[Basic] Active vs. Passive: An attempt by me to explain

[mike dressure]

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I’ve noticed there have been a lot of questions regarding the differences between active and passive crossovers. I am going to attempt to explain the differences between the two and also the advantages and disadvantages of both. I’m not going into a very detailed technical explanation, I just want a basic explanation for the people out there who don’t know the difference between the two.

Just some terms to know before reading:

Low pass filter

A low-pass filter is a filter that passes low frequencies well, but attenuates (or reduces) frequencies higher than the cutoff frequency.

High Pass filter

Exactly the same as a low pass filter but passes high frequencies.

Bandpass Filter

Is a combination of a low pass filter and high pass filter.

Octave

An octave is doubling the frequency. For example going from 50 Hz to 100 Hz is one octave. Going from 50 Hz to 200 Hz is two octaves, ect…

Crossover slope or roll off (reference Basic Car Audio Electronics)

Crossover rolloff (or slope) describes the rate which the audio level increases/decreases per octave as the frequency increases/decreases. Usually a crossover slope is given as 6db/octave, 12db/octave, 18db/octave, 24db/octave, etc. If you want to see a visual of this here is a great picture:

Cyan = 6dB/octave
Red = 12dB/octave
Green = 18dB/octave
Violet = 24dB/octave


As you can see the different types of slopes and also notice that the crossover point is 1k.

Crossover Point

This is where frequency where the roll off or slopes starts to begin. As with the picture from above the crossover point is 1 kHz roll off begins at that frequency.

2 way crossovers

These crossover have two difference types of crossovers: Low pass filter and high pass filter.

3 way crossovers

These crossovers have three different sets of crossovers: Low pass filter, bandpass filter and high pass filter.
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Active crossovers

A key ingredient to an active crossover is that the filtering now occurs before amplification and now hence needs an external power source. This is a basic diagram of what it is:

Head Unit --> Crossover --> Amplifier --> Speakers

This is compared to a passive crossover in which the diagram is:

Head Unit --> Amplifier --> Passive Crossover --> Speakers.

Now basically what an active crossover gives you the ability to do is choose the crossover points and slopes for your speakers rather than a predetermined crossover slope and points determined by a passive crossover. This is a huge advantage because we all know that each car has different characteristics that can alter a speaker’s response. An active crossover can be used to make up for these discrepancies in a car. Here’s an example.

On my first set of components I bought the Rainbow SLCs that came with its own set of passive crossovers. According to their specification sheet the passive crossover is designed to have a crossover point of 4.5 kHz with a 12db/octave slope. I can’t change this at all to adapt to the car environment. But with my Eclipse 8455, which has a built in 3 way crossover I now can choose a crossover point of 4khz for the tweeters at 12db/oct and a bandpass filter for the mids at 3khz @ 12db/oct and 80khz @ 12db/oct. Not to mention I low passed my sub at 80 Hz @ 12db/oct.

I can change these points at any time I want. If I feel now that the mid performs better when low passed 2.5 kHz @ 6db/oct then I will change it to that slope and point. A huge advantage is that you can change these points and slopes to your liking instead of having some company telling the speakers where it sounds best.

Another thing about active crossovers is that each speaker will need its own independent channel so you can't go active by going off of a two channel amp. in order to run a two way active frontstage you must use a 4 channel amplifier or a combination of two 2 channel amps. Why? well if you have your mids and tweets running off a two channel and trying to go active then you can't cross the mids and tweets correctly. So its important to have a channel per speaker to run active.

One thing about going active is that it takes a ton of time to tune. If you are a type that likes to set it and forget it (using passive crossovers), going active isn’t for you. Going active takes a lot of time and effort because you are constantly changing crossover points and slopes to fit your needs and takes a lot of time to find that right combination. Not only that but it takes forever to get your staging and imaging exactly the way you want. Staging and imaging is a totally different animal as well.

Just a quick note on how to choose crossover points and slopes, just simply look at a speaker’s frequency response graph and it can give you a rough idea where to cross everything. Every manufacture has these graphs, so look at them and it can give you a rough idea on where to choose your points and slopes.

So that’s my explanation. Kinda half assed because I’m doing this at school, lol. So if you knowledgeable people who know more about going active please feel free to correct my mistakes.

great thank's go to alphakenny1 for this need to know information