Bryston BP25-MC preamplifier
From left to right, front-panel controls include a Tape/Source toggle switch, the infrared sensor for the remote, and an MM Phono/MC Phono toggle switch. Next is a rotary control to select input sources, which includes settings for Bal 1, Bal 2, Phono, CD, Tuner, Video (for laserdisc), and Disc. At panel center is a rotary balance control. To the right of this is the motor-driven volume control, followed by toggle switches for Polarity/Invert and Mute/Normal. (These switches change settings each time they're switched up or down-rather than muting, for example, only when Mute is pushed down.) The tricolor LED glows red when the preamplifier is not muted or inverting, yellow when non-muted and inverting, and red when muted.
The BP-25MC's build quality and circuit layout are superb. Almost all components are directly mounted on the main system circuit board extending the full width and depth of the chassis. Signals from the unbalanced jacks travel from an input board to this main board via two large ribbon cables. The main board features solder mask and extensive component designators. A written table at pcb center bears the signatures of those installing the capacitors and semiconductors, and the date the system board was completed. Many individual components are used, including very-high-quality film capacitors in a number of places. Looking at the system board from above, I saw no point-to-point wiring.
The BP-25MC's internal power supply—situated at the right when viewing the chassis from the front—seems to be rugged and well-designed. It employs a discrete bridge rectifier (four 1N4004 diodes) feeding two 4700µF electrolytics. Three 3-pin IC regulators are mounted on individual heatsinks, two of them (a 7824 and a 7924) supplying ±24V to the electrolytics, while the third (a 7808) supplies voltage for the logic to control muting on turn-on. Final filtration is accomplished on the low-voltage regulated side by 2200µF electrolytics. A separate source for the power-on muting system shorts all inputs until the preamplifier has settled. Additional 2200µF electrolytic capacitors provide further filtration at key points in the preamplifier, including the phono stage.
The BP-25MC's built-in phono section occupies the left side of the chassis: two matching circuit areas, one per channel, run the depth of the chassis side by side. Two metal-shielded cans, transformers for the moving-coil input, sit just behind the front panel. (These transformers are identical to the company's TF-1 outboard transformer.) The BP-25MC can be ordered with two different gain configurations: the higher configuration of 22.5dB gain, with the transformer wired in series; or 16.5dB gain, with the transformers wired in parallel for a lower source impedance. Behind the transformers sit two identical circuit boards for the moving-magnet cartridges, these including the RIAA equalization circuitry. Total gain for moving-coil cartridges at mid-frequency through the entire phono section of the BP-25MC is 56.5dB.
The line amplification for the BP-25MC is provided by operational amplifier chips, three per channel. Two of these op-amps are to buffer the signal to the tape output and level control, the remaining four for the two balanced outputs. These integrated circuits are specified as delivering high output with low noise. The high 24V rails supplying the op-amps mean that the preamp is relatively resistant to overload.
Listening to the BP-25MC
My interest in the Bryston BP-25MC preamplifier first centered on its remote control—it allowed me to adjust volume during casual listening sessions. This proved to be addictive, as my listening position was 12' from the preamplifier. I soon discovered that the infrared sensor has a narrow angle of reception at that distance, and will not respond if signaled from more than 45 degrees off-axis. To make certain the preamp responded, I turned it so that one corner overhung the edge of the shelf. I also found that when the preamplifier was left on for long periods of time without use, it sometimes went into mute; the remote's Mute and Phase pushbuttons then had to be cycled repeatedly to wake the BP-25MC from Mute.
Even with these minor frustrations, the Bryston BP-25MC proved to be a superb preamplifier for reviewing. The volume-control settings provided stable and reliable output, essential for product comparisons. When reviewing the D/A processors mentioned in the Sidebar, I made a light pencil-mark on the front panel next to the rotary volume control to record the approximate setting needed to produce the same voltage out (read by a multimeter) at the speaker terminals when playing a tone from a test CD. These settings produced the same voltage, within 100mV, each time the appropriate D/A processor was put into the system.
Initial impressions of the BP-25 MC, obtained by listening through its phono section, was that it leaned toward a warm, somewhat distant presentation with an open-sounding top end. As time went on, this sonic signature became clearer and more distinct, particularly in comparison with other preamplifiers at my disposal. With the Quad ESL-63s and Totem Model 1s, which emphasize midrange character, the BP-25 had a warm, smooth midrange. The Snell Type A Reference system revealed the preamp's open, unrestrained top end.