Advantages & Disadvantages of Active & Passive Crossover Systems
Active Crossover
Advantages:
Active loudspeakers are always included in the very good systems. There advantages over conventional systems are something with no doubt. The following text numbers the major advantages of active loudspeakers and a small description of each. These are also mentioned by Vance Dickanson in his very successful books. Recommended for everybody from those who want to know more about speakers till the serious speaker builder.
1. Increased dynamic range.
2. Improved transient response & damping factor.
3. Reduced IM distortion.
4. Easier amp load (constant impedance).
5. Easier control of sensitivity differences, phase, time delay and frequency tailoring
6. Better overall sound quality.
7. Electronic crossover (required) allows for easy adjustment of crossover frequency to optimize system performance.
8. Non linear and lossy effects of passive speaker crossover networks are eliminated (big plus for low frequency drivers).
9. Electronic crossover allows excellent performance with very high filter slope cutoff (keeps unwanted energy out of speakers that can't deal with it).
10. If one amp clips, the other amps in the system are not impacted (resulting in less degradation of the overall sound).
11. Slight increase in audio output capability as compared to one large amp of equal power driving a full range speaker.
12. Audio level to each frequency band can be easily adjusted (to compensate for different speaker sensitivities).
13. Direct control of each driver by its own amplifier.
14. Easier impedance load on the amp.
15. No loss of power or damping factor.
16. Reduced clipping. If a clipping occurs, only one driver/amp is affected.
17. Crossover works at line level maintaining its design properties.
18. Each amp deals with only a specific bandwidth.
19. Reduced harmonic distortion.
20. Reduced intermodulation distortion.
Low intermodulation distortion (IMD)
The IMD is low because the power amplifiers work in a smaller bandwidth. IMD is produced from different frequencies that modulate each other producing new frequency products that don't exist in the original signal. The amplifiers work easier and cant affect the other amplifiers. Also clipping from low frequency overloads of the amplifier in multi-way active systems is not present because here only the loudspeaker can limit the signal.
Larger Dynamic range
A 60W and 30W amplifier in an active system will clip at the same level as a 175W amplifier in a passive system. When a low and high frequency signal are amplified at the same time there is at high levels a sink of the maximum level because the low frequency signal makes the power supply voltages lower due to high current need. That's why active systems always sound dynamic and louder than passive systems.
Better Impulse behavior
Great impulses in music reproduction create sudden and high current transients on the way from amplifier to speaker. These impulses from musical instruments hold almost all of the important information about the instruments sound and character and the room acoustics of the recording. So they carry eventually essential information that is needed to reconstruct the spatial sound image of simultaneously playing instruments in a room and this is very critical and sensitive. Any distortion of these characteristics changes the whole reproduction of recorded music so that's why different speakers sound different.
Better coupling between amplifier and speakers
Amplifier and speakers are better coupled directly and introduced resonances from passive crossovers are avoided. When you use a speaker cable with larger cross section the sound get better. This is always noticeable. The reason is that with lower output resistance the amplifier controls the load better. It has a better damping factor as known. Passive crossovers have a larger share of resistance losses then the cable and make the situation worse since the passive components produce distortions.
Better function of the crossovers that work with a constant load
In a passive crossover the voice coil of the speaker is always a part of the crossover. There is a very big variation in the speakers impedance from low dips to high resistances. This means there has to be a correction circuit that equalizes the impedance to more constant. In an active crossover that is placed before the amplifiers the speaker impedance is of no importance. The cut off frequency and the filter characteristics stay constant whatever the input of the amplifier is. This is very good for the DIYer who wants to experiment with the least mistakes and problems. Even a 24 dB/octave filter is practical without any of the known problem of the passive filters (most important coils and high cost of the passive elements).
Better subjectively sound quality
The sound quality of an active crossover is subjectively higher than a high level passive crossover. There is a big difference in working with millivolt and microampere level signals and with high current, high voltage signals. The second had problems like micro phony effect, a lot of solder points, high currents to handle with the least losses and the quality of the passive elements plays a significant role in the overall performance and quality.
Easy level control
When designing active crossovers is easy and there is a great freedom in controlling the different sensitivities of the speakers. When the tweeter plays too loud you can't just put a resistor in the passive crossover to change the level. It will most possibly change the cutoff frequency or other characteristics of the filter. In an active crossover you can control the various levels through trimmers before the amplifiers like the volume control.
Easy design
With active crossovers it is very easy to manipulate phase, time delays, resonances, amplitude shaping, equalizing etc.
Disadvantages:
1. Expensive: one amplifier is needed for each frequency band, also an electronic crossover is required.
2. Overall system is quite a bit more complicated compared to a single amp full range system.
3. Must be careful of tweeters (there should be some kind of DC block to protect tweeters from low frequency "thump" at amp power up/down).
4. If it is a portable system a lot more cables to hook up (and a lot more stuff that can go wrong)!
5. If not properly wired, ground hum and other noise can be a problem due to so many components to connect up.
6. Residual noise from X/O, less of an issue for digital processors.
7. Greater susceptibility to EMI (electromagnetic interference), and RFI (radio frequency interference).
8. Multiple amps & cables, more complex setup.
The only disadvantage is that the cost is a lot higher than a conventional system since it needs more discrete amplifiers plus an extra active device (the active crossover) . This is relative though. First of all when one is convinced to use a system like this, especially a DIYer I don't think the additional cost will matter so much. DIYers can make less compromises because it more flexible for them to design and build and spend money where it really matters. Cost is very important though in commercial equipment where low cost is a major matter. Apart from that there is one thing that can make the cost lower. Each amplifier as stated before can be a lot smaller than the one very big one needed in a passive system. As conclusion I think it is worth it. The only thing I want to propose here are that no op-amps re used anymore. The sound bad and a discrete op-amp is easier to design. There are a lot of ready ones out there you can use like from the Nelson Pass article or from Erno Borbely.
Passive Crossover
Advantages:
1. Plug & Play simplicity.
2. One amp, one cable, done!
Disadvantages:
1. Passive networks may be expensive if executed properly. They can be bulky and cause power loss.
2. Passive crossover networks are not only frequency-specific, but also load impedance specific. This prevents interchangeability of crossover filters or filter components with speaker systems of different impedances and designs.
3. An ideally working crossover filter including load impedance equalization networks, can be very difficult to design as passive electronic components behave and interact in complex ways.
4. Back EMF (electro motive force) goes back into the X/O, interferes with the input signal from the amplifier.
5. Passive X/O buffers the amplifier from the drivers resulting in loss of damping, loss of direct amplifier control over the drivers.
6. Loading effects, inductors, magnetic coupling, larger Cs, and parameters less adjustable.
7. Passive network wastes power, lowers efficiency, requires higher wattage amplifier to compensate.
8. Differing impedance of various drivers and the resulting phase shifts from the X/O present a difficult load for the amplifier, especially 1st order X/Os.
9. X/O properties and accuracy varies with power and temperature resulting in shifting properties and inconsistent linear response.
10. Low order X/O reduces phase & time shifts but introduces other issues. Greater frequency sharing between drivers and higher strain on drivers due to wider bandwidth demands increases distortion, both THD and intermodulation, induces interference patterns, amplitude irregularities, driver resonances, cone breakup, and hampers off-axis response.
11. High power draw in a specific frequency range, usually the bass, may cause amplifier clipping and possible damage to the woofer, midrange, or most likely, the tweeter. The amp has to deal with the combined complex impedance load and power draw.
