VTL 100W Compact monoblock power amplifier Measurements
On the test bench, you could have been forgiven at first for thinking that this VTL was a solid-state design, the small-signal frequency response (1V into 4 ohms) extending from 2.2Hz to 77kHz (3dB). Noise levels, too, were impressively low, measuring 90dB, unweighted, with the input shorted, this improving by just over 6dB when an A-weighting network was switched in circuit ahead of the meter.
Looking at the output impedance, however, revealed the amplifier's thermionic nature, with a value around 1.1 ohms at 20Hz and 1kHz rising slightly to 1.25 ohms at 20kHz. Though around 10 times the output impedance of a typical solid-state design, and a little higher than some other all-tube designs, this is still excellent for a tube design and the consistency with frequency points to the excellent nature of the output transformer. This higher-than-usual source impedance will, however, be reflected in a loudspeaker's measured frequency response, particularly if the latter is a 4-ohm design, which is something to be borne in mind when auditioning.
The high sensitivity was confirmed, 720mV input at 1kHz giving full output. Maximum power output was a little higher than spec at 125W into 8 ohms in the midband. Into 4 ohms, however, it was current-limited to 90W, which may explain the fact that audible clipping occurred earlier than expected into the low-sensitivity Celestion SL700 compared with the Krell KSA-50.John Atkinson