Bryston BP-173 line preamplifier Measurements

Sidebar 3: Measurements

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!


Fig.1 Bryston BP-173, balanced frequency response with volume control set to maximum at 1V, into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta) (1dB/vertical div.).

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.2 Bryston BP-173, balanced spectrum of 1kHz sinewave, DC–1kHz, at: 2V (left channel blue, right red), 0V into 100k ohms (left cyan, right magenta) (linear frequency scale).

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.


Fig.3 Bryston BP-173, balanced distortion (%) vs 1kHz output voltage into 100k ohms.


Fig.4 Bryston BP-173, balanced distortion (%) vs 1kHz output voltage 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%)!


Fig.5 Bryston BP-173, balanced THD+N (%) vs frequency at 10V into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta).


Fig.6 Bryston BP-173, balanced spectrum of 1kHz sinewave, DC–1kHz, at 10V into 600 ohms (left channel blue, right red; linear frequency scale).


Fig.7 Bryston BP-173, balanced HF intermodulation spectrum, DC–30kHz, 19+20kHz at 1V into 100k ohms (left channel blue, right red; linear frequency scale).


Fig.8 Bryston BP-173, balanced HF intermodulation spectrum, DC–30kHz, 19+20kHz at 10V into 600 ohms (left channel blue, right red; linear frequency scale).

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

Ortofan's picture

... "superbly well engineered" and "it is difficult to see how a preamplifier could perform any better on the test bench", is it sufficient to derive the full benefit of 24-bit hi-res recordings?
A noise and distortion level of 0.001%/-100dB is barely equivalent to 16-bit resolution. Even the -120dB level only equates to about 20-bit resolution. The LSB for 24-bit encoding is on the order of a level of 0.00001%/-140dB. Does any preamp come close(r) to achieving this level of performance?

dalethorn's picture

Mind-boggling. So whatever the preamp is capable of with the best-case 1 khz test tone, you run a typical 24-bit recording through it and try to hear the preamp's contribution to the sound? No argument here, just trying to imagine how all that breaks down in the real world.

Gnib's picture

I`m very curious about this follow-up on the DAC and phono modules.

dalethorn's picture

"Cubed models employ an array of 12 active devices for the first 6dB of gain. Developed by the late Dr. Ioan Alexandru Salomie, this array acts as "a super-linear" input buffer to filter out audio- and radio-frequency noise, particularly anomalies that originate in the power line, reducing the overall noise and distortion to less than 0.001%."

Well, I thought Ortofan's questions were simple enough, but the array (no pun intended) of issues aren't going to make things easy. Ignoring any photo stages (can I?), the "array of 12 active devices" acting on the "power line anomalies" is interesting. I'd prefer to filter my power elsewhere, but who knows? Maybe these guys have tricks that their competitors don't.

allhifi's picture

Cubed's IBG (Input Buffer/Gain) circuit may be advantageous in main amplifier stages but may not be particularly suitable for preamp's 'Input' stage.

Bryston's 'Cubed' series amplifier's clearly have an articulation/ resolution capability previous series lacked.

Interestingly, the "SST" series was a MAJOR improvement over ST series, but by many accounts, the SST2 was a step back?
Apparently, changes were made to the 'Input Stage' in the SST2 -not well received by many accounts. Yet it "measured" better to Bryston ! Go figure.
Come (near) full-circle, the new "3" series employs a circuit that, interestingly, has Bryston talking 'audiophile' language previously dismissed as superfluous, imaginary -not relevant/important -or "measurable" Go figure.

I suspect IBG-circuit doesn't correlate well in line-stage applications.
While many line-stage (preamp) designer's appear to be employing lower-and-lower gain 3,6, 9 db., Bryston continues with 12-18 db.? designs.

Stick with the 3B/4B3 (Cubed) amps; quite an accomplishment for Bryston.

peter jasz

allhifi's picture

dalehorn: I suspect you are referring to Bryston's noise level/vs./source resolution ?

If so, the "breakdown" in the real-world is obvious via listening evaluations.


dalethorn's picture

Interesting - In any case, these proprietary techniques don't reveal enough to do much besides just listening for any anomalies. If I were engineering these things, I'd like to explore ways to both shape and filter noise (if that makes sense), to try to make it less apparent psychoacoustically. How that would look from a whole-system perspective I have no idea.

allhifi's picture

Indeed, what you say makes complete sense (re: noise attenuation)

It's unclear why makers don't consider using 'Balanced' power/ transformers in their designs -or AC Re-generation, as noted in ML's No.52 preamp.

The Balanced (Symmetrical) AC power supply I'm using (Blue Circle MR-800/1200) offers up excellent 'performance' with any/all components connected/powered. (I use on for 'sources', the other for amplification)

I have recently acquired an Exact Power SP-15 -sitting idle currently.
The SP-15 has a single, substantial 1.8-2KVa that may prove beneficial for main amplification -eventually to replace the 1200 watt Blue Circle.

The improvements in (particular lower/mid frequency) resolution is dramatic, and most welcome -when compared to wall AC power.

Yet, at busy AC-usage times of the day, one can easily hear the polluted line's impact upon SQ; a lumpy, slow sound. It happens regularly.

I suspect an AC Re-generator (PS Audio P-3) would work extremely well in low-power applications, particularly 'digital' gear where SQ improvements are incredible.