Balanced Audio Technology VK-P10 phono preamplifier Measurements

Sidebar 3: Measurements

The BAT VK-P10 was measured as delivered: set up for a typical low-output moving-coil. All measurements were made from the balanced inputs to the balanced outputs.

The BAT's voltage gain measured 55dB (1kHz). S/N (ref. 1V) was 64dB from 22Hz to 22kHz, 61.2dB from 10Hz to 500kHz (both unweighted), and 78.7dB A-weighted. Pin 2 of the balanced configuration was positive. The input impedance was 46.7k ohms in the left channel, 47.2k ohms in the right.

Fig.1 shows the VK-P10's frequency response/RIAA error into the Audio Precision System One's 100k ohm load. Except for a relatively insignificant rolloff in the bass beginning at 60Hz, it is very flat. The output impedance of the VK-P10 measured 1885 ohms (L) and 1894 ohms (R). These figures are relatively high. Normally I would recommend that, with such an impedance characteristic, the input impedance of the next device in the chain—in this case, most likely a line-level preamp—should be at least 10 times this value, or more (which would rule out a number of otherwise good preamps). BAT takes exception to this, arguing that the important design characteristics are the ability of the device of high output impedance to deliver current, and the size of its output coupling capacitor. The VK-P10, they argue, has good current capacity although its use of a 1µF output cap means that a preamplifier with a minimum input impedance of 10k ohms should be used to avoid premature bass rolloff.

Fig.1 BAT VK-P10, RIAA error into 100k ohms (0.5dB/vertical div., right channel dashed).

To check this, I ran several additional tests. The preamp's 1kHz gain drops to 42.7dB into a 600 ohm load, and to 32.3dB into a 150 ohm load—both figures inadequate for a low-output moving-coil cartridge. The VK-P10's output impedance at 50Hz measured 6.2k ohms. While it would deliver 2V into a 150 ohm load (sufficient to drive almost any power amp into clipping), it took an input of 65mV for it to do so, which resulted in a THD+noise reading of 10.9% at 1kHz.

A load of 150 or even 600 ohms is relatively unusual, however, rendering the above figures of mainly academic interest. To see what happens to the frequency response of the VK-P10 into moderately low but real potential loads, I loaded it down with 5k ohms, 10k ohms, and 20k ohms. Since we at Stereophile can do this only single-ended, the measurements shown in fig.2 for these loads were taken across one leg of the balanced output. (The slight added rolloff above 20kHz was due to this single-ended tap, not the loading.) As you can see, there is measurable rolloff in the bass with any of these loads. The rolloff with 20k ohm at the output is very slight, but the audible effect of any of the lower loads will depend very much on the capabilities of your loudspeakers and the type of program material you listen to.

Fig.2 BAT VK-P10, RIAA error (from top to bottom) into 100k ohms, 20k ohms, 10k ohms, 5k ohms (all single-ended); 600 ohms, and 150 ohms (balanced) (2dB/vertical div.).

Notice the effect of 150 ohm and 600 ohm loads, however (both of these measurements were taken fully balanced). I would definitely not recommend using this preamp into such low-impedance loads. As noted above, though such loads are rare, they do exist (if rarely in tube products).

The crosstalk in fig.3 is very low, even with the typical increase at high frequencies.

Fig.3 BAT VK-P10, crosstalk (from top to bottom): R-L, L-R (10dB/vertical div.).

The THD+noise vs frequency result shown in fig.4 is good. Two input levels are shown, each of them far higher than you'll ever see with a low-output moving-coil. But, as with most phono preamp measurements, a high-level input is required to minimize the effect of noise on the reading. The minimum noise reading was at an input of 24mV; such a high level often results in an unrealistically high THD+noise result at high frequencies. At a 10mV input, still very high for a low-output moving-coil, you can see the HF distortion starting to come down and the distortion at lower levels increasing—the latter result due simply to noise.

Fig.4 BAT VK-P10, THD+noise vs frequency at 10mV at 1kHz into 100k ohms (bottom above 10kHz) and at 24mV at 1kHz (right channel dashed).

Fig.5 shows the THD+noise percentage plotted against output voltage at 1kHz. The minimum level corresponds to an input of 24mV—the same input used above for the THD+noise vs frequency graph (and for the crosstalk measurement). This result, obtained with both channels connected to the BAT's inputs and outputs, was essentially unchanged whether one or two channels were driven. After all the measurements were completed, I noted that with only one channel connected, the distortion minimum occurred at a lower output voltage (5V) and was lower in level (under 0.1%).

Fig.5 BAT VK-P10, distortion (%) vs output voltage into 100k ohms at 1kHz.

In fig.6 I have plotted the spectral response of the VK-P10 to a 1mV, 50Hz input. This input was not pre-equalized—1mV at 50Hz is a very high input for a high-gain phono stage. The most significant artifacts are the second harmonic and a noise component at 180Hz—both of them at a low -75dB (0.017%).

Fig.6 BAT VK-P10, spectrum of 50Hz sinewave, DC-1kHz, at an input level of 1mV (linear frequency scale). Note that the second harmonic at 100Hz is the highest in level at -75dB (0.017%).

Finally, overload (1% THD+N) for the VK-P10 with an unequalized source was reached at an input of 58mV at 1kHz (an overload margin of 41.3dB referred to the standard MC output level of 500µV), 81mV at 20kHz (24.2dB), and 7.1mV at 20Hz (43dB)—all very good results.

The measurements for the BAT VK-P10 were uniformly good. As for the highish output impedance and the limited size of the unit's output coupling capacitor discussed above: As always, it will be a problem only if care is not taken to match it with the characteristics of the rest of the system. I would certainly not expect it to be a problem when used with BAT's own associated equipment.—Thomas J. Norton

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