For starters, the LS5 is fully balanced from input to output. To understand the difference between a preamp with balanced inputs and outputs and a truly balanced preamp like the LS5, let's look at how most balanced input and output preamps process a balanced input, using the LS2B as an example…
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From the front, the LS5 looks identical to Audio Research's popular LS2: two knobs on either side of the Audio Research nameplate, and a row of toggle switches along the bottom. But that's where the similarities end; the LS5 is a completely different animal from the LS2, or even the balanced LS2B.
Robert Harley returned to the LS5 in November 1994 (Vol.17 No.11):
While I was auditioning the $4495 Audio Research LS5 Mk.I preamplifier that I reviewed in August, I took possession of the $3495 Sonic Frontiers SFL-2. As much as I liked the Audio Research, it was clear that the Sonic Frontiers was serious competition. In some respects, the SFL-2 was the winner. The Sonic Frontiers had a cleaner treble, with less grain. In fact, it was the SFL-2's lack of treble grain that pointed out the LS5's trace of edge on instrumental textures. The SFL-2 revealed that the LS5 had a very slight…
While I was auditioning the $4495 Audio Research LS5 Mk.I preamplifier that I reviewed in August, I took possession of the $3495 Sonic Frontiers SFL-2. As much as I liked the Audio Research, it was clear that the Sonic Frontiers was serious competition. In some respects, the SFL-2 was the winner. The Sonic Frontiers had a cleaner treble, with less grain. In fact, it was the SFL-2's lack of treble grain that pointed out the LS5's trace of edge on instrumental textures. The SFL-2 revealed that the LS5 had a very slight…
Sidebar 1: Robert Harley's System
The LS5 Mk.II has been part of my reference system for the past few months. I also had on hand an original LS5 for side-by-side comparisons at matched levels, to refresh my memory, as well as a Sonic Frontiers SFL-2. The associated components I used to evaluate the LS5 Mk.II included Audio Research's VT150 tubed monoblocks—the best power amplifiers I've heard in my system. While I had some criticisms of the original LS5, I had absolutely no reservations about the VT150's wonderful musicality. Loudpeakers were Thiel CS3.6es—a moderately difficult load,…
Sidebar 2: Measurements
BL2 Input Controller: The BL2 had a very high input impedance of 1.14M ohms, a little lower than the specified 1.5M. At these high impedances, however, an accurate measurement is difficult to obtain. Output impedance was a very low 55 ohms across the band.
The BL2's gain into a 100k load was 5.93dB, fractionally under the 6dB to be expected from converting a single-ended signal into a balanced signal. DC offset levels were quite low, measuring 4.3mV (left channel) and 1.5mV (right channel). The input overload point was greater than 13V RMS.
The…
Sidebar 3: Specifications
LS5 preamplifier: Fully balanced, pure vacuum-tubed line-stage preamplifier. Tube complement: six 6922 dual triodes, six 12BH7 dual triodes. Frequency response: 1Hz–100kHz ±0.5dB (–3dB points below 0.1Hz and above 250kHz). Distortion: <0.01% at 2V RMS output (typically <0.005% in midband). Gain: 30dB/12dB switchable. Input impedance: 200k ohms balanced. Output impedance: 400 ohms balanced. Maximum input level: 20V. Controls: Gain, Input Selector, Power On/Off, 30dB/12dB Gain, Monitor/Source, Mute/Operate. Noise: 40;uV RMS residual IHF weighted balanced…
At the 1992 Winter Consumer Electronics Show in Las Vegas, Audio Research showed a line of reference products that represented the pinnacle of founder William Z. Johnson's life work as an amplifier designer (footnote 1). Although the all-tubed, fully balanced preamplifier and tubed monoblock power amplifiers were shown as works-in-progress, it was clear that these were products aimed at advancing the state of the amplifier art with no consideration for cost. Indeed, the reference preamplifier and monoblock power amplifiers were a return to Audio Research's roots, and a personal statement by…
The VT-150s presented an astonishingly believable and natural rendering of timbre. Instead of hearing a hi-fi representation of the music, I felt I was hearing the music itself. The result was a complete sense of involvement, relaxation, intimacy, ease, and ability to forget the playback system. The listening fatigue I so often experience with lesser amplifiers was replaced by the ability—no, compulsion—to play music for extended periods.
I don't want to give the impression that the VT-150s sounded soft, rolled-off, overly lush, or "tubey"—they didn't. The midrange and treble magic I've…
Sidebar 1: System
The VT-150s drove Thiel CS3.6es—a moderately difficult load, and a loudspeaker that tends to be amplifier-sensitive. Loudspeaker cable was primarily AudioQuest Sterling. Interconnect was Expressive Technologies IC-1 or AudioQuest Lapis.
Source components included a variety of digital processors and transports, but most of the critical evaluation of the VT-150s was done with a Sonic Frontiers SFD-2 driven by Theta Data Basic or Mark Levinson No.31 transports via AES/EBU interface (the excellent Wonder Link). Interconnects between the SFD-2 and Audio Research LS5…
Sidebar 2: Measurements
I measured a voltage gain of 10 (20dB) from the VT-150's 4 ohm output tap, 14 (22.9dB) from its 8 ohm tap, and 18.6 (25.4) from the 16 ohm tap. This increase in voltage gain at different output transformer taps occurs because the taps have different turns ratios (the number of turns of wire on the secondary winding in relation to the transformer primary winding). Input sensitivity (the input voltage required to drive the amplifier to clipping) was 2.4V RMS (balanced), which is ideal for the high-gain LS5. Input impedance measured a high 215k ohms across the band…
Sidebar 3: Specifications
Description: Vacuum-tube monoblock power amplifier. Tube complement: two matched 6550 pairs (output), two 6550 regulators, one 12AX7 regulator, four 12BH7A driver, two 6922 input. Power output: 130W continuous into 16 ohms (24.2dBW), 20Hz–20kHz, with <1% THD (typically !x0.05% at 1W). Approximate power available at clipping: 145W at 1kHz (24.6dBW). Power bandwidth (–3dB): 12Hz–80kHz. Input sensitivity: 2.3V RMS balanced for rated output. Input impedance: 200k ohms. Hum and noise: <0.5mV RMS (98dB below rated output, IHF weighted, input shorted). Power-…