Sonic Frontiers SFL-2 Preamplifier Measurements

Link: written about by Jason Thorpe on SoundStage! Ultra on October 1, 2024

General information

The Sonic Frontiers SFL-2 was released in about 1995. The SFL-2 under test here was owned by SoundStage! writer Jason Thorpe. He bought it in the late 1990s and had used it in his system until late 2023. He’s since sold it to a consumer. But before he sold it, we wanted to measure it to gauge its current level of performance. It was measured in March 2024. Jason reports that the tubes at time of measurements had under 2000 hours on them, which he felt meant they had plenty of life left since they were rated for about 10,000 hours.

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

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

The SFL-2 offers five sets of line-level unbalanced (RCA) inputs, one set of line-level balanced (XLR) inputs, and two sets each of unbalanced and balanced outputs. The balanced outputs offer 6dB more gain than the unbalanced outputs. The volume control is a stepped attenuator with 23 positions (including the minimum position, which is grounded). Based on the accuracy and repeatable nature of the channel deviation (table below), the volume control is in the analog domain but must have been, at one time, carefully level matched between channels at each volume position due to the high level of accuracy. The stepped attenuator offers, for the most part, 3dB increments throughout the entire range. There’s an additional 0dB/-1.5dB switch that can enable 1.5dB of attenuation, for a finer adjustment.  

There is a significant difference in terms of THD between unbalanced and balanced inputs and outputs in the SFL-2 (see both the main table and FFTs below). The lowest THD configuration measured was balanced in/balanced out, although the left channel exhibits much higher THD than the right channel. Unless otherwise stated, measurements were made with the volume set to unity gain, using the XLR inputs and outputs, with a 2Vrms input.

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

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Primary measurements

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 SFL-2 is essentially perfectly flat within the audioband, and 0dB at 5Hz and -1.5dB at 200kHz. The SFL-2 appears to be DC-coupled, as it yielded 0dB of deviation at 5Hz. 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 blue/red traces are with the phase switch set to 0 degrees, the purple/green traces are with the phase switch set to 180 degrees. The SFL-2 does not invert polarity (except when the phase switch is set to 180 degrees), and it yielded a worst-case 5 degrees or so of phase shift at 20kHz.

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 left and right channels into 200k ohms, while purple/green (L/R) are into 600 ohms. As previously mentioned, there are gross differences in THD ratios between the left and right channels into 200k ohms. The right channel ranged from 0.003% at 20Hz, down to 0.0004% at 1 to 2kHz, then up to 0.003% at 20kHz. It should be noted that because this is a tube-based preamp, noise levels are higher than what would be seen in a well-designed solid-state preamp. As such, the limiting factor in assigning THD ratios for the right channel into 200k ohms is largely due to the noise floor (the analyzer cannot assign a THD ratio below the noise floor). THD ratios for the left channel into 200k ohms were relatively constant through the audioband, just above 0.01%. The 600-ohm THD ratio were considerable higher, ranging from 0.03/0.1% (left/right) at 20Hz, then up to 0.3/0.4% (left/right) from 200Hz to 20kHz. The SFL-2 is significantly impacted by a lower impedance load, due to the high output impedance (about 800 ohms) tube outputs. This is also evidenced by the maximum output signals (at 1% THD) measured at the balanced outputs into 200k ohms and 600 ohms: a staggering 90Vrms versus 1.5Vrms.

THD ratio (unweighted) vs. output voltage

The plot above shows THD ratios measured at the output of the SFL-2 as a function of output voltage into 200k ohms with a 1kHz input sinewave. At the 10mVrms level, THD values measured around 0.1% at 10mVrms, dipping down to around 0.0003% at 2Vrms for the right channel and 0.003% at 0.4Vrms for the left channel. Between 1 and 30Vrms at the output, we find a 10 to 25dB difference in THD between the left and right channels. The 1% THD point is reached at around 90Vrms. 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 SFL-2 as a function of output voltage into 200k ohms with a 1kHz input sinewave. At the 10mVrms level, THD+N values measured around 1.5%, dipping down to around 0.003% at 5-7Vrms for the right channel, and 0.01% at 1-2Vrms for the left channel. Between 3 and 30Vrms at the output, we find a 10 to 25dB difference in THD+N between the left and right channels.

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 at around -80dBrA, or 0.01%, for the left channel, and only -120dBrA, or 0.0001% for the right channel. The third harmonic, at 3 kHz, is at -115dBrA, or 0.0002%, for both channels. There are no power-supply-related noise peaks above the relatively high noise floor, which varies from -100dBrA at low frequencies, down to -130dBrA at 20kHz.

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. We see that the signal’s second harmonic, at 2kHz, is at around -70/-75dBrA (left/right), or 0.03/0.02%. The third harmonic, at 3kHz, is at -110dBrA, or 0.0003%, for both channels. There are no significant power-supply-related noise peaks above the relatively high noise floor, but for a very small -110dBrA (left channel), or 0.0003%, peak at 120Hz.

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. We see that the signal’s second harmonic, at 2kHz, is at around -70/-60dBrA (left/right), or 0.03/0.1%. The third harmonic, at 3 kHz, is at -100dBrA, or 0.001%, for both channels. There are no significant power-supply-related noise peaks above the relatively high noise floor, except for the very small -110dBrA (right channel), or 0.0003%, peak at 120Hz.

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. We see that the signal’s second harmonic, at 2kHz, is at around -60dBrA, or 0.1%. The third harmonic, at 3kHz, is at -100dBrA, or 0.001%, for both channels. Using the unbalanced outputs appears to yield the worst-case THD ratios. Again, there are no significant power-supply-related noise peaks above the relatively high noise floor, except for a very small -110dBrA (right channel), or 0.0003%, peak at 120Hz.

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. The only significant non-signal peak is from the signal’s second harmonic (100Hz) at -80/-105dBrA (left/right), or 0.01/0.0006%. The third signal harmonic (150Hz) is at -115dBrA, or 0.0002%. As above, there are no significant-power-supply related noise peaks above the relatively high noise floor, except for the very small -110dBrA (left channel), or 0.0003%, peak at 120Hz.

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 -85/-95dBrA (left/right), or 0.006/0.002%, while the third-order modulation products, at 17kHz and 20kHz, are at -115dBrA, or 0.0002%.

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

Shown above is the FFT of the balanced outputs of the SFL-2 with the APx 32-tone signal applied to the analog balanced input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 2Vrms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and are around the -115dBrA level for the left channel and the -130dBrA level for the right channel (below 15kHz or so).

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 SFL-2’s slew-rate performance. Rather, it should be seen as a qualitative representation of its very 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 SFL-2’s reproduction of the 10kHz squarewave is essentially perfect, with sharp corners and no ringing.

Diego Estan
Electronics Measurement Specialist