So you are wanting to buy an amplifier for your car, but you don't necessarily know what you should look for or what you actually need. No worries, this site is here to help.
The first thing you should look out for is what capacity your subwoofers are using. As you may know, subwoofers and speaker systems require a specific amount of current in order to run and deliver a strong sound. Lesser subs will take up less power, and stronger subs with more bass will use up more power. So what you will need to do is find the model of your subwoofer and your speaker system (they could be the same item, but most are separated) and look up how many 1. ohms it runs at and 2. the capacity of its energy, measured in hz.
If you don't have a speaker set or a subwoofer system, that just means you are going to be golden when picking out your car's amplifier. This is because you get to do the research together, and find out which is compatible with what before you purchase. Also, if you do some minor research online, you will find plenty of people on the forums that have already purchased companion products together and have tested if they are compatible and if they work optimally together. So if you haven't bought yourself a speaker set or just want to upgrade the whole audio system in your car, definitely look around and make sure your next set of speakers and subs will match the power of your amp. This is highly important because if you under- or over-power your speakers, either your amp could blow out or your speakers will. It's a simple matter of power currency being delivered and matching each in a set. You don't want an overly expensive car amp only to match it with a rather cheap speaker set that will only use a portion of the power the amp can deliver.
With that out of the way, here is the meat of your whole audio system. How many watts does your amp need?
With my experience, most people go for mid-range speaker sets, and that typically warrants a wattage of approximately 1500. However, keep in mind that those people already know what they are doing with their car system, and understand what to buy in a speaker and sub set.
Most new people, however, go for a system that is not necessarily the cheapest, but is more budget-friendly, per se. This means the requirements they have for an amplifier is diminished, more around the 500 watt range, which is where most of the budget amps are priced at anyway.
One of the more important measures to look out for when buying an amp, though, is the ohm capactiy. A 1500 watt amp will typically run at 2 ohms. Why is "ohm capacity" important for buying amps? Because that is the true measure of current you will need to look at. You want an amp that runs at less ohm capacity than your speakers do. This means that there is no possibility the amp brings too much power to your speakers and eventually damaging them. Ideally, though, you'd want an amp that runs at the same ohm capacity as both your speakers and subwoofers, so as not to under-power and to use the whole system to their optimal levels. If you can't do this, the next best option is to simply underpower the speakers, as then at least you won't blow them out.
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