Brown Audio Labs SP-1B Preamplifier

Affordable and Damned Good

Equipment report
Solid-state preamplifiers
Brown Audio Labs SP-1B
Brown Audio Labs SP-1B Preamplifier

Nestled in the North Texas hill country, the city of Graham is home to Denny Brown and Brown Audio Labs (BAL). Denny says that he started BAL with the intention of focusing on tube designs, a technology he realizes that for most non-audiophiles “would be considered archaic, ancient, and just plain all washed-up.” His main reason was sound quality, because to his ears tubes capture harmonics and timbre more naturally than do transistors. Now that’s a design philosophy I can totally subscribe to. With a background in electronics and a huge interest in music, both as a performer and a listener, it was only a matter of time before Denny started building his own tube gear, and eventually selling it to other music lovers. It should be noted that BAL is a one-man show, as befits high-end audio’s artisanal roots. It has been said that small operations have the benefit of being nimble. In Denny’s case, it has allowed him to pivot swiftly toward interesting designs. The SP-1B’s design evolved during the summer of 2015, and my review sample represents the final design iteration as of autumn of that year. What captured my attention big-time about the SP-1B was a combination of its circuit topology and its price.

The front panel bears a slight resemblance to some of the classic Audio Research preamps, but otherwise is rather spartan control-wise. Controls consist of an input selector, volume pot, power and mute switches. Inputs on the rear panel comprise one mm phono, three line-level, and one MP3. At an asking price of $850 you can’t expect the chassis to be littered with exotic passive parts. What is surprising though is the consistent use of good-quality parts. However, the volume control is a notable exception. It’s an inexpensive 250K stereo pot that doesn’t track particularly well at low volume settings. But sonically I’m not sure that it’s actually inferior to say, an Alps motorized pot. (Note that BAL will be offering higher-end volume control upgrade options.) Alas there’s no balance control, a feature I’m rather fond of in a preamp given that speaker drivers are rarely matched any better than 1 to 2dB.

The power supply is built around a pair of 50VA toroidal transformers. The additional transformer is needed to satisfy the filament current demand of the tube regulator. Oh yes, that’s not something you see every day. In fact, I would venture to say that this level of power-supply sophistication is unheard of at this price point. A 6080 dual triode is connected in parallel and used as the series-pass voltage regulator. The current through the 6080 is controlled by a 12AT7 dual triode connected as a cascode. The purpose of the regulator is twofold: to reduce residual AC hum from the AC smoothing filter, and to maintain a constant DC voltage (within limits) as the load demand and AC power line vary. The results are a stable DC operating point and an exceptionally low noise floor. Signal-tube filament voltages are rectified using fast-recovery Schottky diodes and are tightly regulated using an integrated circuit voltage regulator.

The phonostage design closely resembles the vintage Dynaco PAS circuit with a few tweaks, the result of subjecting the design to circuit-simulation software. The obvious difference is the use of the 6SL7 dual triode instead of the ubiquitous 12AX7 workhorse. This is essentially an octal version of the 12AX7 with similar electrical characteristics but slightly lower gain. Its larger plate structure tends to add a bit of sonic character. Properly deployed, I find the 6SL7 to offer a more dynamically nuanced presentation than the 12AX7. The line section is a mu-follower using two 6SN7 sections per channel. The mu-follower is a totem-pole topology in which a cathode follower sits on top of a common-grounded cathode gain stage. It combines desirable performance aspects such as high gain, low distortion, immunity from power supply ripple, and low output impedance. No wonder that it has become a popular circuit for audio applications. You would expect the gain of a 6SN7 mu-follower to approach a 6SN7’s maximum possible gain or mu of about 25dB. But in this case, negative feedback is used to lower the line section gain to about 16.5dB. This also lowers the output impedance to about 350 ohms measured at 100Hz. My sample was shipped with vintage 6SN7s, but future production will most likely default to new production tubes, probably Russian-reissue Tung-Sols.

My initial listening sessions were conducted with line-level inputs using digital source material. I found the SP-1B’s presentation to be both detailed and dynamic. The big-tone bravado I expect from and love about the 6SN7 was very much in evidence. Once experienced, it’s a sonic attribute that I find hard to walk away from. It’s first and foremost about tonal balance for me, a priority that my mentor, the late J. Gordon Holt, instilled in me many years ago. Distortion levels were also exceptionally low. There was none of the upper-octave grain and brightness that seem to afflict most entry-level tube preamps. That would be the sort of “industrial-strength” treble range that etches transients and is sometimes mistaken for enhanced resolution. But the real shocker was the phono section, or more precisely, the combination of the phono and line sections. They fit together sonically like pieces of a jigsaw puzzle and pulled the curtains back on the soundstage to a degree I’ve only experienced with much more expensive preamps. Transparency galore! After 30 years of audio reviewing, I’m not often surprised, but the SP-1B’s phono input performance definitely did the trick—it put on a show. And it did so to an extent that qualifies as a “wow” moment in my book. My very first such moment was the QUAD ESL-57’s reproduction of human voice. It redefined for me what a loudspeaker was capable of. In the case of the SP-1B, it destroyed my expectations of what was sonically possible for an affordable preamp. First and lasting impressions were of spectacular dynamics, suave midrange textures, and excellent low-level resolution. What more could you ask of an $850 preamp?

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