Cayin A-50T integrated amplifier Measurements

Sidebar 3: Measurements

The Cayin A-50T has two output transformer taps, 4 ohms and 8 ohms, and two modes of output-stage operation: Triode and Ultralinear. I performed a full set of tests in each mode from both sets of output taps, but discuss only a representative selection here. Before performing any measurements, I checked the bias on each of the four output tubes; it varied from 322mV to 332mV, all well within the recommended range.

The maximum voltage gain into 8 ohms was lower than usual for an integrated amplifier. Even in the higher-gain Ultralinear mode, it was 32.5dB from the 8 ohm tap, 30.6dB from the 4 ohm tap. Triode mode reduced the gain from both taps by 2dB. This won't be an issue with modern line-level sources, such as CD players. The low gain, however, meant that even in the worst case, with the input shorted but the volume control set to its maximum, the Cayin's signal/noise ratios (ref. 1W into 8 ohms) were excellent, at 68dB (unweighted, wideband) and 98dB (A-weighted). All modes and taps preserved absolute polarity. The input impedance was a little lower than specification, ranging from 74k ohms at 20Hz to 59k ohms at 20kHz, but these figures are still usefully higher than average. Channel matching was excellent, but channel separation (not shown) was good rather than great, at >70dB L–R and >80dB R–L below 2kHz.

As is usual for a traditional tube design, the A-50T's output impedance was high. Even in the optimal condition of the 4 ohm tap in Triode mode, this was 1.6 ohms across the band; from the 8 ohm tap in Ultralinear mode, it was a very high 3.3 ohms. As a result, the modification of the amplifier's frequency response by the interaction between this source impedance and the modulus of impedance of the loudspeaker load was large, at ±2dB with our standard simulated loudspeaker from the 8 ohm tap in Ultralinear mode (fig.1, magenta trace). Even with the 4 ohm tap in Triode mode (not shown), the response variation was an audible ±1dB. But into resistive loads, the Cayin's small-signal response was commendably flat, with a high-frequency rolloff that didn't begin until well above the audioband (fig.1, red and blue traces). As a result, the A-50T's reproduction of a 1kHz squarewave was superb for a tube design (fig.2). This amplifier obviously uses well-engineered output transformers, though in the worst case—8 ohm tap into higher impedances—a trace of a well-damped ultrasonic resonance can be just seen with a 10kHz squarewave (fig.3).

Fig.1 Cayin A-50T, Ultralinear mode, 8 ohm tap, frequency response at 2.83V into: simulated loudspeaker load (magenta), 8 (red), 4 (blue) ohms (1dB/vertical div.).

Fig.2 Cayin A-50T, Triode mode, 8 ohm tap, small-signal 1kHz squarewave into 8 ohms.

Fig.3 Cayin A-50T, Triode mode, 8 ohm tap, small-signal 10kHz squarewave into 8 ohms.

With only a pair of EL34 tubes per channel, the A-50T is not going to be a powerhouse. But taking the clipping point as our usual 1% THD+noise point, the amplifier did more than meet its specified output powers—35Wpc (15.3dBW) in Ultralinear mode, 16Wpc (12dBW) in Triode mode—with the output transformer matched to the load in Ultralinear mode or set to half the load impedance in Triode mode. Figs.4–7 show how the THD+N percentage changes with output power with each of the four operating mode/tap combinations. That the A-50T uses a low amount of loop negative feedback is revealed by the way the measured distortion increases linearly with power, though this is more evident in Triode than in Ultralinear mode. Distortion levels are generally higher in Triode mode, but this mode does appear to maintain its performance with loads that are increasingly lower than the nominal transformer tap value.

Fig.4 Cayin A-50T, Ultralinear mode, 8 ohm tap, distortion (%) vs 1kHz continuous output power into (from bottom to top at 10W): 4, 8, 2, 16 ohms.

Fig.5 Cayin A-50T, Ultralinear mode, 4 ohm tap, distortion (%) vs 1kHz continuous output power into (from bottom to top at 10W): 4, 8, 2, 16 ohms.

Fig.6 Cayin A-50T, Triode mode, 8 ohm tap, distortion (%) vs 1kHz continuous output power into (from bottom to top at 10W): 8, 16, 4, 2 ohms.

Fig.7 Cayin A-50T, Triode mode, 4 ohm tap, distortion (%) vs 1kHz continuous output power into (from bottom to top at 10W): 8, 16, 4, 2 ohms.

With high impedances, operating the Cayin in Ultralinear mode (which I note was BJR's preference) gives distortion levels that, other than in the top octave and below 100Hz, are more typical of a solid-state design. Again, however, the amplifier in Ultralinear mode can be seen to be increasingly nonlinear as the load impedance drops, particularly from the 8 ohm tap (fig.8). In Triode mode, the Cayin is less linear overall, but again the rise in THD at the frequency extremes is evident, to levels that, with the lowest loads, might have audible consequences (fig.9).

Fig.8 Cayin A-50T, Ultralinear mode, 8 ohm tap, THD+N (%) vs frequency at 2.83V into: 16 (blue), 8 (red), 4 (magenta), 2 (green) ohms.

Fig.9 Cayin A-50T, Triode mode, 8 ohm tap, THD+N (%) vs frequency at 2.83V into: 16 (blue), 8 (red), 4 (magenta), 2 (green) ohms.

However, the actual distortion is composed almost entirely of the relatively innocuous low-order harmonics (fig.10). At low frequencies, the third harmonic predominates in Ultralinear mode (fig.11) but is the same level as the second harmonic in Triode mode (fig.12)—but the odd-order harmonics drop rapidly in the midrange, leaving the second the dominant harmonic, even in Ultralinear mode (fig.13). With the reduction in linearity in the top octave and above, it was not surprising to find that the A-50T didn't fare particularly well in the demanding high-frequency intermodulation test (fig.14). It is fair to note, however, that this graph was taken with the amplifier operating at a power level just below visible clipping on the oscilloscope screen.

Fig.10 Cayin A-50T, Triode mode, 8 ohm tap, 1kHz waveform at 1W into 48 ohms (top), 0.105% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.11 Cayin A-50T, Ultralinear mode, 4 ohm tap, spectrum of 50Hz sinewave, DC–1kHz, at 10W into 4 ohms (linear frequency scale).

Fig.12 Cayin A-50T, Triode mode, 4 ohm tap, spectrum of 50Hz sinewave, DC–1kHz, at 10W into 4 ohms (linear frequency scale).

Fig.13 Cayin A-50T, Ultralinear mode, 8 ohm tap, spectrum of 1kHz sinewave, DC–1kHz, at 10W into 8 ohms (linear frequency scale).

Fig.14 Cayin A-50T, Triode mode, 8 ohm tap, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 9W peak into 8 ohms (linear frequency scale).

I was impressed both by the Cayin A-50T's build quality and by its measured performance, though it definitely works best when the nominal loudspeaker impedance is equal to or greater than the output transformer tap used. Despite its affordable price, it obviously uses well-engineered output transformers. In a sense, it represents what a tube amplifier from the Golden Age would perform like if it were made using modern materials and parts, yet it costs less in today's devalued dollars than classic tube amps cost 40 years ago.—John Atkinson

COMPANY INFO
Zhuhai Spark Electronic Equipment Co.
US distributor: VAS Industries
1 Bethany Road, Building 1, Suite 16
Hazlet, NJ 07730
(732) 888-3288
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