Benchmark LA4 line preamplifier

Click Here: Fjallraven Kanken Art Spring Landscape Backpacks
Upon hearing about this new product from Benchmark Media, the LA4, my mind turned to Laurindo Almeida, Bud Shank, Ray Brown, and Shelly Manne—the original L.A. Four jazz quartet. No such association was intended, however: the “4” merely designates change from previous Benchmark designs (footnote 1).

The LA4 is housed in a shoeboxy, half-rack chassis that stands apart from the contemporary hi-fi wide-and-flat norm. A large touchscreen, an engraved Benchmark logo, and a single large volume-control knob dominate the front. At the bottom left are a small on/off button and the IR sensor.

The action of the large knob is stepped, and an audible relay click accompanies each 0.5dB step change. I am accustomed, as perhaps many are, to smooth, silent operation, and I hastily concluded that the clicking would be intolerable. Nevertheless, I was wrong, especially as this construction contributes to some of the virtues I observed later.

All the other relevant external features are on the back: two pairs of balanced inputs (XLR), two pairs of unbalanced inputs (RCA), one pair each of balanced (XLR) and unbalanced (RCA) outputs, and a single mono balanced output that sums the stereo pair. There is also a pair of bidirectional 12V trigger jacks and an IEC power inlet.

But inside the LA4 lurks more interesting stuff, most notably that stepped volume control, which is based on arrays of precision resistors and sealed relays.

Control of Volume
The traditional audio volume control is a potentiometer in which a spring contact (wiper) is rotated along a curved, stationary resistive element, changing the amount of resistance in the signal path. For audiophiles, there are two concerns. The first is the reliability of the electrical connection between the wiper and the stationary element: Any compromise in contact integrity could result in distortion and/or nonlinear signal attenuation as the control is operated. The second concern, specific to stereo potentiometers, is how to maintain identical inputs/outputs for two potentiometers on a single shaft; often with potentiometer volume controls, the channel balance varies with the volume setting.

The most common way to deal with this in high-quality audio is to use a stepped attenuator that replaces the potentiometer with an array of discrete resistors and the spring- loaded wiper with a rotary switch. In volume controls like this, the signal still must pass through one or more switch contacts and, like the wiper, all need to maintain their integrity over a long duty cycle. Balanced circuitry has twice as many elements that need to be precisely paired and maintained. For multichannel control, the demands expand further.

The LA4’s 16-step gain stage feeds a 256-step attenuator with 12 gold-contact relays and 64 precision (0.1%) thin-film resistors, switched by an array of 40 sealed, gold-contact precision relays. The use of physically large, metal-film resistors ensures rapid heat dissipation, preventing heat-induced increases in resistance values. The relays are controlled by an FPGA to ensure precise timing of relay switching, which Benchmark says reduces transient currents that can cause pops, clicks, and “zipper noise.”

The LA4’s design, the company says, ensures long-term, low-distortion, low-noise operation with accurate interchannel balance and repeatability. It also means that one can use multiple LA4s and know that they are accurately matched.

Using the LA4
Upon power-up, the LA4 displays the Benchmark splash screen that, after a few seconds, switches to the Main Screen dominated by the bold letters “LA4.” The main screen shows the input selection by number and user-programmable name, the volume level by number and by a bargraph. The touch-sensitive screen has “buttons” for screen dimming, additional settings, balance setting, –20dB reduction, and muting. Tapping Settings takes you to three screens that access optional settings for Display, Input & Volume Setup, Power Control, Remote, and options for locking/resetting all.

The LA4’s remote control is the same one supplied with the DAC3 in all its incarnations; it works with both the DAC and the LA4. When used with either the LA4 or the DAC3, the remote control’s arrows allow the user to step through the inputs in either direction. Six discrete input buttons permit direct-access input selection, but these work only for the DAC3.

Footnote 1: The LA4 follows the HPA4 ($2999)—a very similar product, intended to drive headphones. The HPA4 followed on the heels of the Benchmark HPA1 and HPA2 headphone amps—but Benchmark skipped 3 to indicate a major change in chassis design and to avoid confusion with the company’s DAC-3, one version of which, the HGC, incorporates the HPA2 headphone amplifier.