Bryston B-60R integrated amplifier Measurements

Sidebar 2: Measurements

The Bryston B-60R was hot, but not alarmingly so, following its one-hour, 1/3-power preconditioning test. The B-60R's line input impedance measured 51.6k ohms at 1kHz, a comfortably high value that should be compatible with any consumer program source. The Bryston's output impedance is very low—under 0.04 ohms up to 1kHz, increasing to 0.06 ohms at 20kHz. The output impedance at the B-60R's tape outputs measured 49.5 ohms and was virtually unchanged with changes in source impedance, indicating buffered tape outputs. The output impedance at the preamp outputs measured 72 ohms and the preamp section's gain (full volume setting) is 4.8dB, making the B-60R suitable for use as a preamp, perhaps for future use in driving a more powerful, outboard amplifier.

DC offset at the main outputs measured 4.9mV in the left channel, 3.7mV in the right. Signal/Noise Ratio (ref. 1W into 8 ohms), with the level control set at 12:00, measured 79dB from 22Hz to 22kHz and 74dB from 10Hz to 500kHz, both unweighted, and 84dB A-weighted (all values to the nearest decibel). (The corresponding values at the full setting of the level control are 63dB, 63dB, and 71dB, respectively.) The Bryston is noninverting, a positive-going input resulting in a positive-going output. Its volume-control tracking is good: within 0.27dB over the range measured (9:00 to 3:00). The B-60R's voltage gain at the maximum setting of the level control is 34dB. All of the following measurements were taken at this maximum setting.

Fig.1 shows the frequency response of the B-60R. The response is virtually unchanged as the load impedance changes, due in large part to the amplifier's low output impedance. Fig.2 indicates the Bryston's output with a 10kHz squarewave input, a fine result with a fast risetime and only a slight rounding of the waveform's leading edge. The Bryston's 1kHz squarewave response, not shown, is near textbook-quality.

Fig.1 Bryston B-60R, frequency response at (from top to bottom at 6kHz): 1W into 8 ohms, 2W into 4 ohms, and 2.83V into simulated speaker load. (0.5dB/vertical div., right channel dashed.)

Fig.2 Bryston B-60R, small-signal 10kHz squarewave into 8 ohms.

Fig.3 shows the B-60R's crosstalk, a good though not exceptional result. The slight inconsistency between the channels is unlikely to have audible consequences. The increase at higher frequencies is typical of most two- (or more) channel components, and is generally due to capacitive coupling between channels.

Fig.3 Bryston B-60R, crosstalk, L-R (top) and R-L (bottom) (10dB/vertical div.)

The THD+noise percentage vs frequency results in fig.4 are outstanding, and are typical of Bryston amplifiers I have measured in the past. It is also typically difficult to get a readable THD waveform from this manufacturer's products at low power, and the B-60R was no different. Fig.5 shows the THD+noise output waveform of the Bryston with a 1kHz input and a high 40W level into a 4 ohm load. There is a hint of third-order harmonic dominance plus some higher-order harmonics, but noise is the principal component even at this high output level. (The distortion waveform into 8 ohms was even harder to find, and into 2 ohms the amplifier would shut down at the levels required to get a usable result before the waveform could be captured. No harm resulted from these shutdowns; the Bryston resumed operation after cooling down for a minute or so.)

Fig.4 Bryston B-60R, THD+noise vs frequency at (from top to bottom at 20kHz): 4W into 2 ohms, 2W into 4 ohms, 1W into 8 ohms, and 2.83V into simulated speaker load (right channel dashed).

Fig.5 Bryston B-60R, 1kHz waveform at 40W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 shows the spectrum of the Bryston's output with 50Hz reproduced at a level of 60W into a 4 ohm load. All of the artifacts here are below -90dB (0.003%). Fig.7 shows the intermodulation in the output caused by a combined 19+20kHz signal at 53W into 4 ohms. All artifacts with this input are below -80dB (0.01%), which was also true of the 8 ohm result (at 36W, not shown).

Fig.6 Bryston B-60R, spectrum of 50Hz sinewave, DC-1kHz, at 60W into 4 ohms (linear frequency scale).

Fig.7 Bryston B-60R, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 53W into 4 ohms (linear frequency scale).

Fig.8 shows the THD+noise vs power curves for the B-60R, one channel driven, at 1kHz. The discrete clipping measurements for the B-60R (1% THD+noise at 1kHz), to the nearest watt, are shown in Table 1. With continuous tones, the Bryston marginally exceeds its power rating into 8 ohms, but falls just a few watts shy into 4 ohms (at the power-line voltage used in the test).

Fig.8 Bryston B-60R, distortion (%) vs continuous output power into (from bottom to top): 8 ohms, 4 ohms, and 2 ohms.

JA used the Miller Audio Research "Amplifier Profiler" to check the Bryston's output power on tonebursts, which are more typical of a music signal. Using a 10-cycle-on, 40-cycle-off burst with a fundamental frequency of 1kHz, and plotting the THD+noise against the output power gave the traces shown in fig.9. Into 8 ohms (black trace) just over 100W is available on this toneburst signal, this increasing to 170W into 4 ohms (red) and 265W into 2 ohms (green). Into the punishing 1 ohm load, however, the voltage had dropped sufficiently that the output power was only just greater than it was into 2 ohms (blue trace). At lower powers, the distortion into this extreme load was also significantly higher than into higher impedances. However, repeating this test into a reactive load (8 ohms plus -45 degrees phase angle) gave a result very similar to the black trace in fig.9, implying the Bryston will work well with real-world loudspeakers.

Fig.9 Bryston B-60R, distortion (%) vs dynamic output power into 8 ohms (black), 4 ohms (red), 2 ohms (green), and 1 ohm (blue).

The Bryston B-60R is not truly happy driving a low impedance loads—no surprise for such a compact integrated amplifier. But in every respect that matters for most applications, it turned in a fine set of test-bench measurements.—Thomas J. Norton

Table 1 Bryston B-60R: Clipping (1% THD+noise at 1kHz)

Both Channels DrivenOne Channel Driven
W (dBW)W (dBW)
8 ohms67 (18.2)67 (18.3)67 (18.3)
4 ohms92 (16.6)93 (16.7)93 (16.7)
2 ohms95 (13.8)
677 Neal Drive
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