Review: Tortuga Audio’s LDR3.V25 Passive Preamp And TPB.V1 Tube Preamp Buffer

Review by Noam Bronstein

Transparency – if there ever was an overused word in home hi-fi circles, this one might take top prize. But I’ve noticed a decline in its usage over the years, too. As electronics (and speakers) have improved over time, transparency has become a basic expectation: the days of euphonic and unresolving components is largely a thing of the past. In high-end circles, harmonic distortions today tend to be more a facet of room interactions, poor recordings, and other factors that aren’t in the equipment manufacturer’s control. Nowadays, a tubeophile (for example), can have his warmth and his detail, too. Win win. At least in general terms.

Lately I’ve been thinking about the word transparency differently, mainly thanks to the gear discussed in this review. Pure detail resolution hasn’t usually been higher than #3 or 4 priority on my “must-have” list of a component’s strengths. For me, transparency isn’t so much a sonic attribute, as an outcome that results when a high emphasis is placed on eliminating distortion and noise in audio electronics. Done right, this emphasis adds immensely to music enjoyment (as it relates to tube gear, this ‘epiphany’ goes back to my first couple of Elekit reviews. Maybe further back, but the Elekits, in my memory, were among my first really low-noise tube gear). I don’t know if I had “settled”, as far as no longer expecting more and more veils to be lifted – but Tortuga’s LDR-based preamp and active tube buffer under review today have given me compelling reasons to reassess.

Tortuga Audio, based in Cape Coral, Florida, is the brainchild of Morten Sissener. Morten, and particularly his work in the field of passive attenuation, have become something of a haven for DIY’ers (and regular audio folks) interested in passive preamp technology, and Tortuga now has several units available commercially, in both assembled and kit form. In fact, their passive preamp is now in its fourth generation of production – with quite a fascinating history behind it. When I found out how highly people were speaking of these components, I contacted Morten about a review. Some time later, he sent me the LDR3.V25 Passive Preamp, and the TPB.V1 Tube Preamp Buffer. Both of these review units came pre-assembled, no solder required.

The two components arrived very well packed, in separate compartments within the same shipping box. They’re both simple, black boxes – tastefully clad in aluminum, not overly fancy on the outside, but with a modern appearance, and nicely built. The simplicity is deceptive – these units are sophisticated, and use modular design concepts and firmware (in the case of the LDR) in an effort at longevity and future proofing.

LDR3.V25 Passive Preamp

I initially used the LDR Passive on its own, substituting for the active Aikido 6DJ8 linestage preamp that serves as the ‘daily driver’ in my system. I’ll say right from the outset – the improvement the LDR made in my system was not subtle. When something like that happens, I leave it in for as long as possible. If I tire of it, I know it was just ‘different’. After a couple of months, I had no desire to remove the Tortuga gear!

I’ve used passives at various times in the past – especially the Tubecad/Glass Audio resistor attenuator, which has served me well as a benchmark, when needed – so I had some idea of what to expect. But I really didn’t think all that much was ‘missing’ with the Aikido linestage. To my amazement, the LDR peeled off several layers of haze, grunge, and confusion. And it wasn’t limited to one frequency range, or genre; this was across the board. Music just snapped into focus with the LDR, and I used it successfully with several tube amps from Triode Lab/Finale Audio, as well as a Sugden A21 Class A solid-state amp. The LDR is good enough that even with the extra interconnect, it easily outperformed the preamp sections of the two integrated amps I’ve had on hand – the Sugden as well as my Yaqin MS-20L.

The technology in the LDR is unique, and worthy of discussion. The LDR requires power (supplied by a wall wart) to operate its functions, an extensive list of them including volume, all handled remotely with the included AppleTV remote control. But the attenuation itself, in fact all of the audio processing, is completely passive. And here is right about where LDR attenuation departs from anything else: the resistance used to control (attenuate) the audio signal is based on the light-dependent resistor, known by the acronym LDR.

What’s a Light Dependent Resistor, then?

I’ll quote from Tortuga’s very informative website.

“Light dependent resistors combine a photoresistor and a light emitting diode (LED) inside a small sealed device the size of a piece of M&M candy. The result is an optically controlled variable resistor with no moving parts. Each LDR has 2 pairs of wires. One pair is connected to the LED, the other pair is connected to the photoresistor.

Unlike a mechanical potentiometer (“pot”) or stepped attenuator, an LDR has no mechanical parts or interface point that can wear out or degrade the audio signal. Best of all, the photoresistive semiconductor material used in light dependent resistors does very little to materially degrade or color the audio signal. Photoresistors do add a slight amount of distortion to the audio signal but the level of this distortion is insignificant.

Light dependent resistors have been around for quite a while but are relatively new to audio. Using LDRs in high-end audio applications has only become practical in recent years as the technology has improved.”

The biggest challenge with designing an LDR audio preamp was that these resistors are very nonlinear, and their characteristics aren’t consistent from one sample to the next. This is where patience, parts quality improvements, and plenty of ingenuity were employed, to take Tortuga from a cool concept, to where they are today – in their fourth generation of LDR/controller development. In a nutshell, Morten has been able to code and produce controller boards that manipulate the resistors to perform in a way that exploits their ability in the audio reproduction realm.

To really understand how this progression happened, I highly recommend reading this page Tortuga’s site: LDR Evolution. It’s a fascinating story.

In a nutshell, two things happened over the course of several years: first, LDR resistors became more consistent and reliable – better for critical audio applications. Secondly, Tortuga Audio developed an advanced algorithm and related firmware that “corrects” the mismatched performance characteristics of a given set of installed LDRs such that they perform as if they have nearly identical current vs. resistance curves. This correction also normalizes their operating range between 100 ohms and 100,000 ohms. To achieve this, the user may occasionally initiate a 10 minute LDR calibration cycle. During calibration the resistance of each of set of 4 LDRs is measured vs 100 target resistance levels, and the firmware stores each of these 400 adjusted command voltage levels in an attenuation command table used by the controller during normal operation. 

Quite an elegant solution. It tackles both major issues – first, this adaptive calibration process all but eliminates ‘out of spec’ reject LDRs which lowers costs considerably. Secondly, as the LDR’s gracefully age, Tortuga’s firmware corrects their gradual drift out of spec, so that it becomes a non issue. It reminds me of automated solutions like map-programmable fuel injection. Or, closer to home, microprocessor-controlled tube biasing, as we’ve seen in amps like the Elekit TU8340VK. When you combine a robust firmware solution with the improvements in LDR manufacturing tolerances, Tortuga is now able to make a very reliable, consistently great-sounding LDR preamp.

By the way, a glance at Tortuga’s website shows that between the 1st and current (4th) generation, they’ve brought the cost of an assembled LDR preamp down by some $700. This while, by all accounts, increasing its performance capabilities from “clever ideas” to “stable release”. Not bad!

The LDR3.V25 is a simple, elegant, passive linestage preamp. It’s easy to use, thanks to a menu-based OLED screen and remote. Attenuation can be applied with dB levels, or steps – in 101 increments (0 for mute, 1 to 100 for signal level), which works like a charm – and is invaluable for repeatable testing. For example, if you’re comparing cables, or speakers, you can set the LDR3.V25’s attenuation to 52, listen, switch speakers, set it back to 52, and listen to the other speakers. It takes one big variable out of the equation – there’s no guesswork with matching gain levels (keeping in mind, the Tortuga itself doesn’t produce any gain, it only attenuates).

Functionally, Tortuga has blessed the LDR3.V25 with a huge array of operational settings. It may look simple (and it is), but the little remote with three buttons and a wheel has been programmed to allow you control over volume/muting, balance control, input selection, input impedance (adjustable, with five saveable custom settings), display brightness, auto calibration, mono mode, etc. A USB jack is provided for future firmware updates (a potential future smartphone app could control the LDR via a USB Wifi dongle, as well). I found virtually everything one could want has been thought out carefully. This is a modern unit in every sense. For a full list of the LDR3.V25’s features and specs, click on the product page and expand the Specifications section.

What about the TPB.V1 Tube Buffer?

A tube buffer preamp stage is typically used to add some “tube characteristics” to the sound of a system. A somewhat controversial notion, as many would argue that anything additive is just “pleasant distortion”. And second-order harmonic distortion is exactly that: it’s a sonic byproduct that appeals to the human ear, even while it messes up the engineer’s spec sheet.

But a tube buffer can sort out nasty impedance mismatches, drive subwoofer amps, and more. For example, a common issue with passive preamps is that they can sound “weak” or lifeless if they’re being asked to drive a length of interconnect that’s longer than, say, a meter (some will say a foot! I’ve found it true that the shorter the interconnect, the better). But – with a tube buffer on the end, this issue is almost completely mitigated. So Tortuga set out to build a companion tube buffer, and wow, they’ve done a superb job of it.

Actually, the TPB.V1 isn’t merely a companion to the LDR unit. It can stand alone and be used with other preamps (active or passive), or a DAC or Phono stage that has its own gain control, and so on. The TPB.V1 has two sets of outputs, so for example, driving a stereo power amp alongside a subwoofer power amp is easy to do. It also can be configured either as a single-ended unit (RCA), or balanced (XLR).

The TPB.V1 is a hybrid unit that uses half an input triode per channel (full triode per channel in Balanced), and a Class A JFET output stage operating with a constant current source. The output impedance is a very low 26 Ohms. The tubes chosen for the TPB.V1 are Electro-Harmonix 6H30’s. Any tubes from the 6DJ8/7308/6922/ECC88 family can be readily swapped in. I’ll say this – the new production EH tubes that are included work so well, that when it came time to write the review, I still hadn’t even thought about rolling other tubes in(!). But if rolling is your pleasure, the possibilities are wide open.

So after using the LDR3.V25 on its own for a full month, I finally connected and fired up the TPB.V1 tube buffer. Wow. It was like another epiphany. These two units were “made for each other”. Really (you weren’t kidding, Morten). While you can use either piece on its own in your system, the LDR and TPB form a cohesive whole (*I’ll say more about this toward the end) that just makes music compelling and irresistible. Everything becomes more relaxed, effortless and flowing. I think the impedance matching has as much to do with this as the tube ‘euphonics’. I heard cymbal splashes become “cleaner” and much more vivid (thank you, tubes), and marked improvements in realism, very noticeable with pianos and vocals – everything sounded less like a recording, with more depth and focus. LDR was already very good – say, a solid 8 – and adding the TPB took the performance level up to a 9. As we all know, getting from 8 to 9 is no small feat, it can take a lifetime.

I think I’ll always be curious to try new things, but if I were to stop here, I think I’d be very content, and satisfied. I can say with confidence that this is the best preamplification I’ve lived with to date.

Do you need some gain? I didn’t, but then I did – a pair of Triode Lab 2A3GT push-pull power amps came along, and they want to see a little more than 1x/unity gain. The TPB.V1 has an internal jumper that allows you to switch from 0, to +3dB, or +6dB or gain. I switched the jumpers to the +6dB position, and the difference was very noticeable. Still not quite enough for these amps, which are purposely built as “low gain” (for use with high gain preamps), but I was able to achieve satisfying levels in my small room with the LDR set between 50 and 60.

Do you want more? The list of upgrade options for the TPB.V1 is formidable. Tube rolling, Belleson SuperRegulator, Vishay ‘Naked’ Metal Foil Resistors, V-Caps. All are available to take this unit to another level of performance. And its design is modular, such that if Tortuga develops a new or different output module, it can be swapped in and brought to the most updated spec. My sample included the V-Caps, which no doubt improve its performance considerably.

While it seemed unassuming at the outset, the more I learned about the TPB.V1, the more impressed I became. This is another exciting and versatile offering, proving again that Tortuga Audio are deep specialists in their field of expertise. If you need a tube buffer, it’s well worth considering. If you want the most from your Tortuga (or other) passive preamp, I think it’s a no brainer.

Bringing it All Together

In fact, Tortuga has so much confidence in this pairing, they’ve just announced that their next product release will be a combination of the LDR and TPB. Details are still being finalized, but you can follow that news here. (Edit: there’s an announcement page up on Tortuga’s site now). I like where they’re going with this ‘consolidation’: for people who want both elements, it shortens the signal path (eliminates one interconnect), eliminates a chassis, and potentially lowers the cost and selling price.

Recommended. It’s been a pleasure reviewing these Tortuga Audio products. 

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Tortuga Audio LDR3.V25 Passive Preamp
$1195 USD (assembled)
Product page: https://www.tortugaaudio.com/products/passive-preamps/ldr3-v25-passive-preamp/

Tortuga Audio TPB.V1 Tube Preamp Buffer
$1495 – $1695 USD
Product page: https://www.tortugaaudio.com/product-category/preamp-buffers/

(Prices as of mid-2019)