Posted on Thursday, 13th July 2006 by Auw Jimmy
I have just chat with my friend. He asked me about Active and Passive Crossover System. After gave him some brief explanation, I decided to write down about this topic on my blog. I think this is very interesting topic and most of people often ask about it (usually at car audio). I decide to split this topic to more than one article. The first (this one) should discuss about basic Crossover. Please check other links at Audio categories.
So, enjoy it. This is also my first online blog (I have some of them, but offline).
So now, “What is crossover or X-Over or XO?”
Simply to say, a crossover is used to divide the frequency spectrum. It will cross the frequency to certain range, depends on the needs of the designer.
A passive crossover sample from www.norh.com. The basic idea of a all type of passive crossover is capacitors, coils, and resistors. So, it’s a matter of how you design the crossover. Active crossover uses active component like opamp. We will discuss active crossover in different article.
Why does someone invent crossover?
Human’s ear theoritically can listen frequency from 20-20000 Hz. Some people who usually claim him/herself as and audiophile (or a golden ears) usually can listen beyond this normal range. Sadly, there’s no single driver speaker can produce this wide range perfectly. So, it is very common to see a box of speaker uses two or more drivers (one usually a big driver and the rest should be smaller size). By using more than one driver, each driver should produce only at certain frequency (at their optimum frequency). So, each driver has their own “working range”. Who is in charged to divide the frequency? The crossover!So, actually the crossover is invented because we are going to use more than one drivers in a box of speaker. Crossover will divide this frequency (from 20-20000 Hz) into 2 or more segment. Each segment should be send to each driver. This division will refer to x-way. So, if the crossover divides the frequency into two segment, we call it a 2-way configuration. If the division is 3, so we can call it a 3-way configuration and vice versa.
Common “jargon” in crossover is low pass filter, bandpass filter, and high pass filter.
It is easy to know the function from each filter.
* Low pass filter is used to pass the low frequency (and reject the high frequency).
* High pass filter is used to pass the high frequency (and reject the low frequency).
* Band pass filter is used to pass frequency from a certain range, let’s say from 1000 Hz to 5000 Hz. Frequency less than 1000 Hz and more than 5000 Hz will be rejected.
Every crossover has something called “cut-off frequency”. At this frequency, the volume should be -3 dB lower than before, and getting lower lower and lower after this frequency. Cut-off frequency often called F3 (I will let you know why, but later).
Every crossover also has something called “slope” or “order”. Slope will affect the roll-off frequency for crossover. Let’s say your slope is 6 dB. It means that every octave after the cut-off frequency, the volume will be reduced by -6 dB. Beside slope, we are used to use word “order” to inform the slope. First order crossover has 6 dB slope, second order has 12 dB, third order has 18 dB, fourth order has 24 dB, and vice versa.
Confuse? Yeah, I had same feelings when learning crossover at first time. But let me give you some “easy to understand” example.
You have designed a 2-way 1st order crossover (of course it will has 6 dB slope) with cut-off frequency at 3000 Hz. So, what is this all about?
* 2-way crossover means this crossover has low pass filter and high pass filter.
* 1st order crossover means the slope is 6 dB.
* 3000 Hz means the cut-off frequency.
For the high pass filter, the volume at 3000 Hz is 0 dB, at 1500 Hz (first octave) is -6 dB, at 750 Hz (second octave) is -12 dB, etc.
For the low pass filter, the volume at 3000 Hz is 0 dB, at 6000 Hz (first octave) is -6 dB, at 12000 Hz (second octave) is -12 dB, etc.
Actually, at 3000 Hz the both filter is not at 0 dB, but at -3 dB (so this is why we call the cut-off frequency as F3, it means that at this frequency the volume is -3 dB). But when the low and high pass filter is combined (the -3 dB summation of high and low pass filter) will make the frequency pulled back to 0 dB.
Here’s is a sample of low and high pass filter with cut-off frequency at 3000 Hz and 1st order (6 dB slope). The black line refers to the summation of both low and high pass filter. The line starts at 90 dB (both low and high pass filter). At 87 dB (the F3/cut-off), both lines meet, so we call this as Crossover cut-off or F3.
At 1500 Hz (first octave), the high pass filter should be decreased to around 84 dB (hey it’s a 1st order Crossover or 6 dB slope decrease per-octave), at 750 Hz (second octave) should be at 78 dB, etc.
At 6000 Hz (first octave), the low pass filter should be decreased to around 84 dB (hey it’s a 1st order Crossover or 6 dB slope decrease per-octave), at 12000 Hz (second octave) should be at 78 dB, etc.
Active and Passive Crossover
In this world, there are two types of Crossover. One is active crossover, and one is passive crossover. Active crossover uses active components (opamp/transistor), while passive crossover uses passive components (capacitor, coil, resistor). One important thing to be noted is active crossover’s position is before the amplifier, while passive crossover is after amplifier.
So, active crossover will work in line level (low signal). It divides the frequency spectrum then sends it to the amplifier to be amplified. Active crossover’s cut-off frequency also easy to be adjusted (via analog potentiometer or digitally adjusted). This makes active crossover is very popular among latest digital audio equipment.
Passive crossover work in high level signal. They are positioned after the amplifier. So, they will process the high level signal into two or more frequency band. Passive crossover is hard to be adjusted. Their value is based on the component’s value. So, if you want to change the cut-off frequency, you have to change the component’s value. Not and easy job, eh?
Why do people choose active crossover over passive one?
- Easy to adjust the cut-off frequency.
- Active circuit, produce no loss when processing the source signal.
- Almost unlimited flexibility for higher octave filter (more than 24 dB/octave).
Why don’t people choose active crossover over passive one?
- Requires one amplifier per crossover. If you want to have 2 way active crossover (low pass and high pass), then you will need two amplifiers per channel (four amplifiers for stereo). If you want to have 3 way active crossover (low pass, band pass, and high pass), then you will need three amplifiers (six amplifiers for stereo). Why? Because this type of crossover is positioned behind the amplifier. So, each amplifier should amplify their own band sent from each active crossover only (low/mid/high frequency). The principe is: Divide then amplify (so the amount of amplifier needed is as much as the divided band).
- Quite complicated to be built for you who don’t know electronics.
- People believe this type of crossover has more digital sound (too accurate?).
Why do people choose passive crossover over passive one?
- People believe this type of crossover has more analog sound.
- Easy to design (less components used).
- One amplifier per channel is sufficient (two for stereo configuration). Because this type of crossover is positioned after the amplifier, so you only need one amplifier. The crossover will separate the way (2 or 3 way) then sends it to the correct driver. The principe is: Amplify then divide (so you only need one amplifier to amplify the signal, then after that the passive will divide the frequency). Off course you can use one amplifier per band (like active crossover, so called bi-amp), but this is not a mandatory.
Why don’t people choose passive crossover over passive one?
- Passive circuit. The more components used, the more they will “eat” your source signal. Your 100 Watts amplifier will no longer rated at 100 Watts after passing the passive crossover. This should be no problem for high rated amplifier. But how if I use single ended tube amp (with power output rated at 1-5 Watts only)? The best solution is: Use less components (simple design, 1st or 2nd order only), so they will not cut your power in a big margin.
- Hard to adjust the cut-off frequency. You need to alter the component’s value manually (by replacing them one by one).
- Limited flexibility for higher order crossover. Higher order of passive crossover needs higher value of components, and higher value means higher price.
That is the simple Crossover Tutorial. I’ll continue later with more detailed and deep discussion. Hope this doesnt take your brain’s resources too much