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I was so impressed that I started to explore the idea of designing and marketing an LDR-based passive preamp. This was around 2009. At this time I started hearing about LDR passives becoming available as kits or finished products mostly within the online DIY audio community. But despite the undeniable superior sonic quality of LDRs, they can be challenging to work with. They're not only highly nonlinear, their performance characteristics vary from one part to the next. It soon became clear that to get truly satisfying results with LDRs you had to use closely matched LDRs not unlike a matched set of tubes! This meant developing a testing protocol, configuring a data acquisition system and a means for storing and processing all test data. Even with decently matched LDRs I still wasn't satisfied. A fully commercialized product had to be able to operate via remote and have features like muting and fine-tuning of channel balance. The left/right channel performance had to be precisely matched because that's where your stereo imaging and soundstage quality come from. All of that led to the conclusion that it had to be a software-driven digitally controlled analog design. After going through several prototypes I made the decision to form Tortuga Audio and spent most my time refining the design and working towards launching the business. We finally went live with an online presence/store on October 1, 2012. It's been a very interesting very challenging and enormously exciting past 6 months.

Q: You seem to be a bit of a trailblazer introducing a finished commercial product with LDRs. Why has this technology been mostly confined to DIY quarters and what have been the challenges implementing it successfully?
A: With most new and even evolutionary technology, it's always the early adopters who are willing to give things a first try. Since the DIY community is full of early adopters and the cost threshold isn't that high for playing with LDRs, I'm not surprised that LDR preamps took hold first within DIY and haven't really broken out too far commercially yet. In my view LDR audio technology is at the 'crossing the chasm' stage with no guarantees that it will survive the crossing. If you buy into this view, then you would argue it's fundamentally a marketing and not technology issue.

That said, LDRs are not the easiest components to work with notwithstanding their superior sonic qualities. Their challenges are twofold. First, the relationship between LED current (light intensity) vs. resistance is inherently nonlinear. Secondly, even with LDRs of a given make and model there's considerable variance in this nonlinear relationship from one individual LDR to the next. To implement a reasonable stereo attenuator with LDRs means you have to use 2 per channel. Given both their nonlinear behavior and individual variances, you soon realize that you first have to test each individual LDR and record its current vs. resistance curve. All of that data goes into our LDR database where at any point in time we usually have test results for over 100 LDRs which we haven't already used. We've developed sorting routines that process this data to tell us which next 4 LDRs are most closely matched. If we decide those 4 aren't sufficiently close, we'll test another 50-100 LDRs and add them to the database, then re-run the sort until we get satisfactory matches.

Even with this initial LDR testing protocol we weren't satisfied with the resultant left/right channel attenuation. So we made the decision to design our LDR-based preamps using precision microprocessor control where we could fine-tune both the shape of the taper as well as ensure that left/right channel matching fell within only a fraction of a dB over the entire -60 to 0dB attenuation range. All of that shaping, matching and fine-tuning is ultimately done in software. When we build each LDR board using the 4 best-matched LDRs we load the default software into the board, then run the board through a 'live' integrated left/right channel attenuation test using data loggers and proprietary test software. The results of this integrated test go into another database and give us a precise picture of that build's left/right attenuation curves. That data is then used to create attenuation schedule correction curves which force both left and right channels to have effectively identical behavior. Those correction curves are then programmed into the final software build. The result is that the final software in each LDR preamp board is unique to the LDRs and other components of that board.

Frankly, the development and execution of all the above testing, sorting, retesting, correction and custom software is quite a giant pain. While much of it is automated, it's still labor intensive and not exactly conducive to high-volume/low cost production. But we think it's still worthwhile since the result speaks for itself. We believe this precision yields superior soundstage and channel separation. Together with the inherently open and transparent characteristics of LDRs, the result is a passive preamp with outstanding audio qualities. While a passive preamp may not be suitable for all combinations of sources and amps, there's no reason not to employ LDR attenuation even in active preamps. We plan to eventually offer a companion buffer with optional gain, essentially an active preamp compatible will any audio source or amp. So LDRs are not quite the same thing as grabbing a pot and wiring it in.

Now that I had a clearer picture of man, company and technology, it was time to take a closer look at the finished product. Mr. Sissener considers his LDR1 and LDR6 to be next-gen products superior to what came before. The LDR1 is designed for a minimalist system requiring only a single input with volume control but still offers two outputs and the convenience of a remote. The LDR6 under review is for the audiophile demanding input switching. The unit accommodates 6 RCA inputs but opts for a single output. In an added twist Mr. Sissener dispensed with the traditional mechanical source selector and employed the LDR principle throughout. No mechanical switching contacts are used at all. This is a purist approach.

Is the LDR6 Spartan? No. The designer adhered to the straight wire approach but left the minimalist dogma at the doors. Six inputs serviced without mechanical contacts by a pair of LDRs for each input which operate as resistive analog switches. Balance control because not all recordings are created perfect. This again is accomplished without mechanical contacts. No compromise. Still a straight wire.

Basic box dimensions are 6.25” W x 8.25” D x 3.25” H and the unit weighs a mere 2.5 lbs (5.75 lbs as shipped). The front panel has a row of 7 LEDs, 6 to confirm input selection, 1 for power. There's a single multi-function control knob and a large round IR sensor. When initially turned on the entire row lights up before settling down to power and specific input. The multi-function knob controls volume, balance and input selection with a 'press and hold' group of layered commands. Simpler and more convenient access is available on the remote, which is intuitive and appears to be designed specifically for the product rather than being a repurposed generic. It lacks the heavy metal construction of its upper-end or offshore counterparts but proved effective and reliable.

Operation is well thought out. Switching between sources automatically engages mute. The power button blinks to indicate mute mode. A tap on the mute button or pressing the enter button restores former volume. Volume control is described as digitally controlled analog attenuation and follows in the same fashion from full down indicated by a blinking blue LED up to full unity gain again indicated by a blinking blue LED. The LDR pot uses a custom logarithmic 70-step + mute scheme from approximately -60 to 0dB. The custom nature of the microprocessor digital control provides finer steps of adjustment over the final 6dB as well as consistent channel balance. There are no numerical or visual representations of level so settings are strictly by ear.