Active vs. Passive: An attempt by me to explain

[alphakenny1]

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 www.bcae1.com)

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.

[alphakenny1]

I’m going to do a copy and paste of an explanation of what a passive crossover is that squeak9798 wrote in the sticky “Speaker FAQ.”

Passive Crossovers

Ok, passive crossover networks are unpowered crossovers (i.e. no external power source) that split the frequencies between the speakers in a component set. They send the higher frequencies to the tweeter and the lower frequencies to the mid(s). They accomplish this by using a combination of capacitors and coils to create certain crossover points and slopes. They are wired in line with the speakers, between the speakers and the amplifier. The amplifier's output is connected to the passive's input, then the mid(s) and tweeter are connected to the passive's output.

Now, when two speakers (a mid and a tweeter) are on a passive crossover, the mid and tweeter are not wired in series or parallel. Two 4ohm speakers (a mid and a tweet) do not make a 2ohm load or an 8ohm load. Two 4ohm speakers on a passive crossover network create a 4ohm load on the amplifier. Two 8ohm speakers on a passive xover create an 8ohm load. A 4ohm mid and 8ohm tweeter on a passive xover creates a 4ohm load for the mid's frequencies and an 8ohm load for the tweeter frequencies. Reason for this has to do with the fact that passive's are based on frequency distribution and not power distribution.

When you have components sets with passive crossovers, the power from the amp is not split between the speakers. If you have a 70 watt amp, then each speaker is going to receive 70 watts (assuming all speakers are the same impedance). If you are sending 70 watts @ 4ohm to the component set, and the mid is 4ohm and the tweeter is 8ohm, then the mid will receive 70 watts but the tweeter will only receive 35 watts.

This probably isn't a very technical explanation….but it gets the point across none-the-less Let's pretend theoretically that we are running a 70 watt @ 4ohm amp full range. That amp (theoretically) puts out 70 watts at all frequencies at 4ohms, correct?? Now, pretend that we are running that amp to a component set (with all 4ohm speakers) through a passive crossover with a crossover point of 3500hz. So, we are taking that full range signal from the amp and splitting up the frequencies between the mid and tweet at 3500hz. Now, since we are splitting the frequencies and nothing else, there is still going to be 70 watts worth of power at all frequencies below the crossover point and at all frequencies above the crossover point, just the same as there was before we split the signal (since it was putting out 70w at all frequencies).

Just a summary basically a passive crossover built by a certain company has a set high pass filter for the tweeters and a set low pass filter for the mids. It’s set in stone. You can’t change it. Another key thing is the passives crossovers occur after amplification.

Passive crossovers are for a person who likes to plug and play and that’s it.

[alphakenny1]

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. Any questions you noobs ?

[jujumantb]

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.

BEEP! The crossover point is actually where the rolloff reaches -3dB

[alphakenny1]

BEEP! The crossover point is actually where the rolloff reaches -3dB

like i said, some of my material might be a lil off but thanx.

[AcidicDreams]

how hard is it to make a passive xover for yourself?

[squeak9798]

how hard is it to make a passive xover for yourself?

Depends.

Construction wise; not hard at all.

Design wise....depends on how much work, effort, research and time you want to put into it.

Doesn't take long or much time/effort to whip together a generic 2nd order filter.

To really do it right, and build a great crossover optimized for the speakers you are using, where you are mounting them, etc....it really takes some time, knowledge and measurement equipment that not everyone has access to.

[Eugenics]

good read

[FoxPro5]

Nice work Pete! Now please explain why the differential harmonic relay module circumvents the capacitor limitation on the network reponse curve

[alphakenny1]

Nice work Pete! Now please explain why the differential harmonic relay module circumvents the capacitor limitation on the network reponse curve

[FoxPro5]

Sorry that made no sense and I totally made it up...just wanted to keep you on your toes. Good write up though. We need to sticky that!

[helotaxi]

Or explain how a Besel filter is needed to account for impedance rise at resonance to allow for a smooth crossover frequency responce...

[AcidicDreams]

Depends.

Construction wise; not hard at all.

Design wise....depends on how much work, effort, research and time you want to put into it.

Doesn't take long or much time/effort to whip together a generic 2nd order filter.

To really do it right, and build a great crossover optimized for the speakers you are using, where you are mounting them, etc....it really takes some time, knowledge and measurement equipment that not everyone has access to.

hmmm I think active would be easier.... maybe down the road when I have more free time...

[thch]

BEEP! The crossover point is actually where the rolloff reaches -3dB

well, its really application specific. when you design generalized filters you assign goals. you can define a filter by an arbitrary cutoff point, but for most things, its assumed to be -3dB. The biggest example is the Linkwitz Riley crossover which has a -6dB cutoff.

@OP -- please add info regarding the dB scale. you have information about octaves, but not dB.

a nice guide, not cumbersome. at this point i like to classify crossovers into categories:
1.) preamplifier, active analog. (like amplifier crossovers)
2.) preamplifier, passive analog. (fmods)
3.) preamplifier, digital (HU filters using DSP)
4.) postamplifier, electrical (passive crossover)
5.) postamplifier, acoustic (using acoustics to give a desired response).

maybe i'll add some generalized filter stuff, because it comes up ALL the time:
most filters are defined by a combination of 1st (-6dB/oct) and 2nd (-12dB/oct) networks. 1st order filters are not resonant and cannot really favor one specific frequency more then other. 2nd order filters can either be two 1st order filters (Q < 0.5) or can be resonant (Q > 0.5). a high Q* filter is highly resonant, favoring one frequency more then others -- possibly even more then other frequencies in the passband.

4th order networks (-24dB/oct) can thusly be made from 2 2nd order filters. typically it will be a filter with a higher Q (maybe 1.6) and a filter with a lower Q (maybe 0.6). this ends up not favoring any specific frequencies, but also gives a sharp cutoff.

--------------- main point ---------------
This is important becuase EVERYONE asks if you can combine filters. YES you can. BUT not with the intended results. The final filter will be the product of all the filters. thus if filter A is -10dB at 80hz, and filter B is -0.1dB at 80hz, the final result will be -10.1dB @ 80hz. if both filters A and B are -3dB at 80hz, the result will be -6dB @ 80hz. if you were defining a cutoff at -3dB, then the resulting filter's cutoff is not 80hz.

further, just because you have maybe a 4th order filters (made from 4 1st order stages) doesn't mean it will be good. the slope WILL eventually make it to -24dB/oct, but it may not change from -0dB/oct to -24dB/oct in a small bandwidth. i assume this is why some concider fmods to be -6dB/oct filters.


New developments in crossovers are there in digital filters as well as combining bandstop filters with normal filters. the bandstop creates a high slope but only in a small band. the argument is that you can get a filter that is -60dB/oct, and always at or below -30dB in the stopband. CDT and NTM use these filters. NTM has a patent on applying the hourglass filter to audio systems. I also had this same idea, but never finished working on it.

* -- Q refers to the "quality" of a resonator. basically how well it favors a single frequency. a high Q filter may not be desired if the filter is to affect a wide bandwith.

[thch]

Or explain how a Besel filter is needed to account for impedance rise at resonance to allow for a smooth crossover frequency responce...

actually, i've never heard that term used.

bessel filters are filters that have coefficients based upon bessel polynomials. the result is a filter where the phase response is fairly linear -- giving a simple delay that is constant for all frequencies. (actually not true, but the bessel filter is the best for constant delay).

the speaker has an impedance that can affect the passive crossover. there exists a few solutions to the common problems.

the easy solution is to place a resistance in parallel with the speaker. if you place a 4ohm resistor in parallel with a 4ohm speaker, the impedance will be between 2 and 4ohms. but this uses a lot of power (the resistor gets equal or more power then the speaker).

the other solution is to use a resonantor to effectively place a resistor in parallel with the speaker, but only at one frequency. the idea is to place the resistor in parallel with the speaker only when the speaker has a high impedance and thus to remove the speaker's high impedance, at least as far as the amp/crossover is concerned.

speakers also have an inductance. so a "zobel" network is sometimes added in parallel with the speaker to basically place a lower and lower value resistor in parallel such that the parallel combination of the inductive speaker and capactive zobel ends up with a constant impedance.