Active versus Passive
Home What are they? Active vs Passive Adjustable Crossover Analog or Digital? Frequencies, Slopes Filter Topologies FR Correction   < Tech << T-Line Speakers		There are a lot of opinions on this subject and we will try and summarize some of them. Some people are strongly against active crossovers. Their main reason: less active components in a signal path - less signal degradation - better sound quality. Well, they are correct to some extent. Everything depends on the quality of the crossover itself and an ability of the loudspeakers to reproduce sound with enough definition to hear inserted sound degradation. Pure arithmetic is not working in such a case. You can't say: here we have 20 Integrated Circuits and that is equivalent to 2000 transistors, therefore the sound is not as good as before this device was installed. Usually such statements are made by people without a technical background. They are forgetting, that during recording or in live concerts, signals from the microphones are going through hundreds if not thousands of Integrated Circuits. Microphone preamplifiers, mixers, compressors, limiters, expanders, noise gates, distribution amplifiers, equalizers, and power amplifiers, are always present in a signal path. If we compare the total amount of distortions inserted by the passive crossover (especially at high power levels), to the total amount of distortions inserted by active crossover, we will find that passive crossover is not as good as active crossover. Even passive components can produce a lot of unwanted effects. As an example, we can take the inductor (coil), which is always present in passive crossovers. At high power levels inductors are saturated and produce a large amount of harmonic distortions. Also, inductors always have internal resistance and this resistance inserts losses. With passive crossover, you always loose 1-2 dB of Sound Pressure Level (SPL). It does not sound like much until you calculate this loss in equivalent power losses at the amplifier output. Passive crossovers always reduce signal level. They do not have any amplification of their own and can correct frequency response and other discrepancies of the drivers only by inserting losses. The more complicated the crossover network - the more losses are introduced into the system. One of the important parameters of the power amplifier is the Damping Factor (DF). This parameter gives us a ratio between load impedance and amplifier output impedance at a specified frequency. Amplifiers with lower output impedance have better control over the loudspeaker, because low output impedance works as a shunting resistor across speaker terminals and reduces unwanted cone movements. Passive crossover inserts additional resistance in series with amplifier output and makes high DF useless. With active crossovers, you can get much better separation between different frequency bands. In theory, the same level of separation can be reached with passive crossovers, but in practice, it is not possible because inserted losses will be too high. Not only will losses prevent you from making high order passive filters, but also the cost factor must be taken into account. Large coils and large capacitors are expensive and are not available with high precision values. You have to go to the expense of measuring each component individually and select pairs with close tolerance. The best capacitors available for passive crossovers have +/- 5% tolerance and that is not enough for high quality stereo speakers. Not that 5% of absolute frequency value will be significant for the crossover point, but the difference in frequency value between the left and right speaker will affect stereo image greatly. In active crossovers, you can use components with 1% tolerance and still pay much less, because all parts used in active crossover will cost less than parts used in passive crossover with the same parameters. Even after manual selection of the critical components used in active crossover, your total crossover cost will still be less. The only expense added by the active crossover will be the cost of an additional power amplifier (or amplifiers). That sounds like a major expense until you realize that you do not need as much power for separate drivers as when you have to feed the complete system with passive crossover. If you need a 100 W stereo amplifier for your speakers after you install active crossover, you probably only need a 20-30 W amplifier for your high frequency driver (or drivers). More on this subject you will find in Dennis Bohn's article "Crossovers, Equalizers & Compressors" in LIVE SOUND!