Z-Systems rdp-1 digital preamplifier:
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My greatest success was with the Mravinsky Shostakovich Eighth on Philips. This is a powerful and emotionally draining in-concert performance of Shostakovich's depiction of the "bitter suffering of the times," but it suffers from poor balance and bad digital transfer. The sound lacks weight and detail, has a nasal midrange, shrieks in the treble, and is cramped into a small center space. Not only that, it was mastered at the wrong speed—the pitch is high. Technically, this is a terrible recording, but the performance warrants attention.

First, I used a parametric stage cut (-3.8dB, 3.15kHz, Q=0.4) and boosted the bass with the LF shelf control (+3.4dB, 141Hz). This took the vicious edge off the violins and added a bit of weight to the bass. An additional LF boost (+2.6dB, 50Hz, Q=1.0) helped me delineate the bass drum, and an HF shelf boost (+1.6dB, 1kHz) restored the overall balance. What a difference! Not only was I able to hear into the orchestration, but the instruments were no longer crowded together, and there was a sense of space and place. What a marvelous transformation, even if the pitch is off!

Tweaking Your System (Or Not)
Before I received the rdp-1, I was thinking that it would be a great device for equalizing my system and room in the frequency domain. I'm no longer quite so sure. I've used the rdp-1 to modify the system response with a few speakers I've had in-house for review. For example, the Gershman GA-P 520-x benefited from a 0.8dB HF shelving at 1.41kHz, which knocked down the mid-high response and improved the soundstage spread considerably. The settings of the rdp-1 confirmed and quantified my perceptions; for this, it is a valuable tool.

However, I would not want to rely on the rdp-1 (or similar devices) to equalize speakers for continued use, because frequency response should be determined by design and not by Band-Aid EQ. For example, while the rdp-1 can modify the amplitude response of the speaker, it cannot correct phase interactions between drivers, nor can it change the radiation pattern of the speaker. Moreover, since the rdp-1 operates only in the frequency domain, it cannot affect how the room acoustics filter the reflected/refracted sounds that reach the ear after the direct sound. Thus, changing the amplitude response of a reasonably flat speaker to compensate for room anomalies may result in poor on-axis response. Even with a well-behaved speaker, careful measurements are the only way to determine the EQ settings needed to correct in-room response (see ETF, or How I Learned to Love My Equalizer").

With the rdp-1 in my system and its remote held lightly in my hand, I attempted to solve problems in system/room response. The best place in my room for my Duettas' imaging and overall balance is just where they're likely to excite a 30Hz resonance with some signals. Until I can get off my duff and damp that mode, the rdp-1 did a dandy job of reducing the boom.

Another example: I have a wood console on the left wall of my listening room and an upholstered couch on the right. With small speakers widely spaced, I improved performance by compensating for the slight brightness imbalance due to the unbalanced acoustic. Both of these problems would be better solved in other ways (absorbent/diffusive treatment and/or furniture rearrangement), but the rdp-1 made it quick and easy (if you know what you're doing).

The rdp-1 can serve as a control system for an all-digital system, reduce the need for more than one DAC in such a system, and, to a degree, supplant an anti-jitter device. I have found it transparent, flexible, and useful, and I feel that its performance justifies the asking price. Do you need one? Perhaps not, but I certainly do.

Conclusions
The NAD 118 and the Z-Systems rdp-1, in their quite different ways, successfully bring DSP into high-end audio. Although both perform best in all-digital environments, each has different target users. The NAD, preferably with an external DAC, presents the same interface as traditional preamps and will appeal to those who want flexible control but prefer not to think about such quantities as frequency and Q. The rdp-1 requires an external DAC and demands that the user be analytical and precise with it. Each is a rewarding device, but my heart lies with the rdp-1.