Sidebar 4: Measurements
I measured the MBL Noble Line N11's performance with my Audio Precision SYS2722 system (see the January 2008 "As We See It"), repeating some of the tests with a loan sample of Audio Precision's more-recent, higher-performance APx555 system. As advised in the manual, using the front-panel menu, I deactivated all the N11's inputs other than one of the balanced inputs and the unbalanced CD1 input for the measurements. I performed all the testing with the N11 set to its recommended Unity Gain mode and its volume control set to its maximum in that mode, "70." I then repeated some of the tests with the preamp set to apply its maximum gain with the volume control set to "100."
In Unity Gain mode, the maximum gain for the balanced input to the balanced output was identical, at 0.34dB. The maximum gain for the unbalanced input to the unbalanced output was similar, at 0.59dB. In Gain mode, the maximum gain was 10dB for both sets of inputs and outputs, this slightly higher than the specified 8dB. The preamplifier preserved absolute polarity (ie, was noninverting) with both balanced and unbalanced inputs and outputs. The XLR jacks are wired with pin 2 hot, the AES convention.
The N11's balanced input impedance is specified at 10k ohms, which was confirmed by my measurements at all frequencies from 20Hz to 20kHz. The unbalanced input impedance for CD1 was low, at 2.3k ohms, but this is again the specified figure. The unbalanced Aux inputs are specified as having an input impedance of 50k ohms—I activated this input just for this measurement and measured 47k ohms from 20Hz to 20kHz. The balanced output impedance was a low 198 ohms, the unbalanced impedance 100 ohms; both values were consistent across the audioband and conformed to the specification.
The preamplifier's frequency response in both balanced and unbalanced modes was flat from 10Hz to 200kHz, both into the high 100k ohm load (fig.1, blue and red traces) and into 600 ohms (cyan, magenta). Fig.1 was taken with the N11's volume control at its maximum setting; both the response and the superb channel matching were identical at lower settings of the control. Channel separation (not shown) was excellent, at >100dB in both directions below 2kHz. It decreased to 80dB at 20kHz, presumably due to low-level capacitive coupling between the channels.
I concluded my measurements of the Benchmark LA4 preamplifier in the January 2020 issue by writing "Benchmark's LA4 is the widest-bandwidth, widest-dynamic-range, lowest-noise, lowest-distortion preamplifier I have encountered." Its performance on the test bench reveals that MBL's N11 now takes that crown.—John Atkinson
Fig.1 MBL N11, Unity Gain Mode, balanced frequency response with volume control set to "70" at 1V into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta) (0.5dB/vertical div.).
From balanced inputs to balanced output, the MBL preamp offered extremely low noise, with virtually no power-supply–related spuriae in its output (fig.2). The wideband, un-weighted signal/noise ratio, measured with the balanced input shorted to ground but the volume control set to its maximum in Unity Gain mode, was a very high 93.4dB, left channel, and 97.5dB, right, both ratios ref. 1V output. Restricting the measurement bandwidth to the audioband increased the S/N ratio to an extraordinary 112.6dB for both channels, while switching an A-weighting filter into circuit further improved this ratio, to 114.4dB! Repeating the S/N ratio measurements in Maximum Gain mode, with the volume control set to "100," degraded these ratios by 2–3dB, but in either mode, this is the quietest preamplifier I have encountered. I fed the N11 a balanced 1kHz tone at 10mV and averaged 16 captures calculated with a 1.2M FFT length, the longest offered by the APx555. The FFT bins all lay below –117dB, referenced to the low signal level (fig.3), which again is extraordinary.
Fig.2 MBL N11, Unity Gain Mode, balanced spectrum of 1kHz sinewave, DC–1kHz, at 1V into 100k ohms (left channel blue, right red) and with no signal (left cyan, right magenta) (linear frequency scale).
Fig.3 MBL N11, Unity Gain Mode, AP555x measurement, balanced spectrum of 1kHz sinewave, 20Hz–20kHz, at 10mV into 100k ohms (left channel blue, right red) (linear frequency scale).
Fig.4 plots the percentage of THD+noise in the N11's balanced output into 100k ohms. The THD+N is very low between 2V and 9V. (It rises below 2V output, due to the fixed level of noise becoming an increasing percentage of the signal level.) Neither the N11's balanced nor unbalanced output clips (ie, when the THD+N reaches 1%) until a very high 10.5V. Reducing the load to a punishing 600 ohms reduced the maximum balanced output level to 8.1V (fig.5), which is still much higher than will be needed to drive a power amplifier into clipping.
Fig.4 MBL N11, Unity Gain Mode, balanced distortion (%) vs 1kHz output voltage into 100k ohms.
Fig.5 MBL N11, Unity Gain Mode, balanced distortion (%) vs 1kHz output voltage into 600 ohms.
To be sure that the reading was not dominated by noise, I measured how the N11's distortion changed with frequency at 2V output. The THD+N percentage was extremely low throughout the audioband both into 100k ohms (0.0004%, fig.6, blue and red traces) and 600 ohms (0.0005%, cyan, magenta traces). What is unusual about this graph is that the very low THD+N percentage doesn't increase toward the top of the audioband, which implies that the N11's circuit has an extremely high open-loop bandwidth.
Fig.6 MBL N11, Unity Gain Mode, balanced distortion (%) vs frequency at 2V into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta).
I examined the spectrum of the distortion at the same balanced output level (fig.7). Although the second, third, and fifth harmonics can be seen, these are still below –120dB (0.0001%), which is close to the residual level of these harmonics in my Audio Precision SYS2722's analog signal generator. I repeated the spectral analysis with the APx555 system, whose signal generator features even lower distortion: the second harmonic lay at –130dB in the left channel, –136dB in the right; the third harmonic lay at –135dB in both channels; and the fifth harmonic lay at –128dB in both channels. I offer these results for completeness's sake; these levels are so extraordinarily low that they are of academic interest only.
Fig.7 MBL N11, balanced spectrum of 50Hz sinewave, DC–1kHz, at 2V into 100k ohms (left channel blue, right red; linear frequency scale).
Tested for intermodulation distortion in balanced Unity Gain mode with an equal mix of 19 and 20kHz tones at a peak level of 2V, the second-order difference product at 1kHz lay at just –136dB, and the higher-order products all lay at or below –126dB (fig.8). Commendably, these products didn't rise at the same level into 600 ohms.
Fig.8 MBL N11, balanced HF intermodulation spectrum, DC–30kHz, 19+20kHz at 5V into 100k ohms (left channel blue, right red; linear frequency scale).
