Solid State Preamp Reviews

I measured the Bryston BP-17³'s performance with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). The maximum gain for both the balanced and unbalanced inputs to the balanced outputs, and from the headphone output, was 17.5dB. The maximum gain from unbalanced input to unbalanced output was 11.6dB; ie, an input of 1V results in an output of 3.785V.

The BP-17³'s input impedance from 20Hz to 20kHz was 6800 ohms unbalanced and 10k ohms balanced, both impedances very slightly higher than specified. The preamplifier preserved absolute polarity (ie, was non-inverting) with both balanced and unbalanced inputs and from all outputs. Its XLR jacks are wired with pin 2 hot, the AES convention. The preamplifier's unbalanced output impedance was a relatively low 690 ohms from 20Hz to 20kHz; unusually, the balanced impedance was lower, at 340 ohms at 1kHz and 20kHz, and slightly higher at 20Hz, at 420 ohms. The headphone output impedance was a little on the high side for use with lower-impedance headphones, at 72 ohms.

My measurements confirmed the BP-17³'s frequency-response specification of 20Hz–20kHz, ±0.05dB. In both balanced (fig.1, blue and red traces) and unbalanced modes with its volume control set to its maximum, the output was down by just 0.05dB at 20kHz, and was –3dB at just over 100kHz. The BP-17³'s superb channel matching was preserved into lower impedances (fig.1, cyan and magenta traces) and at lower settings of the volume control. This control had a rather aggressive action, its 12:00 position reducing the gain by 32dB from its maximum!

The channel separation was superb below 2kHz, at >108dB in both directions, decreasing to a still very good 80dB at 20kHz. The BP-17³ is a very quiet preamplifier: the audioband signal/noise ratio ref. 1V output, measured with the input shorted to ground but the volume control set to its maximum, was 89.7dB (average of both channels). Switching an A-weighting filter into circuit slightly improved this ratio, to 92dB. Spectral analysis of the Bryston's low-frequency noise floor indicated some spuriae at 60Hz and its odd-order harmonics in the left channel (fig.2, blue trace), though these are all at or below –99dB (0.001%) and won't be audible.

Fig.3 plots the percentage of THD+noise in the Bryston's balanced output into 100k ohms. The THD+N rises below 15V output due to the fixed level of noise becoming an increasing percentage of the signal level. The actual distortion doesn't rise above the noise floor until the output reaches 15V, when it is just 0.0005%, but rises rapidly above that level. This is of no relevance in actual use, as 15V is way more than needed to drive any power amplifier completely into clipping. When I reduced the load impedance to the current-hungry 600 ohms (fig.4), the BP-17³ still delivered >15V at clipping (ie, when the THD+N reaches 1%). The distortion was a little higher for unbalanced input to unbalanced output: the BP-17³ clipped at >15V into 100k ohms, and at a still-high 7V into 600 ohms.

I measured how the BP-17³'s distortion changed with frequency at a very high level, to be sure that the reading was not dominated by noise. It remained consistently low throughout the audioband into the high 100k ohm load (fig.5, blue and red traces), and was even lower into 600 ohms (cyan, magenta), though it rose slightly at the frequency extremes. At the same high output level into 600 ohms, the spectrum of the distortion comprised the third and fifth harmonics (fig.6), but these are very low in level, at a respective –110 and –120dB. At the same level into 100k ohms, these harmonics were at the residual level in the generator's output. Intermodulation distortion at a typical level at which the preamplifier will be used was unmeasurable (fig.7), though the power-supply harmonics in the left channel can be seen. Even at a peak signal level of 10V into 600 ohms (fig.8), the second-order difference product at 1kHz lay at just –112dB (0.00025%)!

The Bryston BP-17³'s measurements indicate that it is superbly well engineered. It is difficult to see how a preamplifier could perform any better on the test bench!—John Atkinson