CAT Problem Solving and Troubleshooting 2 — Questions and Answers
Question 1: A powered loudspeaker produces a loud 60 Hz hum even when no signal is connected. What is the most likely cause?
- Faulty amplifier output transistor
- Ground loop through the AC power connection (Correct answer)
- Blown high-frequency driver
- Incorrect crossover frequency setting
Correct answer: Ground loop through the AC power connection
A 60 Hz hum present with no signal input typically indicates a ground loop introduced through the AC power path.
Question 2: During a live event, one side of a stereo mix drops to silence intermittently. The mixing console meters show signal on both channels. Where should you check first?
- The FOH EQ settings
- The cable run between the console and the amplifier (Correct answer)
- The stage monitor mix
- The microphone capsule
Correct answer: The cable run between the console and the amplifier
Intermittent dropout on one channel with confirmed console output points to a failing cable or connector in the signal path between the console and amp.
Question 3: An amplifier's clip indicator lights continuously at moderate gain settings with a clean input signal. What is the most probable cause?
- The input sensitivity is set too high (Correct answer)
- The output impedance is too low
- The power supply voltage is too high
- The signal frequency is too low
Correct answer: The input sensitivity is set too high
Continuous clipping at moderate volume with a clean input usually means the input sensitivity (gain) trim is set too high, driving the amp into saturation.
Question 4: You measure 0 VDC on the output of a Class-D amplifier while the input signal is present and the power LED is on. What is the best next diagnostic step?
- Replace the output capacitors immediately
- Check the protection relay and fault indicator (Correct answer)
- Increase the input gain
- Swap the speaker cables
Correct answer: Check the protection relay and fault indicator
Class-D amplifiers have protection relays that disconnect the output on fault conditions; checking the relay and fault LED is the correct first step.
Question 5: A condenser microphone produces distorted audio only at high sound pressure levels. The most likely fault is:
- Insufficient phantom power voltage
- Overloaded mic preamplifier input (Correct answer)
- Defective XLR connector shell
- Wrong polar pattern selected
Correct answer: Overloaded mic preamplifier input
Distortion that appears only at high SPL is a classic symptom of a mic preamp input stage being driven beyond its headroom.
Question 6: A DSP processor shows signal on its input meters but the output meters read nothing. Outputs have been verified with a different device. What should you check?
- The input sample rate setting
- The output routing matrix inside the DSP (Correct answer)
- The microphone phantom power supply
- The amplifier gain structure
Correct answer: The output routing matrix inside the DSP
If the DSP receives signal but produces no output, the internal output routing or mute assignments are the most likely misconfiguration.
Question 7: Feedback occurs in a system only when the presenter stands near the front fill speakers. The most effective corrective action is:
- Reduce the main PA level by 6 dB
- Lower the front fill gain or apply a narrow notch EQ at the feedback frequency (Correct answer)
- Switch the microphone to omnidirectional pattern
- Increase the subwoofer output
Correct answer: Lower the front fill gain or apply a narrow notch EQ at the feedback frequency
Reducing the front fill gain or notching the offending frequency directly addresses the feedback loop between the fills and the microphone.
CAT Problem Solving and Troubleshooting 2
A powered loudspeaker produces a loud 60 Hz hum even when no signal is connected.
What is the most likely cause?