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I've recently bought a new car, and the stock audio system is lacklustre at best. I've replaced the door speakers in previous cars and this has made a decent enough improvement to the overall sound, and I am looking at doing the same in this car.

I know a lot of people install aftermarket amps, but I am unsure what the end-goal of doing this is. As far as I am aware, an amp takes the low-level input and boosts it to a power high enough to drive the speakers, an amp with a higher output can drive bigger speaker allowing for a louder volume.

"More volume" is not something I am aiming for, I want better sound quality. I will not be upgrading the cars OEM head unit, and as such will be using the amps built in to the head unit. Are there any problems I could run in to from powering new speakers using the built-in amp, or are there any significant gains to audio quality to be had for installed upgraded amplifiers?

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Yes, the gain in quality is significant, even when you're not looking for an increase in volume.

Why having an amplifier matters

The stock system can only output about 15 W/channel (*). That's not enough to drive the speakers cleanly at the volume you need when driving at highway speed. At normal listening volume, the sound will often be distorted, or be missing 'punch' because the stock system can't supply enough power at low frequencies.

Sound is a bit weird. Volume is a logarithmic function, so to get a bit more volume you need a lot more power. Also, music has a large dynamic range: the difference between a quiet passage and a loud piece is enormous. So, when playing at low level, the amplifier supplies less than 1 W most of the time (this gets you between 80-90 dB in many systems). But the beat can require 10-100 W in very short pulses. So a big amplifier buys you headroom/dynamic range, even when you don't play at high volumes.

When you're driving at 120 km/h, in many cars the interior noise level is already around 80 dB. Just to make your music audible in those conditions, the stock radio is already running close to its limit.

Efficiency

If you don't want to use an amplifier, try finding a set of speakers with a high efficiency rating. This figure shows you how many dB the loudspeaker produces when fed with 1 W if input power.

*: This applies to car stereos that fit in a 1-DIN or 2-DIN slot and don't use an external amplifier. Some car manufacturers offer a higher-quality sound system as an optional extra, this will usually have an external amplifier.

Car stereo manufacturers advertise with higher ratings for their 1-DIN or 2-DIN units, but they cheat by measuring at very high distortion. Anything more than about 18 W will be distorted. For a good comparison, always look at the RMS power ratings, not 'peak' or (worse) PMPO. The 18 W is a consequence of using a 12 V supply for the amplifier. Without a large, expensive converter stage to increase this voltage, you can only generate 18 W at 4 Ohm.

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  • Good answer. To expand on the dB aspect; the dB scale is logarithmic, so +3dB is a doubling in perceived volume, and +10dB is an increase of 10x the power (animations.physics.unsw.edu.au/jw/dB.htm shows all the math for this). So 60dB is conversation level in a restaurant, 70dB is a Vacuum Cleaner, and 80dB is a freight train at 15meters. 80dB+ for long periods = hearing damage.
    – PeteCon
    Commented Jan 19, 2017 at 17:41
  • The 15W depends heavily upon which car and what sound system it came with. Driving a speaker accurately depends more on signal quality than wattage. A jigawatt amp driving a speaker that doesn't need it - and therefore won't use it, 'cause that's how electricity works - is overkill. A good filter will have more impact if the amp is already sufficient. Basic signals processing.
    – 3Dave
    Commented Jan 20, 2017 at 1:28
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An aftermarket amplifier can (or in most cases, will) be of better quality in general. An expensive 100W amp will give better sound quality than a cheap 100W one. Simply explained: at 100W, a cheap 300W amp will give quality comparable to an expensive 100W amplifier at 100W, its full power.

With all amplifiers the sound quality gets worse when you increase the volume. So a 45W stock radio in your car will only sound reasonable to say, 10W. With the more expensive, established brands it is less so. If you get a Bose, Sennheiser, or JBL amp, it'll still sound ear caressing at full power. However, your average Sony or Pioneer CD player will sound terrible at only half its power.

At 45W it will still make noise, but it'll sound terrible. It'll make cracking sounds where you expect goose bump inflicting bass. If you google for 'harmonic distortion' you'll find out how it works exactly.

Like you, i also wanted fair quality at a normal volume. If you're going for the established brands, you'd need a 15W amp for that. I chose a 100W amp from AliExpress, because at 15W that still gives good quality, and it is still a lot cheaper than a good amplifier.

In other words, if you're going for a cheap amp, you need to heavily compensate on the output power to keep good quality. It's also worth learning the difference between Class A,B,AB,C, and D amps. These classes tell something about the technique, not necessarily the quality.

In my experience, the price of the speakers won't make that of a difference. The only thing is that most cars only have a single wide-range speaker in each door, while it's better to have separate woofers, mid-ranges and tweeters.

Lastly, decibels and output power have a logarithmic relation. If you have an 15W amp of which you think: "i'd like to have twice this volume", then you'd need a 150W amp. Output power increases factor 10 per doubling of the volume.

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    "With all amplifiers the sound quality gets worse when you increase the volume." This is incorrect. Sound quality is constant over most of the volume range. Only when you get close to maximum volume do you get increasing distortion.
    – Hobbes
    Commented Jan 19, 2017 at 16:19
  • @Hobbes For better amps; agreed. For cheap amps though, it is true in my experience, it'll sound fair to half the maximum volume, and then it gets worse and worse. But that's probably just because better amps cap the volume to the point where the sound is still good. If you could drive them further, the same would probably apply, as it does with cheap amps.
    – Bart
    Commented Jan 20, 2017 at 7:34
  • Some cheap Amplifiers sound Ghastly at low volume due to crossover distortion.
    – Autistic
    Commented Jan 20, 2017 at 11:37
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Aftermarket amplifiers are usually necessary when people are installing giant subwoofers and/or want more volume. As long as you replace the speakers with others of the same size but better specs, you can get better quality sound.

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