Bryston B135 Cubed Integrated Amplifier Measurements

Link: reviewed by Philip Beaudette on SoundStage! Hi-Fi on June 15, 2021

General Information

The B135³ (Cubed) was conditioned for one hour at 1/8th full rated power (~17W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The B135³ offers six sets of line-level unbalanced (RCA) inputs; a set of fixed, line-level unbalanced (RCA) outputs; a set of variable unbalanced (RCA) pre-outs; a set of unbalanced (RCA) main-ins; and a pair of speaker outputs. Based on the accuracy of the left/right channel matching (see table below), and 0.5dB volume-step resolution throughout its range, the B135³ volume knob is not a potentiometer in the signal path, but, rather, provides digital control (analog domain) over a proprietary or integrated volume circuit.

All measurements, with the exception of signal-to-noise (SNR) or as otherwise stated, were made with the volume set to unity gain for the preamplifier (about 2 o’clock) as measured at the pre-outputs. Signal-to-noise ratio (SNR) measurements were made with the volume control set to maximum. At the unity gain volume position, to achieve 10W into 8 ohms, 310mVrms was required at the RCA line-level input.

Volume-control accuracy (measured at speaker outputs): left-right channel tracking

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Bryston for the B135³ compared directly against our own. The published specifications are sourced from Bryston’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth is set at its maximum (DC to 1MHz), assume, unless otherwise stated, a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channel.

Our primary measurements for the B135³ integrated amplifier as a whole revealed the following using the RCA line-level inputs (unless specified, assume a 1kHz sinewave, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

For the continuous dynamic power test, the Bryston able to sustain 159W into 8 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (15.7W) for five seconds, for five continuous minutes without inducing a fault or the initiation of a protective circuit. Although the peak power level for the test was just below the 1% THD+N level, the Clip indicator never light up during the high power bursts during the five-minute measurement. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the B135³ heatsinks were quite warm to the touch, where touching for more than five seconds would induce pain.

Our primary measurements revealed the following using the balanced line-level inputs at the headphone output (unless specified, assume a 1kHz sinewave, 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth):

Frequency response (8-ohm loading, line-level input)

In our measured frequency response plot above, the B135³ is perfectly flat within the audioband (20Hz to 20kHz) and beyond. These data come very close to corroborating Bryston’s claim of 20Hz to 20kHz +/-0.05dB (although Bryston claims this for the preamp section, while the graph above is for the integrated amp as a whole). The B135³ is -0.02dB at 5Hz, -0.14dB at 20kHz, and -3dB at about 70kHz. The B135³ should not be considered a high-bandwidth audio device, as the -3dB point is below 100kHz. In the chart above and most of the charts below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8ohms or 2.83Vrms) as a function of frequency, swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink is an actual speaker (Focal Chora 806, measurements can found here), and the cyan is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we find that there’s a total deviation of less than 0.1dB in the flat portion of the curve, which is an indication of a high damping factor, or low output impedance. At 20kHz, the spread is larger, at about 0.15dB. The maximum variation in RMS level when a real speaker was used as a load is very small, deviating by less than 0.1dB within the flat portion of the curve (20Hz to 10kHz), with the lowest RMS level, which would correspond to the lowest impedance point for the load, exhibited around 200Hz, and the highest RMS level, which would correspond to the highest impedance point for the load, at around 5kHz.

Phase response

Above is the phase response plot from 20Hz to 20kHz. The B135³ does not invert polarity, and the plot shows very little phase shift, with a worst case of under +10 degrees at 20kHz.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the output into 8 ohms as a function of frequency (20Hz to 20kHz) for a sinewave input stimulus. The blue and red plots are for left and right at 1W output into 8 ohms, purple/green at 10W, and pink/orange at full rated power (135W). The power was varied using the volume control. At all frequencies and power levels, THD ratios varied between about 0.0005% and 0.005%. As is typical, there is a rise in THD values at high frequencies, however, the differences (about 5dB increase from 10kHz to 20kHz) are small.  The right channel also generally outperforms the left, especially at lower frequencies, by as much as 5dB. The lowest THD ratio values were found at 135W for the right channel between 100 and 200Hz, at 0.0005%. The highest THD values were also found at high power, at about 0.007% at 20kHz also for the right channel. At 1kHz, the 10W and 135W data measured the same at just below 0.002% THD, while the 1W data is worse at 0.003/0.002% (left/right channels).

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD ratios measured at the output of the B135³ as a function of output power for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right), with a 1kHz input sinewave. The 4-ohm data shows consistently slightly higher THD values compared to the 8-ohm data (about a 2-3dB difference). At the 50mW level, THD values measured around 0.2/0.4% (8/4 ohms), dipping down to around 0.001% at 30W to 100W for the 8-ohm data, and 0.0015% for the 4-ohm data from 50W to 150W. The “knee” in the 8-ohm data occurs at around 115W, hitting the 1% THD mark at 159W. For the 4-ohm data, the “knee” occurs near 155W, hitting the 1% THD mark at 250W.

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD+N ratios measured at the output of the B135³ as a function of output power for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right), with a 1kHz input sinewave. The 4-ohm data shows consistently slightly higher THD+N values compared to the 8-ohm data (about a 2-3dB difference). At the 50mW level, THD+N values measured around 0.2/0.4% (8/4 ohms), dipping down to around 0.003% at 100W for the 8-ohm data, and 0.004% for the 4-ohm data at around 150W.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only)

The chart above shows THD ratios measured at the output of the B135³ as a function of frequency and load (8/4/2 ohms) for a constant input voltage that yields 10W at the output into 8 ohms (and roughly 20W into 4 ohms, and 40W into 2 ohms). The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find fairly consistent THD ratio values across all loads up to 1kHz (between 0.001 and 0.003%).  Above 1kHz, the spread between 8/4/2 ohms is obvious, with an increase in THD of about 5dB each time the impedance is reduced. Overall, even with a 2-ohm load at roughly 40W, THD values ranged from 0.003% at 20Hz to just above 0.01% at 20kHz.

FFT spectrum – 1kHz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the line-level input. We see that the signal’s second harmonic, at 2kHz, is at -105/-100dBrA (left/right channels), or 0.0005/0.001%, while the third harmonic, at 3 kHz, is at -100dBrA for the left channel but indistinguishable for the right channel. Odd order harmonics are obvious in the left channel, but not in the right channel, however, these are all below -100dBrA, or 0.001%.  The fourth harmonic at 4kHz is at -110dBrA, or 0.0003%. Below 1kHz, we see noise artifacts, with the 60Hz peak due to power supply noise at -95dBrA, or about 0.002%, and the 180Hz (third harmonic) peak at -100dBrA, or 0.001%. The second noise harmonic (120Hz) is only visible from the left channel, but is low at -105dBrA, or 0.0005%.

FFT spectrum – 50Hz

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W output. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant signal harmonic peak is that of the 3rd harmonic (150Hz) at about -100dBrA, or 0.001%, but only on the left channel, while the signal’s second harmonic peak (100Hz) is visible at -105dBrA, or 0.0005%, for both channels. The most predominant noise peak is at the fundamental (60Hz) at -95dBrA, or 0.005%, then at -105dBrA from the left channel only at the second harmonic (120Hz).

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus)

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is just above -110dBrA, or 0.00003%, while the third-order modulation products, at 17kHz and 20kHz are around -105dBrA, or 0.0005%. Other modulation product peaks can be seen above the -100dBrA, or 0.001%, threshold at 8, 9, and 10kHz.

Square-wave response (10kHz)

Above is the 10kHz square-wave response at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the Bryston’s slew-rate performance. Rather, it should be seen as a qualitative representation of its relatively extended bandwidth. An ideal square wave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The B135³ reproduction of the 10kHz squarewave can be considered clean, with slightly rounded edges devoid of undershoot and overshoot.

Damping factor vs. frequency (20Hz to 20kHz)

The final chart above is the damping factor as a function of frequency. Both channels show a general trend of a higher damping factor at lower frequencies, and lower damping factor at higher frequencies, with roughly a factor of 2x difference at 30Hz compared to 20kHz. Both channels’ damping factors tracked fairly closely, with a peak value of 274/309 (L/R) at around 40Hz, and a low value at 125/112 (L/R) at 20kHz.

Diego Estan
Electronics Measurement Specialist