Part 5
X-Over
Importance: Type, number of X-over frequencies, frequency cut-off, adjustment level-roll off.
Since no linear driver/speaker is capable of reproducing all (20HZ to 20KHZ) frequency flatly and at a desirable level, any decent system will require 2 or more drivers to achieve the desired frequency range and level. In theory, simply hooking up these speakers would work. However, if a driver is designed for a specific range, using a full spectrum signal to power the speaker outside that range can both be damaging and inefficient.
For instance, if a tweeter is only capable of producing frequency from 1.5Khz and up and anything below would cause excessive travel by the voice coil and thus damage, powering it beyond this capability will surely damage the tweeter. To prevent this from happening, a X-over limit frequency outside the designed range from passing through. In practical terms, the linear drivers being the norm and as such, variations on the design are used to produce specific region of the musical frequency. Typically, tweeters for high frequencies (2.5khz to 20khz) midrange/woofers (80hz to 5khz) and subwoofers (20hz to 200hz). To divide these signal to the specific speakers, two methods are used; Active and Passive.
Active X-overs are electronic unit that divide the full spectrum signal into sets of signals and pass it to the designated amplifier while passive X-over are made up of diodes, capacitors, resistors and are designed to allow specific frequency from the amplifier to pass to specific speaker/driver.
Active X-over an efficient way to utilize amplification power as specifically dedicated amplifiers can be use. This provides more flexibility since variable x-over ranges are not fixed and can typically be overlap and level adjustments. With the flexibility comes the cost is more as dedicated amplification must be use.
Passive X-over usually are included separately with the speaker in a typical 2/3 way component set or are already built into 2/3 way system. Rarely would you one find one that come with a subwoofer. Typically, the amplifier that drive the subwoofer would have the necessary frequency removed ahead of the chain. Some don’t but the only harm is higher frequency entering the subwoofer and effecting the soundstage. Levels to individual drivers are set within the Passive network but the adjustability is very limited.
Location in chain:
Active: Usually before an amplifier or line driver.
Passive: After amplifier and before speakers/driver
High Pass: indicate a frequency range from which signal above is allow to pass through - High
Low Pass indicate a frequency range from which signal below is allow to pass through - Mid/Mid Bass
Band Pass indicate a frequency range between two specific frequencies which signal above is allow to pass through - Mid/Bass/Mid Bass/Subwoofer
Amplifiers:
Importance: Number of Channels, Power within the frequency (20Hzto20KHZ) produce, power at different resistive value (power at different Ohms), Current rating, Damping factors, Slew rate, Frequency cut off
When it comes to Amplifiers, the first thing most people look at is the wattage. True, power rating will give you an idea what the amplifier is capable of but be aware of how this specification was achieved. Some rating are measure using a sine wave only at 1KHZ and not full audio spectrum and in short burst. This would be unrealistically favoured to show a rating that is not real world. Music in not a single frequency so this reading without reference is very misleading. Additionally, any rating must also include the percentage of Total Harmonic Distortion (THD) value for leaving it out can have an amplifier whose rating originally is 20W (20hz-20khz with .1%THD) to an rating of 2000W (20hz-20khz with 100%THD) because it is CLIPPING. It is also worth noting to observer what input voltage was use to achieved the rated output. Sometimes manufacturer rate their output using a 14.4 volt rating rather than 12volt. A 14.4 volt rating is typical for all car alternator when the car is running and @around 12 volt when not. Using 14.4 volt as oppose to 12 volt will obviously yield higher wattage output. Most high current, well designed amplifier will rate their output at 12 volts. Remember again to compare same specs at the same variables.
Impedance and Ohms
Giving the above, we must also look at what resistive value that this wattage was achieved and at what duration. Because speaker/drivers are directly driven by an amplifier and have resistive values consider as ‘loads’, the amplifier must be able to provide this power in this context. Car audio nominal ratings are typically rated at a resistive value of 4 ohms. Mindful that this rating is not linear across the frequency spectrum there are typical peaks and valleys. So a 4 ohms speakers might have frequency in which it there are 3 or 6 or 2 ohms area. On average that speaker is 4 ohms and is rated as such. If the amplifier is incapable of producing power at lower resistive rating, it may overheat or clip (produce distorted signal). Either scenario will cause damage. Clipping is the leading cause of speaker damage not overpowering. A high current amplifier often can be stable down to .5 ohm. Remember every time the impedance is half, power demand are doubled and the amp must be able to supply this power effectively otherwise clipping will result (assuming at full volume) or overheat. Often in a badly designed system where the amps are clipping but not completely obvious, the result is “ringing” in the ear of the listener. Most times it is caused by extreme decibels but clipping can also have this affect.
Damping Factors/Slew rate
When a signal is sent to the speaker, its’ movement is based on what that signal is asking it to do. Without sufficient damping factor, the speaker would continue to move because of inertia. As the it bottoms from one end to another base on the design of the drive, this ‘ringing’ effect would make the sound very “muddy or sloppy”. An amplifier with high damping factor rating would minimize this effect thus give it a much tighter control/sound.
Slew rate is a measure of an amplifier's ability to follow its input signal change in comparison to its’ output voltage which relates to an amplifier’s ability to pass complex waveforms without clipping them. This is not typically advertise but if one exist, look for the lowest number as it translate the ability of amplifier that is stable.
Signal to Noise Ratio
We touched briefly above regarding S/N ratio noting that it is a measurement of background noise generated by the unit. As noted, be aware that how this was measure be it at full volume of at 1 watt or full volume. The higher the number the quieter the amplifier all else being equal.
Channel Separation
Channel Separation measured in db is the mount of difference between 2 channels. Mono having 0db separation as both channels share the exact same signal, the higher the number the less likely signal from one channel is heard through another.
Headroom
Music is dynamic in nature and good amplifiers will allow quick bursts of power that is over the rated RMS/Nominal rating. Standard is 3db which equal doubling the power. A amplifier that has high headroom generally sound more dynamic since it can produce more power at a given time.
How much power?
How much money is enough? Keep in mind what your speakers are capable of but more importantly the power ratio when apply to a balance system. I favour the 1-2-4 factors approach in which the mid get twice the amount of power while the bass four times that of the high. This is on the basis on an active system. Many good system utilized a single amplifier while using an EQ and passive x-over network to achieve balance. The rule of thumb is get as much good clean power as you can afford.
Continue to Part 6
Last edited by zakimak; 12-04-2013 at 04:02 PM.
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