VTL ST-85 power amplifier Measurements
The VTL's protective cage was hot following its 60-minute, 1Ú3-power preconditioning test, if not unusually so for a tube amplifier. The bias of the right channel was found to be misadjusted during the distortion measurements. It was recalibrated, and all measurements made prior to the recalibration were rechecked and corrected, as necessary.
The ST-85's input impedance measured a high 114.8k ohms. Its output impedance ranged from 1 ohm to 1.08 ohms, depending on frequency and load impedance. This is not particularly high for a tube amplifier, but still high enough to have a measurable effect on the frequency response with a loudspeaker load.
The ST-85's voltage gain into 8 ohms is higher than usual, at 30.1dB, and the amplifier is noninverting. Its S/N ratio measured 72.3dB from 22Hz to 22kHz, 71.3dB from 10Hz to 500kHz (both unweighted), and 82.2dB A-weighted (all ref. 1W into 8 ohms). DC offset was a very low 1mV in the left channel, 0.6mV in the right.
The frequency response of the VTL ST-85 into various loads is plotted in fig.1. Note the large effect of the simulated real-world load impedance on the frequency response—an effect common in tube amplifiers, with their typically high output impedances. The 10kHz squarewave response in fig.2 has a single cycle of high-frequency ringing that is quickly damped. (It is also visible in the 1kHz squarewave, not shown.)
Fig.1 VTL ST-85, frequency response at (from top to bottom at 6kHz): 1W into 8 ohms, 2W into 4 ohms, and 2.828V into simulated loudspeaker load (0.5dB/vertical div.).
Fig.2 VTL ST-85, small-signal 10kHz squarewave into 8 ohms.
The crosstalk measurements shown in fig.3 are good. The crosstalk from the left into the right channel is significantly lower than in the other direction, but even in the latter case, the level is well below anything that might have audible consequences.
Fig.3 VTL ST-85, Crosstalk (from top to bottom): R-L, L-R (10dB/vertical div.).