Passive vs active subs - is one better than the other?

If you’ve been wondering if equalization is a bad thing, and if we should actually want some limits on how low a sub goes, then this video will satisfy your curiosity. Andrew explains three reasons why our subs have amplifiers.

Passive vs active subs - is one better than the other?

Although there are some so-called active loudspeakers that have built in power amplifiers, most speakers that you'll find out there in the world don't have built in amplifiers, they need an amplifier to drive them. Now, the question is, why our subwoofers usually have built in power amplifiers? It's a very good question. With these days where we've got 11 channels maybe more in home theater receivers for instance, why not just have another amplifier channel to power the subwoofer? If you can fit 10, 11 channels you can certainly fit in another one in there if required. Subwoofers are a little bit of an interesting beast. In the early days, they were typically so-called passive, so they needed an external amplifier. In some cases what would happen is, is that you would connect your left and right speaker outputs from your system to the subwoofer, and then connect wires to your two main speakers.

Sometimes the electronics in the subwoofer there was a passive system to roll off the base from your main speakers, but basically those early subwoofers didn't have amplifiers. So what's changed? Why do we typically see a power amplifier like this one in most subwoofers? Well, there's a number of reasons. If you look at what we're trying to do in a subwoofer, without having the box be the size of a refrigerator and have a massive woofer, normally in a smaller cabinet like this one with an eight inch woofer, 10 inch woofer, whatever it happens to be in a aesthetically pleasing cabinet size, you're not going to get excellent low frequency extension. It's just physically impossible, if you want to have some semblance of efficiency so that the subwoofer can actually have some output and deliver that output. What we need to do is we need to apply some equalization, and the equalization that's in some circles a bad word when it comes to high-end audio.

Nobody uses equalizers. You don't want to do that, that's just going to mess up the sound. Well subwoofers like I said are a little bit of a different beast, and we can put filters and electronic systems in place before the amplifier to even out the frequency response of the subwoofer, and extend the base. So that let's say without an EQ circuit of some sort, this subwoofer may only go down to let's say 50 Hertz naturally, and have any significant output. But with some equalization, we can easily get it down to 30 Hertz. Now, another thing that's critically important along with the EQ, is some form of compression or limiting. You have to remember that subwoofers when we're dealing with low frequencies, in some cases, very low frequencies, these are the most stressful areas of the audio band on any component. We're asking drive units to have high excursion to produce those low frequencies.

We want to in many cases just have one small subwoofer in a massive home theater system, and have it be able to deliver mind-blowing explosions and all of those things. Along with the equalization that's going to tax the driver more because you're boosting some frequencies that it's naturally should be rolling off at, those excursions may even get higher. The one thing that you don't want to happen is you're watching your favorite movie and there's a big explosion. You don't want to hear the subwoofer making horrible noises because some woofer's hit its mechanical limit, it's hit the backstop or whatever it makes a clank or a bang or something like that, that isn't part of the signal that it's being sent.

What tends to happen in subwoofer amplifiers is we have some form of a limiter to make sure that we can squeeze every last ounce of either driver excursion or amplifier power out of the system, and then stop everything past that point, so that you don't get mechanical noises and it prevents the chance of you actually damaging something, whether it's the amplifier or the woofer itself that's in the subwoofer box. Those are the two main reasons that we tend to find powered subwoofers as the norm these days, so that we can get flat base response, deeper extension for a given size cabinet and woofer combination, and also apply some protection in the form of limiting or compression or compressors, to make sure that you can get all the dynamics from your special effects and in your music without fear and risk of damaging anything or making any unwanted mechanical noises. Thanks a lot for watching, and if you have any questions please comment below (silence).

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