The Luminous Audio Axiom II Signature Passive Preamplifier

Roy Harris -- I was attending a meeting of the Audio Syndrome — a local audio club whose members reside in the New York City metropolitan area. The president of the club was the host. The meeting encompasses a discussion followed by listening.

During the discussion portion, a member distributed literature obtained from the Luminous Audio website. The literature described an inexpensive passive preamp, the Axiom II. As I read the material, I realized I had come upon a preamp whose design approaches ‘a straight wire with gain’, considered by some audiophiles to be the epitome of design. In fact the preamp contains three components, a short length of solid core 21 gauge, 4 9’s silver wire, a resistor and an Alps potentiometer.

The company’s website contains a thorough explanation of the design. For those uninterested in the technical aspects of the preamp, I offer some salient facts about the company and describe the three versions of the preamp.

The standard version, the Axiom II, is equipped with a Holco metal film resistor and sells for $195.00 . The Signature version, comes equipped with either a Caddock metal film resistor or Alan Bradley carbon fiber resistor, at a cost of $220.00 . The designer Tim Stinson suggests the metal film resistor for solid state-based systems and the carbon fiber resistor for tube-based systems. I was thinking about providing him with my favorite resistor, the Reiken carbon fiber resistor, but I decided against it, because I thought it might ‘colour’ the sound of the preamp. The deluxe model, The Walker mods, starts at $399.00, depending upon options selected. Changes to the basic design include a one percent metal film resistor-based volume switch, continuous cast copper wire in place of the silver wire, chassis dampening, Vampire Wire female RCA connectors, and two additional inputs, with one additional main out. The suggested benefits from the Walker mods include improvement in micro dynamics, greater bass definition, increased inner detail, wider staging, and a small increment in overall detail, i.e., greater transparency.

As I mentioned, there are only three parts within the preamp. Only one, the resistor is in the signal path. Thus, its affect on the preamp is greatest. The pot and wire are parallel to the signal path. The pot will only have an affect on the sound of a stereo system, when listening at very low volumes, as it will load down a source’s output. Therefore, the resistor brand and value are the two factors that should be considered.

The designer will customize the preamp at anytime — before or after purchase, to satisfy gain requirements or taste. If a customer has a favorite resistor, Stinson will install it for $25.00. If after purchase there is a component change, the value of the resistor may need to be altered. The gain requirement is determined by the output voltage of the source and the sensitivity of the amplifier.

Given my requirements — an output voltage of about 2 volts from a CD player, and input sensitivity of my VTLs of .75 volts, the designer installed a 1.2 K ohm Alan Bradley carbon fiber resistor.

The subject of the review is the Axiom II with a 1.2 K ohm Alan Bradley resistor.

I will be using a product, I suspect is unfamiliar to most readers of this review. It is called the Ophiopogon. It is designed to absorb noise, especially EMI and RFI, as well as reduce mechanical chassis vibration.

Listening Session

First, I shall state that I listen out of phase. I connect the speaker cable out of phase.

Many CDs, e.g., Sheffields are recorded out of phase. However, I have found over the years that out of phase listening is preferable. 

I requested that the preamp be broken in before it was sent to me. When I received the preamp, I listened casually for 24 hours, prior to critical evaluation. I was confident that this preamp would not exhibit a sonic signature, because of its simple design. I realize that a resistor has a sound, but I did not expect to hear it.

It has been said that the two primary problems with passive preamps are a loss of dynamics and bass extension. Thus, I made a mental note to include musical selections to test these parameters. I decided to confront the issues of dynamics and bass first, and then listen for other variables, such as the treble and staging.

My favorite orchestral selection is Ravel’s Alborado Del Gracioso, conducted by Ernest Ansermet, recorded in 1961, from a London CD, 414 046-2, unfortunately out of print at this time. My friend owned two copies of this disc. At a CES show during the 90s, he traded the disk for a pair of Definitive Audio book shelf speakers, after Sandy Berlin auditioned the disk for two minutes.

I generally listen at low volume levels. In this case, the beginning of the composition was measured at 62 DB. After 40 seconds, there was a crescendo. My Radio Shack meter showed an increase of over 18 DB.

Prior to the crescendo, there was a brief clarinet solo in the background. The clarinet was positioned far behind the front of the orchestra, indicative of significant depth of field. A bit later, there was a longer bassoon solo, again deep into the rear of the orchestra. Later into the performance I observed a snare drum. One could hear each strike of the wood drum stick upon the surface of the drum. All percussion instruments were audible, including a small cymbal recorded at a lower level than other instruments

After my initial audition of Alborado, I made a small change to my CD player. I applied two tube dampers to a 6DJ8 tube. The affect of the dampers on the sound of the stereo system was immediate and apparent, Strings were more focused, treble was more extended, but I felt the music sounded ‘digital’ and a bit harsh. I did not wait to long to remove them.

Thereafter, I made another change, substituting the Fusion Audion (Gold) interconnect cables for the Mojo Audio (copper) cables. The spectral balance changed. There was more emphasis upon the mid bass, less on the treble, which seemed a little smoother and less extended. Dynamics were unchanged, there was greater depth, body and naturalness of timbre, but the sound of the orchestra was more diffuse. There was a gain in tactility and fullness.

With the Fusion Audio cable, there was an ease of presentation, with less focus and resolution. The snare drum was less clear, and the cymbal, recorded at a low volume level, could barely be heard.

The Mojo Audio is more suitable for reviewing, while the Fusion Audio may be more suitable for longer term listen ability, especially when recording quality is an issue. The former may be less inaccurate while the latter has more ‘colour’.

I felt satisfied that the preamp passed the test of dynamics, so I next introduced the first of two tests of bass response.

The first featured an electric bass, taken from the CD, Bela Fleck, COSMIC HIPPO, track 4, Warner Bros 9 26562-2. Just for kicks, I left the Fusion Audio in my system for one more comparison. Victor Wooten’s bass solo exhibited articulation and fullness in the mid bass. However, I expected that greater extension was possible, so I brought back the Mojo Audio and listened again. As expected, bass was deeper and very full, and had greater impact. You could hear the instrument vibrate. Cymbals were clearer as well. This result was consistent with what I heard from the Ravel piece.

Accuracy v Musicality, anyone?

I left the Mojo Audio cable in the system for the remainder of the review.

The next selection is an audiophile classic, ADAGIO ALBINONI, featuring Gary Karr, acoustic bass, from a Cisco release GCD 8003. I played the title track, Albinoni’s “Adagio”. The SPL was in the range, 70 to 82 DB.

At the beginning, there is a brief organ solo. The organ sounds distant, full and not lacking bass extension. Thereafter, the acoustic bass is heard in the foreground. One first notices frequencies in the treble range, until a bit later, when Gary Karr descends the scale, in which case, one can appreciate the size of the instrument, observe the vibrating wood body, and appreciate its range, especially the facility to play as low as 40 HZ. As the two musicians are playing together, there is also a sense of space — the ability to hear each instrument distinctly. Kudos to the recording engineer for a fine recording which conveys the natural timbre of both instruments.

The sound of a violin is a good test of treble response. I selected TWO WORLDS, featuring Dave Grusin and Lee Ritenour, Decca 012 157 960-2, track 8.

At the outset, there is a brief piano solo. The piano hits a low note, then the violinist, Gil Shaham, can be heard. The treble was extended, without an etching or thinness in string density. Dave Grusin’s fingering of the piano keys was very clear. The acoustic guitar sounded like its strings were composed of nylon. This is a high resolution rendering of three instruments — violin, piano and guitar, yet there is sufficient body, avoiding frequency response irregularities.

The perspective seems like the fifth row because of the revelation of so much detail. One also can observe micro dynamics, in this arrangement of Bartok’s ‘Stick Dance’. Out of curiosity, I replaced the 12AX7 tube in my CD player and listened again I substituted a Tesla NOS for an RCA. There was less treble extension, a very slight rounding, the strings of the violin and acoustic guitar seemed a bit thicker, lateral sound stage was wider, there was greater fullness, but, of course, less resolution.

I felt that the timbre of the piano, guitar and violin sounded a bit more natural, and, instead of a fifth row presentation, the instruments exhibited a more distant perspective. I expect this tube, like the Fusion Audio cable, to be more forgiving of problem recordings, and conducive to longer term listening.

While I would prefer this tube for personal listening, it seems to offer some coloration and would not be suitable for reviewing purposes. The RCA tube is more appropriate for reviewing.

I always include a harpsichord CD in a review. This time, I decided to play one of my favorite Bach pieces, performed by my favorite harpsichordist. In this instance, I selected Bach’s Chromatic Fantasy and Fugue, performed by Pierre Hantai. I also own other versions of this work, played by Trevor Pinnock and Christopher Rousset. However, I think Pierre Hantai captures the spirit of this work better than the other two players. The CD is on the Virgin Veritas label, 7243 62473 2 1.

First, I could really appreciate his technique — his fingers are fleet. His approach to this music, whether phrasing, accenting and interpretation in general, are first rate. It’s fortunate to be able to hear the harpsichord in all its splendor. The sound is very percussive. He leans on the keys at times, holding them for a second or two and then releases them, at which time, you can hear the inner parts of the instrument resonating. Hantai really digs into the music. At times, he has a heavy touch. One can appreciate the difficulty of the work and the ability of the harpsichordist. There is such resolution, that I could listen for hours spellbound in the presence of a great artist, without experiencing fatigue.

I decided to conclude this section of the review, with a torture test for any stereo system, namely, a big brass band played at volumes exceeding 85 DB, and in some cases over 90DB. This is a well recorded disc, featuring great musicians and complex harmonies.

The CD is titled THE BRASS ORCHESTRA, with the trombonist J.J. Johnson as the leader of the band, Verve 314 537 321-2, track 1. When volumes exceeded 90 DB, it gave one the impression of being present at a live concert. The treble was smooth and even when listening loudly, it was not unpleasant. The brass instruments, especially the lead trombone were rich, rounded and timbrally accurate. The engineer probably decided to deemphasize percussion instruments, as the cymbal was somewhat soft sounding and recorded at a lower level than the other instruments. I listened to other tracks again without incurring fatigue or a desire to leave the listening room.


If I had to describe the preamp in one word, that word would be ‘invisible’, as I did not detect an audible sonic signature. I cannot ascribe the word perfect, for two reasons. First, all components are imperfect, and second, the recording is an unknown.

Components are designed at a price point. They have flaws. However, the designer was clever and smart enough to offer an inexpensive component which did not reveal any flaw. Since the recording is unknown it is not possible to determine that it reproduction is accurate. One can, attempt to test the connotations of accuracy, including the ability to observe differences in the sound of recordings and components.

I initiated changes in cables, tubes and applied tube dampers, admittedly on an impromptu basis, as another test, in addition to the musical selections. The changes in the sound of the stereo system were detected quickly and easily.

This preamp is a very useful component to both tune your stereo system to satisfy your taste, and observe strengths and weaknesses in other components. Ideally, if one desires color, it is best to have one component accomplish the goal, while the others, like this preamp are virtually neutral.

Usually, this is very difficult. The speaker is the least accurate component, the room can be a problem and the quality of the AC may not always be controlled. However, selecting the Axiom II eliminates a source of coloration, if that is one’s objective.

If you wish to configure a stereo system using the Axiom II, select your components wisely. If you don’t like the sound of your stereo system, it is probably not the fault of the preamp, unless the wrong value is selected for the resistor. The preamp is highly resolving, unlike many of the tube preamps I have encountered. It is quiet and free of sonic artificats. It enables you to hear all instruments on a recording, provided the rest of your stereo system is also noise-free. This preamp is suitable for those stereo systems that are well ‘behaved’, especially free of peaks and dips in frequency response. As such, the preamp will not compensate for problems in your stereo system, but, may possibly exacerbate them, e.g., brightness. The preamp is suitable for those whose other components are consistent with one’s sonic preferences, and is appropriate for tuning one’s system, as necessary, as I demonstrated, earlier in the review.

The Luminous Audio Axiom II Signature Passive Preamplifier will be my reference for the near future.

Further information: Luminous Audio