Hey gang, I wrote this article a few days ago and have been asked to post it up here as well.
Choosing an amplifier is about more than just watts.
When speaking to anyone about achieving quality music in car audio most people simply focus on the speakers rather than the amplifier. They often speak about speaker build quality and power handling, enclosures, porting, fibrefill, loading and speaker cable. You don’t really need to worry about the amplifier so long as it’s powerful enough right? Just like when you’re considering the purchase of a new car; the only thing you need to look at is the power output of the car alone right? Wrong.
When choosing an amplifier to power your speakers there are more stats than just power output you need to think about before you splash a couple of grand on an amp that will sound like rubbish. For those among you dreading an upcoming lecture on amplifier classes, resistors, capacitors, transistors, transformers and power supplies relax, I’ll keep this to plain English for the explanation of which stats to look out for. High power output is important but quality amplifiers are not just about sheer power. Mining dump trucks have over 6000 horsepower but that doesn’t make them spectacular performers. Besides power there are certain other important figures that must be taken into consideration when choosing a suitable amplifier. We’ll go through a few of the more commonly found stats.
Damping Factor
Damping factor describes an amplifiers ability to control a woofer cone. It’s the ratio of rated load impedance to the internal impedance of an amplifier. The higher the damping factor the more efficiently an amplifier can control unwanted movement of the speaker coil. High damping factor is crucial for subwoofers and the higher the damping factor the better. It is debatable if anything over 50 is audible. Damping factor is calculated by dividing the speaker impedance by the output impedance of the amplifier. In other words the damping factor will decrease as the speakers impedance decreases. This means an amp optimised at 4 ohms will provide tighter bass than when they’re running at 2 ohms. A lower damping factor will leave bass notes sounding soft and undefined, regardless of the amplifiers power output. You can see by this that a smaller 100 watt amplifier with a high damping factor can often sound better than one twice it’s size with a low damping factor.
Slew Rate
Sometimes referred to as damping factor for tweeters, the slew rate describes the amplifiers ability to accurately control fast direction changes of a speaker cone or dome. Have you even turned your stereo up to discover that your cymbals sound like someone throwing a brick through a glass window? That’s because the amplifier simply wasn’t fast enough to accurately reproduce the high frequency ring of the symbols. Measured in volts per microsecond, a low slew rate softens the definition of a sound signal which blurs transients and causes the sound to appear muddy. A high slew rate means the amp responses faster which ultimately results in crystal clear highs.
Total harmonic distortion
THD is the measurement of the how much the amplifier can distort the sound signal through the introduction of added harmonics or overtones. THD figures are usually given as percentages and a THD figure below 1% are generally inaudible to most people. However, distortion is a cumulative phenomenon so if your head unit, eq, crossover and amplifier are all rated at less than 1%THD each, together they could produce 5%THD which may well be noticeable to most of you.
Signal to noise ratio
Noise leaking into the sound signal is an ever present problem in car audio. The Signal to noise ratio is a measurement of noise level in the amplifier compared to the level of the signal. A higher S/N ratio signifies a greater difference which is better. Technically speaking, it’s the ratio expressed in dB of signal power at a reference point in a circuit to the noise information that would exist if the signal were removed (the noise floor). The maximum signal to noise ratio of the amp can be seen as a measure of realistic fidelity. This ratio is how much absolute noise it produces compared to the highest signal voltage it can pass without distortion. Many companies combat noise by utilising balanced line systems.
Stereo separation
Separation is not spoken about much but this refers to the amplifiers ability to maintain the separation between the right and left channels. This is essentially what allows an amplifier to reproduce an accurate sound stage. Each individual instrument is after all, are recorded in it’s own location in the sound stage and you should be able to hear this in the same way when it’s played in your car.
Just a final few points to remember while you’re looking at specs. You’ll find many are followed by the term ‘A weighted’. Put very simply, ‘A weighting’ is a way of colouring the figures a little to make them appear more attractive. Loading is another issue to consider. Watch the impedance of speakers when choosing them because while most amps are stable at low impedance levels, they’re not overly efficient nor performing 100% when loaded down. Your cars engine is 8000rpm stable but it’s unwise to try and keep it there for long. By the same token many amps are 2 and 1 ohm stable but this is for intermittent spikes (as music is dynamic it causes the speakers resistance to continuously change during playback), not continous everyday running.
These are some of the more important figures to observe when buying amplifiers. It’s not simply just a matter of buying which ever amp outputs the most power. It’s a matter of taking all the figures into consideration and choosing which amplifier best suits your needs.
vti-2 edit: updated title
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