YBA 2 preamplifier Measurements part 2

More important than the level of any distortion present is its spectral content: high, odd-order harmonics—seventh, ninth, eleventh—are much more audible than low harmonics, such as the second or third. (I illustrate this phenomenon on Stereophile's Test CD 2: tracks 21-23 allow you to compare the audibility of various amounts of pure second-, third-, and seventh-harmonic distortion superimposed on a 1kHz tone. Even 0.1% of seventh is easily audible, whereas 1% of second is not.) To look at the YBA 2's distortion signature, I drove it hard into 100k ohms with a 50Hz sinewave; the spectrum of its output is shown in fig.5. You can see from this graph that the innocuous second harmonic, at 100Hz, is dominant, just reaching the -60dB, or 0.1%, level. (The total distortion content at this level was a hair above 0.1%, as can be seen from fig.4.) There is some third-harmonic visible, at -80dB (0.01%), but all other harmonics are below the FFT-plot's noise-floor "grass."

Fig.5 YBA 2, spectrum of 50Hz sinewave, DC-1kHz, at 1.9V into 100k ohms (linear frequency scale). Note that the second and third harmonics, at 100Hz and 150Hz, are the highest in level, 60dB and 80dB below the level of the 50Hz fundamental (0.1% and 0.01%, respectively).

The main output impedance measured a low 37 ohms at 1kHz and 20kHz, but a much higher 450 ohms at 20Hz, perhaps due to a too-small DC-blocking capacitor in series with the output. While this should not present any problems with power amplifiers having high input impedances, if the amplifier's input impedance is significantly lower than 10k ohms, the YBA 2's low frequencies will sound too lean. The tape outputs appear to be unbuffered, meaning that recorders should either be left on or disconnected, to avoid any adverse loading effects. If there were any DC offsets present on the main outputs, they were below the ability of my meter to read.

The MM phono stage featured excellent overload margins, these measuring more than 24dB across the audio band. The overload level (for 1% THD) at 1kHz was 82.5mV; this equates to an output level from the tape output of 4.33V—almost twice as high as the specification would imply. As the MC transformer module plugs right into the YBA 2's phono input jacks, its source impedance is irrelevant. Its measured voltage gain was 25.5dB, and at typical MC output levels, its distortion was dominated by the third harmonic. This, though, was at a low -74dB (0.02%), even at high levels of low frequency (fig.6). (Note the low-level 60Hz component in this graph, due to the transformer picking up some hum.) When I hit the module with 1mV of 50Hz, however, the distortion increased, the third harmonic reaching -50dB (0.3%). Though this is a very high input level at this frequency—remember that the RIAA pre-emphasis reduces the low-frequency content of the groove information by up to 20dB—this behavior implies that it might be possible for recorded warp-wow information, which can consist of high levels of low frequencies, to overload the transformer cores.

Fig.6 YBA 2 MC module, spectrum of 100Hz sinewave, DC-1kHz, at high (500;uV) input level (linear frequency scale). Note that the third harmonic predominates, at -74dB (0.02%).

Fig.7 YBA 2 MC module, spectrum of 50Hz sinewave, DC-1kHz, at very high (1mV) input level (linear frequency scale). Note that the odd-order harmonics predominate and are much higher in level than in fig.6.

All in all, although not all my measurements match the optimistic specifications, the YBA 2 appears to be a very well-engineered preamplifier—as I might have expected from its sound quality.—John Atkinson

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