Tortuga Audio LDR1B.V25 Passive Preamp

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Located in Cape Coral, Florida, USA, Tortuga Audio is known for making well-regarded, hand- crafted, high-end passive preamplifiers–using LDR (light dependent resistor) technology for volume control. It’s owner/designer is Morten Sissener, who started the company 5 years ago. The word ‘tortuga’ translates from Spanish as ‘turtle’, hence explaining Tortuga Audio’s cute green logo of a sea turtle wearing headphones.

I must admit that turtles have fascinated me since childhood. Perhaps the closest living relative of the dinosaurs, turtles are reptiles that are remarkably resilient, have long life spans—and they can hibernate. As a child, I had many different species as pets in the late 1960s and early 1970s, from the common red-eared slider to an alligator snapping turtle, an African sideneck  turtle, and even an Australian snake-necked turtle. At its height, my collection of live turtles easily reached over 30. (How and why my parents allowed this is a mystery—I even had an alligator at one point.)

Many of my turtles were eventually donated to zoos and museums that kept live animals. To this day, although I no longer have any live turtles, I keep a variety of turtle trinkets and art work scattered about my apartment, collected over decades.

So, of course, I was intrigued by Tortuga Audio the first time I was alerted of them (about 1.5 years ago) in part due to my young interest in turtles. But as an audiophile, I was also interested because Tortuga’s specialty has been (until now) passive preamplifiers versus active, a dichotomy in high-end audio that I have occasionally experimented with for sound quality in my audio system over the last several years.

I am very grateful to Sissener for reaching out to me to review his newest ware, the focus of this review, the Tortuga Audio LDR1B.V25 Passive Preamp (USD$1895) and (optional) TPB.V1 Tube Preamp Buffer (USD$1695 basic, with several upgrade options offered). I readily agreed to do so. The LDR1B.V25 is their newest balanced version of a passive preamplifier (a single-ended version is also available). Moreover, if you combine it with the optional TPB.V1 buffer, together they form an active preamplifier; the buffer is a hybrid preamp buffer with a tube input stage and a solid state output stage, but without any built-in volume control, hence the need for an upstream volume control for its use. The TPB.V1 can in fact be used not only with Tortuga’s own passive preamps, but also other passives and even other devices that contain internal volume controls. (More on that later in this review.)

Tortuga Audio offers some of its products as build-it-yourself kits and also sells components for DIY, a wonderful approach which can significantly lower the prices and allow others to experiment. Tortuga only sell products directly from its website.

In case you are as curious as I was: when I asked Sissener why he chose the word ‘Tortuga’ for his company name, he answered:

The choice of 'Tortuga' goes back to a childhood fascination with the Dry Tortugas National Park that I first discovered in adventure books in grade school. When I got my first boat I named it Tortuga Dreamer. When I started my audio company 5 years ago, there was only one choice…Tortuga Audio.

What is LDR? Why is Tortuga Audio using it?

A passive preamplifier for an audio system uses attenuation to control volume; high attenuation yields low volume, low attenuation yields high volume. There is no ‘gain’ involved here as is the case in active preamplifiers; in essence an attenuator’s job is to lower not increase the volume of the output coming from your source (such as the output from a Digital to Analog Converter (DAC)) heading for your amps. Without such attenuation, your speakers (and your ears) would get the full possible blast attainable from the output. Attenuation is typically achieved by using resistors (resistive elements) which can be controlled to lower the output, but there are also non-resistor varieties that instead use attenuation transformers.

In general, because there is no gain involved, such passives do not need to be plugged into a power source to do their job, except possibly (using very minimal power) for some peripheral reasons (lights, remote control, etc.), and they are known for resulting in very low distortion, hence of interest in high-end audio. Some audiophile users tend to describe the sound quality resulting from such preamps as transparent, neutral, and ‘uncolored’.

Passive preamplifiers also tend to be very light in weight; grams as opposed to kilograms in some cases, and less expensive than active ones (there are exceptions), because they do not require heavy/expensive electronic internals. A passive preamp, however, depends entirely on the source component to drive the amps meaning that your source component must already produce a high enough output to justify the use of only attenuation for volume control.

LDR refers to ‘light dependent resistor’. Such a resistor’s resistance level is varied by utilization of a light emitting diode (LED): regulating the brightness of the LED shining on the resistor affects the resistance. The higher the light intensity, the lower the resistance. Tortuga Audio uses (purely analog) LDRs. Attenuation is achieved by varying the resistance levels of a network of 4 LDRs (2 for each stereo channel) to achieve specific output volume levels.

Although LDR based attenuators were around several years before Tortuga Audio (for example, the The Lightspeed Passive Attenuator designed by George Stantscheff around 2009 or so) in Tortuga’s LDR attenuator designs, attenuation is software driven, with remote control, and self-contained LDR calibration; there are no potentiometers (pots), no stepped attenuators (switched resistors), and no transformers. The LDRs in its newest board (V25) are controlled by a software driven 32 bit ARM digital processor that regulates the resistance of the LDRs via voltage signals from multi-channel 12/16 bit DACs.

The Tortuga design of attenuator is still ‘passive’ because, as explained to me by Sissener, there is ‘no active processing, amplification or buffering of the audio signal nor any direct connection to, or manipulation of, the audio signal by a power supply. Signal volume in each channel, however, closely follows that of a potentiometer; it is attenuated by a voltage divider composed of 2 LDRs in a classic series/shunt L-Pad.’

When I asked Sissener why he chose to use LDR technology in passives instead of more conventional technology, he wrote:

I was building a tube preamp based on a well known DIY design and had purchased a highly regarded potentiometer for volume control. Disappointed initially by the sound quality, I was looking for ways to improve it. I’d recently become aware of LDRs for volume control so I decided to try them out in this preamp. I replaced the potentiometer with a very rudimentary LDR attenuator and was immediately blown away by the improvement in sound quality. I had a hard time believing that an attenuator (a voltage divider) could make that much of a difference. This led to experimenting with bypassing the tube preamp altogether and using the LDR attenuator directly - as a passive attenuator/preamp if you will. The sound quality actually sounded better yet! I was so impressed, and frankly excited, by this experience that I spent the next few months studying LDRs and learning how best to use them in a top flight attenuator. Given their nonlinear and variable nature I concluded that an old school analog control scheme was a nonstarter and that I had to go with a software driven digital controller. After another 2 years of development work and testing I launched Tortuga Audio in October 2012 with our first LDR based passive attenuator. Customer reaction and feedback has been very positive since the beginning and in the interim the underlying LDR controller technology has gone through 4 iterations.

What is new in the LDR1B.V25?

The LDR1B.V25 is an upgraded version replacing the LDR1B.V2 Balanced Passive Preamp. As before, it is a unity gain, truly balanced stereo passive preamplifier using LDR. It provides the balanced attenuation using synchronized independent attenuators for each of the 4 balanced signals (2 balanced signals per stereo channel).

But this new version incorporates Tortuga’s new generation V25 LDR preamp controller (e.g., the core LDR attenuator platform board, a software driven audio volume control and input switching device board) denoted simply as the V25; it was released in April 2017 and represents Tortuga’s 4th generation of LDR technology and is a significant rebuild/redesign of the 3rd generation board. It uses LDRs both for attenuation and switching between inputs (the board itself now can support up to 6 stereo inputs, which brings the total number of LDRs to 18.  The V25 board can be bought just by itself for DIY folks starting at USD$299.

The LDR1B.V25 passive preamplifier unit is controlled by an included pre-paired 3rd Generation Apple TV Remote.

The very nice minimalist looking unit is black metal with two blue light monitors on the front displaying the volume level of each channel (from 0–99), and a black control knob for on/off and volume. The knob can be totally ignored by using the included Apple Remote, which controls a variety of functions beyond on/off, including balance, muting, input selection (in the 2-input model) and even impedance settings (from 1-99k ohms) for impedance matching purposes. Very impressive is the ‘Auto-Calibrate’ mode that can be initiated by the Apple Remote; it calibrates the 4 attenuation LDRs, by updating their attenuation table if they were to drift, or lets you know if an LDR might need replacing. The process takes only about 5 minutes.

All else is on the back including a USB port for upgrading firmware, and an input for plugging in the required AC-DC transformer (Output: 12V DC, 500mA) which of course is needed to run the LEDs in particular. The LDR1B.V25 is offered as either a 1 input, dual output or as a 2 input, 1 output; I was sent the 1 input, dual output version. (Some users apparently want dual output to drive both a pair of stereo speakers, and a subwoofer.)

Dimensions: 6” (W) x 9” (D) x 3.25” (H) Weight: 4 lbs

What is the TPB.V1 Tube Preamplifier Buffer?

The TPB.V1 is a hybrid tube/solid-state active line stage without a volume control that can be added to any upstream passive preamp (including of course the LDR1B.V25) or other audio source that has a volume control (such as a DAC that has an internal volume control). The result is active preamplification with some tube euphonics. In the input stage, the tubes used are the 6H30Pi Gold Electroharmonic (2 of them), the premium gold pin high performance version of the classic Sovtek 6H30Pi super twin triode; they provide 4 channels for balanced audio.

The Tortuga Audio TPB.V1 Tube Preamp Buffer.

The Tortuga Audio TPB.V1 Tube Preamp Buffer.

They are beautiful and each comes in a small, alluring golden box. The solid-state output stage is FET. The TPB.V1 comes with a trigger cable that can be connected between it and the LDR1B.V25, which allows the 2 units together to go on and off at once controlled by the LDR1B.V25.

The tubes attach to the top of the case with no cover. To use the TPB.V1 and LDR1B.V25 together to form a standalone active preamp, one need only connect them by a pair of balanced XLR interconnects. Then the outs of the TPB.V1 go to your amp(s), while your source inputs go into the LDR1B.V25.

Although the minimal black case is the same as for the LDR1B.V25, the TPB.V1 is 2 lbs heavier and requires a power cable (it comes with a stock one, but you can use your own higher-end one if you wish). There are no controls of any kind on the front. The unit is powered on by a switch in the back that emits green light when on, and there is an additional toggle switch for turning only the tubes on and off (Standby Mode) while leaving the solid state portion powered on. When all is on, a tiny blue dot shaped light glows in the center of the front. And I must say that the unit looks so cool in the dark with those tubes glowing on top.

Dimensions: 6” (W) x 9” (D) x 3.25” (H) Weight: 6 lbs

The Tortuga Audio TPB.V1 Tube Preamp Buffer rear panel.

The Tortuga Audio TPB.V1 Tube Preamp Buffer rear panel.

Testing for this review.

Since the unit I received only had 1 input, I used only a digital source, a DAC, in this review, fed by a Mac mini using Roon and Tidal HIFi streaming. But since my DAC (PS Audio Direct Stream) contains an internal volume control, and as reference I use the DAC with an active preamp (PS Audio BHK Signature) for volume control, this led to several testing cases for sound quality, even more so since the BHK unit itself is a tube/solid state hybrid. I list below the 4 test cases I examined and include a conclusion for each case.

Before moving on, though, I think it is important to consider the prices of these various components to give the reader a better idea of my evaluations/comparisons. If you sum up the USD$1895 of the LDR1B.V25 with the USD$1695 of the basic TPB.V1 buffer you get USD$3590, versus the USD$6000 of the BHK—a significant difference. I was given for review an upgraded buffer bringing its cost up by USD$401 to USD$2096 (USD$75 for a Belleson SuperRegulator upgrade, and USD$326 for a VCAP coupling cap upgrade—there is also an optional USD$695 for Vishal metal film resistors that my review model did not have) bringing the Tortuga total up to almost USD$4000 at USD$3991 and as high as USD$4686 if you decide to get the Vishay metal film resistors. (The BHK by contrast, comes with 5 pairs of balanced inputs and a built-in headphone amp.)

One also has to use an extra pair of balanced interconnects, between the LDR1B.V25 and the TPB.V1, when using the Tortuga pair together. But then again, one can significantly reduce the Tortuga prices by opting for their DIY routes. As for size and weight comparisons: The LDR1B.V25 paired with the TPB.V1 comes to only 10 lbs, and (if stacked) is 6” (W) x 9” (D) x 6.5” (H), but with the 2 tubes on top adding another 2.5” to the height. The BHK by comparison is 22 lbs, and 17” (W) x 14” (D)x 4” (H).

The 4 Test Cases with conclusions.

        • LDR1B.V25 compared with the DAC’s internal volume control (e.g., DAC connected direct to amps with balanced cables).
This was just to get a sense of balance (no pun intended). Conclusion: Darn close to my BHK if not equal. Impeccable clarity, transparency, and neutrality with a dash of smoothness (that made listening non-fatiguing). That speaks very well for the LDR1B.V25 because in general, it was not (to my ears) until I came across the PS Audio BHK Signature preamp that my sound quality increased; before then adding various preamps decreased the sound quality.

        • LDR1B.V25 alone compared to attaching the TPB.V1 buffer to it. How does the buffer affect the sound quality of the passive LDR1B.V25?
Conclusion: In my opinion the buffer increases sound quality. It tastefully and subtly yields a bit more richness/fullness in the bass and midrange and a bit more air and space around instruments, albeit with a slight loss of clarity. Voices have more body. The tubes are at play here no doubt adding in what some would call ‘coloration’ or distortion of the kind that adds in pleasantries (second and third harmonics?). Nice.

        • LDR1B.V25 with TPB.V1 buffer versus BHK. In essence, this is comparing two different hybrid active preamps.
Conclusion: In the same spirit, but with an edge to the BHK. The Tortuga yielded a leaner overall sound, with less addition of air and space; the BHK yielded a deeper, more 3-dimensional sound stage, and with added textures of instruments and voice, and lingering decays for percussive instruments.

        • Using the TPB.V1 buffer connected directly to the DAC. This means using the DAC’s internal volume control (in lieu of the LDR1B.V25).
This is to test the effect of the buffer itself on other volume control devices besides the LDR1B.V25. Conclusion: Interestingly, this had less of an effect on the sound quality than it did using the LDR1B.V25 with TPB.V1 together as was the case in (2) above. There was less of that added body and so on. Passive purists, who might otherwise shun the use of tubes (or active preamplification) might very well find a friend in this combo.

WARNING: in (1) and (3), as for any such DAC with an internal volume control, I set my DAC’s internal volume output to 100% as I do when using the BHK. One has to remember, however, to lower that volume in advance if going either back to ‘direct to amps’, or to (4), or you will most likely blow your speakers when playing music. In short: Before you start sipping that martini, make sure you have your volume settings right when doing the above kind of experiments with your own equipment.

Setting up.

Setting up was very easy. I had no trouble doing so using no instructions whatsoever except making sure I fully understood the buffer. I never used the trigger cable since I had many permutations of the two units to consider in my testing (together, not together, etc.). I offer here what I went through when using the TPB.V1 as a pair with LDR1B.V25. The other cases are similarly carried out.

Turn DAC and amps in standbye mode (or powered off). Connect an XLR interconnect pair from the output of the LDR1B.V25 to the input of the TPB.V1. After connecting, and plugging in the power cords for the LDR1B.V25 and the TPB.V1 (and making sure they both are off), plug XLR interconnects in from the DAC, connect XLR interconnects from LDR1B.V25 out into amp(s). Turn on the LDR1B.V25 unit using the Apple Remote. Make sure the volume is not too high. Turn on the power switch in the back of the TPB.V1. Toggle the tube switch in back to on
position (toggle right as you look at the back). Turn back on your DAC and amps. When all your system is on play some music and use the Apple Remote to regulate volume.

Some of the music I used for testing.

Having Roon, I easily set up a playlist that helped me with my testing. (I turned off Roon’s Radio Mode so I could really focus.) I mention here a sample of what I included.

        (1) Karrin Allyson’s blues studio album In Blue (2002), particularly the track Moaning in which she remarkably tracks/imitates Steve Wilson’s saxophone playing with her voice--just stunning. That voice/sax part allowed nice comparisons in the testing. Also, the beautiful track, My Bluebird with delicate cymbal work in the beginning that helped discern differences in percussion.

        (2) Janos Starker, Bach Suites for Solo Cello (1965). His breathing sounds and use of bow
(with interesting peripheral sounds as it touched the strings) [‘ghost notes’ — Ed] was very revealing and helpful in testing.

        (3) Johnny Cash, his version of the American classic folk song, The Streets of Laredo (also known as The Cowboy’s Lament) from his album American IV: The Man Comes Around (2002). It is a sad piece not only because of the story behind it (a young cowboy cut down in his youth), but because Cash is clearly getting old and his shaking voice shows it. That shaking voice was very helpful across the listening tests; he has such a unique voice. The song itself has a fascinating history, going back well before the American version; one can find earlier Irish variants from which it was derived such as The Unfortunate Rake in which the cowboy is a young man who had sex with a prostitute and whose life ended by tertiary syphilis. My first introduction to The Streets of Laredo was in my youth; the version by Burl Ives from the 1950s. My favorite fun version is a hilarious spoof on it by the Smothers Brothers from the 1960s; check it out. There is also a lovely version by Suzanne Vega (2011), but of course this is not supposed to be a lovely song–but I can forgive her. There are even songs written using just the melody, such as the beautiful Bard of My Heart by Judy Collins.

       (4) How could I resist: The famous 1967 hit by The Turtles, Happy Together, which while I listened brought back such warm innocent memories of my childhood. Hmmm…hopelessly biased by my turtle past, I proclaim with certainty that it definitely sounded best with the Tortuga duo (LDR1B.V25 with TPB.V1)!

Summary

If you are a fan of passive preamps, and in need of a volume control, I would say that the Tortuga Audio LDR1B.V25 Passive Balanced is clearly among the very best—perhaps even the best—I have ever heard in my system. If you already have volume control built within your source and would like to explore an elegant/tasteful dash of tube euphonics then by all means try out adding in the TPB.V1 to your system. And if in need of an active preamp with at most 2 inputs, check out the LDR1B.V25 together with the TPB.V1 buffer; they make a wonderful pair. As in Happy Together, by the Turtles, the two components indeed are happy together. Highly recommended.

Further information: Tortuga Audio

Associated Reference Equipment

Speakers: Alta Audio Celesta
Amplifiers: 2 Merrill Audio Veritas Monoblocks Special Edition (SE).
Server: Mac-Mini modified by Mojo Audio, with Mojo Audio Illuminati power supply
DAC: PS Audio Direct Stream with Bridge II, using ethernet connection to Mac-Mini
with Roon Labs Music Player, and Tidal streaming
Preamplifier: PS Audio BHK Signature
Interconnects : Anticables Level 4.1 Reference PLUS Xhadow (with cryo option) XLR, Antipodes Reference XLR.
Speaker cables, jumpers, power cords: Waveform Fidelity.
Power generator: PS Audio P3 Power Plant.