The Bryston 11B/12B Pre-Amplifier is a high-fidelity audio component designed to provide exceptional sound quality and versatile control for a comprehensive domestic sound system. Engineered for musical accuracy and long-term reliability, it aims to deliver a clean, transparent, and dynamic audio experience.
Function Description
The Bryston 11B/12B serves as a central signal preamplifier, managing various audio sources and routing them to power amplifiers and recording devices. Its core function is to switch between different inputs, provide necessary gain, and offer volume and balance control without introducing noise or distortion.
The Model 12B features a phono stage with an internal moving-coil (MC) step-up transformer, offering a 22.5dB voltage gain and optimized input impedance for most MC cartridges. This transformer can be bypassed via a front-panel switch for moving-magnet (MM) cartridges, which utilize dedicated MM inputs with approximately 275 pF of input loading capacitance (adjustable by special order). The phono stage in the Model 12B, including the MC transformer, inverts absolute phase, ensuring proper phase alignment. For MM cartridges, phase can be corrected by reversing the cartridge leads.
The Model 11B is identical to the 12B except it lacks the MC transformer, making both phono inputs MM inputs. An MC transformer can be retrofitted to convert an 11B to a 12B.
The preamplifier includes a sophisticated tape monitor function that allows for flexible recording and monitoring. It supports two tape recorders, enabling dubbing from Tape Deck 1 to Tape Deck 2 (when "Tape 1" is selected on the TAPE OUTPUT switch) or vice versa (by reversing connections and selecting "Tape 2"). This design also facilitates the integration of external audio processors like equalizers or noise reducers into the tape loop.
A dedicated headphone jack provides convenient private listening. When headphones are plugged in, the preamplifier automatically disconnects the direct feed to the power amplifier, indicated by a change in the front panel pilot light from green to red. This ensures that headphone listening does not disturb others.
A special remote output, switched via the "Main Output" selector but exiting ahead of the volume control, provides a line-level signal for driving amplifiers in remote locations with their own local volume controls. This allows independent volume control for remote systems. Optionally, this can be replaced by a second set of main outputs for controlling two amplifiers simultaneously (e.g., for subwoofers or bi-amplified systems).
Important Technical Specifications
- Phono Stage (Model 12B):
- MC Step-Up Transformer: 22.5dB voltage gain, optimized input impedance for most MC cartridges.
- MM Input Capacitance: Approximately 275 pF per channel (adjustable by special order).
- Low Filter:
- Inflection Point: 31.7Hz.
- Slope: 6dB/octave (minimum type).
- Effectiveness: Reduces subsonic woofer-wobble and intermodulation effects, particularly in the 2-5Hz range, by an average of 20dB.
- Mute Switch: Reduces preamp gain by 20dB, affecting only the main outputs.
- Turn On Delay: Approximately 3-second delay via an output relay to prevent transient thumps. The relay also opens instantly when powered off and is hermetically sealed for long-term reliability.
- Accessory Outlets: Three 3-prong grounded 120V receptacles on the rear panel (two switched, one unswitched).
- Maximum Power Draw: Caution advised against plugging large power amplifiers (e.g., Bryston 3B and 4B) into these outlets to prevent damage to the preamp's power switch and potential audible problems due to fluctuating voltage. Bryston 2B-LP or Class AB amplifiers up to 50 watts per channel (or Class A up to 15 watts per channel) are permissible.
- DC Power-Supply Output: A DIN connector on the rear panel provides ±24V DC of regulated power, protected against excessive current draw.
Usage Features
- Input Selection: Comprehensive switching facilities for various sources including Phono (MC/MM), Tuner, Aux, CD, Tape 1, and Tape 2.
- Tape Monitor Function: Independent selector switches for "MAIN OUTPUT" and "TAPE OUTPUT" allow for flexible monitoring and recording from any source while listening to another.
- Two Tape Recorders & Dubbing: Easily integrate two tape decks for recording and direct dubbing between them in either direction.
- External Audio Processor Integration: Seamlessly incorporate equalizers, noise reducers, or other audio processors into the tape loop.
- Headphone Listening: Convenient private listening with automatic power amplifier disconnection. Users are advised to reduce volume before removing headphones to avoid sudden high sound levels.
- Remote Output: Provides a line-level output for remote systems with independent volume control.
- Low Filter: Activates in the 'In' position, affecting only phono inputs, to effectively reduce rumble and improve sound clarity.
- Mute Switch: Reduces gain by 20dB for temporary background listening or during record changes, affecting only the main outputs.
- Mono Function: The "Mute" switch also connects the two channels in Mono, aiding in establishing a clear center image and adjusting balance. Users should reduce volume before deactivating the mute switch to avoid a sudden increase in sound level.
- Balance Control: Tailored inflection type with gradual action near the center for precise left-right balance adjustments.
- Power-On Delay: A 3-second delay prevents transient thumps during startup, protecting connected equipment.
- Pilot Light: A green pilot light indicates normal operation; it changes to red when the output relay is open (e.g., during headphone use or power-off decay).
Maintenance Features
- Robust Construction: Engineered for many years of trouble-free operation.
- Hermetically Sealed Output Relay: Ensures nearly infinite lifetime and prevents contact contamination.
- Power Supply Protection: The DC power-supply output is fully protected, with a circuit that opens the output relay automatically if excessive current is drawn.
- Fuse Protection: The AC line fuse is designed for user replacement with the same type and rating.
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