Solid State Preamp Reviews

Thomas R. Marshall's famous phrase, "What this country needs is a good five-cent cigar," should be modified for the 1990s to be "What this world needs is a good $1500 preamplifier."

It's hard to find a good full-function preamp (ie, one that includes a phono stage) at a reasonable price these days. There are plenty of great preamps on the market, but they tend to be expensive or line-stage only—or expensive line stages. Moreover, many manufacturers are omitting phono stages altogether, on the assumption that people don't listen to LPs anymore. This situation leaves the LP aficionado with fewer choices for affordable preamps. Further, many inexpensive products don't deliver the musical goods; it's the rare product that combines great sound with an affordable price.

This is unfortunate, because the preamplifier—the heart of any audio system—exerts a large influence on the system's overall sound quality. Every source component must pass through the preamp, which imposes its sonic signature on the music. No matter how good your source components or your loudspeakers, if a preamp sounds poor, it will degrade the system's overall musical performance. Conversely, if a preamp sounds great, it'll get out of the music's way, letting through to the amplifier and loudspeakers exactly what's coming from the source components. The less the preamp does to the signal, the better.

But are there great-sounding, full-function preamplifiers for under $1500? This question led me to the $1495 Exposure XVII preamp, which incorporates a phono stage with easily switchable moving-magnet and moving-coil phono boards. I was intrigued by the XVII after reviewing Exposure's superb $1295 XV integrated amplifier (Vol.16 No.2). The XV virtually defined the concept of musicality in an affordable product; would Exposure's more ambitious XVII preamp live up to the XV's reputation?

Description
The XVII looks identical to Exposure's integrated amplifiers. The austere front panel has two rotary switches—one source selector and one record-out selector—and volume-control and power rocker switches. The chassis rear holds five pairs of line-level inputs and one phono input, all on gold-plated RCA jacks. The phono and CD line inputs are both higher-quality jacks. The XVII's main output appears on a pair of RCA and XLR jacks in parallel. The XLR jacks are single-ended, with only pins 1 and 2 wired. They're included because Exposure believes XLR connectors sound better than RCAs. Exposure's power amps have unbalanced inputs on XLR jacks for connection to the XVII (footnote 1). Two tape-out pairs are also provided.

The XVII's massive power transformer would look more at home in a power amplifier than in a preamp. A 250VA custom-made toroidal transformer (5" in diameter by 2¾" deep) supplies a bridge rectifier and two electrolytic filter caps. As with all Exposure products, the capacitors are custom-made. These feed four regulation stages: two for the phono section and two for the line section. A pair of large TO-3 devices supplies ±24V to the line stage. This regulated ±24V also supplies the input to the cascaded phono-stage regulators—a pair of Linear Technology 337/317 TO-220 devices—which then supply ±18V to the phono section. These rather high voltage rails assure a high input-overload margin. (The XVII is the first Exposure preamp to include a power supply in the same chassis as the audio circuitry.)

The size of the power supply is surprising considering the minimalist circuitry it drives. The line stage occupies just two 1½" by 2¼" pieces of the large circuit board. A tape output buffer—which uses just a single pair of transistors per channel—adds slightly to the parts population. Each line-stage channel comprises eight bipolar transistors, about 20 resistors, a couple polystyrene caps, and a few electrolytic caps; the last, unusually, are used as coupling caps. The complementary class-A output driver can source a significant amount of current and drive impedances as low as 1k ohm. Although this use is not recommended, the XVII can reportedly drive headphones. Line-stage input impedance is specified at a lowish 10k ohms.

The phono stage occupies a small patch of circuit board on the unit's right side. This is augmented by a removable phono board soldered between the input jacks and the phono stage. Two phono boards are available: one high-gain type for low-output moving-coil cartridges, and one low-gain for moving-magnet pickups. No gain specs are provided for the phono boards, but the gain can be inferred from the specified input sensitivity: The MM board's sensitivity of 150mV referenced to 1V output translates to just over 50dB of gain; the MC board's calculated gain is 64dB. Note that these gain figures include the XVII's line-stage gain. Phono input impedance is 47k ohms with the MM board, 470 ohms with the MC.

Only one phono board is included in the price. However, if you get a different cartridge and need the other phono board, Exposure will exchange them at no cost. The only expense would be the dealer's labor for replacing the board—about a ten-minute job.

As with the line stage, the phono section uses very few parts. All the transistors in the XVII are bipolar types. The MC board uses many transistors in parallel for lower noise. A combination of active and passive circuits provides RIAA equalization.

Other technical features include a muting relay that disconnects the main outputs if the line voltage drops or is shut off. The 100k ohms Alps volume pot is active (in the feedback loop) rather than acting as an input attenuator. The tape buffer stage, line stage, and phono sections are decoupled from each other. A line-stage–only version of the XVII—the XIX—is available for $1295. (Other than the absence of a phono stage, the XVII and XIX are identical.)

Overall, the XVII is an unusual design, with bipolar transistors instead of FETs in the phono front end, electrolytic coupling caps, and a massive power transformer for very little circuitry.

But enough of transformers and coupling caps—what really matters is how the product reproduces music.

Glitches
Before describing the Exposure XVII's sound, I must report a few problems I had with the first review sample. First, the unit produced a low-level buzz through the loudspeakers, regardless of the volume-control position or input selected. The buzz wasn't audible during music, but I could hear it when no music was playing. Lifting the preamp ground at the wall made it worse, as did grounding the preamp and floating the power amplifiers. (Note that both grounds should not be lifted simultaneously.) Disconnecting all inputs to the unit (to break a possible ground loop) had no effect.

Second, the XVII put out a very loud hum through the loudspeakers after the preamp had been transported to Santa Fe for measurement. I traced the problem to a cold-solder joint on one of the large filter caps. I resoldered the joint from the top of the board and was back in action. However, after I moved the XVII a second time, the other solder joint on the same capacitor failed in the identical manner. Note that the large filter caps don't appear to be secured mechanically to the printed circuit board or chassis; they're held in place by the soldered leads and a small dab of glue. A nylon tie might be a better idea here.

After these problems appeared to have been fixed, I continued with the review. My impressions were less than positive; the XVII sounded grainy, hard, lacking in depth, and the unit overlaid music with an opacity that diminished the musical experience. The XVII's bench performance also featured levels of power-supply noise that were much higher than usual. After a preprint of the full text of the review had been sent to Exposure for comment (Stereophile's standard procedure), Exposure North America's Casey McKee suggested that the review sample had been damaged. The loose filter caps—which, according to Exposure, don't come loose unless the unit takes a severe jolt—indicated that the XVII had been dropped in shipping. Finally, the review sample's poor measured performance (high noise and the presence of power-supply–related noise) also indicated that the unit wasn't working correctly.

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Footnote 1: I learned the origin of the term "XLR" from Exposure's John Farlowe: The three-pin XLR connector was originally designed to carry a stereo audio signal, not a monaural balanced signal. Pin one was ground, or "X"; the other two pins were "L" for "left channel," "R" for "right channel."