Field Test: LaChapell Audio Model 992 Tube Preamp

Dec 1, 2006 12:00 PM, By Michael Cooper

ROBUST DESIGN WITH OPTIONAL EXTENDED GAIN

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When inventor Scott LaChapell set out to design the LaChapell Audio Model 992 tube preamplifier, his primary goal was to highlight the sonic character of the rich-sounding 6072 vacuum tube as much as possible. In addition to the two 6072s placed in the input stage for each of the two channels, the company decided to use two 12AU7s in each channel's output stage because of the latter tube's transparency and resistance to overloading. That decision, along with choosing a transformerless mic input topology with very high-input impedance, really put the 6072's sound in the spotlight. LaChapell added Jensen JT-11 output transformers, instrument inputs and a host of other refinements — including breathtaking cosmetic touches — and the dual-channel, all-tube 992 was born.

The downside to using 6072s on the inputs without a transformer was less-than-spectacular gain range. A “standard” 992 ($3,495 list) provides only about 50 to 53 dB of gain, which may not be enough for some applications. For these types of situations, LaChapell also offers an Extended-Gain 992 ($3,895) that substitutes four ECC803/12AX7 tubes, along with a Jensen JT-115k input transformer for the Standard model's four 6072s. I chose the standard 992 for review.

PAINTING A PRETTY PICTURE
The 2U, rackmountable 992 is built like a tank and looks gorgeous. The front panel of its highly ventilated chassis has a deep-red, powder-coated finish and identical controls for each channel. Large, matte-gray aluminum knobs serve input and output gain pots. Large, backlit Sifam VU meters show output levels. Sturdy toggle switches activate the following functions independently for each channel: meter on/off, 48-volt phantom power, 20dB pad, polarity reversal and mute. An additional toggle turns on the power. All toggle switches have red LEDs on their tips that light when their associated functions are active.

The reason for the meter on/off switches is to prolong the meters' life — 0 VU is calibrated to +4 dBu. The meters use isolation amplifiers to preclude adding inter-modulation distortion to the audio signals. The channel-mute switches prevent speakers from popping when you flip phantom power on or off.

High-impedance instrument inputs on ¼-inch jacks are also accessible from the front. This input stage is placed before that for mic signals and uses a Jensen JT-DB-EPC transformer. It has up to 33 dB of available gain and cannot make use of the 20dB pad. All mic input and line output connections are via rear panel XLRs. A channel's XLR input is automatically shut off by a relay whenever a plug is inserted into its corresponding instrument jack.

TEST POSITIVE
My first test of the 992 was recording a strummed Santa Cruz OM acoustic guitar captured with a spaced pair of DPA 4011 condensers. The sound was stunning: sparkly, rich, big and clear, with a tightly controlled bottom end. Compared to the sound of my Millennia HV-3D preamp, the 992 gave a slightly understated reproduction of low-midrange frequencies that enhanced this instrument's clarity; no EQ was needed.

The 992's subtle low-midrange scoop proved to be inappropriate for a female vocalist who had a natural peak in the 3 to 5kHz range. For her, a darker-sounding preamp was needed. On an electric guitar cabinet miked with a Royer R-122 (and adding an LA-2A to the audio path), the 992 sounded wonderfully full-bodied and brimming with subtle nuances. I also got stellar results using the 992 with an R-122 on large wind chimes and Native American wood flutes.

On a rock session, the 992 was used on snare drum (miked with a Shure SM57) with an SPL Transient Designer TD4 placed downstream. The track had lots of body and sizzle, and a deep bottom end that provided the necessary punch.

The 992 sounded killer on male background vocals. On a particular country song, I needed an especially lush texture from only one backing vocal. A Lawson L251 (in omni mode) routed through the 992 and my LA-2A gave me the colorful, sweet, bigger-than-life sound I was after.

The 992's instrument inputs provided plenty of gain for DI'd Strat and passive electric bass, as long as the trims on my downstream Apogee Rosetta were maxed. Both tracks sounded clear, yet had rich overtones that added interesting character. However, the standard 992 didn't provide enough gain for me to get the bass guitar to burp. For an aggressive, overdriven bass track, the Extended-Gain 992 would probably be a better choice.

CLEAR WINNER
I have just a few gripes: My review unit sported a confusing mirror-image control layout, but LaChapell noted that, as of the next production run, all units would present toggle switches for channel 2 in the same left-to-right order as for channel 1. Titles for the I/O gain knobs will also be moved to more easily viewed positions; my review unit had this screening placed underneath the knobs, where it was hard to read. The 992 is expensive, but its opulent sound, robust build quality and breathtaking cosmetic touches justify the price. This is not just another me-too tube preamp; it's the real deal.

LaChapell Audio, 209/383-3486, www.lachapellaudio.com.

Field Test-LaChapell Audio Model 992
Note: Stated specifications are typical measurements, but subject to change without notice. All measurements taken while driving a 40k-ohm load with a 1khz tone.

Standard 992
Internal Gain: 0 to >50 dB
THD+N @ 50dB gain: <0.25 percent
Frequency Response: 15 Hz to 110k Hz
Output Gain: +24 dBu
RMS Noise, 20dB Gain: -89 dBu
Input Impedance: 8.2k-ohm
Tube Complement: four 6072, four 12AU7
Weight: 26 pounds
Extended-Gain 992
Internal Gain: 0 to >74 dB
THD+N @ 50dB gain: <0.25 percent
Frequency Response: 10 Hz to 100k Hz
Output Gain: +24 dBu
RMS Noise, 20dB Gain: -91 dBu
Input Impedance: 1.4k-ohm
Tube Complement: four ECC802/12AX7, four 12AU7
Weight: 26 pounds






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