VAC Renaissance Signature Mk.II preamplifier Measurements

Sidebar 3: Measurements

I first looked at the VAC Renaissance Signature Mk.II's phono stage, taking the output signal from the tape-out jacks. The input impedance and voltage gain at 1kHz were 47k ohms and 43.7dB (MM setting) and 365 ohms and 64dB (MC). The RIAA error, shown in fig.1, is distinguished by a double-humped response curve with an upper-midrange valley some 0.5dB deep. This covers a wide enough frequency range to be audible. The response is sensibly curtailed above and below the audioband, reaching -3dB ref. 1kHz at 11Hz and 76kHz. Channel separation (fig.2) is good rather than great, with capacitive coupling increasing the level of crosstalk above 1kHz, other than a curious notch between 20kHz and 30kHz.

Fig.1 VAC Renaissance Signature Mk.II, phono stage RIAA error (0.5dB/vertical div., right channel dashed).

Fig.2 VAC Renaissance Signature Mk.II, phono stage channel separation ref. 10mV input at 1kHz (10dB/vertical div.).

Noise levels were low, and the overload margin at 1kHz was excellent at 28.5dB for both MM and MC inputs (ref. 5mV and 0.5mV, respectively). This worsened a little at the frequency extremes, to 16.75dB at 20Hz and 19.7dB at 20kHz (the same margins for both inputs), but this is still good measured performance. It should be noted that the phono stage "latched up" at low frequencies; ie, once it had clipped, it stayed clipped even when the driving signal was then reduced to a much lower level.

The line stage offered a maximum voltage gain of 11.7dB for unbalanced inputs measured at the unbalanced and balanced outputs, and balanced inputs measured at the balanced outputs. The input impedance at 1kHz was high at 67.5k ohms unbalanced and 61.4k ohms balanced, and the line stage inverted signal polarity via both sets of inputs, with pin 2 of the XLR source and load wired as "hot." The specification says that the preamp is non-inverting. The output impedance was 149 ohms at 1kHz from both balanced and unbalanced jacks, this dropping slightly to 136 ohms at 20Hz and 143 ohms at 20kHz.

The frequency response varied according to the volume-control setting and the inputs and outputs used. Fig.3 shows the balanced-in-to-balanced-out response. Flat within the audioband, it rolls off by 1dB at 16Hz and 45kHz with the volume control set at its maximum position. As the volume control is reduced in level, the ultrasonic bandwidth increases, reaching -1dB at 60kHz with the volume control at its unity-gain setting (9:30). Perhaps more important, the level of a peak in the ultrasonic response at 150kHz rises as the volume control is lowered. The peak was not affected by the 300 ohm output-loading resistors.

Fig.3 VAC Renaissance Signature Mk.II, balanced line-stage frequency response with volume control at (from top to bottom at 150kHz): unity gain, 12:00, maximum (1dB/vertical div.).

A similar picture could be seen when the VAC preamp's line stage was fed an unbalanced input and measured at its unbalanced outputs (fig.4). However, the 150kHz peak is higher in level, and a second peak appears between 30kHz and 50kHz (the amplitude of this peak and its center frequency vary with the volume-control position). This suggests a latent instability in the preamp's circuit. Both of the response graphs reveal excellent gain matching between the channels at all settings of the volume control, however.

Fig.4 VAC Renaissance Signature Mk.II, unbalanced line-stage frequency response with volume control at (from top to bottom at 150kHz): unity gain, 12:00, maximum (1dB/vertical div.).

The Renaissance's line-stage channel separation (fig.5) again showed the upward slope with frequency that is indicative of capacitive coupling. While the separation was acceptably high in the audioband for both modes of operation, the unbalanced performance had a similar notch to the phono-stage curves seen in fig.2, though it was slightly lower in frequency.

Fig.5 VAC Renaissance Signature Mk.II, channel separation, from top to bottom: unbalanced, balanced (R-L dashed, 10dB/vertical div.).

Figs.6 and 7 plot the THD+noise percentage present in the output signal against frequency for an output level of 1V into 100k ohms. The distortion in both graphs is acceptably low above 100Hz, but rises at lower frequencies. As the balanced behavior (fig.6) is not appreciably better than the unbalanced (fig.7), I assume that the Lundahl input transformer is to blame. Note the small peak evident in the unbalanced distortion curves at the same frequency where there is a notch in the channel-separation curves.

Fig.6 VAC Renaissance Signature Mk.II, balanced line stage, THD+noise vs frequency at 1V into 100k ohms.

Fig.7 VAC Renaissance Signature Mk.II, unbalanced line stage, THD+noise vs frequency at 1V into 100k ohms.

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