VTL IT-85 integrated amplifier Measurements

Sidebar 3: Measurements

As might be expected, the VTL IT-85 runs hot, but not too hot to touch, even after the one-hour preconditioning. A clone of the input signal was available from the processor outputs, this unaffected by the volume-control setting, and the amplifier didn't invert signal polarity from these outputs, the preamp outputs, or the main speaker outputs. The VTL's input impedance was a moderate 19k ohms, and maximum voltage gain into 8 ohms was a high 40.2dB.

As CS tried biamping his speakers using the IT-85 and a sample of VTL's ST-85 power amplifier, I checked the gain structure of this arrangement. The IT-85's preamp section has a moderate maximum gain of 9.8dB, its power amp a gain (into 8 ohms) of 30.4dB. By contrast, the ST-85 has an 8 ohm gain of 30.65dB, meaning that vertical biamping (one stereo amplifier for each speaker) would result in an 0.25dB imbalance between the channels. Best, therefore, to do what Chip did and horizontally biamp: ie, use the ST-85 for both woofers and the IT-85 for both tweeters.

The IT-85's output impedance was moderately high, at 0.85 ohms across the audioband. (The ST-85's source impedance was slightly lower at 0.75 ohms, which probably explains the slight difference in gain.) As a result, there will be a mild, ±0.6dB interaction between this impedance and the manner in which the partnering loudspeaker's impedance changes with frequency, which can be seen in fig.1 (top trace at 2kHz). The response into resistive loads is basically flat down to well below 10Hz, with only a mild (-0.4dB) rolloff at 20kHz. (Note the excellent volume-control channel match in this graph, by the way.) The exact rolloff was somewhat dependent on the volume-control setting; the traces in fig.1 were taken with volume set to 1:00. At the maximum setting, the rolloff decreased to -0.2dB in the left channel, -0.05dB in the right. Though the 1kHz squarewave (not shown) was basically perfect, the 10kHz squarewave response (fig.2) reveals a damped ultrasonic resonance.

Fig.1 VTL IT-85, frequency response at (from top to bottom at 2kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, 2W into 4 ohms, and 4W into 2 ohms (0.5dB/vertical div., right channel dashed).

Fig.2 VTL IT-85, small-signal 10kHz squarewave into 8 ohms.

The unweighted, wideband S/N ratio (ref. 1W into 8 ohms) was okay at 66.7dB, improving to 77.7dB when A-weighted. Channel separation (fig.3) was excellent in the L-R direction, but merely good in the other direction.

Fig.3 VTL IT-85, channel separation (10dB/vertical div., R-L dashed).

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