked mode to affect each channel equally. Then there's the Transparent Tone Control (TTC), the heart of the RDP-1.

We all know that the practice for the past 20 years of eschewing tone controls is down to the degradation they caused in analogue components. Digital technology changes all that. TTC is a "complete digital-domain parametric equaliser" which can be used to correct either minor of grave anomalies in the system itself or the room. Generously, the literature also says the RDP-1 can "make it sound the way want it to sound," the most libertarian remark I've heard in an audio context in years. As PM's explains, the RDP-1 corrects tonal imbalances using bell and shelf filters. With judicious use of these controls (and a lot of patience), it's possible to deal with problems which no amount of component swapping would cure.

Does it work? In Florida, the system sounded so good in bypass mode that I couldn't imagine where one would find a need for signal correction. Most impressive was the way the RDP-1 made the Grand SLAMMs 'fit' the listening room by removing bumps, dips, overhang and resonances I hadn't really noticed until their cumulative removal made them evident. A by-product of the tone-shaping was an enhancement of image positioning, while the sound opened up and transparency increased rather than suffered.

In a far cruder vein, I used the RDP-1 as much to 'cure' recordings as I did to correct the listening room. With far less precision than the spectrum-analysed set-up in Florida, it was still possible to reduce traces of sibilance or boom without corrupting the rest of the sound. The RDP-1 allows the user to zoom in on a specific problem without destroying everything else. This is not to suggest that the RDP-1 will turn an awful system into a dream, and the best solution to system creation still involves the sensible selection of primary components to suit a room. But the RDP-1 will the sound in a way which was never available to us before with crude graphic equalisers.

Probably the most fun to be had is through using the RDP-1 to 'improve' trashy recordings; this device makes some of the worst bootlegs on earth almost listenable. Given that the RDP-1 offers 99 presets, you could set it up to deal with your room and a variety of hardware combinations, and still have plenty of pre-sets left over to dedicate to favourite recordings with less-than-spectacular sonics. For that alone, the music lover will hunger for an RDP-1. I hated seeing this go back to the importer.

But the best news, for those of you of limited means or who are quite content with your existing set-ups (especially your pre-amps), Z-Systems has announced a model which will consist only of the TTC section, so you can add this digital tone-shaping wizardry to any system which will accept a component within its digital signal path. As it costs half the price, I'm putting one on my wants list.

Input/output precision: up to 24-bits
Gain control: -95 dB to +12 dB
Gain resolution: 0.2 dB increments from +12 dB to -12 dB
1 dB increments from -12dB to -20 dB
2 dB increments from -20 dB to -50 dB
3 dB increments from -50 dB to -70 dB
5 dB increments from 70 dB to -95 dB
Filter types: 4 parametric, 2 shelving
Center frequency resolution: 1/6th octave ISO from 28 Hz to 18 KHz
Filter gain/cut: from -95 dB to +12.0 dB
Filter bandwidths: Q=0.4, 0.8, 1.3, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0,7.0, 8.0
Shelf filter slopes: 6 dB/octave
Dither types: 20-bit and 16-bit proprietary floating-point techniques
Dynamic range: better than 144 dB
Number of presets: 99
THD+Noise: better than -135 dB
Processor type: TM5320C31 32-bit floating-point DSP
Processor performance: 60 MFLOPS
Sample rates supported: 32kHz, 44.1kHz, 48kHz (88.2kHz/96kHz upgrade
available from factory