November 2008

Bryston BP 6 C-Series Preamplifier: Measurements

All  preamplifier measurements are performed independently by BHK Labs. Please click to learn more about how we test preamplifiers there. All measurement data and graphical information displayed below are the property of SoundStage! and Schneider Publishing Inc. Reproduction in any format is not permitted.

• Measurements were made at 120V AC line voltage and on the left channel unless otherwise noted.
• This preamplifier does not invert polarity.
• AC line current draw: 0.12A.
• Input impedance at 1kHz: 47.0k ohms.
• Output impedance at 1kHz: 75 ohms.
• Gain, volume at maximum:
  • Instrument loading, Lch/Rch = 3.366X, 10.5dB / 3.371X, 10.6dB
  • IHF loading, Lch/Rch = 3.341X, 10.5dB / 3.346, 10.6dB
• IHF Sensitivity, input volts for standard IHF output of 0.5V, IHF loading:
  • Lch/Rch = 149.7mV / 149.4mV
• Output noise versus bandwidth and volume-control position, wideband/A weighted in µV, input termination 1kHz, Lch | Rch:
  • At maximum: 132.1µV / 7.5µV | 132.8µV / 8.3µV
  • Unity gain: 132.8µV / 8.4µV | 138.5µV / 8.4µV
  • At minimum: 133.1µV / 9.0µV | 133.5µV / 9.0µV

General

The Bryston BP 6 C-Series line-level preamplifier, like so many units these days, is of a simple, basic design with a minimum of extra features.

A note on the measurements of this unit: Both channels measured very close to each other in all respects with the exception of the volume-control tracking, which, while better on some other preamps tested, was still quite acceptable.

Chart 1 shows the frequency response with the volume control set for unity gain for 0.5V input with instrument loading. In this chart, both channels are shown. The effect of the IHF loading is essentially negligible. The two channels are tracking within 0.4dB at this point on the volume control. The bandwidth of this unit (as defined by the -3dB points) is less than 10Hz to about 90kHz. Further, because the frequency response did not change noticeably with the volume set at maximum down to -70dB of attenuation, there is no Chart 2 to show these variations. Volume-control tracking between channels was within about 0.7dB down to -70dB of attenuation.

Chart 3 shows how total harmonic distortion varies with input level and frequency for both IHF and instrument loading with the volume control set for unity gain. In this design, there was no difference with instrument or IHF loading, a desirable attribute of circuit performance.

A spectrum of the distortion and noise residue of a 1kHz test tone at 0.5V output with IHF loading is plotted in Chart 4. AC-line-hum harmonics here are relatively low. At this output level, the signal harmonics are so low as not to be visible.

Both channels shown with instrument loading
Red line = left channel
Blue line = right channel

N/A

Instrument or IHF loading
Red line = 20Hz and 1kHz
Blue line = 20kHz

Balanced output

IHF loading
Red line = spectrum of 1kHz test signal distortion and AC-line harmonics at 0.5V input and output at unity gain.

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