Accuphase C-2850 Preamplifier Measurements

Link: reviewed by Doug Schneider on SoundStage! Hi-Fi on July 15, 2021

General information

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

The Accuphase C-2850 was conditioned for 30 minutes at 2Vrms at the output before any measurements were taken. All measurements were taken with both channels driven.

The C-2850 (as tested) is an analog line-level preamp offering several balanced (XLR) and unbalanced (RCA) inputs and outputs, and a headphone output (¼″ TRS). The volume control is implemented using a proprietary process Accuphase calls “Accuphase Analog Vari-gain Amplifier (AAVA).” This system works by converting the incoming analog signal from a voltage to a current in 16 weighted steps. Each step is digitally controlled and switched in or out of the circuit depending on the encoded position of the volume knob. The current from each step switched into the circuit is summed and converted back to a voltage. The 16 circuit steps are analogous to on/off bits, and therefore, the volume system allows for 65536 (2¹⁶) discrete positions. Accuphase has configured the volume control to provide 251 steps ranging from -95dB to 0dB. Between -95 and -85dB, step sizes are 5dB; between -80 and -74dB, 3dB; -74 to -60dB, 2dB; -60 to -50dB, 1 dB; -50 to -30dB, 0.5dB; -30 to -8, 0.2dB; and finally between -8 to 0dB, 0.1dB. Considering both the exquisite channel tracking (see table below) and the variable, ultra-fine adjustments, this may be the finest digitally controlled analog volume control available in a consumer product.

The C-2850 also offers three gain settings, both for line-level (12, 18, and 24dB) and for the headphone output (Low, Mid, and High). The preamp gain setting affects the headphone gain, where Low is -10dB relative the preamp setting, Mid is 0dB, and High is +10dB. This means there are nine possible gain settings for the headphone amp: 2, 8, 12, 14, 18, 22, 24, 28, and 34dB. Unless otherwise stated, all measurement data below were taken with the 12dB gain setting for the preamp, and the Mid gain setting for the headphone amp.

When using the unbalanced and balanced inputs and outputs, the C-2850 provides the same gain regardless of combination. That is to say, with the volume set to unity gain, if I fed 2Vrms into the unbalanced input, I measured 2Vrms at the unbalanced and balanced outputs. If I fed 2Vrms into the balanced input, I measured 2Vrms at the unbalanced and balanced outputs. It’s also important to highlight that Accuphase assigns pins 2/3 on their XLR connectors as inverting/noninverting, which is the opposite to what we typically find in North-American or European products. For example, if I fed an unbalanced input and measured phase at the balanced output, it was 180 degrees out-of-phase. To compensate for this, Accuphase provides a polarity-inverting switch on the front panel, which was tested and flips the polarity as advertised.

I found small differences in THD and noise between the RCA and XLR inputs and outputs for the same output voltage. The RCA outputs exhibited about 11dB (unweighted) more noise than the XLR outputs, while the RCA inputs (when measured at the XLR outputs) measured slightly worse in terms of THD compared to the XLR inputs (0.0005% vs 0.0003% at 1kHz). Unless otherwise stated, all measurement data below are with the balanced inputs and outputs, at 2Vrms with volume set to unity gain (-12dB). Signal-to-noise ratios (SNR) were measured with the volume at maximum position.

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 Accuphase for the C-2850 compared directly against our own. The published specifications are sourced from Accuphase’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 is set at its maximum (DC to 1MHz), assume, unless otherwise stated, a measurement input bandwidth of 10Hz to 90kHz, and the worst case measured result between the left and right channel.

* The discrepancy in balanced output impedance may be due to Accuphase specifying this value for the inverting and noninverting pins separately. Our measurement considers both inputs on the balanced connector together. Treated separately, our measurement would be halved, or 48k ohms.

Our primary measurements revealed the following using the balanced line-level inputs (unless otherwise specified, assume a 1kHz sinewave, 2Vrms output into 200k ohms load, 10Hz to 90kHz bandwidth, 12dB gain setting):

Our primary measurements revealed the following using the balanced analog input and the headphone output (unless specified, assume a 1kHz sinewave at 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth, 12dB and Mid gain setting):

Frequency response

In our measured frequency response plot above, the C-2850 is near perfectly flat within the audioband (20Hz to 20kHz). The blue/red traces are without the 10Hz filter engaged, the purple/green traces with the 10Hz filter. These data do not quite corroborate Accuphase’s claim of 3Hz to 200kHz +0/-3dB (measured down to 5Hz). While at the upper end of the frequency spectrum, the -3dB point was measured at 200kHz, at low frequencies, Accuphase’s claim would imply that the C-2850 is DC coupled, whereas our measurements indicate AC coupling. Nevertheless, at the extremes of the audioband, we measured only -0.35dB at 20Hz (-1dB with filter on) and -0.04dB at 20kHz. The C-2850 can be considered a high-bandwidth audio device. 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.

Frequency response (Compensator dial 1, 2, and 3 positions)

Above are four frequency response plots for the balanced line-level input, with the Compensator control set to Off (blue/red), 1 (purple/light green), 2 (pink/cyan), and 3 (brown/dark green). We see what appears to be conventional bass-control EQ with various degrees of gain. At position 1, just under +3dB at 20Hz, position 2 yields about +5.5dB at 20Hz, and position 3 about +8.3dB.

Phase response

Above is the phase response plot from 20Hz to 20kHz, with the Phase control disabled (blue/red) and enabled (purple/green). The C-2850 does not invert polarity, while setting the Phase control to Invert does exactly that—it provides -180 degrees of shift. Since these data were collected using the balanced input and output, there is no phase inversion. However, since Accuphase assigns pins 2/3 on their XLR connectors as inverting/noninverting, the opposite to what we typically find in North American or European products, feeding the signal into an unbalanced input and measuring on the balanced output would yield the exact opposite of what is shown above.

THD ratio (unweighted) vs. frequency

The chart above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a 2Vrms sine-wave 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 are very low, near 0.0001% around 50-60Hz 20Hz, and around 0.0003-0.0004% through most of the audioband. The worst-case THD values are at 20Hz (0.001%) and 20kHz (0.001% into 600 ohms and 0.0007% into 200k ohms). Overall, the 600 and 200k-ohms load THD data are nearly identical.

THD ratio (unweighted) vs. output voltage at 1kHz

The plot above shows THD ratios measured at the output of the C-2850  as a function of output voltage into 200k ohms with a 1kHz input sine wave. At the 10mVrms level, THD values measured around 0.003%, dipping down to around 0.00009% at 0.4Vrms. The “knee” occurs at around 7Vrms, hitting the 1% THD just past 8Vrms.

THD+N ratio (unweighted) vs. output voltage at 1kHz

The plot above shows THD+N ratios measured at the output the C-2850 as a function of output voltage into 200k ohms with a 1kHz input sinewave. At the 10mVrms level, THD+N values measured around 0.05%, dipping down to around 0.0005% from 1.5 to 5Vrms.

FFT spectrum – 1kHz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus at 2Vrms, measured at the output into a 200k-ohm load. We see that the signal’s second harmonic, at 2kHz, is at -110dBrA or 0.0003%, while the third harmonic, at 3 kHz, is at -125dBrA or 0.00005%. Below 1kHz, we see some noise artifacts, with the 60Hz peak due to power supply noise visible at -145/-130dBrA (left/right), or 0.000006/0.00002%, and the 120Hz (second harmonic) peak just below -130dBrA.

FFT spectrum – 50Hz

Shown above is the FFT for a 50Hz input sinewave stimulus at 2Vrms 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. Here we find the second harmonic of the signal (100Hz) and the third harmonic of the signal (150Hz) at -120/-125dBrA respectively, or 0.0001/0.00006%. The worst-case power supply peak is at 120Hz measuring just below -130dBrA, or 0.00003%.

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

Shown above is an FFT of the intermodulation (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 and just above -120dBrA, or 0.0001%.

Square-wave response (10kHz)

Above is the 10kHz square-wave 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 C-2850’s slew-rate performance. Rather, it should be seen as a qualitative representation of its high bandwidth. An ideal square wave 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 C-2850’s reproduction of the 10kHz square wave is squeaky clean, with very sharp edges devoid of undershoot and overshoot.

Diego Estan
Electronics Measurement Specialist