When buying car stereo equipment on E-bay, you can save yourself alot of money and time if you know what is compatible with any of your existing equipment. If you’re starting a setup from scratch, it will help you on your decision making if you are familiar with some simple terms like Ohms, Voice Coils, Channels etc. Knowing some of those terms and how they relate to what you want to build will help you better understand if a particular amplifier will work with a particular subwoofer or set of speakers.

When browsing, you may see items that say “Dual 1 Ohm voice coils”. Would you know if that subwoofer will work with an amplifier is rated as “2 Ohm Mono stable” The answer is yes, because if you’re familiar with Ohms Law, you would know that you can series the two coils to give the amplifier a 2 Ohm Load.

Another example is, if you have 2 sets of speakers in your car (4 total speakers), do you know what the optimal number of channels an amplifier should have for you to power your speakers? The answer is 4 channels, however you can run the 2 sets off a 2 channel amplifier. The problem with running the two sets of speakers off a 2 channel amplifier is that you again run into the possible problem of Ohms Law. If each speaker has an impedance of 4 ohms and you are putting 2 speakers per channel on the amplifier, the amplifier is seeing 2 ohms per channel, and you have to make sure your amplifier is 2 ohm stable otherwise damage can occur to the amplifier.

The number of channels on an amplifier usually range from 1 channel (also known as Mono), 2 channel, 4 channel, 5 channel. I say usually because amplifiers are rapidly changing and being designed for all kinds of applications, so you may see others like 3 channel.

Mono amplifiers are usually used to power subwoofers and can range anywhere from a few hundred watts up to 5,000 or more watts. Some mono amplifiers can also run well below the usually Ohm loads you may be familiar with, for example, you may see a high current amplifier listed that states it can be dropped to .25 ohm. That is extremely low, but it is certainly not unheard of when dealing with higher powered competition quality amplifiers. You can run multiple subwoofers off of an amp like that by using the parallel wiring and showing the amplifier the low ohm load. To give you an example of running multiple subs off of a mono amplifier, you can take 2 subwoofers that are listed as “Dual 4 Ohm” and parallel the voice coils down to 1 ohm, because ohms law states that two 4 ohm voice coils in parralel will equal 2 ohm, and if you were to parallel the voice coils on each subwoofer to 2 ohm, and then parallel the two individual subs together, you will have a final ohm load of 1 ohm, because two 2 ohm subs in parallel equals 1 ohm.

Now I know it can be confusing at first and that why it is important that you understand Ohms law, series, parallel, channels, voice coils etc. before buying an amplifier or subwoofers that may not work with some of your existing equipment.

Subwoofers usually come in SVC (single voice coil), DVC (dual voice coil) or even QVC (Quad voice coil). To match subwoofers with amplifiers, you need to know how many voice coils your subwoofer/s have and what their impedance is. You don’t want to buy an amplifier that is 1 ohm stable if you have a pair of subwoofers that are dual 2 ohm voice coils, because you will either be showing the amplifier too low of an ohm load, or you will show the amplifier a higher ohm load that will cause the amplifier to not put out as much power as it is capable of. With a pair of dual 2 ohm voice coils, and using Ohm’s law, you’ll see that your coils can be paralleled down to either .5 ohm (too low for that particular amp) or a 2 ohm load (to high to get the most power out of the amp). So you would want to look for an amplifier that puts out most power at either .5 or 2 ohm.

Subwoofers also come in different sizes and for different applications. Sizes usually vary from 8″ up to 18″, but some manufacturers build subwoofers even bigger then that. Some of the applications you may hear are “Sound quality”, “SPL” or “SQL”. If a subs is listed as an SQ or Sound Quality sub, it means that the sub has particularly good sound quality, and may not be the most suited subwoofer for an “SPL” setup. SQ subwoofers usually have a lower power handling capability when compared to “SPL” subwoofers. SPL subwoofers are usually designed for one thing and that is to handle lots of power and get extremely loud. These subwoofers are usually used if you’re competing in sound off competitions where the loudest system wins. They may not have particularly good sound quality. If you want a mixture that will give you good sound quality, and have enough uuummmph for the occassional sound off competition, an SQL sub might be what you’re looking for.

To determine what type of subwoofer you need or want, it might be a good idea to frequent some of the car audio forums on the internet and ask some of the seasoned competitors.

Head units or In-Dash CD players also come in a variety of flavors. Some offer great expansion capabilities for future expansion. For example, some of the lower end units may only have a set of front and rear outputs, whereas some higher end units have front / rear / subwoofer / auxillary outputs. The higher end head units also tend to offer the user more control of the functions, for example, you can control the cutoff or crossover frequency to your subwoofer outputs and adjust them accordingly. Looking at the specs for the head unit should give you enough information, but if it’s not listed in the description, you should always ask the seller and not assume.

I will write more and try to get even more specific as time permits, but for now, happy shopping and don’t be afraid to ask the seller questions.