No circuit is better than a bad circuit...so a passive preamp can give you great sound at a lower price. And this one from Antique Sound Lab is...different.

When it comes to improving a music system, we have always favored starting at the front and moving toward the back -- the speakers. The argument is well-known: how can even the best speakers correct problems suffered by the source? The corollary to this theorem: the preamplifier should take priority over the power amplifier.
     Not everyone needs to make that distinction, of course, because -- as you'll see from reviews in this very issue -- some companies are building integrated amplifiers that make you wonder whether preamps will survive as a category. Still, perhaps you have (or have your eye on) some killer power amp, whose hefty purchase price will leave little cash to add a preamp. Now what?
     Well, how badly do you need a preamplifier anyway? A typical one provides perhaps 6 dB of gain, and perhaps you can do without it. If your speakers are efficient enough (89 dB or better sensitivity), and if your power amp has enough gain (as most do), that 6 dB is expendable. Of course you do need something in its place. At the very least you need a volume control, and unless you have only one music source you also need an input selector.
     That's what you get with a typical passive preamplifier, which has the selector and the volume control and that's all. No tubes, no transistors, and no need to plug the unit into the wall. Two such units can be seen on the cover of UHF No. 54.
     And now here are two more, including this unusual one from China. It is different from all of the others we have seen, and perhaps we should start by explaining why.
     Most passives have a configuration that looks like this:
The switch, at left, lets you select the source you want, and the potentiometer lets you adjust the volume. That's it, that's all. If the unit has been properly designed, turning the unit all the way up gives you close to unity gain: it has about the same effect as plugging in a length of wire. Turn the knob counterclockwise, and the signal at the output will drop. Simple. What can go wrong (famous last words)?
      Of course you would expect performance to depend on the quality of the parts. A good volume control, a high quality switch and well-made input and output jacks do affect what you hear, just as they do in active preamplifiers.
      There's a down side to passive control boxes: high impedance.
     The output impedance of a preamp, measured in ohms, is the load that the power amplifier and the connecting cable "see." If the impedance is 200 ohms (a not untypical value in an active amplifier), the power amplifier will "think" it has a 200 ohm resistor across its input. That impedance yields low noise and minimum distortion.
     But the output impedance of a passive preamp is much higher, and it even varies. It is highest when the volume control is all the way up, typically about 10,000 ohms. It drops as you turn the volume down, but passive preamps tend to be run at the top of their range. The high impedance can magnify certain cable flaws. It may result in noise. And unless the power amplifier itself has a considerably higher input impedance (which most do, fortunately), there can be considerable distortion.
     This Antique Sound Lab unit works differently, with a transformer instead of a volume control:
The transformer changes impedance, from high at the input to low at the output. The volume control is actually a switch, selecting different transformer windings depending on the volume desired. Of course the choices are coarse, as we shall see. Curiously, the volume knobs are not calibrated from 0 to 10, as is the custom, but from 1 to 12. We mused that they go one digit higher than the famous guitar amplifier in This is Spinal Tap.
     The box is full-width, looking more like a "real" preamp and not a mere accessory. There are four inputs, including "tape," and separate volume knobs for the two channels.
     We substituted the T1DT for our Alpha system's reference preamp and listened to a track on Sources, the new CD by Brazilian singer Bïa (Audiogram ADCD10132). We were pleased with what we heard. The accordion introduction was pleasant, and the guitar somewhat soft but quite satisfying. Bïa's voice remained warm, though it was slightly harder and more sibilant.
     Buddy Bolden's Blues (Opus 3 CD19703) was pleasantly coherent, with rich textures. The bottom end was slightly light, though the sousaphone still sounded impressive. The interplay between sax and clarinet was a delight.
     Papa John was also softer than with our reference preamplifier, and everything seemed more distant as a result. The slight lightness made Doug McLeod seem less "round," but there was no shift toward high frequencies. Indeed, the violin, which occupies a major solo role in this song, was smooth but not somnolent. We liked the strong dynamics, and also the considerable refinement.
     We ended with Fanfare for the Common Man (Reference Recordings RR-93CD). It sounded lighter than with our reference, but only a little. The impact of the large bass drum and the other percussion instruments was powerful. The brass sounded quite natural, without the somewhat "etched" distortion you could expect from an active preamplifier not quite up to the task.
     Still, Reine thought that the softness required more volume. We listened again with the volume control turned up two notches (about 8 dB). We liked it even more that way.
     In the lab we discovered that, instead of having an insertion loss (that is, losing signal even with the volume all the way up) this preamp had a very slight gain of 0.1 dB. This is voltage gain, not the sort of gain a "real" preamplifier provides. The volume controls were indeed quite coarse, with steps of 4 dB toward the top, but more like 7 dB further down. There are, of course, no half steps.
     Unlike most passives, this transformer-equipped device has relatively low impedance. Even all the way up it is just 1.5 kilohms. Two steps down, it drops to 600 ohms.
     We were pleasantly surprised by this inexpensive product. It offers advantages over the usual passive preamp design, and its performance is not likely to make you regret you bought it. Not unless you have a lot more money to spend than the company is asking for this neat unit.

Model: Antique Sound Lab T1DT
Price: C$600/US$400
Warranty: 5 years on most parts, 1 year on labor, transferable
Dimensions: 38.5 x 15 x 8 cm
Most liked: Strong performance for a pittance
Least liked: Slight softness, coarse volume adjustments
Verdict: An often ignored solution to audiophile budget problems

CROSSTALK

     The first thing that struck me was how refined the midrange was. It made voices quite expressive and lyrics crystal clear. Guitars were surprisingly well rendered, and I enjoyed the contrast between the different timbres of instruments playing within the same range, such as the clarinet and soprano sax. As the listening session progressed I analyzed less and enjoyed more.
     The music was there. It was sweet and rich, from the unexaggerated sound of brushed cymbals to the lovely voices of the different brass sections playing in harmony. Voices were round and naturally warm and reflected facial expressions. Bass seemed a bit less deep, but I didn't pay much attention to that difference, so blended was it with the rest of the music.
     I remain quite surprised by the quality displayed by this passive preamp. We used difficult pieces to test it, and it reproduced them with confidence. Actually it was transparent enough to let them be finely reproduced by the rest of the system...as a passive preamp should do.
--Albert Simon

     Despite its unprepossessing dimensions, this passive preamp has numerous qualities. It presents depth and image that are quite satisfying. I appreciated the richness of gorgeous detail: inflections, nuances, and all sorts of effects. The bottom end is solid, and the very lowest notes are even surprising, but never upsetting the well-balanced image. There's harmony, rhythm, and punch! The dynamics are remarkable, and the virtuosity of the artists comes across.
Considering the very reasonable price, this is a tempting product.
--Reine Lessard

     There is so little inside a typical passive preamplifier that you could expect them to sound nearly alike. Well, they don't. And therein lies a caution for designers who pay attention only to their circuit designs...there's more to the sound of a product than its amplifying circuit.
     Of course the design of this particular passive preamp is different from that of others I've heard, but I like the result. In every way, it delivers the promise of the no-circuit principle. It sounds truly excellent, much better than any "real" preamplifier you could find anywhere near that price.
--Gerard Rejskind

(To read the entire issue, just order issue 62 at our secure server.)

Complete articles from this issue:
Vecteur I-4 integrated amplifier, Antique Sound Lab passive preamp, State of the Art

Excerpted articles from this issue:
Copy Right!, DVD for Your Future, Vecteur L-4 player, Musical Fidelity Nu-Vista amplifier, Moon Attraction processor, Creek OBH-12, Two Interconnects, Antique Sound Lab amplifier