Review: Audio Logic 2400 DA converter

Category: Digital

I have owned the Audio Logic DAC in various iterations since 1994, when I sold my Krell MD-1/SBP 64X combination and acquired my Forsell transport and a modified Altis Reference unit which had four tubes sticking out of holes in its top plate, which in fact was one of the original prototypes of the Audio Logic. I eventually bought a demo unit late production Audio Logic Model 34, which I upgraded a few years later to 2400 status, and have recently updated this unit with Jerry Ozment’s latest thinking for his and the Altis DACs, the replacement of the two 12AU7 tubes in the analog stage (which provide the gain) with transformers. Jerry advised me that this type of gain stage is common in the pro audio industry, and he has been thinking of doing it on his commercial units for a number of years. The principal reason for this post is to report my impressions of this latest upgrade, as a number of AudiogoN members had asked me about it in recent threads.

The overriding word I have used in the past when describing the Audio Logic has been that it sounds much more “alive” than other DAC’s I’ve heard from the likes of dCS, Levinson, Krell, ARC, and Wadia. This probably has to do a lot with the AL’s natural presentation of harmonics, timbres, dynamics and soundstaging, which I find have more of an energy and dimensionality about them than most competing brands, which have sounded sterile, generic and uninvolving to me in comparison. The highest accolade I can give to this DAC, in all its versions I’ve been through, is that I can listen to it in the same listening session as vinyl without feeling much of a letdown in my involvement with the music.

I had approached this latest upgrade with a little trepidation, as the 12AU7 tubes I used in my DAC are Telefunkens, not the standard Mullards that Jerry likes and supplies with the unit. The Mullards have more high-frequency energy in my system than the Teles, which are a little warmer tonally and probably a little rolled in the upper-mids and treble. However, and a problem for me, there was an upper midrange glare that I would get from the Mullards through my speakers which ultimately made them almost unlistenable for me. My preference would have been to have the Mullards’ energy in the highs with the Teles’ overall neutrality; my concern was that the transformers would sound exactly like the Mullards. On the other hand, Jerry’s an excellent designer, and my experience with a properly-designed step-up transformer in the Lamm LP2 phono stage (where the MC step-up is handled through a transformer) has been an eye-opener, to say the least.

In connection with my evaluation of the updated unit, in addition to my aural memory from many years listening to my AL unit, I also have on hand a Sony SCD 777ES which was modified by Jerry Ozment to add the Audio Logic analog stage, so I could make comparisons between the Sony unit by itself with the old AL fully tubed output stage and the updated AL DAC fed by the Sony transport using its digital output. While not quite an ideal bypass test, it was nonetheless helpful in identifying differences. Most of my listening was done using my Forsell Mk.IV transport and dCS Purcell, the combination I’ve used with the AL for the past several years. Cables used were Siltech G-5 Golden Ridge (digital) and Siltech Compass Lake and NBS Omega interconnects. The units feed a Jadis JP200MC preamp which uses all EF86 tubes; the rest of my system is described under my user name at Audio Asylum.

The upgrade definitely changed the tonal balance of my unit, for the better in my view, as it added the high frequency energy that I got from the Mullard tubes to the otherwise neutral midrange of the Telefunkens, but without a hint of the glare of the Mullards. The bass reproduction was significantly improved as well, being better controlled and having more impact. I would characterize the tonal balance as far more neutral, not as warm or “dark” as before, and far more extended and delicate in the high frequencies. If you use the stock Mullard tubes in your Audio Logic, the increased upper frequency energy or change in tonal balance may not be as noticeable as if you use Tele’s or Teslas; if you use the latter, it will seem brighter overall, not objectionably so but certainly something you will notice.

The biggest difference made by the upgrade is in terms of the unit’s resolution of low-level detail. In the past, I have always recognized that there are many digital units out there which could present more detail than the AL, but I’ve always felt that it was either due to a tilt towards the upper frequencies or else done in a way that somehow lost the music in the process. The upgrade significantly lowers the noise floor of the unit (much as I love tubes, they do have some noise, no matter how quiet), but it does not change the character of the unit or make it “analytical” by any means. The most noticeable result of the increased resolution is that images in the soundstage are much clearer (no fuzzy images) and spookily three-dimensional, or more “palpable,” sort of the trick that some phono cartridges (such as the Benz Ruby and Koetsu RSP) can do in spotlighting individual performers within the soundstage without losing the overall picture of the full ensemble. I had thought this was solely due to the use of tubes as the gain stage, as the unit previously had been quite good at presenting three-dimensional images, but this was a whole new level. Ambient cues are much more clearly reproduced, and evenly throughout the frequency range, not just in the highs; in fact the low frequency reproduction of ambience and instrumental textures is startlingly good. Layering of images in the soundstage and overall depth of image are also improved markedly, as is the clarity and focus of images at the edges of the soundstage.

Dynamics were also improved by the upgrade, principally on the macro-dynamic level; micro-dynamics were also improved, but not to as great a degree (the latter has always been a strength of the AL unit, I’ve felt). There is absolutely no sense of strain or compression when the music gets loud or complex. The transformers Jerry uses are quite large and heavy for the application (although they’re certainly nowhere near the size of the torroidal power transformers you might be familiar with) and I observed no saturation problems even on extremely demanding piano and full orchestral pieces.

Summing up, I’d say that the upgrade has made the unit’s music reproduction seem more “real” than before. In comparison to the Sony using the old analog stage, there has been a loss of the slight warmth the Telefunkens provided (I think those who use the stock Mullards in the unit would not notice the difference as much), but in all other respects the sound produced by the unit has improved substantially. While you do lose some ability to “flavor” the sound of the unit by changing the 12AU7 tubes, you still can do so, to perhaps a lesser extent, by playing with the 6DJ8/6922 tubes that remain in the output stage. I’m not bothered by the changes in the tonal balance, though, to go through that exercise. I’m very sensitive to brightness and glare in my system, and while I think that on some source material with excessive brightness (old DG’s, for example) the updated unit can sound a little bright, I view that as more accurately reproducing the source material, and even then the brightness is never to the point that I would find unlistenable.

` Overall I’m very pleased with the results of the upgrade – it has significantly raised the performance of a unit that I’ve always felt was among the top rung of digital processors without losing the inherent musicality that made it so special in the first place. As evidence, I’m going to be giving Jerry my SACD player to make the upgrade to that unit, assuming the tranny’s can fit in the rather small box he attached to the side of the Sony to house the output stage. However, it is not your father’s tubed player in terms of sonic character; those looking for additional warmth in CD playback may want to audition an upgraded unit before doing the upgrade. I am, though, becoming more and more convinced through my experience with the Lamm phono stage and the upgrades made to my Basis turntable as well as this upgrade that a highly resolving front end component which is true to the music (not an easy balancing act) is preferable to a colored source – I still have the ability, using cables and different tubes/components downstream, to tailor the sound to my liking, but I’m not losing musical information at the start which can never be retrieved. But I guess that’s another thread….

I’d be interested in hearing the impressions of others out there who have had this upgrade performed on their Audio Logic or Altis units.
Bravo! A very good review. Thanks for sharing your thoughts with all of us.

Thanks for the update. You mention something about Jerry and the Altis DAC. What is the difference between this DAC and the Audio Logic?

Peter: Afraid I know very little about the Altis, although I'm reasonably sure that Ozment was involved with its design (his name was on the Altis Reference's digital board in my prototype, and he has been handling repairs and upgrades on them since Howie Mandell passed away). Their top of the line DAC was reputedly quite good.
Thanks for a nice article. I always find the use of passive transformer gain to be an interesting topic to consider, though I've never used passive gain in my system. Anyway, I have been making some changes lately to eliminate tube colorations and noise from my preamplification, but that has meant going SS, power amps notwithstanding. I don't know if this will turn out to be satisfactory in the long run, and my gear substitutions haven't exactly been done on a level playing field (the SS replacements are newer and costlier), but so far, so good. I'm glad to hear that your pursuit of overall neutrality, extension, and quietness hasn't either had to take a back seat to the cliched audiophile desire to play with sonic 'flavoring', nor wound up preventing you from enjoying even more everything that was already good about your tube-based system. (And I agree with your determination that disks with flawed sound must be heard for what they are, and not become the lowest common denominator that we permanently compensate for, an approach which dictates throwing out the baby with the bathwater IMO.)
So which tubes did you eventaully settle on for the new unit?

How long did it take to break in? I just got one in on audition and it has only a handful of hours on it as well as the stock tubes (Sovteks - yuck!).

Out of the box, the bass is a little subdued and the midrange a little closed in compared to my Electrocompaniet ECD-1. On a positive note, there is more detail and the top end is much better.

I've been using Siemens Ccas in the DAC and Telefunken Ccas in the little box attached to my modded Sony SCD 777ES (same output stage as the Audio Logic) to good effect. There are a lot of people who like Philips or NOS Sylvanias as well. Joe Kubala (I think he goes under the user name kubalasosna cables or similar) has done a lot of experimenting with tubes, he certainly has an opinion if he's reading this one (I think he likes the Sylvanias).

Did you find that there was some sibilance in the vocals and some lightness in the bass with the stock tubes?

I like many things this dac does, but I am not sure if I will pull the trigger yet.

George: The version I have originally used Mullard/Brimar(?) 12AU7s and Sovtek 6922s. Jerry always liked this combination, but I felt it was a bit too bright for my tastes, so I switched to Telefunkens, which eliminated the brightness/glare I heard in some discs and added a fuller sound in the bass, which may be what you're talking about when you speak of lightness in the bass. I'd call Jerry and ask him what his recommendations might be, he was very helpful with me when I called to ask. I always felt the 12AU7s (now eliminated in the new versions with output transformers) affected the sound the most, but there still is a substantial difference when you replace the Sovteks I had with Siemens or Telefunkens.