2
performed at normal room temperature. In case the power amplifier had been operated before, it has to be given
several hours for cooling off.
4.2 VCA - OFFSET
Periodically (rhythmical) opening and short-circuiting the CNS2.1 and CNS2.2 on the main PCB printed board
assembly for channel A; respectively CNS2.3 and CNS2.2 for channel B. Using VR100 respectively VR300 to
adjust the power amplifier outputs to their minimum offset (with oscilloscope to minimal peak value or to the
audible minimal volume setting).
5. Testing of functions
5.1 OUTPUT - offset voltage
DC-voltage measuring at the SPEAKER A/B outputs with U(DC)
≤
±10 mV.
5.2 LIMITER
5.2.1 damping test
Channels separately driven with a 1 kHz signal up to U(A) = 49 volts, respectively U(A) = 42.2 volts (without
load). Increase the input voltage by 10 dB. The LIMITER LED lights and the output voltage increases by about
2 dB to approx. 64 volts, respectively 56 volts; with slight clipping. The distortion rate of the limited signal is
at THD = 1.0 … 1.5 %. Increasing the input signal up to a value of +20 dBu should not result in remarkably
higher clipping.
5.2.2 attack and release times
perform the test of both power amplifier channels separately:
testing without load resistors
1.)
Drive the power amplifier input with a burst signal (f = 1 kHz, 10 cycles, rate:
≈
0.5 sec.) and U(E) = +10
dBu.
2.)
Use an oscilloscope to monitor the output signal. After 3 - 4 signal periods, the limiter controlled the
major distortion down to a minor residual distortion (THD = 1.0 … 1.5 %).
attack time: 3 - 4 ms
release time: 30 - 40 ms
5.3 POWER-ON DELAY
Signal present at the power amplifier input. Use the power-on switch to switch the power amplifier's power on.
Approximately 2 seconds after switching the power on, the signal is present at the amplifier's output.
5.4 FAN CONTROL
Upon powering-on the power amplifier, the fans will run for about 2 seconds and then stop; presuming that the
amplifier's temperature is not too high. In idling condition (power-on, no signal present) the fans are switched
between the SLOW and OFF (0 V) mode, depending on the temperature of the heat sink. When CNS1.8 is
connected via a 51 kohms resistor with +15 V, the fans will run in SLOW mode. Fan-voltage of 16.8 V DC
measured between Pin CNS2.4 and Pin CNS1.4. Disconnecting the plug CN5 results in the fans running at
FAST speed. Fan-voltage of 28 V DC measured between Pin CNS2.4 and Pin CNS1.4.
5.5 SOAR PROTECTION CIRCUIT TEST
Channels separately driven on 4
Ω
up to 49 volts, respectively 42.2 volts. Parallel connect a 1
Ω
resistor. The
protection circuit reacts and tries continuously to re-start! The protect-LED lights. Repeat the test with a 2
Ω
resistor; the power amplifier should not switch off.
5.6 SHORT-CIRCUIT CURRENT-LIMITING TEST
perform the test for both power amplifier channels separately and without load:
-
drive the channel with a burst signal (f = 1 kHz, 1 - 3 cycles, rate
≈
1 sec.) with U(E) = 0 dBu
-
connect a load resistor of 1
Ω
-
the short-circuit current-limiter limits the output voltage at the load resistor symmetrically (monitor via
oscilloscope) to a peak voltage value of 35 V, respectively 27 V ( approx. 35 A respectively 27 A maximum
peak output current).
5.7 DC-VOLTAGE PROTECTION CIRCUIT TEST
perform the test for both power amplifier channels separately:
-
drive the power amplifier with a test signal (f = 7 Hz) and without load
