Description: Hybrid transistor-input/vacuum tube-output monoblock power amplifier. Tube complement: three matched pairs 6550C, one matched pair 6L6GC, one matched pair 6AS7G, two 6N1P. Output power: 200W continuous, 20Hz–20kHz (23dBW). Power bandwidth: 10Hz–200kHz. Frequency response: 0.5Hz–240kHz, -3dB points at 1W. THD: typically 0.5% at 200W, <0.02% at 1W. Input sensitivity: 1.7V RMS balanced for rated output (27dB voltage gain into 8 ohms). Input impedance: 200k ohms balanced. Slew rate: 40V/µs. Risetime: 1.2µs. Hum & noise: <0.2mV RMS, 109dB…
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Sidebar 2: Associated Equipment
Analog source: Simon Yorke, Rega P3 turntables; Graham 2.0, Immedia RPM2, Rega RB300 tonearms; Lyra Parnassus D.C.t, Grado Statement, Lyra Helikon cartridges.
Digital source: Audio Alchemy DDS•Pro CD transport, DTI•Pro 32 resolution enhancer, EAD DSP9000 Mk.3 HDCD D/A processor, Musical Fidelity X-Ray CD player.
Preamplification: Ayre K-1 and Audio Research Reference Two preamplifiers, Audio Research Reference phono section.
Loudspeakers: Sonus Faber Amati Homage, Audio Physic Rhea subwoofer.
Cables: Interconnects: Hovland Music Groove DIN/RCA…
Sidebar 3: Measurements
I made sure the bias currents had all been properly set after an hour's warmup before I started testing the Audio Research VTM200. The amplifier was non-inverting and appeared to be wired with pin 2 of its XLR jack wired as "hot," the modern convention. Its input impedance was a very high 185k ohms and its voltage gain into 8 ohms measured a high 30.2dB from the 16 ohm output-transformer tap, 27.5dB from the 8 ohm tap, and a low 24.8dB from the 4 ohm tap. Its output impedance also varied according to which tap was used: a reasonably low 0.35 ohm from the 4 ohm…
Until recently, all problems in digital audio systems were blamed on either the analog/digital converters (ADCs) used in mastering or the digital/analog converters (DACs) needed for playback (footnote 1). As the performance of both ADCs and DACs improved, however, a previously unrecognized mechanism for distortion was unmasked: jitter. As we shall see, jitter—or, more correctly, word-clock jitter—can be a significant limitation in the technical and sonic performance of digital audio systems (footnote 2).
This article is a primer on jitter in digital audio. We will look at what jitter is…
To give a sense of the magnitude of the variables involved here, let's consider a multi-bit converter reproducing a 20kHz sinewave, and a clock with 200ps RMS of white jitter measured in the DC-40kHz bandwidth. In this case, the converter would produce white noise at an amplitude of -95dB relative to the input signal amplitude in the DC-20kHz band, plus a -95dB white-noise product in the 20-60kHz band. A clock jittered with 200ps RMS at a frequency of 1kHz applied to a DAC reconstructing a 10kHz sinewave will produce spurious tones at 9kHz and 11kHz, each with a relative amplitude of -101dB…
A word about optical links. Still using the example above with digital signals A and B, a low-pass filter at 5MHz—typical of TosLink—causes a time difference of 121ps. A 6MHz low-pass filter causes a time difference of 33ps. For adequate performance, optical links must have a bandwidth of 9MHz minimum. To operate at 48kHz and have a 15% margin for speed adjustments, the interface bandwidth must be at least 11MHz.
Moreover, calculations show that jitter caused by interface band-limiting is correlated with the signal. In fact, frequency analysis, both theoretical and measured on equipment…
Jitter & the input receiver
Let's take a closer look at how the digital processor's input receiver affects the jitter at the DAC. First, the input receiver has an intrinsic jitter level, defined as the jitter on the recovered clock when the input signal is jitter-free. The amount of intrinsic jitter is the figure quoted by input receiver manufacturers (and by high-end companies using that receiver in their products) as the device's "jitter." For example, the popular Crystal CS8412 input receiver is quoted in the data sheet as having 200ps of jitter. The older Yamaha YM3623 input…
Let's take a closer look at how the digital processor's input receiver affects the jitter at the DAC. First, the input receiver has an intrinsic jitter level, defined as the jitter on the recovered clock when the input signal is jitter-free. The amount of intrinsic jitter is the figure quoted by input receiver manufacturers (and by high-end companies using that receiver in their products) as the device's "jitter." For example, the popular Crystal CS8412 input receiver is quoted in the data sheet as having 200ps of jitter. The older Yamaha YM3623 input…
There has been much argument in audiophile circles about whether an LP or a CD is a more faithful representation of a master tape. Although we recorded Robert Silverman's thrilling performance of the Liszt B-Minor Piano Sonata for CD release, we also had in mind to issue an LP. As the source for both would be the same, the question we can answer is: Will an LP cut straight from a 20-bit master tape via a Class A 20-bit DAC sound closer than a CD noise-shaped to 16 bits from the same 20-bit original?
Although we did record backup analog tapes at the 1993 Sonata sessions, the edited four-…
Cutting at Camarillo: Wes Phillips
John Atkinson and I flew to Camarillo in November 1996 to test a pet theory: Given the audible superiority of the 20-bit data coming off the Nagra-D to the 16-bit CD standard, would an LP cut from the 20-bit digital master tape preserve enough of that quality to justify pressing a vinyl disc? Our partners in this project were RTI's Don MacInnis and Analogue Productions' Chad Kassem, who had recently purchased David Wilson's disc-cutting facility and had installed it within RTI as AcousTech (footnote 1), under the direction of legendary mastering…
John Atkinson and I flew to Camarillo in November 1996 to test a pet theory: Given the audible superiority of the 20-bit data coming off the Nagra-D to the 16-bit CD standard, would an LP cut from the 20-bit digital master tape preserve enough of that quality to justify pressing a vinyl disc? Our partners in this project were RTI's Don MacInnis and Analogue Productions' Chad Kassem, who had recently purchased David Wilson's disc-cutting facility and had installed it within RTI as AcousTech (footnote 1), under the direction of legendary mastering…
Examining the cutter-head assembly, Stan was stunned to find that the new stylus was, in fact, defective. We all breathed a sigh of relief. Everyone knew that something would go wrong; we'd gotten our disaster out of the way early, we figured, and it was relatively minor.
"Whew," sighed Don MacInnis. "I thought we'd blow the whole day trying to link up our gear with yours. This is nothing compared to that."
Stan set the reference levels and the preview time, made sure the cutter-head was functioning smoothly with the new new stylus, and we were ready to cut our first real lacquer…