Preamplifiers

Bryston BP17 Stereo Preamplifier

ARTICLE INDEX

The Design

The BP17 is fully balanced from input to output, meaning that each channel actually has two preamplifier circuits, with one path inverted. The two signals are combined at the output, and any electrical interference that gets into the circuitry along the way is cancelled out. I see more and more preamps and power amps in a true balanced configuration, and this is good.

All inputs use relays instead of the less expensive, but noisier, CMOS switch. Analog and digital circuits (digital in the BP17DA) have their own power supply for isolation of the high frequencies in digital signals from the audio band signals in the analog circuit.

Chris Russell, Chief of Engineering at Bryston, describes the circuit design: "The signal comes in through the selector relays, to discrete input buffers. The input buffers have a gain of 6 dB. (That approach places the gain before the volume control to reduce output noise, and the input buffers have +/- 24 V of headroom in two polarities, a total of +/- 48 V, so there’s no chance of overload.) There are in fact two buffers per channel, one inverting and one non-inverting, resulting in the signal being in balanced mode internally. The signal goes to the volume control chips (integrated resistor ladder networks) in differential mode because doing so results in a roughly 10X reduction in distortion. After the volume control chips, the signal routes through a discrete differential amplifier (which cancels any even harmonics that the volume control chips could contribute), to output buffers which are unity gain, then to the output connectors. With respect to the balanced XLR output connectors, the - output is routed through a discrete inverting unity gain buffer."

The inputs are all RCA (single-ended), so after the signal comes into the BP17, it is converted to a balanced signal by an inverter, so that one path continues on non-inverted, and the other path along side is inverted, the two paths being the balanced circuit.

On the front panel, from the left, are buttons to select the input, and if you purchase the BP17DA, you can select one of the digital inputs. Two balance buttons are off by themselves next to the volume control. There is also a convenient headphone jack, which is nice, because headphones, and vinyl for that matter, are making a huge comeback. Speaking of vinyl, the BP17P has a phono stage.

The chassis is very attractive with a thick silver-colored brushed aluminum front plate.

The rear panel has a mirror image arrangement of the inputs and outputs for each channel. The AC receptacle is three-prong grounded.

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