True Dimensional Sound Passive A/T and Passive Audiophile Sound-Enhancement Devices

by Mike Masztal

One of the most impressive products Jim Saxon and I heard at last summer’s CEDIA show in Atlanta was the True Dimensional Sound TDS-II analog audio processor (see my review and Greg Weaver’s follow-up in the SoundStage! archives). This affordable accessory brought about a transformation to my system that I explained in my initial review. The active unit was powered by a wall transformer, but better passive units were in the design process. Well, they’ve arrived.

I was rather anxious to hear the new units because although I liked the TDS-II a great deal, I found it to thicken the sound while playing back some CDs on my reference system. However, this was not the case with lesser equipment, which also happened to be less expensive, and in summary, I thought the TDS-II was a product music lovers needed to try in their systems.

Since my review, TDS has been busy upgrading their technology and expanding their product line into the commercial, audio, home-theater and car-audio sectors. TDS is also gaining increasing commercial usage of its products. The technology was used in the recording process of the movies Jerry Maguire and My Best Friend’s Wedding. In addition to home/car audio applications, TDS is focusing on licensing their technology to specific OEMs in various markets to integrate into their products. This integration can be done with TDS in its current passive form, as an analog IC, or as a DSP-based program.

TDS’s technology has not gone unnoticed either. The ’98 CES in Las Vegas awarded TDS the Innovations ‘98 award, and their latest recognition comes from Discover magazine, with TDS being picked as a finalist for Discover’s annual Technological Innovations Award covered in the July 1998 issue.

The product

As usual, new-product development takes more time than planned, but TDS finally sent me samples of the new Passive A/T and Passive Audiophile units. Measuring 2"H x 4 1/8"W x 6 1/2"D, the A/T unit retains the in/out switch on the faceplate like the active unit. The Passive Audiophile, 2"H x 7"W x 7"D, is offered both with and without the in/out switch -- a wise decision in my opinion. The Passive Audiophile also uses larger upgraded internal components and connectors that the Passive A/T doesn’t have. I used the Passive Audiophile almost exclusively for evaluation.

The product literature recommends placing the units between the preamp and power amp. I did this in my system; however, this was not the way to go. I found this configuration tended to make the sound somewhat woolly. I believe the large amount of gain provided by the units seemed to overdrive my amps. The best way I can think to describe the effect is by comparison to something that happens with a tube-powered guitar amplifier. By cranking up the gain section, you can get an aggressive, hard sound that makes the guitar sound cool, but in a music-playback system, it mucks up the sound. Placing the TDS unit between the DAC and preamp corrected this aberration. I discussed this phenomenon with co-designer Art Garcia, who stated that this might occur depending on the amp’s input impedance. Art recommends that the unit be placed in between preamp and power amp, but users should experiment to determine the optimum location in their systems.

Reference system

Source: CAL Delta transport, CAL Alpha DAC retubed with Brimar CV4004. Preamp: Aronov LS-9000 retubed with Mullard 12AX7s. Amps: Aronov LS-9100 monoblocks sitting on Golden Sound DH cones and squares. Speakers: Silverline Panatella sitting on top of Osiris Giza II bases. Interconnects: JPS Superconductor. Speaker cables: Silverline Sterling Signature. Power cords: JPS Labs. All components were plugged into a homemade, Hubbell-equipped, non-filtering outlet strip powered by a dedicated outlet. My listening room is 12'W x 18'L x 8.5'H, complimented by Corner Tunes and homemade room-tuning devices.

The sound

Well, was the Passive Audiophile unit all that I hoped it would be? In a word, YES. All the benefits of the active unit were there: improved harmonics, timbre, and better tonality -- particularly in the bass. Images became more focused and a had a lifelike density. Low-level detail, the resolution of the back-end of notes, and decay were also substantially improved. The "clean window" and live-performance analogies I used in my initial review still applied.

However, there were distinct differences between the active and the passive units. First, the gain of both passive units was much greater than that of the active unit. Lab tests performed by TDS confirm this. How can this be -- for passive units to produce more gain than their active counterpart? The full technical explanation is very involved, but in a nutshell the passive units rely on the available current of the source and its own high input impedance (200k ohms at 20kHz) to produce a gain in voltage, which is even higher than if the unit had a power supply like that of the TDS-II. Those with inexpensive CD players having smallish power supplies should find this added gain beneficial to their system’s performance. Second, there was no overloading or woolly sound on the CDs I had previously listened to with the active unit. I believe this effect was likely due to the wall-wart power supply of the TDS-II. Lastly, both passive units were more refined, and the noise floor for both was also noticeably lower than that of the active unit. In my opinion, this degree of improved refinement is what you might expect by upgrading to better components, not merely what you might discover with better interconnects or speaker wire. I’ve also heard the Passive Audiophile in other high-end systems with the same beneficial effect.

Conclusion

I consider myself somewhat an audio purist -- I like simple controls and generally shy away from equalizers and other black boxes. About 15 years ago, a Linn dealer enlightened me about the deleterious effects on sound of each additional pot, solder joint, metal-to-metal contact, etc. But the stunning performance of the TDS Passive Audiophile device has shown me that all add-ons need not be subtractive to a system’s performance. This thing is going to stay in my system. In fact, I bet the Passive Audiophile device is even appropriate for mega-expensive, reference-quality systems. The cost is very reasonable, and TDS still offers a 30-day money-back guarantee. The Passive A/T unit, at $195, is a steal considering the improvement it can give to affordably priced systems.

So which one should you choose? Well, there is a significant degree of difference between the two units, with the Audiophile version being more refined than the A/T. If you’ve been bitten by the audio bug and are going to be around for a while, I’d spring for the Passive Audiophile version as you can build your system around it. For the casual listener who is happy with his or her inexpensive system (I sometimes wish I were in this group -- I’d be a lot richer), the Passive A/T will suffice. All in all, both TDS passive units still get my highest recommendation.

...Mike Masztal
mikem@soundstage.com

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