ANSWERS-------
1*.) line drivers are devices that increase the signal's voltage on its way to the amp. the gain on the amp can be turned down and the amplifier doesn't have to amplify the signal as much, which of the following might be (an) advantage(s)?
A -- the amp runs cooler
B -- the amp will play less noise
C -- Both
D -- None of the above
ANSWER -- B
Amplifier efficiency is based upon current gain -- the amp's "class". the question baits the taker into thinking that voltage gain consumed power. realistically, the voltage gain stages are isolated from the input level. no amp would allow the user to modify loop gain, especailly to lower it dangerously low. In some cases where noise is picked up on the transmission, line drivers can be helpful. especially if it allows for baleneced transmission. thus B.
This Q is one thing that irks me because it is an easy way to trick someone into spending $100 on something for no real reason.
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2*.) Two woofers are installed in a custom fiberglass box in the trunk of a car. the woofers are aimed towards each other.
A -- both woofers should be wired out of phase.
B -- both woofers should be wired in phase.
C -- one woofer should be wired in phase the other out of phase
D -- Either A or B
E -- Different for every install
ANSWER -- D
SPL -- Sound PRESSURE Level. if you want sound you want pressure. the way to make pressure is to change the volume in the cabin. to do this, you need both woofers to move outward at the same time, and inward at the same time. A lot of people get tricked by the speakers faceing each other. People will often tell you that bass is omnidirectional. so that would mean that the direction would never matter. (the position might though.)
This Q is often missed. you may choose A or B based upon the same reasoning as #3.
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3*.) A user has a component set with the tweeter in the dash, and the mid in the door. the speakers are angled differently.
A -- both speakers should be wired out of phase.
B -- both speakers should be wired in phase.
C -- one speaker should be wired in phase the other out of phase
D -- Either A or B
E -- Different for every install
ANSWER -- E
In this case the speakers are distant and have different path lenghts. this difference is signifigant for non-woofer speakers. because of this, one way of hooking up the speaker might be better then the other.
Any installer who would test the system out with various speaker polarity combinations is a conciderate installer who i would want helping me.
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4.) Component sets are placed with the tweeter in the dash, and the mid in the door. what crossover is most likely the best?
A -- 12dB/oct Linkwitz-Riley
B -- 24dB/oct Linkwitz-Riley
C -- 12dB/oct Butterworth
D -- 24dB/oct Butterworth
ANSWER -- D
Linkwitz-Riley filters are a buzzword. they are not the best choice for car audio becuase the reason they are good is phase coherence. unfortunantly the path difference issue keeps that from happening in a car. oh well. This same path difference will cause cancelation at higher frequencies. most likely, a high order filter would be good to prevent this from being an issue.
This Q was among the most missed overall, and likely won't come up in a real install situation. more of a Q for hobbiest. I also wanted to point out design challenges for audio in cars.
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5*.) a subsonic filter is used in a ported box setup. it should be set to what frequency:
A -- 5hz below tuning
B -- 1/3rd octave below tuning
C -- Different for each system
D -- Same for each system, but not A or B
ANSWER -- C
This one is often answered A or B. both will say it is "well known", but have no reasons as to why. Realistically the Xmax of the speaker would be important, as would the box and woofer and required system bandwidth. some systems might not need a subsonic filter and the filter could be set much lower.
This forum favors B, but a few went with A. i think only one person said C. This Q was asked becuase i've seen a few arguments of X hz lower vs 1/Y octaves lower, neither with any reasons. there actually are equations for these things (ugly equations).
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6.) a class AB amplifier is running at 1/3rd of its rated power output. what would be a typical efficiency?
A -- 16%
B -- 33%
C -- 47%
D -- 60%
ANSWER -- B
The key is the output level being 1/3rd of maximum. I had to change C from 42% to 47% to make it more fair. A classB amp's ideal efficiency (@1/3rd output) can be found with calculus and is about 40%. real amps would be lower, especially with the class A biasing.
This Q shows the reason you shouldn't run a low impedance load off of an amp by simply reducing gains. it can be done of course, but this explains why its a bad idea -- you lose efficiency because you're amps needs voltage rails for the high impedance speakers, but now needs to put out more current. you can, but get diminshing returns. Most people answered C.
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7.) MOSFETs are almost universally found in what part(s) of the amplifier:
A -- Audio Amplification
B -- Audio Filtering (crossovers)
C -- Power Supply
D -- Two of the above
E -- All of the above
F -- None of the above
ANSWER -- C
I think Tru is the only company not using MOSFETs in the PSU... MOSFET, another buzzword, is a type of transistor used extensively in computers. they are used in other apps mainly because they are cheap, not because they are good. for audio apps, the power PFET is a limitation. many amps use "triple darlinton" BJT configurations.
This Q was asked because some people think that all transistors must be mosfets or something. no one said B.
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8.) Some EQ filters have a high Q setting, standing for "Quality". what defines a high "Quality" filter?
A -- compatibilty with SACD/DVD-A formats
B -- High priced components
C -- affects a large range of frequencies
D -- affects a small range of frequencies
E -- None of the above.
ANSWER -- D
Contrary to popular beleif, Q actaully does stand for "Quality" and dates back to radio tuners. A high quality inductor would allow the radio to tune to a specific frequency. modern filters are either FFT based or PLL based allowing for Qs in excess of 1000... EQs use similar filters, just scaled down to lower frequencies.
This Question really was just filler. I think i will take it off the next revision because i've been informed some manufacturer's have gone against the standard and defined Q as it is in C.
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9.) will setting the gains on an amplifier all the way up cause it to clip?
A -- Yes, in all cases.
B -- Not always, but in some cases.
C -- Never.
ANSWER -- B
everyone knows that amplifiers amplify voltage. Vo = K * Vi. if you know K, but not Vi, you can't determine if the amp will clip or not. people who turn the gains all the way up will typically remark at how loud the system gets at low HU volume settings. they avoid clipping by keeping the HU at low volume settings, but can easily get high levels of distortion.
This Q was related to #10. this Q was "missed" most because of grammer -- people would say C then explain why it was B. since it might cause clipping, B would be the correct answer. C would mean that it would _never_ cause clipping.
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10*.) DMMs should be used as the definitive method for setting gains?
A -- Yes
B -- No
ANSWER -- B
No, the DMM cannot show if the speaker is overexurting, or too loud. The DMM is a tool in the grand scheme. The ear must be used as well. some people think an OScope must be used. there are actually ways to determine clipping with a good DMM and creativity, but that's not the point. the gains can be set to blend the system together.
This Q was asked because too many people tell new audio entheusists about the DMM method and make it out to be the ONLY way to do things. i was suprised when not everyone on this forum answered A though.
