28 QSC Audio Products, LLC
Signal Indicators Test
1. Disconnect the load resistors.
2. With a 1.6 V rms, 1 kHz input signal into the channel, turn the
gain control to maximum. The three signal LEDs (Signal, -20 dB,
and -10 dB) should be be lit. Set the analyzer’s 0 dB reference to
this level.
3. Turn down the signal by 10 dB. The -10 dB LED should be dim.
4. Turn down the signal another 10 dB (-20 dB to the reference
level). Both the -10 dB and -20 dB LEDs should be off.
5. Turn the amp gain control to minimum. All three signal LEDs
should be off. Repeat this test with the other channel.
Bridge Mode Test
1. Turn off the power switch.
2. Set the mode switch to the bridge position. The bridge LEDs on
the front and rear panels should light when you turn the
amplifier back on.
3. Connect the load to the two red output binding posts (channel 1
positive and channel 2 negative).
4. Select an 8 ohm load resistance and apply a 1.6 V rms, 1 kHz
sine wave signal to amp channel 1’s XLR input. Adjust the amp
gain control to obtain 1700 watts output (116.5 V rms). Verify
that the THD is below 1%.
5. Turn the amplifier’s gain controls to minimum.
6. Turn the amplifier off and set its mode switch back to stereo.
Connect a separate load resistance to each channel’s output.
Bias (Crossover) Adjustment
NOTE: The bias should not need readjusting unless the amplifier is
overheating or draws excessive idle current.
1. Let the amplifier cool down to room temperature.
2. Turn off the analyzer’s 80 kHz filter.
2. Set the analyzer signal sine wave output to 0.16 V rms at 20 kHz
and set the amplifier gain controls to full gain. Put the signal into
the input of one channel.
3. Put an 8-ohm load on the channel’s output.
4. Adjust the bias trimpot VR43 (channel 1) or VR166 (channel 2) for
a total THD+N figure of 0.07% or slightly less. Figure 2.4 shows
what the residual crossover spike should look like when the bias
is properly set.THD+N.
This adjustment must be done quickly, before the amplifier starts
to warm up significantly. If the amplifier begins to feel warm to
the touch before you complete the bias adjustment, you must
turn it off and allow it to cool down to room temperature before
trying again.
5. Turn the channel’s gain control to minimum. Verify that the AC
line current is no more than 1.0 A (120 V model) or 0.5 A (230 V
model).
6. Turn off the amplifier to let it cool down, then repeat the
procedure for the other channel, if needed.
7. Turn the analyzer’s 80 kHz filter back on.
Frequency Response Test
1. Connect an 8-ohm load resistance to channel 1’s output. Apply a
1.6 V rms, 1 kHz sine wave signal to amp channel 1’s XLR input
and turn its gain control to full gain. The output voltage should
be about 64 V rms.
2. Reduce the signal by 10 dB to 20 V rms (51 watts output). Set the
analyzer’s 0 dB reference to this point.
3. Check the frequency response from 20 Hz to 20 kHz by sweeping
or spot-checking frequencies between these extremes. Verify
that the output voltage’s amplitude at each frequency is 0 dB,
±0.20 dB.
4. Repeat steps 1 through 3 for channel 2.
4 Ohm Power vs. Distortion Test
1. Connect a 4-ohm load resistance to channel 1’s output. Apply a
1.6 V rms, 1 kHz sine wave signal to amp channel 1’s XLR input .
2. Adjust the gain control to obtain 850 watts output power
(58 V rms). Verify that the THD is below 1%. Check the output
power at 20 Hz and 10 kHz; it should also be 625 watts, with
THD less than 1%.
3. Turn the gain control down 3 dB to obtain 425 watts (41 V rms).
Verify that the THD is below 0.02%. Check the output power at
20 Hz and 20 kHz; it should be 425 watts, with THD less than
0.02%.
4. Repeat steps 1 through 3 for channel 2.
2.5 PL325 Test and Calibration Procedure (continued)
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