Premier Service Center Certification: Basic Amplifiers
Thank you for taking the time to participate in the QSC Premier Service Center Certification program.
To become a QSC Premier Service Center, you must pass four tests built off the online training material courses. This certification program is intended to allow your Service Center become fully capable and knowledgeable in servicing QSC products professionally and reliably.
This second test will cover some general knowledge about the service test bench. We will also be asking questions about the different amplifeir classes and specifically about our RMX 850 Class AB amplifier. Good luck!
Questions and Answers
- 1.
How many 8 ohm non-inductive resistors should be used per test workstation when testing a 2 channel amplifier?
- A.
2
- B.
4
- C.
8
- D.
16
Correct Answer
C. 8Explanation
To test a 2 channel amplifier, you would need to connect a resistor to each channel. Since the resistors should be non-inductive and have a resistance of 8 ohms, you would need to use 8 of these resistors per test workstation. This is because each channel requires one resistor, and there are two channels in the amplifier. Therefore, the correct answer is 8.Rate this question:
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- 2.
Which test equipment is required for servicing QSC amplifiers?
- A.
Sine-wave generator
- B.
2 channel oscilloscope
- C.
Variable transformer
- D.
Load resistor bank.
- E.
Everything listed here is required.
Correct Answer
E. Everything listed here is required.Explanation
To service QSC amplifiers, all the listed equipment is necessary. A sine-wave generator is needed to generate a clean and stable signal for testing and troubleshooting the amplifiers. A 2-channel oscilloscope helps in analyzing the waveform and voltage levels. A variable transformer is used for adjusting the input voltage during testing. Lastly, a load resistor bank is required for simulating different load conditions on the amplifier. Therefore, all the mentioned equipment is necessary for servicing QSC amplifiers.Rate this question:
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- 3.
Which frequency range do QSC amplifiers pass?
- A.
All frequencies
- B.
20 Hz - 20 kHz
- C.
40 Hz - 60 kHz
- D.
10 Hz - 1 kHz
Correct Answer
B. 20 Hz - 20 kHzExplanation
QSC amplifiers pass a frequency range of 20 Hz to 20 kHz. This means that these amplifiers are capable of amplifying and reproducing audio signals within this range, covering the entire audible spectrum for human ears. By passing all frequencies in this range, QSC amplifiers ensure that the full range of audio content, from deep bass to high treble, is accurately amplified and reproduced.Rate this question:
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- 4.
Which lab tools are recommended for amplifier repair? Mark all that apply.
- A.
Temperature controlled soldering station
- B.
Diagonal cutters
- C.
Needle nose pliers
- D.
Hammer and nails
Correct Answer(s)
A. Temperature controlled soldering station
B. Diagonal cutters
C. Needle nose pliersExplanation
The recommended lab tools for amplifier repair are a temperature controlled soldering station, diagonal cutters, and needle nose pliers. A temperature controlled soldering station is necessary for precise and controlled soldering of components. Diagonal cutters are used for cutting wires and leads, while needle nose pliers are useful for gripping and manipulating small components. However, a hammer and nails are not necessary for amplifier repair.Rate this question:
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- 5.
Along with 8 ohm non-inductive resistors, what else should the test load include?
- A.
(1) 2 ohm resistor
- B.
(3) 2 ohm resistors
- C.
Built in load resistance detection
- D.
A short circuit
Correct Answer
D. A short circuitExplanation
A short circuit should be included in the test load along with the 8 ohm non-inductive resistors. A short circuit allows for testing the behavior of the circuit under fault conditions, where the current bypasses the normal load path and flows directly from the source to the ground. This helps in identifying any potential issues or abnormalities in the circuit's response to such faults.Rate this question:
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- 6.
Repair decision: If the board is burned, with its fiber and resin damaged, what type of repair should be performed?
- A.
Component-level repair
- B.
Board replacement
- C.
Rework of all broken traces
Correct Answer
B. Board replacementExplanation
If the board is burned and its fiber and resin are damaged, the best course of action would be to replace the board entirely. This is because the damage to the fiber and resin indicates significant structural damage, making it difficult to effectively repair the board. Board replacement ensures that the damaged components are completely removed and replaced with a new board, ensuring the optimal functioning of the device.Rate this question:
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- 7.
When is it okay to substitute components with ones from Mouser, Digi-Key, or any electronic parts distributor?
- A.
Whenever possible
- B.
If the unit needs immediate repair
- C.
Only with the advice of QSC Technical Services Group
- D.
When it's cheaper to do so
Correct Answer
C. Only with the advice of QSC Technical Services GroupExplanation
It is only okay to substitute components with ones from Mouser, Digi-Key, or any electronic parts distributor when you have the advice of QSC Technical Services Group. This suggests that QSC Technical Services Group has the expertise and knowledge to determine if the substitution is appropriate and will not cause any issues or damage to the unit. It emphasizes the importance of seeking professional advice before making any component substitutions.Rate this question:
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- 8.
When ordering parts or assemblies from QSC Technical Services, what type of part number should be used?
- A.
QSC Part #
- B.
Electronic Manufacturer Part #
- C.
Value or part number on the device.
- D.
Peanut butter jelly sandwich
Correct Answer
A. QSC Part #Explanation
When ordering parts or assemblies from QSC Technical Services, the correct part number to be used is the QSC Part #. This is because QSC Technical Services uses their own part numbering system to identify and track their products. Using the QSC Part # ensures that the correct parts or assemblies are being ordered and reduces the chances of any confusion or errors in the ordering process.Rate this question:
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- 9.
Where can most QSC technical service resources be located?
- A.
From the QSC website via service center login page
- B.
From a google search on the internet
- C.
From the QSC forums
- D.
Requesting them by mail
Correct Answer
A. From the QSC website via service center login pageExplanation
Most QSC technical service resources can be located on the QSC website via the service center login page. This suggests that QSC provides a dedicated section on their website where users can access technical service resources. This is likely a secure area that requires users to log in, ensuring that the resources are only accessible to authorized individuals such as QSC customers or service technicians.Rate this question:
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- 10.
What is the primary purpose of the output section of the power amplifier?
- A.
To multiply the input voltage of the amplifier by any gain.
- B.
To attenuate the voltage present on the output.
- C.
To differentiate the difference between a positive and negative signal.
- D.
To multiply the input voltage of the amplifier by a fixed gain.
Correct Answer
D. To multiply the input voltage of the amplifier by a fixed gain.Explanation
The primary purpose of the output section of the power amplifier is to multiply the input voltage of the amplifier by a fixed gain. This means that the output voltage is amplified by a constant factor, regardless of the input signal. The fixed gain ensures that the amplifier consistently provides the desired level of amplification, which is important for maintaining the integrity and quality of the output signal.Rate this question:
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- 11.
A high quality power amplifier has which features? Mark all that apply.
- A.
Extremely low voltage gain
- B.
Low distortion
- C.
Flat frequency response
- D.
Low noise
Correct Answer(s)
B. Low distortion
C. Flat frequency response
D. Low noiseExplanation
A high-quality power amplifier is expected to have low distortion, meaning it accurately reproduces the input signal without introducing any unwanted changes or artifacts. It should also have a flat frequency response, meaning it can accurately reproduce all frequencies in the input signal without any significant changes in amplitude. Additionally, a high-quality power amplifier should have low noise, meaning it does not introduce any unwanted background noise or interference into the output signal. These features ensure that the power amplifier can faithfully reproduce the input signal with high fidelity and without any significant degradation in quality.Rate this question:
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- 12.
Gain control on a power amplifier is usually adjusted by which electronic component?
- A.
Potentiometer
- B.
Capacitor
- C.
Standard resistor
- D.
Diode
Correct Answer
A. PotentiometerExplanation
A potentiometer is a variable resistor that allows for precise control of the gain in a power amplifier. By adjusting the position of the potentiometer's wiper, the resistance can be changed, which in turn adjusts the amount of signal that is allowed to pass through the amplifier. This allows for fine-tuning of the gain to achieve the desired output level. Capacitors, standard resistors, and diodes do not have the same level of control over gain and are not typically used for this purpose.Rate this question:
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- 13.
Which amplification class is the least efficient?
- A.
Class A
- B.
Class B
- C.
Class AB
Correct Answer
A. Class AExplanation
Class A amplifiers are the least efficient among the given options. In Class A amplifiers, the output transistor is biased to operate in the active region, where it conducts current continuously, regardless of the input signal. This means that even when there is no input signal, the amplifier still consumes power. As a result, Class A amplifiers have low efficiency, typically around 25%. In contrast, Class B and Class AB amplifiers are more efficient because they utilize push-pull configurations, where each transistor conducts only during one half of the input signal cycle, reducing power consumption.Rate this question:
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- 14.
Which amplification class sees high distortion at zero crossings?
- A.
Class A
- B.
Class B
- C.
Class AB
Correct Answer
B. Class BExplanation
Class B amplification sees high distortion at zero crossings because it operates by using two complementary transistors, one for each half of the waveform. Each transistor is biased to conduct only during one-half of the input signal cycle, resulting in a push-pull configuration. However, this configuration causes a small gap or overlap between the two halves of the waveform, leading to distortion at the zero crossing points. This distortion is commonly known as crossover distortion, which can be minimized by using a class AB amplifier that combines the characteristics of class A and class B amplifiers.Rate this question:
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- 15.
Which amplification class is capable of very high signal integrity?
- A.
Class A
- B.
Class B
- C.
Class AB
Correct Answer
A. Class AExplanation
Class A amplification is capable of very high signal integrity because it operates in a way that allows the entire input signal to be amplified by the transistor, without any cutoff or distortion. In Class A amplifiers, the transistor is biased to operate in its linear region, ensuring that the output signal is an exact replica of the input signal. This results in minimal distortion and excellent fidelity, making Class A amplification ideal for applications where signal integrity is of utmost importance, such as in high-end audio systems or professional audio equipment.Rate this question:
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- 16.
The RMX 850 power amplifier is which amplifier class?
- A.
Class A
- B.
Class B
- C.
Class AB
Correct Answer
C. Class ABExplanation
The RMX 850 power amplifier belongs to Class AB. Class AB amplifiers are a combination of Class A and Class B amplifiers. They operate with both positive and negative signals, allowing for a more efficient use of power compared to Class A amplifiers. Class AB amplifiers are commonly used in audio applications where a balance between efficiency and sound quality is desired.Rate this question:
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- 17.
In the following schematic (p19) of an input section of an RMX 850 amplifier, why are resistors R100, R101, R105, and R106 precision 1% values?
- A.
To increase the gain of the differential amplifier.
- B.
Because 1% resistors are expensive and they make the amplifier high quality.
- C.
To ensure a high rejection of common-mode noise.
- D.
To reduce RF interference.
Correct Answer
C. To ensure a high rejection of common-mode noise.Explanation
Precision 1% resistors are used in the input section of the amplifier to ensure a high rejection of common-mode noise. Common-mode noise refers to unwanted signals that are present on both input lines of a differential amplifier. By using precision resistors with a tolerance of 1%, the amplifier can accurately balance and match the resistances in the input section, which helps to cancel out common-mode noise and improve the overall performance and signal quality of the amplifier.Rate this question:
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- 18.
In the following schematic (p19) of an input section of an RMX 850 amplifier, what is the purpose of capacitors C101 and C103 togther with R105 and R106?
- A.
To form a low-pass filter to reduce sensitivity to RF interference.
- B.
To electrically isolate the differential input from ground.
- C.
To reduce the gain of the amplifier.
- D.
To capacitate.
Correct Answer
A. To form a low-pass filter to reduce sensitivity to RF interference.Explanation
Capacitors C101 and C103, together with resistors R105 and R106, are used to form a low-pass filter. This filter helps to reduce the sensitivity of the input section of the RMX 850 amplifier to RF interference. By allowing only low-frequency signals to pass through and attenuating high-frequency signals, the low-pass filter helps to maintain the integrity and quality of the audio signal being amplified by the amplifier.Rate this question:
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- 19.
Which high-pass filtering settings does the RMX 850 amplifier have? Mark all that apply.
- A.
30 Hz
- B.
40 Hz
- C.
50 Hz
- D.
75 Hz
Correct Answer(s)
A. 30 Hz
C. 50 HzExplanation
The RMX 850 amplifier has high-pass filtering settings at 30 Hz and 50 Hz. These settings allow frequencies above the specified values to pass through the amplifier, while attenuating frequencies below them. This can be useful for filtering out low-frequency noise or unwanted signals, and ensuring that only the desired frequencies are amplified.Rate this question:
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- 20.
Which amplification class has the best efficiency?
- A.
Class A
- B.
Class B
- C.
Class AB
Correct Answer
B. Class BExplanation
Class B amplification has the best efficiency among the given options. In Class B amplifiers, the output transistors are biased to operate in cutoff mode when no input signal is present. This means that no power is consumed when there is no input signal, resulting in higher efficiency compared to Class A amplifiers. Class AB amplifiers also have good efficiency but slightly lower than Class B, as they have a small bias current flowing through the output transistors even when no input signal is present.Rate this question:
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- 21.
What is the primary purpose of the driver transistors in the amplifier section?
- A.
To supress RF and oscillations.
- B.
To turn on the front LEDs when signal is applied.
- C.
To provide protection for the amplifier section.
- D.
To invert signal polarity and provide some voltage gain.
Correct Answer
D. To invert signal polarity and provide some voltage gain.Explanation
The primary purpose of the driver transistors in the amplifier section is to invert the signal polarity and provide some voltage gain. These transistors are responsible for amplifying the input signal and ensuring that the output signal has the opposite polarity. Additionally, they also provide some voltage gain to boost the strength of the signal before it is further amplified by the power transistors.Rate this question:
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- 22.
In the following bias circuit, increasing the resistance in VR increases the ___.
- A.
Quiescent base currents in the driver transistors.
- B.
Gain of the amplifier.
- C.
Frequency range of the amplifier.
- D.
Maximum power the amplifier.
Correct Answer
A. Quiescent base currents in the driver transistors.Explanation
Increasing the resistance in VR will increase the quiescent base currents in the driver transistors. This is because the bias circuit determines the operating point of the transistors, and increasing the resistance in VR will increase the voltage across it. This increased voltage will result in a higher bias current flowing through the base of the transistors, leading to an increase in the quiescent base currents.Rate this question:
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- 23.
In the RMX 850 amplifier, like what electronic component does the transconductance amplifier (clip limiting circuitry) behave like?
- A.
An op-amp
- B.
A capacitor
- C.
A variable resistor
- D.
A diode
Correct Answer
C. A variable resistorExplanation
The transconductance amplifier in the RMX 850 amplifier behaves like a variable resistor. This means that it can adjust its resistance value based on the input signal, allowing for control over the amount of current flowing through the circuit. This feature is commonly used in clip limiting circuitry to prevent distortion and protect the amplifier and speakers from damage. By acting as a variable resistor, the transconductance amplifier can effectively limit the amplitude of the output signal, ensuring that it stays within safe levels.Rate this question:
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- 24.
In the following bias circuit, too little bias current results in ___.
- A.
High gain.
- B.
Excessive crossover distortion.
- C.
A hot amplifier.
- D.
High current consumption.
Correct Answer
B. Excessive crossover distortion.Explanation
Too little bias current in a bias circuit can result in excessive crossover distortion. Crossover distortion occurs when the output signal of an amplifier has a distorted waveform due to the switching between the positive and negative halves of the input signal. Bias current is necessary to keep the amplifier's output at a stable operating point, and when it is too low, the amplifier may not be able to accurately reproduce the input signal, leading to crossover distortion.Rate this question:
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- 25.
In the RMX 850 amplifier, when one of the amplifier's channel is driven into a short circuit, what reduces the current through the output transistors?
- A.
The input section.
- B.
The op-amp because the +/- 15 V supply rails are not replenished during a short circuit.
- C.
The clip limiting section senses a short and reduces the current.
- D.
The Zobel network reduces the current in the output transistors.
Correct Answer
B. The op-amp because the +/- 15 V supply rails are not replenished during a short circuit.Explanation
The op-amp reduces the current through the output transistors because the +/- 15 V supply rails are not replenished during a short circuit. This means that the op-amp cannot provide the necessary voltage to drive the transistors, resulting in a reduction in current flow.Rate this question:
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- 26.
Check all amplifier topologies that QSC has used for several decades.
- A.
Active high flyback diode power rail
- B.
Flux capacitance
- C.
Floating supply
- D.
Grounded collector
Correct Answer(s)
C. Floating supply
D. Grounded collectorExplanation
QSC has used the floating supply and grounded collector amplifier topologies for several decades. The floating supply topology allows for the amplification of signals without a direct connection to ground, which can help reduce noise and improve performance. The grounded collector topology, on the other hand, provides a low output impedance and high voltage gain, making it suitable for driving low impedance loads. By using these two topologies, QSC has been able to design amplifiers that meet their performance and reliability requirements.Rate this question:
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- 27.
What is the purpose of a Zobel network on the output of a power amplifier?
- A.
To reduce the impedance on the load.
- B.
To decrease the output voltage of the amplifier.
- C.
To suppress RF frequencies and oscillations.
- D.
To increase the maximum voltage of the amplifier.
Correct Answer
C. To suppress RF frequencies and oscillations.Explanation
A Zobel network is used on the output of a power amplifier to suppress RF frequencies and oscillations. RF frequencies and oscillations can cause interference and distortion in the output signal, so the Zobel network is designed to provide a high impedance at RF frequencies, effectively blocking them from reaching the load. This helps to ensure a clean and stable output signal from the power amplifier.Rate this question:
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- 28.
In the RMX 850 amplifier, what are the collector pins of the main output transistors tied to?
- A.
Ground and the heatsinks.
- B.
High and low voltage rails.
- C.
Emitter resistors
- D.
Driver transistors.
Correct Answer
A. Ground and the heatsinks.Explanation
The collector pins of the main output transistors in the RMX 850 amplifier are tied to ground and the heatsinks. This connection ensures that any excess heat generated by the transistors is efficiently dissipated through the heatsinks, while also providing a reference point for the amplifier's output signal. By grounding the collector pins, any potential voltage fluctuations or noise in the output signal can be minimized, resulting in a cleaner and more reliable audio output.Rate this question:
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- 29.
In the RMX 850 amplifier, where are the op-amp supply rails derived from?
- A.
Low voltage windings on the transformer.
- B.
From extremely thin air
- C.
Main power supply rails
- D.
Straight from the transformer
Correct Answer
C. Main power supply railsExplanation
The op-amp supply rails in the RMX 850 amplifier are derived from the main power supply rails. This means that the op-amp is powered directly from the main power supply, which provides the necessary voltage and current for the amplifier to function properly. The main power supply rails are typically designed to provide stable and regulated power to all the components in the amplifier, including the op-amp, ensuring optimal performance and audio quality.Rate this question:
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- 30.
In the example of the RMX 850 amplifier, are the main DC supply rails shared?
- A.
Yes
- B.
No
Correct Answer
B. NoExplanation
In the example of the RMX 850 amplifier, the main DC supply rails are not shared. This means that each channel of the amplifier has its own independent DC supply rail. Sharing the supply rails would mean that both channels would draw power from the same source, which could potentially cause interference or power issues. By not sharing the supply rails, each channel can operate independently and ensure optimal performance without any potential conflicts.Rate this question:
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- 31.
In the example of the RMX 850 amplifier, which component converts the AC coming from the transformer winding to DC.
- A.
Bridge rectifier
- B.
It is never converted to DC.
- C.
The transformer converts it.
- D.
Capacitors and resistors.
Correct Answer
A. Bridge rectifierExplanation
The bridge rectifier is the component that converts the AC coming from the transformer winding to DC. A bridge rectifier is a type of diode network that uses four diodes to convert the alternating current (AC) into direct current (DC) by allowing the positive half cycles of the AC signal to pass through while blocking the negative half cycles. This process rectifies the AC signal, resulting in a DC output. Therefore, the bridge rectifier plays a crucial role in converting the AC signal from the transformer into DC in the RMX 850 amplifier.Rate this question:
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.JPG "In the following schematic (p19) of an input section of an RMX 850 amplifier, what is the purpose of capacitors C101 and C103 togther with R105 and R106?")
.JPG "In the following bias circuit, too little bias current results in ___.")
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Mar 20, 2023Quiz Edited by
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