Z-Systems rdp-1 digital preamplifier Page 3

Transparent Tone (?) Control
Okay, the rdp-1 was transparent. So what? It was supposed to change the sound. This it did with great finesse. The LF and HF shelf controls may have fixed 6dB/octave slopes, but the nominal frequency is variable, as is the magnitude of the boost/cut. (Remember that the slope also determines how far from the hinge frequency the full effect of the boost/cut will occur. A 6dB cut from a frequency of 500Hz will reach -6dB at about 50Hz. A 12dB cut won't achieve that level until about 10Hz!) Consequently, these controls can mimic standard bass and treble controls, but with added flexibility and precision.

It is the four parametric stages that distinguish the rdp-1 from a tone control. One can select center frequency (1/6 octave ISO frequencies from 28Hz to 18kHz), magnitude (from -95dB to +12dB), and bandwidth (Q=0.4 to 8). As Q is increased, the width of the frequency band affected is narrowed and the steepness of the cut/boost increased. Thus, one can attack specific spectral problems with specifically tailored correction and have minimal contamination of the rest of the audible bandwidth. With the four parametric stages and the two shelf controls, the rdp-1 has greater flexibility than octave or half-octave graphic equalizers of fixed, or preset, Q. (Graphic equalizers with 1/3-octave resolution approach this precision.) Moreover, the independence of each stage of the rdp-1 makes it easier to focus on one acoustic problem at a time. The learning curve is steep but short: Use the rdp-1 for a few recordings and you'll develop skill quickly.

The rdp-1 offers a wide range of settings for input source, output mode, and frequency response, and setting these for a particular application takes some time and effort. Fortunately, once you've done this, you can store your preferences as a Preset in any memory location from 1 to 99, for easy recall when needed. Preset 99 is the user-default location and should be programmed to contain the settings you wish to be applied on power-up. Preset 0 is an unmodifiable "flat" stereo setting with 24-bit output. Your choice of any two Presets can be defined as A and B for making rapid comparisons between them. This is particularly useful for comparing an EQ setting against a modification of it as you incrementally approach your optimum EQ setting. In addition, the Bypass mode (filters flat, 24-bit output, no dither) is always accessible for reference. However, the 24-bit output associated with Bypass can create truncation problems with many DACs, so it's best to create your own flat or reference setting with the appropriate output mode.

The tools are there; how can they be used?

Remastering Your Recordings
I witnessed a most impressive demo a few months back in which an experienced mixing engineer offered to re-equalize any recording submitted by a member of the audience. One of those submitted was a (typically) murky old Doors CD. While listening to the recording, the engineer accessed one of the parametric stages, gave it a boost, and slid it up and down, searching for the voice fundamentals (he knew where they were), then adjusted the gain/Q to make Jim Morrison just a bit more present and live. He followed up by using other parametric stages to re-balance individual instruments. Then, a little tweaking of the HF and LF shelf controls to establish fairly neutral overall response, et voilèa!—the recording sounded new and modern! Not everyone liked the results ("That's not the way the Doors sound!"), but in less than three minutes the recording had been transformed into what the controller/listener wanted it to sound like.

You can do this too. For example, Holly Cole's CDs are common demo discs because of how well her voice has been captured, and also for some really potent LF sounds. In fact, I find the LF on these CDs a bit overbearing. With a fixed bass control (or, indeed, with just the LF shelf control of the rdp-1) it was impossible to reduce the bottom boom without losing the slam. No problem with the parametric, though. I put in a 6dB cut, swept it down to find the offending range, adjusted the cut to suit my taste, and, finally, trimmed the Q to limit the effect to just the range needed. Now that I have just what I want for my system in my room on this recording, I can save that setting as one of the rdp-1's 99 Presets. Now, every time I haul out that CD, I call up that memory site and I'm back on track.

One of the most problematic issues for me is the relative prominence of solo voices. With the rdp-1, I can use one or two parametric stages to grab and adjust the voice to what I regard as an appropriate balance. Diana Krall's Love Scenes CD (Impulse! IMPD-233) is just a bit more breathy and soft than her earlier discs. I pushed up the low bass around 31Hz with a low Q (0.4) to get some whack on the bottom end, and, starting at 2.8kHz, added an HF shelf cut of -0.6dB to minimize the breathiness and let the voice fundamentals come up. Am I right? I can't say, but I made it sound right to me.

Patently inappropriate sounds are also attackable. You know the (literally) subterranean noise on Classic's otherwise wonderful reissue of the Witches' Brew LP (LSC-2225)? I attacked it by feeding the signal through the NAD ADC, and then the rdp-1 via the tape-monitor loop of my Klyne preamp. Since the lowest frequency setting on the rdp-1 is 28Hz, I used a 45Hz LF shelf cut that took full effect at subway levels, and a 31Hz boost with a parametric stage to compensate for what little the shelf cut took out of the musical range. With a bit of interactive tweaking of gain and Q, I was able to wipe out the underground noise without any discernible effect on the bass in the music.

4641-F NW 6th Street
Gainesville, FL 32609
(352) 371-0990