Meitner Audio PRE Preamplifier Measurements

Link: reviewed by Phil Gold on SoundStage! Hi-Fi on January 15, 2024

General information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The PRE was conditioned for 30 minutes at 2Vrms at the output before any measurements were taken. All measurements were taken with both channels driven.

The PRE offers two sets of line-level unbalanced (RCA) inputs, one set of line-level balanced (XLR) inputs, one set each of unbalanced (RCA) and balanced (XLR) outputs (both always on). The PRE offers a maximum of 6dB of gain from input to output for the same input type. That is to say, if the volume is set to unity gain, an input of 2Vrms will yield 2Vrms at the output for the unbalanced input/output scenario, and the balanced input/output scenario. For the unbalanced in/balanced out scenario, 12dB gain is available. For the balanced in/unbalanced out scenario, 0dB of gain is available.

Based on the accuracy and non-repeatable nature of the channel deviation (table below), the volume control is in the analog domain, but digitally controlled. It offers between 2 and 3dB step increments for the first 12 volume steps. From steps 12 to 22, 1dB steps were measured. Beyond level 22 up to 100, the volume control offers 0.5 dB steps.  Overall gain was measured at -68.7dB for volume step one, up to +6dB at the maximum position (100). Volume channel tracking proved exquisite, ranging from 0.000dB to 0.008dB.

There is a difference in terms of THD and noise between unbalanced and balanced signals in the PRE (see both the main table and FFTs below). The balanced outputs have about 6dB more uncorrelated thermal noise, whereas using the balanced inputs yields about 10dB less THD compared to the unbalanced inputs. Unfortunately, the lower distortion is only apparent in the FFTs, because they allow averages over multiple data runs, which averages out and lowers the noise floor, making the very low distortion peaks visible. During normal real-time THD measurements, the analyzer is set to measure for 2-3 seconds (maximum) and cannot assign a THD value below the measured noise floor. This explains why in the primary table below, THD appears lower for the unbalanced input/output compared to the balanced input/output. The true THD ratio figure for the balanced configuration, based on the balanced input/output FFT, is an astounding 0.00002% (about -135dB), compared to the 0.00007% (about -123dB) or so for the unbalanced input.

Unless otherwise stated, balanced input and output was evaluated, with an input and output of 2Vrms into a 200k ohm-load, with the analyzer’s input bandwidth filter set to 10Hz to 22.4kHz (exceptions include FFTs and THD vs frequency sweeps where the bandwidth is extended to 90kHz, and frequency and squarewave response where the bandwidth is extended from DC to 1MHz).

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Meitner for the PRE compared directly against our own. The published specifications are sourced from Meitner’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 was set at its maximum (DC to 1MHz), assume, unless otherwise stated, a 1kHz sinewave, 2Vrms input and output into 200k ohms load, 10Hz to 22.4kHz bandwidth, and the worst-case measured result between the left and right channels.

*SNR measured with unbalanced in/out = 115.3dB
*SNR calculated with residual noise (volume at 0) and unbalanced in/out = 118.6dB

Our primary measurements revealed the following using the balanced line-level inputs (unless otherwise specified, assume a 1kHz sinewave, 2Vrms input and output into 200k ohms load, 10Hz to 22.4kHz bandwidth):

Frequency response

In our measured frequency-response plot above, the PRE is perfectly flat within the audioband (0dB at 20Hz and 20kHz). The PRE appears to be DC-coupled, as it yielded 0dB of deviation at 5Hz. The PRE can certainly be considered an extended-bandwidth audio device, as it is only 0.14dB down at 200kHz. In the graph above and most of the graphs 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.

Phase response

Above is the phase-response plot from 20Hz to 20kHz. The PRE does not invert polarity, and exhibited zero phase shift within the audioband.

THD ratio (unweighted) vs. frequency

The plot above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a sinewave input stimulus. The blue and red plots are for the left and right channels into 200k ohms, while purple/green (L/R) are into 600 ohms. THD values were flat across most of the audioband at 0.0001% into 600 ohms and 200k ohms, with a small rise to 0.0002% at 20kHz. This shows that the PRE’s outputs are robust and would yield identical THD performance feeding an amplifier with either a high or low input impedance.

THD ratio (unweighted) vs. output voltage

The plot above shows THD ratios measured at the output of the PRE as a function of output voltage into 200k ohms with a 1kHz input sinewave. At the 10mVrms level, THD values measured around 0.03%, dipping down to around 0.00006% at 6-8Vrms, followed by a rise to 0.0003% at around 18Vrms. The 1% THD point is reached at 20.2Vrms. It’s also important to mention that anything above 2-4Vrms is not typically required to drive most power amps to full power.

THD+N ratio (unweighted) vs. output voltage

The plot above shows THD+N ratios measured at the output of the PRE as a function of output voltage into 200k ohms with a 1kHz input sinewave. At the 10mVrms level, THD+N values measured around 0.2%, dipping down to around 0.0003% at 10-18Vrms.

FFT spectrum – 1kHz (balanced in, balanced out)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load, for the balanced inputs and outputs. We see that the signal’s second harmonic, at 2kHz, is extremely low at around -135dBrA, or 0.00002%, and subsequent signal harmonics are not visible above the -145dBrA noise floor. Below 1kHz, we can see very small peaks at 60, 120, 148, 180, and 300Hz. These peaks are all below the -130dBrA, or 0.00003%, level. This is a very clean FFT.

FFT spectrum – 1kHz (unbalanced in, balanced out)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load for the unbalanced inputs and balanced outputs. The main difference here compared to the FFT above is the higher second signal harmonic, at -125dBRa, or 0.00006%, versus the -135dBrA 2kHz peak seen when the balanced inputs are used. Noise peaks left of the signal peak are at similar levels as the FFT above.

FFT spectrum – 1kHz (unbalanced in, unbalanced out)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load for the unbalanced inputs and outputs. The same distortion profile with the higher 2kHz peaks can be seen here as with the FFT above. The common denominator is the use of the unbalanced inputs. The overall noise floor is at its lowest here, at -150dBrA.

FFT spectrum – 1kHz (balanced in, unbalanced out)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load for the balanced inputs and unbalanced outputs. The same distortion profile with the lower 2kHz peaks can be seen here as with the first FFT above. The common denominator is the use of the balanced inputs.

FFT spectrum – 50Hz

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output into a 200k-ohm load. 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. Signal-related peaks can be seen at the second (100Hz) and third (150Hz) harmonics, at an extremely low -140dBrA, or 0.00001%. Noise-related peaks are all below -135dBrA, or 0.00002%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output into a 200k-ohm load. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -125dBrA, or 0.0006%, while the third-order modulation products, at 17kHz and 20kHz, are at roughly the same level. This, like the 1kHz FFTs, is an extremely clean result.

Intermodulation distortion FFT (line-level input, APx 32 tone)

Shown above is the FFT of the output of the PRE into 200k ohms with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 2Vrms into 200k ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier. Distortion products are at a vanishingly low -140dBrA, or 0.00001%. Thus, even with a complex input signal, the PRE does not add any audible coloration to the input signal.

Square-wave response (10kHz)

Above is the 10kHz squarewave response at the output into 200k ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the PRE’s slew-rate performance. Rather, it should be seen as a qualitative representation of its relatively high bandwidth. An ideal squarewave 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 PRE’s reproduction of the 10kHz squarewave is extremely clean with sharp corners.

Diego Estan
Electronics Measurement Specialist