Angela-Gilbert Yeung C312 Preamplifier

Link: reviewed by Jason Thorpe on SoundStage! Ultra on November 15, 2023

General information

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

The Angela-Gilbert Yeung C312 was conditioned for 30 minutes at 2Vrms at the output before any measurements were taken. All measurements were taken with both channels driven.

The C312 under test offers three sets of line-level unbalanced (RCA) inputs, two sets of line-level balanced (XLR) inputs, one set of unbalanced outputs, two set of balanced outputs, and a set of fixed line-level unbalanced outputs. There was no difference in terms of gain between unbalanced and balanced inputs, while there was a 6dB increase in terms of gain for the balanced outputs compared to the unbalanced outputs. There was effectively no difference in terms of THD and noise between balanced and unbalanced inputs and outputs; however, 1kHz FFTs are included in this report with all four i/o combinations for comparison purposes. The volume control does not have a numerical display. Based on the accuracy and non-repeatable nature of the channel deviation (table below), the volume control is a potentiometer operating in the analog domain.

The C312 is a very unusual preamp, as it offers three different adjustments on the front panels via three dials. These are labeled Warm, Tube S, and SS. Unless otherwise stated, measurements were made with the volume set to unity gain, using the XLR inputs and outputs, with a 2Vrms input, and the three control dials set to the same positions as were used by the reviewer Jason Thorpe (for the most part): Warm and Tube S at the 10 o’clock position (about 1/3 of full deflection), and SS at the 9 o’clock position (about ¼ of full deflection). The short description as to what these dials do is to control the gain of various stages in the preamp. If the dials are set to minimum, there is no usable output from the preamp with the volume at maximum, while the total gain measured from the preamp with all dials at maximum is an astonishing 52dB (in order to avoid clipping, a very small input signal of 10mVrms was applied). At the end of this report, an attempt was made to characterize the measured difference (if any) to the output signal that the dials have when adjusted.

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

Primary measurements

Our primary measurements revealed the following using the balanced line-level inputs (unless otherwise specified, assume a 1kHz sine wave, 2Vrms input and output into 200k-ohm load, 10Hz to 90kHz bandwidth):

Frequency response

In our measured frequency response (relative to 1kHz) plot above, the C312 is essentially flat within the audioband (0dB at 20Hz, less than -0.1dB at 20kHz). The C312 appears to be AC-coupled, as it yielded about -0.2dB 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 C312 does not invert polarity, and it yielded a worst-case -20 degrees 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 into 200k ohms, while purple/green (L/R) are into 600 ohms. THD values range from 0.0003-0.0005% from 20Hz to 200Hz, then up to 0.02% at 20kHz into 200k ohms. Into a 600-ohm load, THD ratios were nearly identical, but 2-3dB higher through most of the frequency sweep.

THD ratio (unweighted) vs. output voltage

The plot above shows THD ratios measured at the output of the C312 as a function of output voltage into 200k ohms with a 1kHz input sinewave, with the volume set to maximum. At the 10mVrms level, THD values measured around 0.1%, dipping down to around 0.0004% at 5-6Vrms, followed by a rise to 0.0007% at the “knee,” at around 12Vrms. The 1% THD point is reached at 13.5Vrms. 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 C312 as a function of output voltage into 200k ohms with a 1kHz input sinewave, with the volume set to maximum. At the 10mVrms level, THD+N values measured around 1%, dipping down to around 0.0015% at 12Vrms.

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 -100dBrA, or 0.001%, while the third harmonic, at 3kHz, is much lower at -125dBrA, or 0.00006%. Higher order harmonics are non-existent above the -130dBrA noise floor. Below 1kHz, we can see power-supply-related noise peaks at the fundamental (60Hz) and second harmonic (120Hz) at -110dBrA, or 0.0003%, and higher harmonics at -115dBrA, or 0.0002%, and below.

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 FFT is essentially identical to the balanced-in/balanced-out 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. Again, the FFT is essentially identical to the balanced-in/balanced-out FFT above.

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. Yet again, the FFT is essentially identical to the balanced in/balanced out FFT above.

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 most predominant non-signal peaks are from the power-supply-related noise peaks at 60/120Hz at -110dBrA, or 0.0003%. The second (100Hz) and third (150Hz) signal harmonics are very low at -125dBrA, or 0.00006%.

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 -110dBrA, or 0.0003%, while the third-order modulation products, at 17kHz and 20kHz, are at -120/-115dBrA, or 0.0001/0.0002%. This is a clean IMD result.

Intermodulation distortion FFT (line-level input, APx 32 tone, 24/96)

Shown above is the FFT of the speaker-level output of the C312 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the preamplifier and below the -120dBrA, or 0.0001%, level. This is another clean IMD result. The peaks that reach the -110dBrA level at lower frequencies are not IMD products but power-supply-related noise peaks.

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 C312’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 C312’s reproduction of the 10kHz squarewave is clean, with only mild softening in the corners.

What do the Warm, Tube S, and SS control dials do?

Each dial controls the gain in different stages of the preamp. The Warm dial provides the most significant changes in gain: from -42dB to +27.8dB (with the other two dials held at the 9 o’clock position). Both the Tube S and SS dials varied the gain from about -10dB to +15dB (in each case with the other two dials held at the 9 o’clock position). The effects of changing each dial, while maintaining the other two dials at the 9 o’clock position, were explored. When varying the dial positions, we found no appreciable changes in terms of: frequency response, phase, crosstalk, and output impedance. Because each dial affects gain, we predictably found changes in terms of noise and distortion (and IMD). In terms of the dials yielding differences in noise and distortion from one dial to the other, we found the effects of varying Tube S and SS to be essentially identical, while Warm yielded more distortion, with more high frequency harmonics.

We first explored changing the dials while maintaining low distortion, and below are the 1kHz FFTs with each dial at the 3 o’clock position, while maintaining the other two dials at the 9 o’clock position. In each case, an input voltage of 1Vrms was maintained and an output of 2Vrms (using the volume control). We found that with the Warm dial set to 3 o’clock, there was more distortion with a clear peak at the third harmonic (3kHz) at -110dBrA, or 0.0003%, that was not there when Tube S and SS were set to the same position. Having the Warm dial set to the 3 o’clock position did yield less noise compared to when Tube S and SS were set to the same position; however, this may be due to having the overall volume set to a lower position. There was absolutely no difference in the 1kHz FFTs between Tube S and SS set to the 3 o’clock position.

FFT spectrum—1kHz (Warm at 3 o’clock)

FFT spectrum—1kHz (Tube S at 3 o’clock)

FFT spectrum—1kHz (SS at 3 o’clock)

We then explored changing the dials to achieve high distortion (~5% THD). This was done with a baseline of maintaining all dials at the 12 o’clock position with a 2Vrms input, and 2Vrms output, then adjusting one dial at a time to achieve 5% THD, all the while adjusting the overall volume to maintain 2Vrms at the output. We also included a scope capture to display the shape of the 1kHz waveform. In addition, we show an FFT and scope capture for the scenario where Warm is set to maximum. We found that the Warm dial yields “harder” clipping, which can be seen in the distorted peaks of the sinewaves compared to when Tube S and SS were adjusted to yield 5% THD. Once again, no differences were seen between Tube S and SS in the 5% THD scenario.

FFT spectrum—1kHz (all dials at 12 o’clock—the baseline)

FFT spectrum—1kHz (Warm causing 5% THD)

Scope—1kHz (Warm causing 5% THD)

FFT spectrum—1kHz (TUBE S causing 5% THD)

Scope—1kHz (TUBE S causing 5% THD)

FFT spectrum—1kHz (SS causing 5% THD)

Scope—1kHz (SS causing 5% THD)

FFT spectrum—1kHz (Warm at maximum)

Scope—1kHz (Warm at maximum)

Because adjusting Tube S and SS seemed to yield identical results, we explored this further by maintaining Warm at the 10 o’clock position and alternating between Tube S at maximum with SS at minimum, and vice versa, while maintaining the input at 1Vrms and the output at 2Vrms. Below you will find FFTs for a 1kHz sinewave, IMD (CCIF, 18+19kHz, 1:1) and 32-tone, as well as frequency response. We also explored (not shown) IMD (SMPTE, 60Hz+7kHz, 4:1), crosstalk, phase, and output impedance. With the exception of a very small difference in frequency response, there were absolutely no differences between Tube S and SS adjustments. With the Tube S set to maximum, there was a small dip at very low frequencies (-0.2dB at 5Hz), whereas with SS set to maximum, we measured 0dB at 5Hz. This would be inaudible.

FFT spectrum—1kHz (Tube S maximum, SS minimum)

FFT spectrum—1kHz (Tube S minimum, SS maximum)

FFT spectrum—IMD (Tube S maximum, SS minimum)

FFT spectrum—IMD (Tube S minimum, SS maximum)

FFT spectrum—32-tone (Tube S maximum, SS minimum)

FFT spectrum—32-tone (Tube S minimum, SS maximum)

Frequency response (Tube S maximum, SS minimum)

Frequency response (Tube S minimum, SS maximum)

Diego Estan
Electronics Measurement Specialist