Tortuga Audio LDR1 Passive Preamplifier

Roy Harris -- I have a friend who lives in Manhattan who frequently visits audio forum websites. He has discovered many products designed by manufacturers, who are not well known, who sell direct to the consumer, via the internet. I spoke to him recently and asked him if he were aware of any components which he considered worthy of review. He consulted his list of amps, preamps, CD players and speakers, and suggested I investigate the Tortuga Audio LDR1 passive preamp.

I called the designer, Morten Sissener, and he explained below, the basis for the development of the LDR1 preamp:

He was in the process of designing a tube preamp and became dissatisfied with the available analog volume pots, He read information about light dependent resistors (LDRs) on the internet. He applied the technology and developed LDRs to serve as a volume control.

An LDR is a combination of a photo resistor and an LED in a sealed package. Power is required to control the resistance level of an LDR. The external power supply is used solely to control brightness running through the LED. It is completely isolated from the audio signal. The signal from a source only passes through an LDR and a short length of copper wire. Sissener believes that the LDR and short length of wire do not have an audible affect upon the signal.

Tortuga Audio is the only company using a digitally controlled analog LDR. There is a microprocessor inside the preamp which controls the LDR. Tortuga Audio is one of a few which uses LDRs to control volume.

Break In

Sissener burned in the preamp for 72 hours prior to sending it to me, and I auditioned the preamp for 24 hours prior to critical evaluation. He believes that LDRs do not require a break in period. His personal experience is that the LDR1 sounds the same on day one as it does one month later. He also suggested that he has not encountered anecdotal evidence from others regarding the need for break-in.

Listening Results

I always begin a review with the goal of testing for errors in frequency response. I use two CDs for this task. The first is Holly Cole, DON’T SMOKE IN BED, Alert Z2 81020, track 1. The song “I Can See Clearly Now”, commences with an acoustic bass solo. Immediately, I noticed a change in the quality of the bass response. What I heard reminded me of a solid state amplifier. The sound of the strings and wood exhibited greater clarity and focus. However, there was a slight loss of fullness, accompanying the increased resolution. Cole’s voice elicited sibilance to a slight extent, but it did not sound exaggerated. As I have stated in other reviews, her voice is closely miked and it is expected that some sibilance will ensue. However, I did not perceive a peak in the treble. In addition, I heard some decay, following the word “sky”. I had not noticed such decay in previous auditions, on my system, or at audio shows. Further, for the first time there was a hint of ambience retrieval from the recording studio, as one could observe space behind the singer.

The second disc I rely upon as a test of frequency response is Bela Fleck, FLIGHT OF THE COSMIC HIPPO, Warner Brothers 9 26562, track 4. I again observed greater clarity with the LDR1 in my system. The banjo and cymbal exhibited greater articulation and more natural timbre. It seemed that a subtle veil had been removed, although I was not aware of any veiling when my reference tube preamp was in my stereo system. The bass was more controlled, consistent with what I heard on the first disc. The fingering of notes was more in focus, and the sound of the vibrating wood body was more taut. This created an impression of a slightly smaller instrument, and a reduction in fullness. One could also say that any overhang, or sloppiness in the bass had been eliminated. Thus greater resolution was associated with “less meat on the bones”.

The rest of the musical selections were chosen to demonstrate the sonic effects of increased resolution.

The next selection was “Cappricio Italien”, conducted by Alexander Gibson, from a Chesky disc, CD-12. I noticed greater clarity from the trumpets and french horns, without an edge. The string section sounded smoother and less strident than previous auditions on other stereo systems, as well as in my own. There was, as well, greater separation between the trumpets and string sections. Finally, there was greater decay following a cymbal crash.

One of the criticisms of passive preamps is its treatment of dynamic contrasts. A visit to my local library provided an opportunity to test the aforementioned hypothesis. I discovered a recent recording of Chopin’s Ballades by a Polish pianist, whose name I do not know, recording on an obscure Polish label. The pianist in question is Wojciech Switak, and the label is Nurodony Instytut Fryeryka Chopina. The catalogue number is NIFCCD 026. I was impressed by the performance—a combination of poetry and technical excellence. I selected Chopin’s “Ballade in F, opus 38”, and set my Radio Shack meter at 70 DB. I was startled after several minutes, when the pianist used the loudness pedal. My Radio Shack meter registered in excess of 90 DB, for a fraction of a second. In addition to the pianist’s unique interpretation, I was able to detect breathing, obviously recorded at a low SPL.

The last selection was Britten’s “A Young Person’s Guide to the Orchestra”, conducted by the composer, LON 417 509. At the onset, the orchestra sounded like a cacophonous mess—ragged and constricted. Shortly thereafter, the brass, winds and string sections were heard individually. Each ensemble exhibited a more natural timbre, than the initial sound of the orchestra. Next, the instruments of the percussion section—tympanum, cymbal and triangle, had a brief solo. The instruments were introduced in the aforementioned order. Again, I observed additional clarity, relative to my reference tube preamp.

I could now hear the cymbal and triangle as separate entities. There was greater space between instruments. Thus, I heard each instrument, followed by a brief rest. The triangle exhibited greater treble extension and more natural timbre. I also observed that I could sense the metallic bowl of the tympanum. This was the first time I experienced such a phenomenon.


The LDR1 did not exert itself upon the signal, as far as I could discern. As such, it represents the least colored preamp I have ever heard in my system. It also provided greater resolution than any other preamp in my experience. After auditioning the LDR1, I thought of the dichotomy facing serious listeners, namely truth, or beauty. This preamp is truthful. An implication of this dichotomy would be a choice of richness, with some loss of resolution, or resolution with some loss of richness.

Since the LDR1 eliminates a source of distortion, the performance of a stereo system is more dependent on the recording and the other components in your system. You may prefer not to listen to certain recordings because of their flaws. Occasionally, some recordings, usually, popular or jazz, bordered on the analytical.

The benefits of increased resolution have been previously mentioned in the context of my discussion of recordings selected for this review, namely, greater sense of space, greater instrumental separation, improved ambient retrieval and the revelation of detail recorded at sound pressure levels less than that of the instruments on a recording. The main consequence of increased resolution was the absence of euphonic coloration and a slightly “leaner” presentation.

Since all stereo systems have a “sound”, it is easier to alter that “sound”, when you minimize the number of components that have a “sonic signature”, otherwise, the interaction between complementary or reinforcing colorations will make the task of attaining a sonic preference more difficult. A preamp, such as the LDR1, could help to achieve a sonic preference more efficiently.

The LDR1 is best suited for stereo systems having a balanced frequency response, where one’s goal is an increase in resolution. You can determine for yourself, whether this preamp furthers your sonic objectives, as it is available for a home trial direct from the manufacturer. If you are not satisfied, your only expense is shipping costs. Price: $1,195 (includes a remote control which controls volume and channel balance).

Further information: Tortuga Audio