Musical Fidelity X-10D line-level preamplifier John Atkinson offers some comments
It's an appealing idea, using a purpose-built buffer to isolate the possibly wimpy output stages of inexpensive CD players. Many of the op-amp chips commonly used have only a limited ability to source current, for example, even if they can swing the necessary volts. In fact, back in the early '80s, I experimented with using first a discrete class-A headphone driver, then a class-A line-stage buffer, both designed by John Linsley Hood, to isolate my preamp from the very long interconnects I was then using.
I also tried reducing the power-amplifier input impedance to less than 600 ohms to see whether forcing the buffer to source more current would reduce the sonic signature of the cables. It took me some time to realize that although there were some sonic gains in specific areas, particularly in low-frequency definition and extension, the overall sound quality was diminished with the extra circuitry. When it comes to electronic circuits, more can sometimes mean less.
Muse Kastanovich found that neither of the two units under test was transparent, something I found particularly worrisome in the case of the Musical Fidelity X-10D, which has been highly recommended in these pages by Sam Tellig. Following the measurements, I decided to perform my own listening tests. I used a Meridian 500 transport driving a Parts Connection Assemblage DAC-1 processor via a Meridian 518 used as a digital volume control. The output of the DAC-1 was fed either straight into a Pass Labs Aleph 3 driving B&W Silver Signatures or via the Musical Fidelity X-10D. Interconnects were all AudioQuest Lapis x2, 1m between DAC and buffer and 15' between buffer or DAC and power amplifier.
The Assemblage DAC-1 uses an Analog Devices AD847 op-amp chip as its output driver, with a series 76.8 ohm resistor both to define its output impedance and to limit the output current into a short circuit. The DAC-1 has a measured source impedance of just 77 ohms across the audio band, while the Aleph 3's measured input impedance at 1kHz is 22.3k ohms. The DAC shouldn't therefore have any problems driving the power amplifier direct.
And it didn't. The sound was thrillingly direct, with a superbly transparent view into the soundstage. Low frequencies, however, sounded a little exaggerated and lacking in ultimate definition. This wasn't unpleasant on well-recorded rock and classical orchestral CDs. Plugging the Musical Fidelity X-10D and the extra 1m length of AudioQuest Lapis into the chain (and reducing the Meridian 518's digital output level by 1dB) was initially rather frustrating as any difference was, as MK noted in his system sidebar, small. But after trying several recordings, I started to get a handle on what was happening.
Basically, I feel MK was correct when he stated that the X-10D very slightly reduces music's sense of pace and rhythm. I also felt both that the lower treble moved slightly forward in the mix, reducing the sense of soundstage depth, and that the delicious sense of high-frequency transparency I got with the D/A plugged straight into the power amplifier was slightly obscured. I also agree with MK that the midbass sounded a touch rolled off with the Musical Fidelity. However, in my system, this was actually a plus point. The rather ill-defined low frequencies with the direct connection cleaned up nicely with the X-10D in the chain, allowing me to more easily groove on the system's excellent bass extension. There was no doubt in my mind that the Musical Fidelity's tubes more effectively drive long cables at low frequencies than the Assemblage's solid-state op-amps.
With the Musical Fidelity in circuit, there was also more of an analog feel to the presentation, in that the individual soundsources within the image bled into one another slightly. The result was definitely an increased palpability, particularly with voices which sounded more robust through the X-10D. I am sure that many listeners will find this effect an improvement with typical CD sound. But every time I removed the X-10D from the chain, it was actually a relief not to hear that slight melding of sounds. Even though I felt that I also lost an octave of bass extension!
Paradoxically, though I consistently preferred its low-bass presentation, it was the apparently better-engineered Musical Fidelity X-10D that had more of a sonic signature than the Z-Man Audio Signal Enhancer.
With both these units, it will be very hard to predict whether or not they will work an improvement on the sound of any specific system. Despite MK's and my experiences, it is always possible that some components will benefit from their use, particularly if long interconnects are involved. Wes Phillips, for example, reports excellent sonic results using the Musical Fidelity X-10D to buffer the audio output of his DSS satellite receiver. And anything that would help the woefully underspecified analog output stages of PC soundcards would be a boon. But more than usual, you should try to audition these units in your own system before you make a purchase decision.—John Atkinson