APPENDIX B: HOW IS MEASURED THE AMPLIFIER POWER?
measure directly the output voltage on the screen.
Precision of the measurement
•
All the measurement tools (Digital scope and distortion analyzer) have been
recently calibrated (less than one year).
•
We’ve made some measurements on some batch of NXAMP to have a clear idea of
the precision of the amplifier itself, from one unit to the other (small differences in
the manufacturing of the custom transformer of the larges power supplies are the
main factor for having different power capability).
•
The precision on the dummy loads has also been checked with a calibrated
ohmmeter.
Due to all this tolerances, we can compute a value for the power output of each channel of
the amplifier in a precision of (+/- 10 %).
Measurement method
The input signal is a burst sine wave at 1 KHz, for 20 ms, every 500 ms. We increase the
input level up to reaching 1 % THD+N on the distortion analyzer. We measure the signal
with the scope cursor as shown in the picture bellow:
The last one wave of burst signal
shall be measured.
Close-up
The center of the emission line shall
be adopted as the measurement value.
Signal
20 msec
1 msec
500 msec
Vburst
Cursor
We do 4 measurements for each output, so we do 16 measurements per unit. Then we
average on 4 units of NXAMP4X1C or NXAMP4X4C.
Then we redo the same with 4 units of NXAMP4X1U or NXAMP4X4U, mains being 120 Volts
(+2, -0 Volts at 60 Hz).
Finally we average all the measurements and this is the values which are written in the
datasheet of the NXAMP Powered TDcontrollers.
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