Back in the old days, at the dawn of the hi-fi age, most audiophiles built some, if not all, of their systems themselves. Fresh from service stints during WWII and the Korean War, veterans used the knowledge they’d gained in military electronics courses to assemble their own audio components. While a far lower percentage of audiophiles take advantage of this do-it-yourself approach today than in the early days of hi-fi, there are still a few companies whose business models include DIYers, such as the speaker-kit maker GR Research, and Tortuga Audio, a company that manufactures passive preamps but also sells related electronics to the DIY community.
Cape Coral, Florida-based Tortuga Audio opened its Internet portal in October 2012, and sells its products exclusively through its website. From the outset, Tortuga specialized in a unique type of passive preamp (more correctly called a "passive attenuator") design that utilizes light-dependent resistors. Tortuga Audio’s latest single-ended passive preamp is the LDR3.V3 priced at $1195 fully assembled (you can also buy a kit version for $695). The LDR3.V2 has the potential to completely fulfill half of the audiophile dream—a straight wire with gain—just without the gain part.
A Better Volume Control Than Stepped Resistors?
The most commonly used method for adjusting the volume in a preamp, be it passive or active, is with a potentiometer. Another approach is to use resistors, either in an array or in a stepped configuration. Tortuga’s Audio’s president and designer Morten Sissener spent years exploring alternatives to conventional resistive volume control before he produced his first preamplifier.
One such alternative that Sissener considered was the use of light-dependent resistors (LDR)—the combination of a photo-resistor with a light-emitting diode (LED). The resistance of an LDR varies in proportion to the brightness of the LED, is, in turn, is dependent on the electrical current passing through it. Attenuation is achieved through the variation of the resistance levels of the series and shunt LDRs to achieve specific resistance ratios.
The LDR3.V2 uses two LDRs on the input side of each channel. They are connected in a series-shunt configuration and are controlled via a software-driven digital control unit (DCU). The DCU uses proprietary software to maintain minimum input impedance while delivering a 60dB attenuation range from 0 or unity gain to -60dB in 70 steps of slightly less than1dB per step. LDRs are also used as on/off switches in lieu of conventional relays to select the input source. Because LDRs have a life of only 50,000 hours or 5.7 years (if on continuously at max rating), the LDRs in the LDR3.V2 are socketed so they can be easily replaced.
The reason few firms have used LDRs in a preamplifier design is because the relationship between current and resistance within LDRs is nonlinear, and can vary considerably from one LDR to the next. Because of these nonlinear variations, Tortuga initially instituted rigorous testing to match LDRs in pairs. However, as DCU (digital control unit) technology evolved Tortuga developed proprietary, adaptive, auto-calibration technologies that eliminated the need to pre-match LDRs and that also corrected for aging and drift. According to Tortuga, “Auto-calibration is a closed-loop system employing both DACs (digital-to-analog converters) and ADCs (analog-to-digital converters) to calibrate each LDR against an absolute 70-step attenuation schedule. Calibration results are stored in memory tables and then used to accurately control each LDR during normal operation. Overall accuracy is generally within 0.2 percent.”
The same software that controls the LDR3.V2’s input impedance also allows the impedance to be adjusted anywhere from 1k to 99k ohms. A user can configure five different impedance settings and switch among them while playing music to determine which setting is optimal.
As is the case with a conventional potentiometer, the output impedance of the LDR3.V2 is the effective paralleled resistance of the series and shunt LDRs. Its output impedance starts at approximately 100 ohms and remains relatively constant until step 16 of the 70-step scale; it then begins to increase along with the shunt resistance, and peaks at approximately 5000 ohms (25 percent of the nominal input impedance) at step 59 or -6dB of attenuation (specific values will vary depending on the input impedance setting).
Fit, Finish, and Ergonomics
If you prefer understated cosmetics, the LDR3.V2’s straightforward appearance, such as a single knob located on the right half of its 6"-wide front panel, will appeal to you. The left half of the front panel also looks basic, occupied by a pair of blue LED displays that supplies info on volume level and input choices. Overall, the diminutive LDR3.V2 measures only 10" deep by 6" wide by 3.25" high. The chassis is what I would call a high-quality generic project box, although you can choose from a variety of finish options: a lacquered carbonized bamboo or custom-wood front panel ($100 upcharge), as well as anodized silver or black.
The LDR3.V2’s rear panel has room for three pairs of single-ended RCA inputs, two pairs of RCA outputs, a power connector, a 12-volt connector, and a USB slot. The RCA inputs and outputs are spaced so that even my largest barreled connectors attached easily.