Sidebar 3: Measurements
Like the lower-power Burmester 216, which Jim Austin reviewed in January 2024, the Burmester 218 can be operated as a conventional two-channel amplifier or, by bridging the two output stages with the supplied adapter cable, as a monoblock. I performed a complete set of measurements in both modes on one of the review samples (serial number 2180142) with my Audio Precision SYS2722 system, repeating some of the testing with the magazine's higher-performance APx555 system. I preconditioned the 218 by following the CEA's recommendation of running it in stereo mode at one-eighth the specified power into 8 ohms for 30 minutes. At the end of that time, the temperature of the top panel was 95.6°F (35.3°C), that of the side panels 105.9°F (41.1°C).
In stereo mode, the Burmester inverted absolute polarity, indicating that the balanced inputs are wired with pin 2 negative, the opposite of the AES standard. However, in mono mode with the positive speaker input taken from the left channel binding post, as indicated in the manual, the 218 preserved absolute polarity. The 218's input impedance in stereo mode was a low 1850 ohms from 20Hz to 20kHz. (Burmester specifies the balanced input impedance as 1.9k ohms.) It was half that value in mono mode, as the two inputs are now connected in parallel. The voltage gain at
1kHz into 8 ohms in stereo mode was close to the specified 34.2dB, at 34.0dB. The gain in mono mode increased by 5.9dB.
Footnote 1: See fig.3 here.
Fig.1 Burmester 218, mono mode, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (blue), 4 ohms (magenta), and 2 ohms (red) (1dB/vertical div.).
Fig.2 Burmester 218, mono and stereo modes, small-signal 10kHz squarewave into 8 ohms.
The output impedance in stereo mode was a low 0.065 ohms from 20Hz to 20kHz. Even though the two output stages are in series, the output impedance in mono mode was only slightly higher. As a result, the variation in the frequency response with our standard simulated loudspeaker (fig.1, gray trace) was minimal. The response into resistive loads was flat in the audioband, not reaching –3dB until >200kHz into 8 ohms (blue) and 4 ohms (magenta), and at 160kHz into 2 ohms (red). With its wide small-signal bandwidth, the Burmester's reproduction of a 10kHz squarewave into 8 ohms featured very short risetimes in both modes (fig.2), with no overshoot or ringing.
Fig.3 Burmester 218, mono mode, spectrum of 1kHz sinewave, DC–1kHz, at 1Wpc into 8 ohms (linear frequency scale).
Channel separation in stereo mode (not shown) was >70dB in both directions below 1kHz. In stereo mode, the unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms), taken with the input shorted to ground, was an excellent 81.2dB in the left channel, 77.2dB in the right. These ratios improved to 91.2dB in both channels when the measurement bandwidth was restricted to the audioband, and to 93dB when A-weighted. The ratios were similarly excellent in mono mode. Spectral analysis of the low-frequency noisefloor while the Burmester drove a 1kHz tone at 1Wpc into 8 ohms in mono mode revealed that while power supply–related spuriae at 60Hz and its harmonics were measurable, these all lay at or below –104dB (fig.3).
Fig.4 Burmester 218, stereo mode, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.5 Burmester 218, stereo mode, distortion (%) vs 1kHz continuous output power into 4 ohms.
Fig.6 Burmester 218, stereo mode, distortion (%) vs 1kHz continuous output power into 2 ohms.
Burmester specifies the 218's maximum power in stereo mode as 165Wpc into 8 ohms (22.17dBW), 275Wpc into 4 ohms (21.38dBW), and 400Wpc into 2 ohms (20dBW). With our usual definition of clipping, which is when the THD+noise reaches 1%, the 218 with both channels driven clipped at 170Wpc into 8 ohms (fig.4) and 272Wpc into 4 ohms (21.33dBW; fig.5), and with one channel driven, at 480W into 2 ohms (20.8dBW; fig.6).
Fig.7 Burmester 218, mono mode, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.8 Burmester 218, mono mode, distortion (%) vs 1kHz continuous output power into 4 ohms.
In mono mode, the Burmester clipped at 550W into 8 ohms (27.4dBW, fig.7) and 790W into 4 ohms (26dBW, fig.8), both of which are close to the specified maximum powers in this mode of 565W into 8 ohms (27.52dBW) and 785W into 4 ohms (25.94dBW). I didn't examine the power at 1% THD+N into 2 ohms, as the amplifier isn't specified into this load in mono mode.
Fig.9 Burmester 218, mono mode, THD+N (%) vs frequency at 15.5V into: 8 ohms (blue), 4 ohms (magenta), and 2 ohms (red).
Even at high powers, the distortion below waveform clipping in figs.4–8 was very low. I examined how the percentage of THD+N varied with frequency at 15.5V, equivalent to 30W into 8 ohms, 60W into 4 ohms, and 120W into 2 ohms in both stereo (not shown) and mono (fig.9) modes. The THD+N percentage was higher into 2 ohms (red trace) than it was into the higher impedances (blue and magenta traces) but was still low. The increase in THD+N in the top audio octaves will be due to the usual limitation in open-loop bandwidth reducing the amount of corrective negative feedback available (footnote 1).
Fig.10 Burmester 218, stereo mode, 1kHz waveform at 30W into 8 ohms, 0.002% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.11 Burmester 218, stereo mode, spectrum of 1kHz sinewave, DC–10kHz, at 30W into 8 ohms (linear frequency scale).
Fig.12 Burmester 218, mono mode, 1kHz waveform at 30W into 8 ohms, 0.0003% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.13 Burmester 218, mono mode, spectrum of 1kHz sinewave, DC–10kHz, at 30Wpc into 8 ohms (left channel blue, right red; linear frequency scale).
The distortion waveform in stereo mode was predominantly the second harmonic (figs.10 & 11) and in mono mode it was the third harmonic (figs.12 & 13). (Bridging two output stages tends to cancel even-order distortion.) Higher-order harmonics were much lower in level in both modes.
Fig.14 Burmester 218, mono mode, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 60W peak into 4 ohms (linear frequency scale).
Intermodulation distortion was also very low, the 1kHz difference product resulting from an equal mix of 19 and 20kHz tones at 60Wpc peak into 4 ohms lying at –98dB (0.0011%, fig.11). Because of the rise in distortion in the top octaves, higher-order intermodulation makes an appearance in this graph, but the highest in level of these still lay at –84dB (0.006%). This graph was taken in stereo mode; the levels of the intermodulation products with the same signal in mono mode were slightly lower.
The Burmester 218 offers high power with very low distortion and noise, though it deals slightly better with very low impedances in stereo mode than as a bridged monoblock.—John Atkinson
Footnote 1: See fig.3 here.
