VAC PA90 - Manual (Page 16)

16
SPECIFICATIONS
The VAC System has been developed with the critical ear as the major arbiter of quality, with
both conventional and unique measurements providing insight and guidance as necessary. The
lack of emphasis on measurements is due to the fact that engineering's arsenal of equipment
and techniques do not operate on the pattern recognition principals that control human
perception of sound.
In the immortal words of Daniel von Recklinghausen, if it measures good and sounds bad, it
is bad. If it measures bad and sounds good, you've measured the wrong things.
For those concerned with test bench performance, the following describes typical measured
performance of a PA90C1/PS90C 90 Watt combination operated at 120 VAC, 60 Hz.
Power Output: 96 watts continuous average power at 1 kHz with less than .35%
THD into 8 ohms connected to the 8 ohm tap. 120 watts continuous
average power at 1 kHz with less than .45% THD into 5.33 ohms
connected to the 8 ohm tap. Triode mode reduces power to 60
watts.
Frequency Response: down 0.5 dB at 6 Hz and 90 kHz, ref 0 dB = 1 watt @ 1 kHz.
down 3.0 dB at 2 Hz and 103 kHz, ref 0 dB = 1 watt @ 1 kHz.
(Note: response peaks will be observed at approximately 68 kHz and 190 kHz.
Such peaks are present in all tube type transformer coupled power amplifiers, and
are normally suppressed by the use of a phase compensation network in the
negative feedback loop. VAC has deliberately avoided this technique, finding that
it is sonically inferior to the slight ultrasonic peaks encountered with an excellent
output transformer. The compensation technique looks good into a resistive load,
but falters in the real world. Interested audiophiles should see Some Defects in
Amplifier Performance Not Covered by Standard Specifications by Norman H.
Crowhurst, published in the October 1957 Journal of the Audio Engineering
Society.)
Power Bandwidth: down 0.5 dB at 12 Hz and 58 kHz, ref 0 dB = 90 watts @ 1 kHz.
down 3.0 dB at 7 Hz and 72 kHz, ref 0 dB = 90 watts @ 1 kHz.
Slew Rate: Approximately 10 volts per microsecond.
Energy Storage: Approximately 217 Joules per pair.
(Note: This figure is difficult to compare with other designs, as the majority of the
energy storage is not shared among stages but dedicated to individual stages or
tube sections. Thus interactions within the amplifier are less than with
conventional amplifiers with more Joules.)
Negative Feedback: Loop feedback is fixed at approximately 8.3 dB regardless of output
tap selected.
Absolute Polarity: The PA90 does not invert signal polarity.
Input Impedance: 100k ohms
Sensitivity: .7 volts RMS input produces 90 watts output
S/N ratio: > 94 dB